Reducing the Risks of Herbicide Resistance: Best Management Practices

and Recommendations

Jason K. Norsworthy, Sarah M. Ward, David R. Shaw, Rick S. Llewellyn, Robert L. Nichols, Theodore M. Webster,

Kevin W. Bradley, George Frisvold, Stephen B. Powles, Nilda R. Burgos, William W. Witt, and Michael Barrett*

Table of Contents

Executive Summary ??????????????????????? 31

I. Introduction ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? ? ? ? ? ? ? ? ? ? 32

II. Best Management Practices ?????????????????? 34

III. General Challenges to Adoption of Best Management

Practices ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 49

IV. Current Levels of Adoption of Best Management

Practices ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 50

V. Recommendations for Adoption of Best Management

Practices ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 53

Literature Cited ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? ? ? ? ? ? ? ? ? 56

Executive Summary

Herbicides are the foundation of weed control in commercial

crop-production systems. However, herbicide-resistant (HR)

weed populations are evolving rapidly as a natural response to

selection pressure imposed by modern agricultural management activities. Mitigating the evolution of herbicide resistance

depends on reducing selection through diversification of weed

control techniques, minimizing the spread of resistance genes

and genotypes via pollen or propagule dispersal, and

eliminating additions of weed seed to the soil seedbank.

Effective deployment of such a multifaceted approach will

require shifting from the current concept of basing weed

management on single-year economic thresholds.

Programs for herbicide-resistance management must consider use of all cultural, mechanical, and herbicidal options

available for effective weed control in each situation and

employ the following best management practices (BMPs):

1. Understand the biology of the weeds present.

2. Use a diversified approach toward weed management

focused on preventing weed seed production and

reducing the number of weed seed in the soil seedbank.

3. Plant into weed-free fields and then keep fields as weed

free as possible.

4. Plant weed-free crop seed.

5. Scout fields routinely.

6. Use multiple herbicide mechanisms of action (MOAs)

that are effective against the most troublesome weeds or

those most prone to herbicide resistance.

7. Apply the labeled herbicide rate at recommended weed sizes.

8. Emphasize cultural practices that suppress weeds by using

crop competitiveness.

9. Use mechanical and biological management practices

where appropriate.

10. Prevent field-to-field and within-field movement of weed

seed or vegetative propagules.

11. Manage weed seed at harvest and after harvest to prevent

a buildup of the weed seedbank.

12. Prevent an influx of weeds into the field by managing

field borders.

The long-term economic benefits of avoiding additional

costs associated with managing HR weeds are clear.

Nevertheless, widespread adoption of these BMPs must

overcome several real barriers. In particular, growers’ focus

on immediate economic returns must be overcome as well as

their beliefs that the evolution of herbicide resistance in weeds

is unavoidable and that continued availability of novel

herbicide technologies will solve the problem. There is, at

present, no single database collating information on weed

management practices employed by U.S. growers, so the

extent of the adoption of BMPs for HR weeds must be

inferred by combining data from multiple sources. Available

survey data show that, although many U.S. soybean [Glycine

max (L.) Merr], corn (Zea mays L.), and cotton (Gossypium

hirsutum L.) growers employ at least some BMPs, a significant

proportion of growers are not practicing adequate, proactive

herbicide-resistance management. Two key recommendations,

in particular, must be more widely implemented: diversifying

weed management practices and using multiple herbicide

MOAs. Growers need to be educated about MOAs and be

made aware that discovery of new herbicide chemistries is rare,

that the existing herbicide resource is exhaustible, and that

indiscriminate herbicide use leading to rapid evolution of HR

weeds may result in the loss of herbicide options for all.

To address the increasingly urgent problem of herbicide

resistance, we make the following recommendations:

1. Reduce the weed seedbank through diversified programs

that minimize weed seed production.

2. Implement a herbicide MOA labeling system for all

herbicide products and conduct an awareness campaign.

3. Communicate that discovery of new, effective herbicide

MOAs is rare and that the existing herbicide resource is

exhaustible.

DOI: 10.1614/WS-D-11-00155.1

* First, second, and tenth authors: Associate Professor, Associate Professor, and

Professor, Department of Crop, Soil, and Environmental Sciences, University of

Arkansas, 1366 West Atheimer Drive, Fayetteville, AR 72704; third author: Giles

Distinguished Professor, GeoResources Institute, Mississippi State University,

Mississippi State, MS 39762; fourth author: CSIRO Sustainable Ecosystems,

Private Bag 2, Glen Osmond, South Australia 5064, Australia; fifth author:

Cotton Incorporated, 6399 Weston Parkway, Cary, NC 27513; sixth author:

Research Agronomist, Crop Protection and Management Research Unit, USDA–

Agricultural Research Service, Tifton, GA 31794; seventh author: Assistant

Professor, Division of Plant Sciences, University of Missouri, Columbia, MO

65211; eighth author: Professor and Extension Specialist, Department of

Agricultural and Resource Economics, 319 Economics Building, University of

Arizona, Tucson, AZ 85721; ninth author: Professor, Western Australian

Herbicide Resistance Initiative, University of Western Australia, 35 Stirling

Highway, Crawley, WA 6009, Australia; eleventh and twelfth authors: Professor

and Professor, Department of Plant and Soil Sciences, 105 Plant Science

Building, University of Kentucky, Lexington, KY 40546. Corresponding author’s

E-mail: jnorswor@uark.edu

Weed Science 2012 Special Issue:31–62

Norsworthy et al.: Best practices for reducing herbicide resistance N 31
4. Demonstrate the benefits and costs of proactive, diversified

weed-management systems for the mitigation of HR

weeds.

5. Foster the development of incentives by government

agencies and industry that conserve critical herbicide

MOAs as a means to encourage adoption of best practices.

6. Promote the application of full-labeled rates at the

appropriate weed and crop growth stage. When tank

mixtures are employed to control the range of weeds

present in a field, each product should be used at the

specified label rate appropriate for the weeds present.

7. Identify and promote individual BMPs that fit specific

farming segments with the greatest potential impact.

8. Engage the public and private sectors in the promotion of

BMPs, including those concerning appropriate herbicide

use.

9. Direct federal, state, and industry funding to research

addressing the substantial knowledge gaps in BMPs for

herbicide resistance and to support cooperative extension

services as vital agents in education for resistance

management.

In some instances, short-term costs may not favor

implementation of BMPs that provide insufficient immediate

economic benefit, even though their adoption will delay the

evolution of HR weed populations over time. In such cases,

consideration should be given to providing incentives and

expert advice for growers to develop and implement riskreducing weed management plans, following the precedent set

by similar incentives for the conservation of soil and water

resources in agriculture.

Introduction

The evolution of herbicide-resistant (HR) weed populations is a natural response to selection pressure imposed by

modern agricultural management activities. This response in

weeds was predicted by Harper (1956), when he stated ‘‘the

most effective intensity of selection is that which reduces a

population to a small resistant residue which is capable of

rapid reproduction’’ (page 182). The two key principles

implicit in Harper’s observation are fundamental to herbicideresistance management today: (1) reducing the intensity of

selection, and (2) preventing reproduction by the surviving,

resistant individuals. Herbicide applications that eliminate

susceptible weeds before they reproduce create a selective

advantage for any rare, resistant individuals in the weed

population. Reproduction by these escapees then transmits the

resistance trait to their offspring, facilitating their survival

when exposed to the same herbicide mechanism of action

(MOA). Sustained, unidirectional selection pressure, which

occurs with repeated use of either the same herbicide or, in

some cases, the same MOA, favors survival and reproduction

of these resistant biotypes, leading to a weed population in

which resistant plants predominate. Repeating the same

control tactics at a given timing, whether a herbicide

application or a nonchemical control methods, may also

result in the evolution of avoidance mechanisms in a weed

population by selecting for biotypes that have not emerged, or

are outside the optimal growth stage, when control is

implemented (Reddy and Norsworthy 2010). For example,

repeated hand-weeding in rice (Oryza sativa L.) systems in

Asia has resulted in selection for barnyardgrass [Echinochloa

crus-galli (L.) Beauv.] populations that are phenotypically

similar to rice, making them harder to differentiate at

vegetative growth stages (Barrett 1983). Thus, the evolution

of resistance or avoidance is favored when a particular

management practice is repeated without any diversification.

For herbicides, factors that affect how rapidly weed

populations shift to predominantly resistant individuals

include (1) the intensity of the selection (which is a function

of herbicide dose, frequency, and timing of application), (2)

mutation rate and the initial frequency of resistant individuals

in the population exposed to the herbicide, (3) the genetic

basis of the resistance (mode of inheritance, dominance), (4)

life-history characteristics of the weed species (such as annual

vs. perennial life cycle, self fertilization vs. cross-pollination,

fecundity, extent of seed dormancy), and (5) the rate of

reproduction and potential for recruitment of susceptible or

resistant individuals from outside the population (e.g., from

the soil seedbank or by immigration).

Among these factors, producers can influence intensity of

selection and, to a lesser extent, seed production of escapees,

seedbank dynamics, and recruitment of resistant individuals.

Effective management of herbicide resistance—whether

proactive to mitigate resistance evolution or reactive to restore

weed control after resistance has evolved—depends on two

key strategies: (1) reducing herbicide selection pressure and

diversifying weed control techniques so as to minimize

preferential survival and reproduction by resistant individuals,

and (2) minimizing the spread of resistance by means of

pollen movement, seed production, and propagule dispersal.

Strategies that eliminate annual additions of weed seed to the

soil seedbank will be particularly effective, although such an

approach is incompatible with the use of management based

on single-year economic thresholds (Coble and Mortensen

1992; Cousens 1987).

The first incidences of evolved herbicide resistance were

reported in wild carrot (Daucus carota L.), which evolved

resistance to the auxin analog class of herbicides after several

seasons of consecutive use of 2,4-D (Switzer 1957; Whitehead

and Switzer 1967), and common groundsel (Senecio vulgaris

L.), which evolved resistance to simazine and atrazine after

receiving either one or two annual applications of these

herbicides for 10 consecutive yr (Ryan 1970). Since then, 362

confirmed instances of weed resistance have been reported in

198 species (115 dicots and 83 monocots)(Heap 2011b).

More than one-third of these resistant weeds are found in the

United States, primarily in the north-central and southern

regions, where arable crop production is concentrated (Heap

2011b; Vencill et al. 2011). Persistent herbicide selection

across a vast crop area on multiple populations of genetically

diverse weeds has led, and will continue to lead, to the

widespread evolution of HR weed populations (Powles and

Yu 2010). Although herbicides will continue to dominate

weed-control practices because of their economic utility, the

sustainability of herbicide use depends on their diversification

and integration with nonchemical weed-control strategies.

Only by implementing diverse weed-management practices

will herbicides be conserved as a valuable agricultural resource.

Transgenic Crops. Since 1996, glyphosate-resistant (GR)

crops have had a major effect on agriculture, particularly in

the United States, Brazil, Argentina, and Canada (Brookes

32 N Weed Science 12, Special Issue 2012
and Barfoot 2011). The introduction of GR crops in the

United States helped solve a major weed-management

problem that was developing at that time—the evolution of

weeds resistant to the acetolactate synthase (ALS)–inhibiting

and protoporphyrinogen oxidase (PPO)–inhibiting herbicides. However, the introduction of GR crops also prompted

concerns about potential transfer of herbicide resistance to

weed populations via crop-to-weed gene flow (Dale 1994;

Warwick et al. 1999). This has been experimentally

demonstrated for a small number of weed species in North

America that can hybridize with a closely related crop:

examples are described later in this document (see BMP 1).

However, for full introgression of a herbicide-resistance gene

from a crop into a weed population, the initial weed by crop

hybrid must be fertile and repeated transmission of the HR

gene via backcrossing to the weedy parent must take place.

There are few reports of this occurring in the field. Warwick

et al. (2008) described stable introgression of a HR transgene

from GR canola (Brassica napus L.) to weedy birdsrape

mustard (Brassica rapa L.) under field conditions, and Zapiola

et al. (2008) reported persistence of a GR transgene in

populations of weedy creeping bentgrass (Agrostis stolonifera

L.) following accidental pollen and propagule transfer from

experimental plantings of GR ornamental bentgrass (Agrostis

palustris Huds.). However, none of the confirmed field reports

of glyphosate resistance in North American weed species listed

to date by Heap (2011b) have been traced to gene transfer

from GR crops. Although there is a possibility of weeds

acquiring HR genes from crops in a few cases, the

accumulating evidence to date strongly suggests that a much

greater risk is posed by spontaneous evolution of herbicide

resistance in weed populations under selection pressure from

repeated herbicide applications in HR cropping systems. The

wide-scale adoption of any single herbicidal MOA contributes

to the evolution of resistance to that MOA, and the

unprecedented scale of glyphosate use in GR crops has clearly

contributed to the number of GR weeds identified in recent

years (Powles et al. 1997; Vencill et al. 2011).

In parts of North and South America, transgenic GR crops

have dramatically reduced the diversity of herbicides used for

weed management in those crops (Young 2006). Glyphosateresistant crop cultivars now dominate corn (Zea mays L.),

soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum

L.), and canola production in the United States (Nichols et al.

2003). As new HR crops become available, management of

novel HR weeds will continue to be a challenge. Glufosinateresistant soybean, corn, cotton, and canola are now

commercialized, and, in the near future, crops resistant to

the herbicides 2,4-D, dicamba, hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors, and possibly to the PPOinhibiting herbicides are expected to reach the marketplace

(Green et al. 2008). Further, transgenic crops with resistance

to more than one herbicide MOA (i.e., stacked traits) have

also been commercialized in recent times. Strategies for

managing resistant weed populations in the context of these

new technologies must be developed and adopted (Duke and

Powles 2008; Owen 2008; Owen and Zelaya 2005).

Although glyphosate resistance in weeds currently has a

large effect on U.S. agriculture, the discussion and recommendations in this document pertain to all current and future

herbicide technologies. All herbicide technologies, if used

repeatedly, are at risk of losing efficacy because of herbicide

resistance in weed populations. Two recent examples are the

identification of goosegrass [Eleusine indica (L.) Gaertn.]

populations with resistance to glufosinate (Jalaludin et al.

2010; Seng et. al. 2010) and tall waterhemp [Amaranthus

tuberculatus (Moq.) Sauer] populations with resistance to the

HPPD-inhibiting herbicides (Hausman et al. 2011; McMullan and Green 2011); resistance had not previously been

reported to either of these two herbicide MOAs.

Consequence of Overreliance on a Single Mechanism

of Action. Although a number of factors determine the

frequency of resistance events in weed populations, consideration of reported incidents strongly suggests that the single

most important factor leading to the evolution of herbicide

resistance is overreliance on a single herbicide (or group of

herbicides with the same MOA) without using other weed

management options (Heap 2011b). For example, GR rigid

ryegrass (Lolium rigidum Gaudin) and Italian ryegrass

[Lolium perenne L. ssp. multiflorum (Lam.) Husnot]

populations were identified in orchards where glyphosate

had been used continually for at least 14 consecutive yr

(Perez-Jones et al. 2005; Powles et al. 1997; Simarmata et al.

2005). Glyphosate-resistant horseweed [Conyza canadensis

(L.) Cronq.] and common ragweed (Ambrosia artemisiifolia

L.) were confirmed after continuous applications of

glyphosate on GR soybean for 3 and 6 yr, respectively

(Pollard et al. 2004; VanGessel 2001). Likewise, glyphosate

resistance in Palmer amaranth (Amaranthus palmeri S. Wats.)

and common waterhemp (Amaranthus rudis Sauer) was

discovered after only 4 and 6 yr of consecutive glyphosate use

(Culpeppper et al. 2006; Legleiter and Bradley 2008). This

phenomenon is not unique to glyphosate. Prickly lettuce

(Lactuca serriola L.), for example, evolved resistance after 5 yr

of consecutive application of chlorsulfuron (Mallory-Smith

et al. 1990). Other evidence includes, but is not limited to,

2,4-D resistance in wild carrot (Switzer 1957; Whitehead

and Switzer 1967), common groundsel resistance to atrazine

(Ryan 1970), and barnyardgrass resistance to propanil (Carey

et al. 1995).

Costs of Resistance. Because herbicides are the primary

means of weed management in many economically developed countries, herbicide resistance causes greater short-term

cost to manage a weed population, especially with the

absence of new herbicide chemistries. Implicit in the

measurement of the costs of HR weeds are crop yield loss,

reduced commodity prices because of weed-seed contamination, reduced land values, costs of mechanical and cultural

controls, and ultimately the additional expense of alternative

herbicides or cropping systems or both for managing the

resistant weed. Several recent studies have described the

added costs associated with the management of HR weeds. In

the United States, the occurrence of GR horseweed resulted

in a net increase in production cost of $28.42 ha21 in

soybean (Mueller et al. 2005). The additional cost of

managing GR Palmer amaranth in Georgia and Arkansas

cotton production systems was estimated at $48 ha21

(Vencill et al. 2011). In Missouri, Legleiter et al. (2009)

reported similar net cost increases in controlling GR

common waterhemp in soybean. Similarly in Arkansas rice,

the additional cost involved in controlling propanil- and

quinclorac-resistant barnyardgrass was estimated at $64 ha21

(Norsworthy et al. 2007a).

Norsworthy et al.: Best practices for reducing herbicide resistance N 33
Limitations to Implementation of Practices to Prevent

Resistance. Although there is a significant cost to managing

resistant weeds, growers are often hesitant to implement

proactive measures to reduce the risk of resistance evolution in

their fields. A key factor negatively influencing producer

adoption of practices that will mitigate herbicide-resistance

evolution is the expectation that there will be new herbicides

available in the future (Foresman and Glasgow 2008; Llewellyn

et al. 2002, 2007). With invasive plant populations, Finnoff et

al. (2007) demonstrated quantitatively how managers who are

cautious or financially risk averse are less likely to adopt

preventive measures because prevention only reduces the risk,

rather than eliminating it. This perception is perhaps true with

growers weighing the value of prevention vs. control of HR

weeds. For example, in a survey of more than 1,000 corn,

cotton, and soybean growers in the United States, Frisvold et al.

(2009) found that using multiple herbicides with different

MOAs was one of the least-adopted practices for resistance

management, despite this strategy being frequently advocated

by weed scientists as an effective means to reduce the risk of

herbicide-resistance evolution. A number of related factors may

account for this. The overarching reason is that using diverse

MOAs can increase current weed-control costs (Hurley et al.

2009), whereas the benefits of delaying resistance accrue in the

future and are more uncertain. Some growers believe that

mitigating weed resistance is beyond their control, depending

more on their neighbor’s behavior or natural factors (Llewellyn

2006; Wilson et al. 2008). Growers may also believe that

industry will develop new chemistries, reducing the benefits of

resistance management (Llewellyn et al. 2002, 2007). Finally,

even when using diverse MOAs provides short-term returns

comparable to current weed-management programs, growers

will be less certain about the new, diversified programs.

Herbicide resistance-management strategies are typically

implemented reactively when a resistant weed population has

grown to problematic levels and must be controlled. Far less

often are proactive management strategies implemented to

mitigate the evolution of herbicide resistance. Introduction of

new HR crop cultivars can provide options for managing

weed populations resistant to other herbicide MOAs, but

good resistance-management practices must be employed to

avoid evolving resistance to the new herbicide as well.

Rationale for Best Management Practices (BMPs) and

Recommendations. Herbicides constitute the key means of

weed management and are the primary means for reducing

soil tillage practices. Just as conservation of soil, water, and

genetic diversity are fundamental for agricultural production,

conservation of herbicide resources underlies the sustainability

of contemporary agriculture. As such, herbicide molecules and

MOAs (and other pesticides) are scarce resources upon which

the public largely relies for food, feed, fiber, and energy

security. Management practices that mitigate the evolution of

resistance have been identified and will be invaluable in

preserving these vital resources. Resistance management

begins with good agronomic practices, including the

implementation of integrated weed management (IWM)

strategies that use diverse control tactics to reduce the

frequency of herbicide applications and lessen the selection

pressure they exert on weed populations. The BMPs we

propose are discussed below in the context of the currently

available literature documenting their effectiveness.

Best Management Practices

BMP 1: Understand the Biology of the Weeds Present.

Integrating an understanding of weed biology into current

weed-management systems, with emphasis on reducing the

soil seedbank, is paramount for developing sustainable weedmanagement programs (Bhowmik 1997; Walsh and Powles

2007). By understanding weed emergence patterns, length of

developmental stages, fecundity, dispersal mechanisms, and

persistence of weed seed in the soil seedbank, practitioners can

devise a strategy that targets the life stages most sensitive to

management. Knowledge of weed biology in agricultural

systems allows development of weed management programs

that leverage weaknesses in the life cycle of the weed to alter

the competitive relationship with the crop (Neve et al.

2003a,b). Agroecosystems typically contain diverse weed

species, each with different life histories and different

responses to control measures. Effective herbicide-resistance

management combines a variety of chemical and nonchemical

management tactics to diversify selection pressure on weed

populations and minimize spread of resistance genes.

Development of such strategies requires an understanding of

the biology and ecology of weeds and the agroecosystems in

which they occur (Mortensen et al. 2000).

In any growing season, the soil weed seedbank governs the

size and species composition of the weed community (Forcella

et al. 1993). In Iowa, efficacy of weed control in corn is

inversely correlated with population size the previous year

(Hartzler and Roth 1993). Hence, reducing current and

future weed populations is not only a general goal of weed

management, but also a key component in herbicideresistance management. The greater the number of plants

exposed to a herbicide, the higher the probability of selection

for resistance, enriching the gene pool with an increased

frequency of previously rare resistance alleles (Gressel and

Levy 2006).

Long-term herbicide-resistance management, therefore,

requires more than weed control aimed only at minimizing

crop loss in any one season. It is also essential to reduce the

number of individual weeds exposed to herbicide selection, to

prevent propagation by HR weeds, and to reduce the

seedbank in future years. Knowledge of the following

biological characteristics of weed species allows targeted

management of life stages that affect population and seedbank

size, as well as more-accurate assessment of the risk of

resistance evolution.

Germination Requirements and Emergence Pattern. Delayed

emergence or emergence over an extended period can allow

later weed cohorts to escape control measures, especially when

in-crop herbicide applications lack residual control (Neve et

al. 2003a; Reddy and Norsworthy 2010). Late-emerging

weeds may also be subjected to selection by progressively

diminishing doses of soil-applied herbicides that dissipate

slowly, or they can be exposed to sublethal doses of POST

herbicides because of spray interception by the crop, reducing

coverage of the targeted weeds (Zhang et al. 2000). Recurrent

selection under reduced or sublethal herbicide doses over

several generations has been shown to shift the weed

populations toward higher tolerance levels (Manalil et al.

2011; Norsworthy 2012).

Understanding how environmental factors affect seed

germination and emergence is vital to knowing at what time

34 N Weed Science 12, Special Issue 2012
in the growing season weeds are most likely to emerge and the

length of time that weed-control measures must be maintained to provide effective control. Seasonal changes in seed

dormancy influence seed germination and ultimately emergence patterns under field conditions (Baskin and Baskin

1977, 1987; Honek et al. 1999; Omami et al. 1999). Many

weeds have an annual cycle of dormancy, with the

requirements for germination changing over time (BenechArnold et al. 1988; Gallagher and Cardina 1998a,b; Jha et al.

2010; Leon et al. 2007; Norsworthy and Oliveria 2007a,b).

With an understanding of germination and emergence

requirements, emergence in the field has been predicted for

many weeds with great reliability (Forcella 1993; Forcella et

al. 2000; Harvey and Forcella 1993; King and Oliver 1994;

Masin et al. 2005; Norsworthy and Oliveira 2007c). Such

knowledge can be used to determine the appropriate timing

and selection of control measures to prevent the escape of

resistant or potentially resistant weeds. For example, information on Palmer amaranth emergence pattern incorporated

in a resistance-simulation model was vital in revealing the

need for residual herbicides in achieving season-long weed

control and preventing seed production in GR Palmer

amaranth plants (Jha and Norsworthy 2009; Neve et al.

2011a).

The initiation of weed emergence can differ among years

and locations; however, the emergence sequence among weed

species is generally consistent (Cardina et al. 2007; Hartzler et

al. 1999). The initiation of emergence is often predictable

when temperature and moisture availability in the microsite

are known. The base temperature requirement typically varies

among weed species and ecotypes, leading to differences in the

initiation of weed emergence in the field. For instance,

Forcella et al. (1997) observed that the initiation of emergence

of giant foxtail (Setaria faberi Herm.) across years and

locations is closely linked to the time at which the soil

temperature reaches 16 C (i.e., base temperature) at a 5- to

10-cm depth.

Weed communities also differ in emergence periodicity,

with some exhibiting continuous emergence throughout the

growing season, requiring weed management tactics to be

extended well beyond crop establishment (Hartzler et al.

1999). Late-emerging weeds cause less interference with the

crop and experience lower reproductive success than early

emerging cohorts; however, seed production from these lateseason escapees, even at low levels, must be considered when

developing resistance-management strategies (Hartzler et al.

2004; Jha et al. 2008; Norsworthy et al. 2007b). For some

weeds, such as Benghal dayflower (Commelina benghalensis

L.), whose emergence typically occurs after cotton is planted

and growing, manipulation of planting date could be effective.

The benefits of planting cotton before the peak emergence of

Benghal dayflower are twofold: (1) it reduces potential cotton

yield loss by 50% (Webster et al. 2009) and (2) it allows for

cotton emergence before the use of metolachlor, leading to

effective control of germinating Benghal dayflower seed while

avoiding injury to cotton (Culpepper et al. 2004). As such,

knowledge on weed-emergence requirements and emergence

patterns is fundamental to devising management programs.

Reproductive Biology and Life Cycle. Life histories of a weed

species influence the rate of both herbicide-resistance evolution

and spread. Herbicide resistance evolves far more frequently in

annual than in perennial weed species (Heap 2011a). This is at

least partly due to the predominance of annual weeds in

cropping systems with intensive herbicide application, especially in tilled systems where perennial weeds are less common.

However, repeated herbicide application on a highly prolific,

annual weed with short life cycle exposes a large number of

individuals to selection for rare resistance alleles (Gressel and

Levy 2006). In perennial weed species, fewer individuals are

exposed to such selection within the same period, potentially

slowing the rate of resistance evolution.

Breeding behavior (i.e., the degree of self-pollination vs.

outcrossing) is another key factor affecting the evolution and

spread of herbicide resistance. In self-pollinating populations,

dominant or recessive resistance alleles are likely to equally

increase in frequency under selection pressure (Jasieniuk et al.

1996). However, in outcrossing weed populations, the

frequency of dominant resistance alleles will increase more

rapidly than will recessive alleles exposed to herbicide selection

pressure, especially highly dominant resistance alleles that confer

a significant fitness advantage in heterozygotes (Gould 1995).

Weed species with a high degree of outcrossing and genetic

recombination are also at greater risk of evolving multigene

herbicide resistance (Neve 2008; Neve and Powles 2005b).

Herbicide-resistance alleles can be transferred to susceptible

weed populations via pollen movement (Busi et al. 2008) or

exported from resistant populations in seed or other

propagules (Dauer et al. 2007). Such transport also includes

gene flow among overlapping generations of weeds with

extended seed dormancy, potentially allowing resistance alleles

selected for in one generational cohort to be transmitted to the

next. An understanding of pollen and seed biology, factors

affecting pollen or seed viability and longevity, and dispersal

distances, including vectors, is, therefore, essential for

managing gene flow that might lead to resistance acquisition

or export by a weed population (Roux et al. 2008).

Stages within the life cycle of weeds most sensitive to

management must be identified for the most resistant-prone

or troublesome weeds within a field. In general, annual plants

tend to have more weedy characteristics and are more likely to

evolve resistance than are those having longer life spans (Heap

2011a; Sutherland 2004). For annual weeds, seed production

and seedbank persistence is of more importance for

population growth than it is for that of biennial or perennial

weeds. As a result, seed rain by uncontrolled annual weeds has

the greatest influence on population size. Selective spraytopping of weeds, i.e., preventing seed maturation, through

application of herbicides at early stages of reproductive

development has been effective at killing weed seed and

minimizing viable seed return to the soil seedbank (Medd et

al. 1992; Steadman et al. 2006; Walker and Oliver 2008),

although the reproductive stage at which weeds are most

sensitive to spray-topping differs by herbicide and species

(Steadman et al. 2006).

Duration of Reproductive Development. Rapid completion of

reproductive stages favors perpetuation through the soil

seedbank. Initiation of different developmental stages within

a growing season is linked to time of emergence, with the

duration of reproductive development correlated to accumulated heat units or photoperiod or both. Delayed emergence

typically hastens the onset of reproductive development

(Norsworthy et al. 2010a; Shrestha and Swanton 2007). In

Norsworthy et al.: Best practices for reducing herbicide resistance N 35
addition, photoperiodic sensitivity also influences the transition to, and the duration of, reproductive development.

Weeds that flower in response to short days may quickly

produce seed before the first killing frost when emerging in

late summer. Under these conditions, prevention of weed seed

production by late-emerging weeds that can complete their

life cycle in as few as 6 wk can be extremely challenging. For

example, tall waterhemp needs less than 10 d to produce

viable seed following pollination (Bell and Tranel 2010).

Likewise, prickly sida (Sida spinosa L.) and Benghal dayflower

are capable of producing viable seed within only 12 to 16 d

after anthesis (Egley 1976) and 42 d after seedling emergence

(Webster et al. 2005), respectively.

Although a shorter reproductive development ensures

reproductive success and seedbank replenishment, an extended flowering period provides greater opportunity for the

spread of resistance genes via pollen movement in outcrossing

weed species. In situations in which a weed species and a

related HR crop are cross-compatible, extended periods of

flowering synchrony increases the risk of resistance-gene

transfer via weed–crop hybridization. Such transfer of

herbicide-resistance genes has been reported from canola to

birdsrape mustard, wheat (Triticum aestivum L.) to jointed

goatgrass (Aegilops cylindrica Host), ornamental bentgrass to

weedy creeping bentgrass, and cultivated rice to weedy rice

(Brown and Brown 1996; Mallory-Smith and Zapiola 2008;

Seefeldt et al. 1998; Shivrain et al. 2009c).

Mode of Dispersal. Some weeds possess specialized adaptations

that aid long-distance dispersal. For instance, horseweed and

dandelion (Taraxacum officinale G.H. Weber ex Wiggers)

seed are equipped with a pappus that allows for dispersal by

wind. Horseweed seed regularly disperse 500 m from source

populations and have been recovered from atmospheric

currents at about 140 m above ground level (Shields et al.

2006). Barnyardgrass seed typically float in water and can be

transported through irrigation water for up to several miles

(Norsworthy, unpublished observations). The role of birds

and herbivores on weed seed dispersal has also been well

established (Vander Wall et al. 2005). This evidence

challenges the idea that single-field management practices

would limit introduction of HR weeds. For weeds lacking

such long-distance dispersal mechanisms and those with

limited pollen movement, single-field resistance-management

practices can be effective. In addition, anthropogenic processes

can lead to weed introduction, and efforts must be put forth

to minimize the risks of introducing HR weeds, as discussed

in later sections.

Persistence in Soil Seedbank. The length of time that seed or

other propagules remain viable in the soil dictates how long

control measures must be maintained to reduce the overall

seedbank and any reservoir of resistant seed (Matthews 1994;

Powles et al. 1992). Management requires knowledge of how

factors, such as crop type, canopy formation, crop rotation,

soil type, tillage, and irrigation, affect seedbank dynamics,

including key processes, such as germination, herbivory, and

decay (Gallandt 2006). Small-seeded grass and broadleaf

weeds, especially those that lack physical dormancy, often

exhibit shorter persistence in the soil seedbank compared with

large, hard-seeded weeds. Out of 20 weed species buried for

5.5 yr, Egley and Chandler (1983) found that velvetleaf

(Abutilon theophrasti Medik.), purple moonflower (Ipomoea

turbinata Lag.), spurred anoda [Anoda cristata (L.) Schlecht.],

hemp sesbania [Sesbania herbacea (P. Mill.) McVaugh], and

pitted morningglory (Ipomoea lacunosa L.), all large, hardseeded broadleaves, and johnsongrass [Sorghum halepense (L.)

Pers], a large, hard-seeded grass, were more persistent than

most small-seeded weeds.

Challenges. A continuing challenge in weed management is an

incomplete understanding of the biology of weeds across the

myriad of environments they occur. Grouping weed species

into functional groups (e.g., small-seeded broadleaf weeds vs.

large-seeded broadleaf weeds) allows for generalizations about

each group to be evaluated. An example is the longer

persistence of large-seeded broadleaf weeds in the soil

seedbank as opposed to the relatively shorter persistence of

small-seeded broadleaf weeds (Egley and Chandler 1983). The

challenge to this is to determine the appropriate biological

characteristics by which weeds can be grouped (e.g.,

germination requirements, flowering duration, breeding

systems, mode of pollination, seed size, seed dispersal, and

seedbank persistence), and then selecting representative

species from each group to study. Additional research on

the biology and ecology of a diverse group of problematic

weeds is critical for the development of effective and

economical weed management strategies. In the context of

herbicide-resistance management, such knowledge is vital for

parameterizing herbicide-resistance simulation models that aid

in devising suitable resistance management strategies (Maxwell et al. 1990; Neve et al. 2011a). Evolution of HR weed

populations has highlighted substantial gaps in our understanding of weed biology and ecology, especially factors that

affect composition and persistence of the weed seedbank, the

germination and emergence for different species, and the

extent of gene flow via pollen and seed dispersal among weed

populations. Improved knowledge of these areas will be

critical for effective and proactive resistance management.

BMP 2: Use a Diversified Approach to Weed Management

Focused on Reducing Weed Seed Production and the

Number of Weed Seeds in the Soil Seedbank. Frequently

HR weeds evolve in crop production systems where weed

management strategies have not been diverse (Nichols et al.

2009; Talbert and Burgos 2007; Walsh and Powles 2007).

One of the most recent weeds to affect a vast area of crops

across the southern United States is GR Palmer amaranth

(Nichols et al. 2009). Palmer amaranth is a genetically diverse,

obligate, outcrossing weed, and the adoption of no-till or

reduced-tillage production practices; minimal crop rotation,

especially in cotton; and almost sole reliance on a single

herbicide (i.e., glyphosate) has led to the rapid evolution of

glyphosate resistance. Recent simulation modeling has shown

that integration of diversified management tactics, with

particular focus on reducing the soil seedbank, greatly reduces

the risk of new GR Palmer amaranth populations evolving

(Neve et al. 2011a,b).

For a production system to remain sustainable, the soil

weed seedbank must be static or declining. An increasing

seedbank is evidence of a weed that is escaping the current

management regime through herbicide resistance or some

other adaptation. Furthermore, the risk of resistance evolution

is shown to be positively associated with the initial seedbank

36 N Weed Science 12, Special Issue 2012
size (Neve et al. 2011a); therefore, keeping the soil seedbank

at low levels reduces the risk of future evolution of herbicide

resistance. Soil seedbank size varies considerably among fields,

often because of the relative effectiveness of weed management

practices. For example, overall weed seedbank densities across

the Corn Belt varied from 600 to 162,000 viable seed m22

(Forcella et al. 1992). In Arkansas, barnyardgrass seedbank

size in cotton, soybean, and rice fields varied from 0 to

121,000 viable seed m22

, indicating the highly dynamic

nature of soil seedbanks (Bagavathiannan et al. 2011b).

Farming practices directly influence weed populations by

affecting quantity of seed removed from and returned to the

soil seedbank (Wilson 1988). The soil seedbank could be

effectively reduced through preventing weed seed production

and reducing the germinable fraction of weed seed in the soil

through specific tillage techniques, such as stale seedbed, along

with practices that encourage seed loss via herbivory.

The lasting consequences of allowing weed seed production

have long been acknowledged, as stated in an old English

proverb: ‘‘One year’s seeding makes seven years’ weeding.’’

Nevertheless, the idea that a limited amount of weed seed

production each year is acceptable is strongly embedded in

weed management strategies based on annual economic

thresholds. Problems with this approach, especially the longterm effects of subthreshold weed populations, have been

repeatedly demonstrated, independent of the increased risk of

herbicide-resistance evolution (Cardina and Norquay 1997;

Hartzler 1996; Jones and Medd 2000; Norris 1999). For

example, Cardina and Norquay (1997) observed a linear

increase in the soil seedbank when the annual subthreshold

densities of velvetleaf plants were allowed to escape control in

corn. The risks of herbicide-resistance evolution have been

shown to increase with every increase in initial seedbank size

(Neve et al. 2011a). Although economic thresholds have

typically been calculated only to achieve an effect on crop

returns on an annual basis, the long-term effects of

subthreshold, noncontrolled weed populations on the soil

seedbank and resistance evolution requires management

considerations that aim beyond a single growing season

(Gallandt 2006).

A few studies have followed an ecological approach to

understanding and establishing thresholds. Jones and Medd

(2000) suggested a population-based approach to weed

management decisions that views weeds as more than an

annual production problem. In their ‘‘economic optimum

threshold’’ approach, seed production and subsequent

additions to the soil seedbank are evaluated along with future

management implications and costs. Although economic

optimum thresholds are lower than annual economic

thresholds (Bauer and Mortensen 1992), this approach still

fails to consider the effects of weed population dynamics, such

as the initial frequency of resistant alleles in the soil seedbank

before herbicide use and changes in the genotypes under

selection on herbicide-resistance evolution. As such, economic

optimum thresholds ignore future costs associated with

herbicide-resistance management. Thresholds that fail to

consider the evolution of HR weeds are unsound and have

long-term economic and environmental consequences. The

concept of what constitutes an acceptable level of weed seed

production must be abandoned in favor of a zero or near-zero

threshold if the current rate of herbicide-resistance evolution

is to be slowed. An increasing number of weed scientists have

emphasized the need for a near-zero threshold, especially for

highly competitive, prolific seed-producing weeds (Cardina

and Norquay 1997; Norris 1999; Sattin et al. 1992; Swanton

et al. 1999). However, with the exception of work by a small

number of researchers, such as Shrestha (2004), and modeling

by Neve et al. (2011a), management of the soil seedbank to

delay resistance evolution has not been a major consideration

in planning or assessing long-term weed management

programs, which is regrettable. Modeling and field evidence

indicate the long-term value of preventing weed seed

production to minimize weed selection to herbicides and

reduce the risk of herbicide-resistance evolution and spread

(Griffith et al. 2010; Neve et al. 2011a).

The rapid spread of GR Palmer amaranth in cotton

illustrates the fallacy of permitting subthreshold weed

populations to produce seed. Although a density of 0.13

Palmer amaranth plants m22 will not cause yield loss in

cotton (Keeley et al. 1987; Smith et al. 2000), those Palmer

amaranth plants can produce in excess of 500,000 seed per

female plant, with an annual seedbank addition of more than

16 million seed ha21 at that subthreshold level of infestation.

Within 3 yr of a single GR Palmer amaranth plant escaping

control in cotton, the crop field can be overwhelmed by

resistance (Griffith et al. 2010). Hence, at its minimum, the

goal of all weed management programs must be an aim to

eliminate weed seed production from the most competitive,

resistance-prone weeds in a field, including any weeds on the

global list of resistance-prone weeds as identified by Heap

(2011a).

Intensive use of herbicides alone may not be sufficient to

prevent weed seed production and to maintain a low soil

seedbank (Griffith et al. 2010; Hartzler 1996; Jones and

Medd 2000; Schweizer and Zimdahl 1984). Proactively, some

U.S. producers are hand-roguing fields as part of a zero seedproduction approach to address the increasing occurrence of

GR weeds, mainly Palmer amaranth. The expenses these

producers have incurred are far less than those who allowed a

few plants to escape and are later confronted with sizeable soil

seedbanks of resistant weeds that must be rogued multiple

times each year to achieve a harvestable crop (Culpepper et al.

2010). Methods to minimize late-season weed-seed production, such as elevated crop seeding rate and crop-topping and

removing weed seed from equipment during harvest, are

widely practiced in areas of Australia with major HR weed

problems (Walsh and Powles 2007). Although preventing

weed seed production adds to short-term management costs,

it is instrumental in mitigating the evolution of herbicide

resistance. Considering the long-term costs associated with

future resistance management, these short-term expenses are

still economically viable.

Challenges. Eradication of weed seedbanks is a goal that

growers have tried to attain and failed at throughout history.

The largest challenge involves the practicality of preventing

seed return over large areas of land that contain diverse soil

seedbanks filled with dormant, and often long-lived, weed

seed (Buhler et al. 1998). The prolific reproduction potential

of weeds, with some species producing hundreds of thousands

of seed per plant, makes it difficult to effectively eliminate

seed return to the soil seedbank. Previous studies have

demonstrated that the soil weed seedbank reserves can be

reduced by 95% after 5 yr of weed-free management, but

quickly revert to 90% of their original weed seedbank

Norsworthy et al.: Best practices for reducing herbicide resistance N 37
population after only a single year of no herbicide use

(Burnside et al. 1986). A total prevention of seed return to the

soil seedbank is an enormous challenge that will increase

management costs and will often require the use of multiple

tools (e.g., herbicides, mechanical control, cultural strategies,

physical weeding, and plant residue burning, among others).

Unavailability of the full spectrum of tools and a lack of

knowledge on weed biology may limit the efforts to prevent

weed seed production. For instance, late-season herbicide

applications were shown to reduce seed production and seed

viability of Palmer amaranth, sicklepod [Senna obtusifolia (L.)

H.S. Irwin & Barneby], prickly sida, pitted morningglory,

barnyardgrass, and rigid ryegrass (Isaacs et al. 1989; Steadman

et al. 2006; Taylor and Oliver 1997; Walker and Oliver

2008); however, current knowledge of the efficacy of this

practice is limited to a small number of weed species, and

additional research is needed before making suitable recommendations. In particular, knowledge of the earliest time in

the growing season when viable seed are produced is vital for

timing late-season applications. Some nonherbicidal approaches to weed-seed prevention create additional management problems. For example, weeds must be removed

mechanically or physically before seed maturity, and rogued

weeds should be buried in a deep pit or otherwise disposed of

appropriately, not stacked along field margins because

reinfestation can occur due to seed shatter during periods of

rainfall (see BMP 10). Diversified weed management systems

are, by nature, more complex, requiring a larger set of tools

and skills than were required in herbicide-dominated systems.

These systems will often constitute greater costs (real or

perceived) and labor requirements, demand increased sophistication on the part of the practitioner, and require fieldspecific or cropping system–specific strategies based on weed

species composition and risk of herbicide resistance.

BMP 3: Plant into Weed-Free Fields and Keep Fields as

Weed Free as Possible. Herbicide-resistance management

plans must commence before planting because control options

become limited once the crop is planted and weeds begin to

emerge. At crop planting time, crop fields should be free of

weeds, achieved by a range of control tools. Crops planted

into established weeds are risky because even herbicide

mixtures with multiple MOAs may not provide effective

control. Failure of preplant measures to control emerged

weeds is devastating to crop yield and even more devastating if

resistance evolves.

To keep crop fields as weed-free as possible, especially in

conservation-tillage systems where preplant tillage is not

feasible, residual herbicides can be used before or at planting

(PRE). Additional applications of residual herbicides in slowgrowing or open-canopy crops, before the efficacy of the

initial residual herbicide has dissipated, reduces selection

pressure associated with sole reliance on POST herbicides

(Neve et al. 2003b, 2011b). Crops differ in rates of growth;

thus, some crops will require longer periods of residual control

than will others for efficacious weed control. Once a dense

crop canopy has formed, emergence of most weeds typically

ceases because of changes in thermal amplitude and in the

quality and quantity of light in the zone of seed germination

(Egley 1999; Norsworthy 2004a). Throughout the remainder

of the growing season, focus should be on removing any weeds

that have escaped control measures or have emerged in safe

sites after the final late-season herbicide application, including

those that emerge after crop harvest, if seed production is

possible before a killing frost (see BMPs 2 and 11).

Challenges. The nature of the production system can

complicate weed control by limiting the number of tools

available. For instance, tillage as a means of weed control is

not a viable option in many dryland systems and may be

prohibited by government-sponsored programs aimed at

conserving soil resources. Similarly, burning fields may pose

an environmental hazard limited by local regulations.

Preplant, weed-smothering techniques, such as mulches or

cover crops can be cost-effective for small areas of high-value

production, but are not economically viable for large areas.

Where herbicide options are available, there may be time

restrictions on how soon the crop can be planted following

application because of potential crop injury. Furthermore,

environmental conditions, such as temperature, rainfall, and

soil moisture, may influence the application or activation of

residual herbicides. Finally, once the crop is planted, the

number of weed management options becomes more limited.

BMP 4: Plant Weed-Free Crop Seed. Planting weed-free

crop seed is the first step to preventing the introduction of

new weeds into fields and also an important tactic in

preventing the spread of herbicide resistance into new areas

(Thill and Mallory-Smith 1997). Separating weed seed from

crop seed can be costly and often challenging because of the

similarity in size and shape of some weed and crop seed. For

example, jointed goatgrass and wild oat (Avena fatua L.) seed

are often difficult to separate from wheat seed, making them a

common contaminant in wheat seed lots. Through an

extensive seed lot survey in Utah, Dewey and Whitesides

(1990) found that wild oat seed contaminated about 14% of

seed samples collected, with an average of 21 seed kg21 of

wheat seed. Herbicide-resistant wild oat in southeastern Idaho

is thought to have immigrated from an adjacent state via

contaminated crop seed (Thill and Mallory-Smith 1997). A

recent survey of grower-saved crop seed in Australia revealed

that about 73% of samples were contaminated with weed

seed, with an average of 62 weed seed 10 kg21 of wheat,

barley (Hordeum vulgare L.), or lupine (Lupinus spp.) seed

(Michael et al. 2010). Among the weed seed recovered from

these samples, rigid ryegrass had resistance to four different

herbicides, whereas wild radish (Raphanus raphanistrum L.)

and wild oat were each resistant to one herbicide (Michael et

al. 2010). Herbicide-resistant rigid ryegrass seed were also

found as a contaminant of Australian wheat exported to

Japan, demonstrating the potential for international dispersal

of resistance genes via weed seed contamination of grain

shipments. In these shipments, about 4,500 resistant ryegrass

seed were detected in a 20-kg wheat sample in 2006 and

2007, about 35 and 15% of which were resistant to diclofopmethyl, 5 and 6% were resistant to sethoxydim, and 56 and

60% were resistant to chlorsulfuron, respectively (Shimono

et al. 2010).

Challenges. The grower practice of saving seed from one

harvest to plant the following year could be a potential

challenge to ensuring that weed-free seed is planted. Seed

from some crops, including cotton, soybean, rice, and wheat,

can be saved, but convincing farmers to purchase weed-free

38 N Weed Science 12, Special Issue 2012
seed from companies rather than saving their own can be

challenging. There are also challenges associated with cleaning

grower-saved seed, especially if appropriate equipment is not

available. As resistance becomes an increasing problem, our

challenge is to educate producers about the importance of

weed-free crop seed and to discourage use of saved seed that

contains weed seed.

BMP 5: Scout Fields Routinely. Effective scouting (i.e.,

moving through fields systematically to evaluate weed

populations) is a vital component of any successful weed

management program, including one aimed at mitigating or

managing HR weeds. Timely scouting and maintaining an

inventory of weed species, location, density, and size allow

for subsequent assessment of the effectiveness of prescribed

practices. Numerous field-sampling schemes or strategies can

be used when scouting, with the strategy chosen based upon

how and when the data will be used (Clay and Johnson 2002;

Gold et al. 1996). Regardless of the technique used, records

should document the management tools used during previous

seasons along with the effectiveness of each tool on some of

the most problematic weeds present. These data will be

instrumental in determining the efficacy of existing weed

management programs and in monitoring whether weed

species shifts are occurring. A single herbicide is rarely

effective against all weeds present within a field. As a result,

effective scouting allows for recognizing the most efficacious

herbicide for the weed spectrum present and following it with

additional treatment options.

Many chemical weed-management programs aimed at

reducing the risk of herbicide resistance begin with a residual

herbicide, which should be chosen based on the previous

year’s weed spectrum from recorded inventory. In particular,

weeds present at harvest are a good indicator of species that

are not effectively controlled season-long and that may need

further attention in the following year’s crop (Reddy and

Norsworthy 2010). It is imperative that in-season scouting

begin soon after crop planting to determine whether

preplanting measures, such as tillage or residual herbicide

application, have provided the anticipated level of weed

control. Further action, such as additional residual herbicide

application, can be taken before the first residual herbicide has

dissipated. However, timely scouting is vital in making such

decisions.

The effectiveness of most POST herbicides is a function of

the application rate, application timing, and size of the weed

at application; in most field crop settings, a large weed is one

that exceeds 10 cm in height. To ensure proper timing of

POST herbicides, fields should be scouted routinely for weed

sizes and spectrum. Failure to scout and apply POST

herbicides in a well-timed manner could reduce the efficacy

and increase the risk of herbicide resistance (see BMP 7).

Scouting allows for early identification of a lack of

herbicide efficacy, which may indicate the evolution of

resistance. In such instances, samples should be collected

and appropriate tests conducted to determine whether

herbicide failure is a result of resistance. Indicators of possible

herbicide resistance include (1) failure to control a weed

species normally controlled by the herbicide at the dose

applied, especially if control is achieved on adjacent weeds; (2)

a spreading patch of noncontrolled plants of a particular weed

species; and (3) surviving plants mixed with controlled

individuals of the same species. If herbicide resistance is

suspected, seed production from all noncontrolled plants of

the species concerned should be interrupted to prevent the

perpetuation of the resistance problem, even, in extreme cases,

if that means destroying the crop in the process. Timely

scouting is key to making appropriate management decisions.

Challenges. Although thorough scouting of fields provides

growers with valuable information about the level and

spectrum of weed populations in their fields, there are several

limitations. Field scouting is a time-consuming process, and

any delay in scouting or implementation of control measures,

including herbicide applications, will be costly. The efficacy of

many of the herbicides used against rapidly growing HR

weeds (e.g., amaranth species) becomes more limited as

growth stage advances. Most labels recommend that specific

rates be used until a particular weed growth stage is reached;

after which, control will probably be only partial. The cost

associated with proper field scouting can also be an obstacle.

Further, the lack of suitable expertise to properly identify

small weed seedlings (i.e., cotyledon to one-leaf stage) poses a

significant challenge. There exist resources that can assist

scouts in identifying weed species (e.g., Bryson and DeFelice

2009, 2010; Stubbendieck et al. 1994; Uva et al. 1997;

Whitson et al. 2009), but greater education concerning proper

weed identification and the critical characteristics that separate

similar species will be required to adequately train field scouts.

BMP 6: Use Multiple, Effective MOAs against the Most

Troublesome Weeds and Those Prone to Herbicide

Resistance. Herbicides are often the strongest selection agents

for weed species present within a cropping system (Booth and

Swanton 2002). Use of herbicides across vast areas containing

large, genetically diverse weed populations causes intense

selection pressure, often resulting in the evolution of resistance

in weed communities; repeated application of effective

herbicides with the same MOA is the single greatest risk

factor for herbicide-resistance evolution (Beckie 2006; Powles

et al. 1997). Targeting the most troublesome and resistanceprone weeds within a field by using different herbicide MOAs

in annual rotations, tank mixtures, and sequential applications

can delay the evolution of resistance by minimizing the

selection pressure imposed on those weed populations by a

particular herbicide MOA. In an annual herbicide rotation,

crops in two or more subsequent cropping seasons receive

different herbicide MOAs. Mixtures and sequential applications can be used as a resistance management technique by

applying different herbicides to the same crop during the same

growing season, simultaneously, in the case of a mixture, and

at different times for a herbicide sequence.

The occurrence of resistant individuals in a weed

population not previously exposed to the target herbicide is

rare (Gressel and Levy 2006; Gressel and Segel 1990).

However, the proportion of resistant individuals will rapidly

increase with repeated use of the same herbicide MOA. For

example, Beckie (2006) showed that significant levels of

resistance to ALS-inhibitor herbicides can evolve in weed

populations with as few as five applications.

Combining recommended rates of different herbicide

MOAs, simultaneously, sequentially, or annually, greatly

reduces the likelihoods of survival and reproduction of the

resistant individuals. In most weed populations, individuals

Norsworthy et al.: Best practices for reducing herbicide resistance N 39
naturally resistant to more than one herbicide MOA will be

rare. In most cases investigated to date, herbicide resistance is

conferred by a single dominant or semidominant allele

(Jasieniuk et al. 1996; Vila-Aiub et al. 2009). If different

resistance alleles are at unlinked loci, the probability of an

individual plant being resistant to multiple MOAs can be

estimated as the product of the frequencies of resistance to

each single MOA (Gressel and Segel 1990; Wrubel and

Gressel 1994). This assumes that no single allele confers crossresistance to multiple herbicides by mechanisms such as

general detoxification or sequestration.

Modeling studies predict that strategies involving herbicide

rotations or mixtures will be most effective at preventing

resistance evolution in species with limited pollen movement,

where recombination of resistance alleles transferred via pollen

is limited, and with limited seed dispersal (Beckie et al. 2001;

Diggle et al. 2003). Single-gene, target-site resistance evolves

more slowly in weed populations with continuing establishment of susceptible individuals from persistent seedbanks,

keeping the frequency of resistance at low levels (Powles et al.

1997). However, sequential applications in such cases may

expose more than one weed cohort to different selection

pressures, contributing to the longer-term evolution of

genotypes resistant to multiple herbicide MOAs.

Rotations, Sequential Applications, and Mixtures to Mitigate

Resistance. For any herbicide combination strategy to be

effective in mitigating resistance, the herbicides must have

different MOAs because minimizing herbicide selection

pressure on the weed population is the key to delaying

resistance. Annual herbicide rotations are useful but not

sufficient because they subject a weed population to a single

MOA at a time, potentially allowing the progeny of resistant

individuals to survive and acquire additional resistance via

gene flow before the next herbicide application. Annual

rotations and sequential applications may, therefore, select for

multiple resistance over time, especially if the interval between

applications is long enough to accommodate more than one

weed generation or if gene flow is high. In contrast, herbicide

mixtures apply multiple selection pressures simultaneously,

theoretically allowing only the survival of very rare individuals

that have already evolved resistance to these multiple herbicide

MOAs via mutations at multiple loci or gene flow. Modeling

work by Diggle et al. (2003) predicted that the rate of

herbicide-resistance evolution will be slowed more with

herbicide tank mixtures than with annual herbicide rotations.

Likewise, Powles et al. (1997) showed that mixing two

herbicides with different MOAs delays resistance longer than

rotating between them. Similarly, Beckie and Reboud (2009)

reported that mixing an ALS herbicide with a photosystem II

(PSII) inhibitor or auxinic herbicide is more effective than

annual herbicide rotation in delaying resistance to ALSinhibiting herbicides. Wrubel and Gressel (1994) noted that

mixtures containing triazine and chloroacetamide herbicides

were effective in preventing triazine resistance during a 20-yr

period in lambsquarters (Chenopodium L.) and amaranth

weed species, whereas a triazine herbicide applied alone

resulted in widespread resistance in these species. However,

herbicide mixtures are effective at delaying resistance only

when the mixture components target, and are effective on, the

same weed species. For example, a mixture in which one

MOA acts on grass weeds and the other on broadleaf weeds

may provide good broad-spectrum control, but will not

reduce the rate of resistance evolution in either of the weed

groups (Wrubel and Gressel 1994).

For maximum benefit, herbicide mixtures must not only

have different MOAs, but also have similar efficacy and

persistence, allowing them to act simultaneously on the same

weed cohort. Efficacy is also improved when the mixture

components are degraded differently (Powles et al. 1997;

Wrubel and Gressel 1994). Some herbicide mixtures have

synergistic action, in which the activity of the combined

herbicides is greater than the additive effect of each. For

instance, Woodyard et al. (2009) reported synergistic activity

between HPPD inhibitors and PSII inhibitors on common

lambsquarters (Chenopodium album L.), giant ragweed

(Ambrosia trifida L.), and common waterhemp in corn.

Contact and systemic herbicides are often not effective

tank-mix partners because of the contact herbicide antagonizing the activity of the systemic herbicide. When both types

of herbicides are needed as part of a resistance-management

strategy, sequential applications will be superior to mixtures

for reducing the risks of resistance. For example, an effective

sequential HR management strategy developed in Australia

through simulation modeling and later successfully implemented is the ‘‘double knock’’ tactic, in which an application

of glyphosate is followed by paraquat up to 7 d later to control

rigid ryegrass plants that survived the glyphosate application

(Neve et al. 2003b; Walsh and Powles 2007).

Using Rotations and Mixtures After Resistance Has Evolved.

Although the most desirable strategy is to proactively use

annual herbicide rotations, sequential applications, or tank

mixtures before resistance evolves, that requires growers to

apply multiple herbicides even if weed densities are low

(Powles et al. 1997). More commonly, growers prefer to use

one herbicide that still provides good control on susceptible

weeds while adding a second product to control resistant

plants. Jacquemin et al. (2009) found that applying mixtures

to weed populations after resistance has evolved could be

effective if the resistance mechanism imposes a significant

fitness penalty via negative cross-resistance (i.e., resistance to

one herbicide makes individuals more susceptible to a second

herbicide). However, that scenario is not common. More

research is needed on the use of combined herbicide strategies

on already-resistant weed populations, as well as on the

potential for such strategies to select for cross resistance

(Preston 2004).

Rotating Crops with Different or Multiple Herbicide-Resistance

Traits. Rotation of HR crops without the diversity of

herbicide MOA (e.g., GR cotton followed by GR soybean)

will have no effect on reducing the evolution of HR weeds

(Reddy and Norsworthy 2010). However, rotating different

herbicide-resistance crop traits (in the same, or a different,

crop) is one more way of increasing herbicide MOA diversity

within cropping systems, such as rotating GR crops with

glufosinate- or auxin-resistant crops. Based on modeling

studies by Neve et al. (2011b), rotating GR cotton with

glufosinate-resistant or non-HR cotton reduces the number of

glyphosate applications required in a single year during the

course of the rotation. As a result, the predicted risk of GR

Palmer amaranth evolving in cotton would diminish by

approximately 50% and resistance would be delayed by 2 to

40 N Weed Science 12, Special Issue 2012
3 yr. This prediction was partially validated in the field where

large plots of continuous GR cotton were found to have high

densities of GR Palmer amaranth after 4 yr of continuous sole

use of glyphosate, unlike a system in which GR cotton was

annually rotated with glufosinate-resistant cotton (Johnson et

al. 2011). Another option for producers is to grow crops with

multiple herbicide-resistance traits (i.e., stacked traits),

allowing them to use multiple, often broad-spectrum herbicide simultaneously. Cotton cultivars and corn hybrids have

been developed to confer high levels of resistance to glyphosate and glufosinate, and several other crop varieties are in

commercial development with various combinations of

resistance to glyphosate, glufosinate, ALS inhibitors, dicamba,

2,4-D, and HPPD-inhibiting herbicides (Green 2009). This

has the added advantage of allowing the development of

products with MOA mixes for application convenience and

cost effectiveness.

Challenges. A challenge to using multiple MOAs for weeds

prone to evolving herbicide resistance is the need to readily

identify the MOA for each herbicide used. This may be

difficult for weed managers because many herbicide labels do

not include the MOA designations. Additionally, use of

multiple herbicides increases the complexity of the weed

management system and requires some level of expertise for

properly matching herbicides and MOAs with target species.

Finally, it is not clear whether there are enough different

herbicide MOAs available to responsibly target different weed

species in various crops, in part because of the increasing

number of HR weeds.

Another key challenge is the paradigm shift that will be

required in weed management, a change in the farmers’ point of

view. Before the evolution of HR weeds, an established priority

of IWM was to maximize herbicide use efficiency and minimize

the number and frequency of herbicide application, a goal

aimed at mitigating off-target movement and the effect on the

environment. In the wake of HR weed evolution, an increase in

the diversity of herbicide MOAs is a vital component of

stewardship programs aimed at conserving existing herbicide

MOAs while managing HR weeds. Despite its potential, this

strategy may incur additional costs to growers, which is often an

impediment to adoption. Using more herbicides, or more

expensive herbicides with different MOAs, may be perceived to

reduce profitability in the short-term. However, recent evidence

indicates yield increases associated with a diversity of herbicide

MOAs result in equivalent economic benefits, even in the year

of application, making this BMP economically viable in the

medium to long term (Weirich et al. 2011a,b).

BMP 7: Apply the Labeled Herbicide Rate at Recommended Weed Sizes. In the 1990s, reduced herbicide rates

were seen as an environmentally sound practice and were a

part of many integrated approaches to weed management in

crops (Bhowmik 1997; Buhler et al. 1992; Mickelson and

Renner 1997; Steckel et al. 1990). This fit well with the

concept of weed economic thresholds (i.e., weed density at

which cost of control equals the financial return on the

recovered crop yield) to optimize crop yield with submaximum weed control (O’Donovan 1996; Zhang et al. 2000).

Weed management programs often integrate herbicide

application at below-label rates with other control measures,

such as tillage and enhancement of crop competitiveness, to

keep weed densities below economic threshold levels as

defined by crop yield loss models (Blackshaw et al. 2006;

O’Donovan et al. 2007). The underlying assumption is that,

although herbicide manufacturers’ labeled application rates

are set sufficiently high to control a range of weed species

across different growth stages and site conditions, economically acceptable weed control at a given location can often be

achieved with reduced herbicide rates, especially if weed

pressure is low and weeds are smaller than sizes listed on the

label. However, as suggested by Gressel (1995, 2002) and later

established through studies (Busi and Powles 2009; Manalil et

al. 2011; Neve and Powles 2005a), recurrent exposure to low

herbicide doses that allow a proportion of the weed

population to survive may have unintended evolutionary

consequences. For example, recent evidence indicates that

applying herbicides at below the labeled rate was a major

contributing factor to the evolution of diclofop-resistant rigid

ryegrass in an Australian wheat field (Manalil et al. 2011).

A weed population under selection from repeated low doses

of a herbicide risks accumulation of minor genes, which

individually confer only a slight increase in fitness but, when

combined in one individual, provide agronomically significant

levels of herbicide resistance. Manipulation of this type of

polygenic phenotypic response in populations showing continuous trait variation is a long-established foundation of crop

breeding, but few investigators have explored multigene

herbicide resistance in weeds. A dominant or semidominant

allele at a single locus has been found to confer herbicide

resistance in most weed species investigated to date. This may

only reflect that simple, major gene systems are the easiest to

identify experimentally, although rapid adaptation to extreme

environmental events (such as exposure to high doses of

herbicide) is theoretically most likely to involve single genes with

large phenotypic effects (Lande 1983; Macnair 1991). Population response to constant, but less-intense, selection pressure is

more likely to involve selection in many minor genes, each with

a small, individual effect, which accumulates over multiple

generations to produce adapted phenotypes (Orr and Coyne

1992). This type of polygenic evolutionary-resistance response

to sublethal drug or pesticide doses is well documented in

bacteria (Olofsson and Cars 2007), insects (Roush and

MacKenzie 1987), and fungi (Shaw 2006); evidence is emerging

for a similar response in weeds, as described below.

Within-population variation for herbicide resistance has

been reported in some weeds (Jasieniuk et al. 1996; Patzoldt

et al. 2002), including significant survival differences among

individuals at low herbicide doses, although they are

uniformly susceptible to the labeled rate (Neve and Powles

2005a). Such variation may be attributed to different alleles at

minor loci contributing to polygenic resistance or to different

allele combinations at modifier loci affecting expression of a

major resistance gene (Neve and Powles 2005a; Uyenoyama

1986). Regardless of the molecular mechanism, repeated

exposure of these weed populations to reduced herbicide doses

risks incremental enrichment of the gene pool with minor

resistance alleles, a phenomenon termed creeping resistance by

Gressel (1995).

In some weed species, individuals surviving below-label

herbicide rates can initiate a rapid population response.

Experimental exposure of rigid ryegrass populations to

sublethal glyphosate doses resulted in a population shift to

resistance in three to four cycles of selection (Busi and Powles

2009). Using low doses of the acetyl coenzyme A carboxylase

Norsworthy et al.: Best practices for reducing herbicide resistance N 41
(ACCase)–inhibitor diclofop-methyl in an initially susceptible

rigid ryegrass population and then bulk-crossing survivors,

Neve and Powles (2005a) showed that recurrent selection

generated field-level resistance in only three generations.

Increased levels of cross resistance to other ACCase inhibitors

and to ALS-inhibiting herbicides were also reported in the

selected populations, suggesting that gene pool enrichment

had occurred for multiple alleles controlling nontarget siteresistance mechanisms (Neve and Powles 2005b). Enhanced

herbicide detoxification as a widespread cross-resistance

mechanism, as proposed by Preston (2004), is one possible

outcome of repeated selection for minor resistance alleles.

Knowledge of reproductive biology as well as patterns of

herbicide use can help identify weed populations that are

more likely to evolve polygenic resistance through recurrent

selection by low-dose applications. The risk is theoretically

higher in outcrossing species that repeatedly recombine

genotypes across family lineages and in populations that

already possess a diverse array of minor resistance alleles or the

capacity to acquire them from an external source via gene flow

(Holsinger 2000; Manalil et al. 2011; Neve 2008). As such,

threshold-driven weed management strategies using low-dose

herbicide applications increase the risk of polygenic resistance

evolution (Busi and Powles 2009; Neve and Powles 2005a,b).

However, even when used at the registered rate, any volatile or

slowly dissipating herbicide that repeatedly exposes a given

weed cohort to a reduced dose could pose a selection risk. For

example, slow-degrading, soil-applied herbicides may deliver

sublethal doses to later-emerging weeds and allow for the

gradual accumulation of resistance alleles, if the same residual

herbicide is applied over multiple growing seasons (Maxwell

and Mortimer 1994; Zhang et al. 2000). Other situations

where weeds may be subjected to below-labeled rates include

spraying plants larger than those recommended on the label;

inadequate coverage of weeds because of size, density, or crop

cover; and low rates caused by imprecise sprayer calibration or

inaccurate mixing.

Challenges. The use of full-labeled doses of herbicides requires

a change in the approach to weed control. There are perceived

and real risks to using herbicides (i.e., unwanted environmental side-effects, adverse effects on human and animal

health), which led to a concerted effort to reduce herbicide use

in agroecosystems (Mortensen et al. 2000). In the Netherlands, there has been research to support the use of reduced

herbicide doses for weed control (Kempenaar and Lotz 2004;

Ketel and Lotz 1998), contrary to the recommendation in this

document. The ability to reduce weed management costs also

has been a prominent factor in grower adoption of reducedrate herbicide applications. Although there is a perception that

reduced rates provide short-term economic benefits to the

growers, especially on lands that are leased, the costs of

evolving herbicide resistance exceed monetary savings

achieved through reduced herbicide doses.

Another important limitation to the use of full-labeled rates

is the potential for phytotoxicity to the crop. Before

application of a herbicide, labels will need to be evaluated

to ensure that recommended rates are consistent with soils and

growing conditions that are integrated into extension

recommendations among the states in which the crop is

grown. This is of particular concern when different MOAs are

combined in a single application.

Additionally, there are potentially serious consequences to

adopting a full-rate application policy in minor-use crops (i.e.,

vegetables and fruits) where crop tolerances to various

herbicides will vary within a region, depending on cultural

practices and the use of other pesticides. The risks associated

with registration of herbicides in minor-use crops are

problematic; in response to this, the Interregional Project 4

(IR-4) program was created to mitigate some of the cost

associated with pesticide registration in minor-use crops. With

the elimination of methyl bromide use for preplant pest

control in vegetable crops, herbicides have become an

important weed control tool in these systems. Requiring the

use of full-labeled rates in minor-use crops may increase the

risk-to-benefit ratio for many registrants to the point that

herbicide labels may be cancelled in these crops. This could

cause significant problems for weed management in these

high-value, fresh-market crops.

Although not typically viewed as such, herbicide applications will be below-rate if weeds exceed the growth stage or

size listed on the registration label for a particular herbicide

rate. Herbicide application to appropriate weed sizes is critical

but can be a challenge because of the limited availability of an

available skilled labor force or unfavorable weather conditions.

In addition, large farm sizes make it difficult to thoroughly

scout fields and to make timely herbicide applications. Under

these circumstances, it is likely that at least some weeds may

exceed labeled sizes before herbicides can be applied.

BMP 8: Emphasize Cultural Management Techniques that

Suppress Weeds by Using Crop Competitiveness. Cultural

practices can be directed to exploit crop competitiveness,

thereby reducing weed emergence and growth. Some of these

practices include cultivar selection (Jordan 1993; Wax and

Pendleton 1968), narrow row spacing (Norsworthy and

Frederick 2005; Wax and Pendleton 1968), high seeding

rates (Norsworthy and Oliver 2001, 2002; Walsh and Powles

2007), adjusting planting dates (Gill and Holmes 1997;

Walsh and Powles 2007; Webster et al. 2009), irrigation

management (Walker and Buchanan 1982), crop rotation

(Johnson and Coble 1986; Schreiber 1992; Schweizer and

Zimdahl 1984), and judicial application of fertilizers with

appropriate placement. By hampering weed emergence and

growth, each of these practices lessens weed fecundity

(Norsworthy et al. 2007b), which is critical to reducing the

soil seedbank and risks of herbicide resistance (Neve et al.

2011a). Most of these practices also result in increased crop

yields (Howe and Oliver 1987; Norsworthy and Frederick

2005; Norsworthy and Oliver 2001).

Cultivar Selection. Crop genotypes differ in competitive

ability, as measured by the ability to suppress weeds (Jordan

1993; Monks and Oliver 1988; Norsworthy and Shipe 2006;

Rose et al. 1984; Wax and Pendleton 1968). Suppression of

weed growth is desirable in that limiting seed production of

those weeds that escape control would diminish future weed

problems, including HR weeds. For soybean, early-season

growth rate is an important characteristic for competitive

genotypes (Rose et al. 1984). Additionally, plant height,

tillering, leaf angle, and canopy formation are other common

criterion for cultivar selection for improved weed suppression.

Hybrid rice lines with greater tillering and taller growth

characteristics are more competitive with weedy rice than

42 N Weed Science 12, Special Issue 2012
nonhybrids (Shivrain et al. 2009a). Additionally, choosing an

HR crop cultivar that does not flower at the same time as, or

is less cross-compatible with, the weed relative will prevent

transfer of the resistance trait to the weedy species (Shivrain et

al. 2009b). Selection of a full-season cultivar to maximize the

period with crop cover will suppress weed emergence for

prolonged periods, whereas planting an early maturing

cultivar may broaden the window of postharvest seed

production of escaped weeds that potentially contribute to

seedbank persistence and herbicide-resistance evolution

(Reddy and Norsworthy 2010)(see BMP 11). In the southern

Rice Belt, late-emerging or late-maturing weedy rice that

escapes in-season weed control tactics will continue to grow

after rice harvest and produce seed before the first killing frost

(Shivrain et al. 2010). In an ALS-resistant rice system, these

escapes may be first-generation crosses with the HR crop,

given the highly outcrossing nature of weedy rice with

cultivated rice (Shivrain et al. 2007).

Planting Date. In some areas and for certain crops, planting

dates can be manipulated to take advantage of lengthy frostfree growing periods without sacrificing optimum yields. In

such environments, altering planting dates (early or late

planting) may place the crop at a competitive advantage over

specific weeds. For instance, a delay in crop planting may shift

peak weed emergence before crop establishment, allowing for

effective control before crop emergence. Controlling an early

emerging cohort means an ultimate reduction in the soil

seedbank and number of in-crop weeds that must be

controlled. One such example is the implementation of a 2-

wk delay in planting of wheat in some regions of Australia

(Gill and Holmes 1997; Walsh and Powles 2007). Delayed

planting allows for control of a large proportion of rigid

ryegrass that emerges after the initial spring rains, using

nonselective herbicides before crop establishment. A similar

tactic is employed in the southern United States with a double

cropping of June-planted soybean following winter-wheat

harvest, rather than planting soybean in May as is the most

widely used system. The double-cropping system has been

successful at reducing the density of GR Palmer amaranth in

soybean because the 1-mo delay in planting and the

suppressive nature of the preceding wheat crop greatly reduces

Palmer amaranth emergence (DeVore et al. 2011).

Altering planting date, if possible, can mitigate the

movement of resistant traits from crop to weedy relative,

from weed to weed, and from weedy relative to the crop by

minimizing the flowering synchrony between cross-compatible species. This is the case with red rice, which can acquire

resistance to ALS-inhibiting herbicides from ALS-resistant rice

and further spread resistance among compatible populations

(Shivrain et al. 2009a,c). This principle may be applicable to

other HR crop systems where gene flow to weedy relatives is a

potential source for the introduction of HR weeds.

Narrow Rows and Seeding Rates. For many annual weed

species, emergence ceases or diminishes as a result of crop

canopy closure (Jha and Norsworthy 2009; Norsworthy

2004a; Norsworthy and Oliveira 2007c). Narrowing crop-row

widths or increasing seeding rates hastens canopy closure and

increases crop competitiveness, eventually suppressing the

emergence and seed production of late-season weeds and

cohorts (Bagavathiannan et al. 2011a; Howe and Oliver 1987;

Mickelson and Renner 1997; Norsworthy et al. 2007b;

Webster 2007; Yelverton and Coble 1991). Although high

seeding rates increase input costs, this tactic has proven

beneficial for the management of HR weeds. One example is

suppression of HR rigid ryegrass in Australia, where higher

seeding rates have been readily adopted by wheat producers to

achieve more-rapid crop canopy closure and increased crop

competitiveness with ryegrass, in turn reducing ryegrass

growth and number of seed returned to the seedbank (Walsh

and Powles 2007).

Crop Rotation. Choice of crops in a rotation partially dictates

the intensity of tillage and choice of planting dates and

herbicides. In some experiments, crop rotation was less

influential than tillage on the soil seedbank (Ba ́rberi and Lo

Cascio 2001), whereas in others, rotation was a stronger

determinant of the soil seedbank density (Cardina et al.

2002). Lack of crop rotation results in a weed community

with less diversity and hence reduces herbicide options for

controlling those weeds. Risks of resistance are generally

greater in systems with limited crop rotation compared with

those where crop rotation (including rotation of HR traits) is

regularly practiced (Neve et al. 2011b). For example, model

simulations have shown that rotating GR cotton with corn

can reduce the risk of GR Palmer amaranth by almost twofold

vs. growing GR cotton alone (Neve et al. 2011b).

Nutrient Management. From a weed management perspective,

nutrient management can be an essential tool. Factors such as

timing, amount, and placement of fertilizers can be tailored to

maximize crop growth and minimize weed growth. Inappropriate fertilization can be favorably exploited by weeds,

leading to greater weed growth and competition with crops.

In rice production, weedy rice takes up about 60% of applied

N (Burgos et al. 2006), giving it a competitive advantage over

the crop, leading to greater seed production and increased risk

of herbicide-resistance evolution.

Irrigation Management. Irrigation before crop planting can be

used to induce weed germination and allow use of tillage or a

broad-spectrum herbicide to control emerged weeds (i.e., stale

seed bed), reducing the germinable fraction of the soil seedbank

that will otherwise emerge in the crop. Flooding has long been

used as an effective way of managing weeds in rice (Smith

1979). Flooding rice fields as early as possible will improve

control of many weed species, and flooding of fields is

recommended immediately after a preflood herbicide application because the emergence of many weeds ceases after flooding

(Baldwin and Slaton 2001). Furthermore, water-seeded rice

culture, in which fields are flooded and pregerminated rice seed

are spread over the field, is a technique that has been used for

weed management, especially for control of red rice in

conventional rice (Baldwin and Slaton 2001).

Challenges. Altering row width may require expensive

equipment modifications. Increased seeding rates increase

production costs and would be adopted only if producers see

economic benefit. The ability of a grower to alter planting

dates depends on the crop, the size of the farming operation,

other crops produced, availability of labor, length of cropping

season, and whether harvesting has to be scheduled. For

Norsworthy et al.: Best practices for reducing herbicide resistance N 43
agronomic crops planted on a large number of hectares,

planting dates could be restricted by the scheduling of harvest

if commercial harvesting is hired. Alteration of planting dates

will likely be implemented only if there is a perceived weed

control benefit and a lack of observable yield loss associated

with this practice. The implementation of crop rotations may

be challenged by crop price, crop preference of the landowner

on leased farms, management costs, available land, appropriate soil, and availability of equipment among other challenges.

Integration of these cultural practices into current weed

management programs does not guarantee an improvement in

weed control or reduced weed fecundity. For instance,

Norsworthy and Oliver (2002) often found poorer weed control

in irrigated, than in dryland, soybean as a result of more rapid

early season weed growth when soil moisture was not limited.

However, dryland culture is often weedier because of reduced

herbicide activation and reduced crop growth, which results in

slow crop canopy coverage and allows weeds to thrive in between

rows. In irrigated fields, control is reduced if herbicide

application or mechanical practices are not timely, causing

higher weed seed production. Ultimately, irrigation increases

crop competitiveness, but the challenge is integrating irrigation

with a timely, effective weed-control program. Furthermore,

weeds respond differently to cultural management. For example,

terrestrial weeds can be controlled by flooding, but some weeds

are adapted and survive seasonal flooding.

BMP 9: Use Mechanical and Biological Management

Practices Where Appropriate. Tillage. Tillage practices vary

by cropping system and region, and even from grower to

grower, with timing, intensity, and method of tillage

influencing the size and composition of the weed seedbank

(Ba ́rberi and Lo Cascio 2001). Tillage practices must be

regularly changed, in a manner similar to that of other

production practices, to prevent buildup of any particular

species or group of weeds in the soil seedbank. Perennial

weeds and shallow-emerging annual grasses and small-seeded

broadleaves are prominent in reduced-tillage systems (Buhler

et al. 1994; Mt. Pleasant et al. 1990; Schreiber 1992), whereas

annual large-seeded broadleaves are more prominent as tillage

intensity increases (Bararpour and Oliver 1998).

The vertical distribution of weed seed in the soil profile and

the rate of seedbank decline are influenced by tillage practices

(Ball 1992; Ba ́rbari and Lo Cascio 2001; Buhler et al. 1996,

1997; Cardina et al. 1991; Schreiber 1992). As soil

disturbance decreases, there is generally an increase in the

size and diversity of the soil seedbank (Cardina et al. 1991,

2002; Feldman et al. 1997). Warnes and Andersen (1984)

studied the annual population of wild mustard (Sinapis

arvensis L.) over a 7-yr period and reported that the final

population size was reduced to 3% of its original size with

mechanical cultivation three times each year but was reduced

only by half in grass sod or chemical fallow. However, species

shifts are affected less by tillage than by herbicides because

herbicides are a stronger constraint than tillage on the

assembly of communities (Booth and Swanton 2002; Derksen

et al. 1998). When combined with proper timing of herbicide

application, tillage or cultivation can effectively control

perennial weeds such as bermudagrass [Cynodon dactylon

(L.) Pers.](Etheredge et al. 2009), johnsongrass (Bruff et al.

1996), and redvine [Brunnichia ovata (Walt.) Shinners](Castello et al. 1998).

Tillage can be an effective means of weed control through

burial of small weeds, disrupting roots leading to plant

desiccation, and cutting or severely damaging broadleaf weeds

below the apical region, leading to plant mortality. Historically, crops have been planted in rows mainly to allow for

interrow cultivation as a means of weed management. Recent

modeling efforts have shown the value of interrow cultivation

in reducing the risks of herbicide-resistance evolution in row

crops (Norsworthy et al. 2011a). For example, the risk of

barnyardgrass evolving resistance to glyphosate in cotton was

68% over a 30-yr period when a continuous glyphosate

system with no interrow cultivation was used; however, the

risk was reduced to 38% over the same period when

integrating a single midseason interrow cultivation into the

existing system (Norsworthy et al. 2011a).

Tillage can also be used to reduce the soil seedbank by

stimulating weed seed germination. In the absence of weed

seed production, tillage results in more-rapid loss of weed seed

from the soil seedbank compared with a no-till system

(Bhowmik and Bekech 1993; Bridges and Walker 1985;

Buhler 1995). With repeated tillage and no seed input, seed

loss is initially rapid and continues over time, at a rather

decreased rate (Gallandt 2006). In Australia, an early shallow

cultivation, referred to as early tickle, before planting is used to

increase weed emergence (Pannell et al. 2004). This strategy

requires a delay in planting and any weed control benefit must

be weighed against crop yield reduction. For example in

wheat, the yield penalty was 5% for a 10-d delay and 10% for

a 20-d delay in planting. Using germination as a method of

seed loss, however, requires that germinating weeds are

effectively controlled (Gallandt 2006).

Deep-tillage implements that invert soil layers, such as a

moldboard plow, bury weed seed deep enough to prevent

successful germination and emergence (Bennett 2011). For

instance, in a double-crop winter-wheat/soybean system that

integrated a one-time use of a moldboard plow, the emergence

of GR Palmer amaranth was reduced by at least 95% over two

growing seasons (DeVore et al. 2011).

Cover Crops and Synthetic Mulches. Establishing a physical

barrier or mulch (living or nonliving) between crop rows to

reduce light availability and suppress weed growth is an

effective weed management tool that has attracted increasing

research attention, especially for organic production systems

and for high-value crops grown on small areas (Bangarwa et

al. 2011; Norsworthy et al. 2007c; Patterson 1998). In

addition to their weed control benefits, compostable mulch

materials, such as straw, hay, and stubbles, enhance soil

nutrient status and organic matter content, provide nitrogen

benefits if a leguminous cover cop is used, reduce loss of

unused soil nitrogen from the crop root zone, improve soil

structure and texture, and control soil erosion (Salmero ́n et al.

2010). Alternative compostable mulches that can be incorporated into the soil include wetted and rolled shredded paper,

an expensive but good weed-control option that is effective for

up to 2 yr (Pellett and Heleba 1995), and cornstarch-based

biodegradable film (Waterer 2010).

Black polyethylene sheet mulch is effective for high-value

crops, especially against tough-to-control weeds, such as yellow

nutsedge (Cyperus esculentus L.), reducing their shoot biomass

by 72% (Webster 2005a), but it may need to be combined with

additional barrier layers for effective suppression of much more

44 N Weed Science 12, Special Issue 2012
aggressive weeds (Daugovish and Mochizuki 2010). Translucent plastic sheets are less effective at suppressing weed growth

than black polyethylene sheets (Chase et al. 1998). However,

when used for soil solarization to raise soil temperature, clear

plastic mulch has been shown to induce 97% mortality in

kudzu [Pueraria montana var. lobata (Willd.) Maesen & S.M.

Almeida] crowns over two seasons in South Carolina (Newton

et al. 2008). It has also been used to eradicate the seedbank of

weedy golden wreath wattle [Acacia saligna (Labill.) Wendl. f.]

in trials in Australia by inducing germination and then exposing

the seedlings to lethal soil temperatures (Cohen et al. 2008).

Polyethylene mulches have also been successfully integrated

with herbicides for weed control in bell pepper (Capsicum

annuum L.) and other high-value vegetable crops (Bangarwa et

al. 2011; Culpepper et al. 2009; Grey et al. 2007, 2009).

For field crops, the use of synthetic mulches over a larger

production area is impractical and not cost effective. However,

cover crops can be effectively integrated into weed management

programs. A cover crop can be interplanted or undersown

within a cash crop (i.e., living mulch) or sown as a relay or

double crop immediately after harvest and then terminated

before the next cash crop is planted. The cover crop residue is

then incorporated into the soil or left on the surface as a mulch.

In addition to soil improvement and erosion prevention, cover

crops suppress weeds for a substantial period into the cropping

season, delaying the need for an early season herbicide

application (Bergkvist et al. 2010; Kruidhof et al. 2009;

Norsworthy et al. 2010b, 2011b; Teasdale et al. 1991; Yenish

et al. 1996). In addition to competing with, or physically

suppressing weeds, some cover crop species provide weed

control through allelopathy (Ateh and Doll 1996; Hartwig and

Ammon 2002; Putnam et al. 1983).

Allelopathy, the ability of a plant to influence growth of

other plants through biochemicals (i.e., allelochemicals), has

been noted in several cover crop species, including cereal rye

(Secale cereale L.), oat (Avena sativa L.), sorghum [Sorghum

bicolor (L.) Moench ssp. bicolor], wheat, common sunflower

(Helianthus annuus L.), several legumes, and in several

Brassicaceae species (Caamal-Maldonado et al. 2001; Leather

1987; Norsworthy 2004b; Norsworthy et al. 2010b, 2011b;

Putnam and DeFrank 1983; Steinsiek et al. 1982). However,

cover crops should be chosen carefully because some cover

crops may have negative allelopathic effects on the main crop.

For instance, cotton yields were reduced by turnip (Brassica

rapa L. ssp. rapa) cover crop (Norsworthy et al. 2011b), and

corn yields were reduced by oat (Norsworthy 2004b) and rye

(Johnson et al. 1993) mulch. The level of allelopathic activity

is also dependent on a number of factors, including location,

environmental conditions, biomass, and management practices (Schomberg et al. 2006), again, suggesting the need to

make a good cover crop choice for a particular set of

conditions. In addition, some living mulches may adequately

compete with the main crop for resources.

Mowing. Mowing is another mechanical option that can be

used before seed set to prevent weed seed production (Pannell

et al. 2004). Mowing is more effective on annual broadleaf

weeds than it is on grasses and perennial broadleaf species

because mowing effectively prevents propagation in annual

broadleaf weeds by removing the growing point (Meiss et al.

2008); by contrast, many low-growing grasses escape mowing

and will produce seed. Timing of mowing is critical. For

example, mowing spotted knapweed (Centaurea stoebe L.) at

early bolting or budding stages results in new seedhead

production, whereas mowing at flowering greatly reduces

subsequent seed set (Story et al. 2010).

Grazing. Grazing as a means of weed control is feasible in

pasture systems. The effectiveness of grazing for weed control

depends on pasture type, duration of grazing, preferences by

grazing animals, and numbers of grazing animals (grazing

intensity)(Lacey and Sheley 1996; Olson and Wallander

1998; Pannell et al. 2004). Sheep grazing has been shown to

reduce the number of ryegrass plants setting seed by 60 to

80% or 90 to 95% after 3 yr of standard or intense grazing,

respectively (Pannell et al. 2004). An unintended result of

grazing, however, is the likely changes in plant community

composition because of preferential grazing by animals. For

example, leafy spurge (Euphorbia esula L.) was grazed by sheep

but not by cattle (Lacey and Sheley 1996). In general, cattle

grazing can shift the plant community to weedy broadleaf

species, whereas intensive grazing by sheep or goats can shift

the community toward grass species (Lacey and Sheley 1996).

Challenges. Conservation-tillage practices that were established to

prevent soil erosion and promote water conservation will restrict

the incorporation of tillage for weed management. Use of

mechanical methods of weed control will be limited by the

availability of equipment and skilled labor and by the expense of

implementing these practices. In addition, many U.S. Department

of Agriculture (USDA) programs specifically discourage and, in

some instances, prohibit, tillage operations on lands vulnerable to

soil erosion. Other programs provide financial incentives for

conservation tillage, creating an economic disincentive for

incorporating tillage as a tool for resistance management. Once

growers have adopted conservation tillage programs, they may no

longer have a need for retaining tillage equipment.

Although synthetic mulches are effective in high-value

crops, using that system is very expensive. The mulch could be

reused for multiple crops, but some aggressive weeds

deteriorate the integrity of the mulch and limit the number

of seasons that it can be used (Webster 2005b). Further,

removal and disposal of synthetic mulches are also expensive.

For biological mulches, the mulch material itself may be a

source of weed seed (see BMP 10)(Menalled et al. 2004).

Incorporating cover crops and living mulches into a weed

management program can reduce selection for herbicide

resistance, although herbicides may still be used for cover-crop

burndown or to suppress a living mulch and reduce

competition with the main crop (Hartwig and Ammon

2002). Repeated glyphosate treatment of horseweed, a winter

annual weed present in many fields in the spring when cover

crops are burned down, is thought to have led to the evolution

of GR horseweed (Koger and Reddy 2005; Main et al. 2004).

Cover crops can be used to inhibit early season weeds, but

producers must be aware that herbicides used in the

desiccation of the crop should be included in any herbicideresistance management plan.

Although the soil conservation benefits of cover crops are

undeniable, there is often no direct cash return in the form

of a marketable product to offset costs associated with

establishment and termination. One exception is when winter

wheat is planted before soybean, a routine practice in the

southern United States. Additionally, determining the

Norsworthy et al.: Best practices for reducing herbicide resistance N 45
appropriate time for cover-crop termination is complex (Price

et al. 2009) because it requires balancing the ease of killing the

cover crop, amount of biomass produced, and amount of soil

moisture that is being consumed by the cover crop. The

optimum balance will be regional- and often farm-specific and

will vary from year to year, depending on soil type, cover crop

growth, and availability of supplemental irrigation. Thus, the

greatest challenge to the use of any mulch or cover crop is the

added complexity associated with integration into existing

cropping systems.

Repeated use of mowing may lead to a shift in the weed

spectrum, often to one comprising more grasses and lowgrowing broadleaf weeds. Furthermore, mowing all weed

species present within a rangeland or pasture before seed set

can be quite challenging, and mowing is not feasible for most

agronomic or horticultural crops.

Grazing can be an effective way of managing weeds;

however, mature seed of some weed species can pass through

the digestive tract of grazing animals without losing viability

and be redistributed (Lacey et al. 1992). In addition, because

of the increasing specialization in farming operations, the

number of farmers integrating field crops with animal

production has declined, limiting the adoption of grazing

between cash crops.

BMP 10: Prevent Field-to-field and Within-Field Movement of Weed Seed or Vegetative Propagules. The potential

for field-to-field and farm-to-farm movement of weed

propagules, resulting in introduction of new weed species or

HR weeds into noninfested areas must be recognized.

Preventative weed management incorporates practices that

avoid introduction, infestation, or dispersion of weed species

into areas currently free of those weeds. Natural seed dispersal

in the absence of a specialized dispersal mechanism or agent is

generally limited to less than 5 m from the mother plant

(Verkaar et al. 1983), and often, human activities are an

important means of weed seed dispersal (Burton et al. 2006;

Gonzalez-Andujar et al. 2001). Hence, preventing the

immigration of propagules or pollen movement from adjacent

infested areas and preventing propagule dispersal from

infested areas to noninfested areas should be an important

consideration to all weed management programs.

Most weed populations exhibit an aggregated or patchy

distribution (Wiles et al. 1992), attaining spatial stability over

time (Beckie et al. 2005; Marshall and Brain 1999; Rew and

Cussans 1997; Rew et al. 1996; Wilson and Brain 1991). The

degree of spatial stability is more pronounced in perennial

species and in large-seeded weeds, such as wild common

sunflower and velvetleaf, which exhibit shattering and

localized seed dispersal before crop harvest (Burton et al.

2005; Colbach et al. 2000; Dieleman and Mortensen 1999).

Although short-distance, localized dispersal of any weed is still

undesirable, the opportunities for long-distance dispersal

events, followed by rapid colonization, even if less frequent,

are of particular concern for herbicide-resistance management.

In an effective herbicide-based weed management system,

patches of low-density weed populations that do not form a

persistent seedbank can be driven toward extinction (Humston et al. 2005). The key to successful eradication of an HR

weed population is early recognition of resistance, when the

infestation is still minor and manageable, with low seedbank

density. However, when a few resistant plants survive within a

field, their occurrence is often overlooked until the population

builds up to a level that herbicide failure is recognizable; by

which point, seed and pollen carrying resistance alleles may

have dispersed great distances, exacerbating the problem from

a single incidence for an individual producer to one that

affects an entire county, state, or region.

Crop Residues, Manures, and Plant By-Products. Crop residues

and by-products, including composts, organic mulches, straw,

hay, harvest cleanings, or grass clippings, are sometimes used

as amendments, mainly in vegetable crops or home gardens,

to improve soil fertility or reduce weed emergence through

physical suppression. Introduction of new weed species into

fields that previously were free of certain weed species is

possible with introduction of these weed seed-containing

materials. One source of weed seed is livestock food pellets

processed with crop residues (Cash et al. 1998). A crop byproduct that can carry viable weed seed is gin trash, a waste

material from the cotton-ginning process, whereby the cotton

fiber is separated from both the seed and other plant material

adhering to the lint during harvest. Weed seed from plants

remaining at cotton harvest are common components of gin

trash (Norsworthy et al. 2009). In a survey of ginning facilities

in the southern United States, raw gin trash was found to be

routinely applied to crop fields as a disposal method, and it is

likely that the gin trash contains seed from HR weeds present

in cotton fields at harvest (Norsworthy et al. 2009).

Additionally, gin trash is sometimes fed to livestock. A recent

introduction and successful establishment of GR Palmer

amaranth in Michigan is believed to have resulted from gin

trash or hay purchased in the southern United States and then

fed to cattle (C. Sprague, personal communication). Because

the effects of composting gin trash are often variable, viable

weed seed can survive composting (Gordon et al. 2001;

Norsworthy et al. 2009). Therefore, caution should be used

when applying these amendments.

Application of animal manure and open grazing are also

important sources for the introduction of additional weeds to

the soil seedbank, including weed seed with herbicide

resistance (Blackshaw and Rode 1991; Mt. Pleasant and

Schlather 1994; Wiese et al. 1998), because some weed seed

can pass through the digestive tract of ruminants without

losing viability. For example, about 52% of redroot pigweed

(Amaranthus retroflexus L.) seed remained viable after

digestion in the rumen of dairy cattle (Blackshaw and Rode

1991). Additionally, intraregional and interregional transport

of animals that have consumed weed seed–contaminated feed

or ingested weed seed through grazing may contribute to the

movement of weed seed into new areas. For instance, it has

been suspected that cross-state transportation of cattle has

greatly contributed to the spread of tropical soda apple

(Solanum viarum Dunal), a federally listed noxious weed

capable of remaining viable in the rumen of cattle (Byrd et al.

2004). Weed seed dispersal by wildlife, such as deer, birds,

and rodents, is also possible (Goddard et al. 2009; Traveset

et al. 2001).

Manure spreading allows for movement of weed seed that

lack specialized dispersal mechanisms (Thill and MallorySmith 1997). Menalled et al. (2005) reported an addition of

up to 169 weed seed m22 in the soil seedbank when

composted swine manure was applied. Eberlein et al. (1992)

found triazine-resistant Powell amaranth (Amaranthus powellii

46 N Weed Science 12, Special Issue 2012
S. Wats.) in seven fields treated with manure from feedlots,

whereas no resistance was observed in fields on the same farm

that did not receive the manure, suggesting the possible source

of contamination. Introduction of HR weed seed through

manure could rapidly lead to future management problems

for growers. As such, manure should be used cautiously

considering the potential for long-distance dispersal of new

weed species or HR weeds.

Challenges. Preventative weed management can be complex

and challenging. As a result, farmers are often unwilling to

adopt costly preventative practices, especially if the land is

rented or leased (see section on General Challenges to

Adoption of BMPs). In some cases, the cost of actions

preventing the movement of weed seed and propagules in

crop residues and by-products (e.g., straw, stubble) is

prohibitive. As farms grow larger and machinery is moved

greater distances, the importance of proper sanitation to limit

the movement of weed seed, especially HR weed seed, is

essential (Buhler et al. 1998). Effective sanitation of

equipment between fields demands substantial amounts of

time and labor, often affecting farm operations during critical

times, such as during planting, tillage, and harvesting. It can

also be difficult to confirm the effectiveness and quantify the

cost–benefit ratio; only after a weed establishes will the

presence of weed propagules in the machinery be apparent. In

addition, some potential mechanisms for weed seed movement among fields are largely out of the growers’ control, such

as dispersal by wildlife (Goddard et al. 2009), wind (Dauer et

al. 2009; Shields et al. 2006), and water (Kelley and Bruns

1975; Wilson 1980). In a survey of Ohio farmers, most of the

respondents attributed the introduction of new weed seed to

their farm to natural elements, including wind (97%), wildlife

(87%), and birds (80%)(Wilson et al. 2008).

BMP 11: Manage Weed Seed at Harvest and Postharvest

to Prevent a Buildup of the Weed Seedbank. At Harvest.

Unshed weed seed that are present at crop harvest are often

retained in the harvested grain or expelled from the harvester

along with the debris (i.e., straw, chaff, weed seed, among

others)(Shirtliffe and Entz 2005; Walsh et al. 2012). Research

in Australian wheat fields has demonstrated the value of

collecting and destroying weed seed at harvest as a herbicideresistance management tactic (Walsh et al. 2012). However,

the fraction of seed entering the harvester differs considerably

by weed species, depending on maturity characteristics and on

time of crop harvest relative to weed seed maturity. McCanny

and Cavers (1988) found that only 45% of wild oat seed

shatter before crop harvest, whereas about 30 to 45% are

expelled from the harvester and about 11 to 25% are retained

with the harvested grain. The fraction of seed expelled from

the harvester along with the debris could be collected and

destroyed, substantially reducing weed seed additions to the

soil seedbank. Shirtliffe and Entz (2005) found that the chaff

fraction of the debris contained about 74% of the wild oat

seed that were expelled from the harvester, and for ryegrass, it

was up to 95% (Walsh and Parker 2002), suggesting that the

chaff needs greater attention.

Collecting and destroying weed seed at harvest is one of the

most promising strategies to reduce the perpetuation of the

soil seedbank (Maxwell and Ghersa 1992; Walsh et al. 2012;

Westerman et al. 2006;). Maxwell and Ghersa (1992)

suggested that a combine attachment acting as a weed seed

‘‘predator’’ would significantly reduce weed seed dispersal and

limit new introductions. Since then, a number of tactics have

been evaluated in wheat for destroying weed seed at harvest,

the first being the collection of chaff using a cart attached to

the rear of the harvester, which achieved up to 85% removal

of rigid ryegrass from fields (Walsh and Powles 2007; Walsh

et al. 2012). Another tactic involves the use of baling

equipment attached to the rear of the harvester, allowing weed

seed–containing straw and chaff to be baled directly as they

exit the harvester (Walsh and Powles 2007; Walsh et al.

2012). The bales can later be fed to confined livestock. In

Australia, equipment recently developed to destroy weed seed

is the Harrington Seed Destructor, which uses a cage-mill

device comprising bars spinning in opposite directions to

grind weed seed–containing chaff as it exits the harvester

(Walsh et al. 2012). Because up to 95% of ryegrass seed that

enter a harvester exit in the chaff fraction (Walsh and Parker

2002), this equipment has been highly effective in reducing

the amount of HR ryegrass that reenters the soil seedbank.

Similarly, in Oklahoma, hammer and roller mills were tested

and proven effective in destroying the seed of cheat (Bromus

secalinus L.) (Gossen et al. 1998).

After Harvest. After crop harvest, producers often allow weeds

to grow uncontrolled. Such lapses in weed management can

lead to increases in the soil seedbank if sufficient time elapses

between harvest and a weed-killing frost, even if a high level of

weed control was achieved during crop production. Postharvest weed-seed production must be prevented to effectively

manage soil seedbanks for the long term (see BMP 2). In some

weeds, such as common waterhemp, viable seed production

can occur as early as 7 d after pollination (Bell and Tranel

2010), meaning that these weeds are capable of producing

seed after crop harvest, especially in southern climates.

Burning of stubble or chaff that contains weed seed is a

postharvest tactic that has been used effectively in Australia to

reduce weed seed return to the soil seedbank (Walsh et al.

2012). Disengaging the chaff spreaders in harvesters allows

expelled debris to be windrowed, reducing in-field dissemination of weed seed (Maxwell and Ghersa 1992) and, most

importantly, concentrating weed seed in a narrow band of

chaff that can be burned later. Harvesters in Australia have

been fitted with chutes that direct the chaff into bands much

narrower than those produced solely by disengaging the straw

spreader (Walsh and Newman 2007). The narrow bands of

chaff burn at much hotter temperatures over longer periods,

resulting in greater weed seed mortality. For instance, up to

99% of annual ryegrass seed were killed by narrow, windrow

burning, whereas burning of standing stubble was found to

kill only 80% of the seed (Walsh and Newman 2007).

Encouraging postdispersal weed-seed herbivory and microbial decay is another important tactic for reducing seedbank

replenishment. Understanding the effect of production

practices on herbivory and microbial decay of weed seed is

necessary to make informed management decisions that

promote reductions in the soil seedbank. Degrees of herbivory

may differ among weed species and may also be influenced by

postharvest field management, such as tillage, residue cover,

etc. Annual weed seed losses due to herbivory can exceed 80%

(Harrison et al. 2003; Kremer and Spencer 1989). Although

tillage does not always deleteriously affect seed herbivory

Norsworthy et al.: Best practices for reducing herbicide resistance N 47
(Cardina et al. 1996; Cromar et al. 1999), soil disturbance

that buries weed seed can limit herbivory by surface-dwelling

organisms (Hulme 1994). The amount and duration of

vegetative cover can also affect herbivory rates (Chauhan et al.

2010), with systems having some degree of residue cover

generally being more attractive to seed herbivores than are

fully exposed fields (Povey et al. 1993; Reader 1991).

Factors that influence seed decay include tillage intensity,

depth of burial, and interactions with seed herbivores (Kremer

1984). For example, greater numbers of foxtail species occurred

near the soil surface in a no-till system, whereas the number of

buried seed was greatest in a plowed system (Pitty et al. 1987).

Additionally, microbial action on seed can interact with

herbivory, especially when insect feeding leads to increased

fungal infection of weed seed (Kremer and Spencer 1989).

Challenges. One challenge of targeting harvest and afterharvest seedbank replenishment is the need to know the

growth and reproductive biology, including the duration of

vegetative and reproductive stages, time of weed maturity, and

initiation of seed shattering, of multiple weeds (Norris 2007).

Such knowledge is currently lacking for many arable weed

species in different crops and geographical regions. Although

collection of weed seed at harvest is relatively easier,

concentration of weed seed in windrows presents engineering

challenges that have not been studied in U.S. cropping

systems. In Australia, chaff carts or baling were not widely

adopted for weed seed removal because of reduced harvest

efficiency; machinery breakdowns; management costs of

having to remove the residue, usually by burning; and

increased capital costs (Walsh and Newman 2007). However,

new engineering efforts have overcome several of these

limitations (Walsh et al. 2012).

Adoption of stubble burning for weed seed removal may be

limited by factors such as local or regional environmental

regulations, the need for crop stubble to increase soil organic

matter, and the potential reduction in herbicide efficiency by

creating a carbon layer that can readily bind and limit

herbicide activity (Banks and Robinson 1982; Potter et al.

2008). Postdispersal weed-seed herbivory is a potentially

important, but often overlooked, component of preventing

seed additions to the soil seedbank (Harrison et al. 2003;

Jacob et al. 2006; Seaman and Marino 2003). Evaluation of

factors that enhance seed herbivory is still a critical research

need (Cromar et al. 1999; Gallandt 2006; Menalled et al.

2001). Likewise, a thorough ecological understanding of how

production practices can be used to encourage microbial decay

of soil seedbanks (Chee-Sanford et al. 2006), and the

influence of management practices on the combined effects

of herbivory and microbial decay has been limited. Research

on enhancing herbivory and microbial decay as a weed

management tool is still in the descriptive stage and yet to be

extended to growers.

BMP 12: Prevent an Influx of Weeds into the Field by

Managing Field Borders. Weeds in unmanaged habitats,

including field margins, roadsides, ditchbanks, railroads, and

riparian zones can serve as a corridor for the introduction and

movement of new weed species (Boutin and Jobin 1998),

including HR weeds. The density, diversity, and fecundity of

wild and weedy plant populations are generally greatest along

field edges and decrease with increasing distance into crop

fields (Marshall 1989; Wilson and Aebischer 1995). Although

research emphasis on field borders as a source of weed seed

introduction is limited in the United States, there has been a

wealth of research in this area in European countries, mainly

as an opportunity for preserving farmland biodiversity (Kiss et

al. 1997; Marshall 1989; Noordijk et al. 2011; Wilson and

Aebischer 1995). In the context of herbicide-resistance

management, allowing resistance-prone weeds to produce

seed in field borders may exacerbate the risk of resistance.

Most crop producers in the United States place emphasis

on maximizing crop yields by management for weed removal

within fields but often neglect to manage field borders, from

which some of the most invasive and noxious weeds can find

ingress via seed dispersal or through gradual movement of

vegetative propagules, especially in the absence of tillage.

Allowing weed seed production in field borders can have longterm effects on the seedbank of crop production fields,

especially when outcrossing occurs with resistant populations

near a field, allowing spread of resistance through pollen and

seed movement. Simulation models have shown that weeds

with high dispersal potential located in or near field borders

have greater potential to deleteriously affect crop yields across

a field compared with more-competitive weeds that have a low

potential for dispersal (Maxwell and Ghersa 1992). Several of

the tactics discussed previously can be used to minimize weed

entry from field borders, roadsides, and ditchbanks (see BMPs

1 and 2).

Sowing field borders to a less-weedy, preferred species that

can be managed later is another tactic for reducing

immigration of agriculturally important weeds from field

edges. However, Noordijk et al. (2011) concluded that regular

haying as a field-edge management tactic is more effective in

preventing weed seed immigration than sowing wildflower

mixtures along field margins. Planting and maintaining a

dense grass cover in field borders may prevent the

establishment of arable weed communities through shading

and regular mowing. In many parts of the United States,

roadsides and ditchbanks are managed by county and state

entities through mowing, haying, and herbicide applications.

In the southern United States, where GR Palmer amaranth

is a serious problem on many farms, weeds in ditchbanks and

along roadsides cause serious concern because of potential seed

entrance into adjacent crop fields (Bennett 2011). Glyphosate

was traditionally used to manage field margins, but because of

the widespread occurrence of resistant Palmer amaranth, other

means, such as mowing, followed by repeat applications of

paraquat or complete disking have been followed to prevent

seed production in these populations.

Challenges. The willingness of producers to manage field

borders is often limited by time constraints and added

management costs for which there is no direct economic

return. Additionally, the management programs used to

control weeds in the adjacent habitats can hasten the selection

pressure for herbicide resistance, if few herbicides are

continually used. For instance, use of an ALS-inhibiting

herbicide for grass weed control on roadsides (Goatley et al.

1998) increases selection pressure for the evolution of ALSresistant weeds that may coexist in managed rights-of-way and

production fields in the vicinity. Although farmers diversify

their approaches to weed management in field edges, they

have limited knowledge of, or control over, such practices in

48 N Weed Science 12, Special Issue 2012
ditchbanks and rights-of-way, railroads, power lines, and

other similar habitats, which are usually managed by

government entities or third parties. Coordination of control

efforts with other owners, local or state governments, and

third parties is difficult to achieve.

A detrimental effect of managing field borders can be a

reduction in farmland biodiversity (Noordijk et al. 2011).

Field borders could greatly contribute to the preservation of

biological diversity, particularly as sanctuaries for beneficial

insects and rare plants and as food sources for birds and other

organisms (Wilson and Aebischer 1995). It appears that the

goals of herbicide-resistance management to encourage

prevention of weed seed production in field borders may

be incompatible. As such, management decisions will be

discretionary, depending on the specific goals of the grower.

General Challenges to Adoption of BMPs

Patterns of Land Ownership. The long-term economic

benefits of BMPs are certain; however, many growers, out of

necessity, focus on immediate economic benefits. Nationwide,

approximately 38% of U.S. farmland is operated by tenants,

and in parts of the Midwest and Mississippi Delta regions of

the United States, more than 50% of the farms are tenant

operated (USDA-NASS 2007), creating an enormous challenge for promoting BMPs with a long-term view. Farmers

usually approach weed management on these rented farms

differently than they do on land they own and plan to farm

for many years (Soule et al. 2000). Growers on rented land

often focus their attention on the economic returns of the

current crop and are much less concerned with the long-term

effects of management decisions on land they may not farm

the following year. Also confounding the issue of rented land

are restrictions imposed by the landlord on the tenant

concerning crop rotations or, more specifically, concerning a

particular crop to be grown annually without rotation.

Educational programs that target landowners could be

beneficial in altering that behavior; however, there may be

economic issues involving vertical integration and investment

of infrastructure of the farming operation that facilitates those

management decisions. For instance, a land owner who

operates a cotton-ginning facility will often rent only to

tenants who will grow cotton to supply the raw materials for

that business. Regardless of the motivation behind this

practice, weed management strategies available to the tenant

because of these stipulations can be significantly hampered in

a monoculture system.

Grower Perceptions of Future Technology and Effectiveness of Current Practices. There are at least two additional

factors that may hinder the adoption of practices that

contribute to herbicide stewardship: (1) the expectation that

the release of a new technology to solve the HR weed

problems is imminent, and (2) the belief that resistancemanagement strategies will be futile (Webster and Sosnoskie

2010). In Australia, a survey revealed that growers who

believed new herbicide chemistries would be available soon

were less likely to adopt strategies designed to delay the

occurrence of HR weeds, compared with those who were

uncertain about those alternatives (Llewellyn et al. 2007). The

basis for this belief is rooted in recent history (Webster and

Sosnoskie 2010). For instance, triazine-resistant weeds were

effectively managed with ALS-inhibitor herbicides following

their release. Once weeds subsequently evolved resistance to

ALS-inhibitors and ACCase inhibitors, GR crop technologies

allowed growers to effectively manage those HR species, and

many farmers believe that this trend will continue.

Although it is a common perception that new herbicides or

herbicide technologies will be developed to combat HR

weeds, there has not, unfortunately, been a new herbicide

MOA introduced commercially since 1998. In fact, the

dominance of glyphosate and GR crops resulted in less

investment in research and development from agrichemical

companies (Duke 2011). A significant decline in herbicidediscovery research occurred during the period from 1995

through 2010 (Gerwick 2010).

The assessment that herbicide-resistance management

strategies will be unsuccessful is based on the belief that

herbicide resistance in weeds occurs randomly and cannot be

contained. Not only can resistance traits be moved through

seed dispersal by natural elements (e.g., water, wind, and

fauna) and agricultural equipment, but they can also be

transferred through pollen movement (Busi et al. 2008;

Hidayat et al. 2006; Murray et al. 2002; Sosnoskie et al.

2009). Transfer of herbicide-resistance genes among weed

populations through pollen or seed movement has significant

implications for resistance prevention and management. In a

survey conducted in Australia, about 70% of the growers

suspected that their neighbors were the ultimate source of HR

weeds currently on their farms, through either the movement

of seed or the movement of pollen (Llewellyn and Allen

2006). Given the potential for the movement of herbicideresistance traits among weedy populations, many growers

reject the assumption that investment in resistance management programs will delay the occurrence of herbicide

resistance on their farm (Llewellyn and Allen 2006).

Hardin (1968) first proposed the concept of the ‘‘tragedy of

the commons,’’ whereby a commonly held resource is

depleted by those who exploit it for individual gain, even

though the ultimate loss of the resource is collectively

disadvantageous. This idea has been expanded and applied

to agricultural settings by others (Gould 1995; Webster and

Sosnoskie 2010). Hardin provides the example of a shared

resource, such as a body of water, spoiled by individuals or

businesses who found discharging wastes into that water was

more economical for them than the costly treatment of the

waste. Likewise, indiscriminate herbicide use leading to the

rapid evolution of herbicide resistance in weeds may result in

the loss of herbicide options for all growers, especially if

resistance spreads rapidly across a region. Spoiling of the

common resource of pesticide susceptibility by some growers

can make others reluctant to spend time and money to

implement management programs that deter the evolution of

pesticide resistance (Gould 1995). Additionally, it is expected

that the adoption of BMPs for resistance management will be

slow because of the steep learning curve associated with

proficient use of new and often complex technologies and

because of the challenge of clearly demonstrating the benefit

of practices involving short-term costs for longer-term

prevention. The key challenge is to develop and implement

a sustained education and awareness campaign to overcome

the various deterrents to adoption of resistance-management

practices and to establish or strengthen local networks of

farmers willing to work in concert.

Norsworthy et al.: Best practices for reducing herbicide resistance N 49
Current Levels of Adoption of BMPs

Given the growing number of confirmed HR weeds in crop

production systems and the BMPs that can be used to delay

the evolution of resistance, it is important to assess the extent

to which growers are adopting BMPs. Unfortunately, there is

no single national database available that addresses this

question directly. It is possible to infer levels of grower

adoption of some BMPs from published sources, but, for

many BMPs, no accessible data exist. Although the BMPs

described above apply to managing resistance regardless of the

herbicide in question, most of the available information on

grower implementation of resistance management practices

focuses on glyphosate because of the magnitude of its use in

row-crop agriculture and the consequent concern about the

growing problem of weed resistance to glyphosate. However,

these data still provide insights into the current state of

adoption of BMPs for herbicide-resistance mitigation and

management by growers in the United States.

The USDA Agricultural Resource Management Survey

(ARMS) provides data on trends in adoption of HR seed

varieties and weed control practices for cotton, corn, and

soybean in the United States. The most recent survey years for

which results are available for those crops are 2005, 2006, and

2007. Data recorded for each crop include whether (1) HR

varieties were planted, (2) fields were scouted for weeds, (3) a

burndown herbicide was used, (4) PRE herbicides were used,

(5) POST herbicides were used, and (6) tillage was employed

for weed control. Collectively, this information provides some

insight into the adoption of BMPs for herbicide-resistance

management, especially in HR crops, and the extent of

diversification implemented in weed management programs.

However, some caveats apply when interpreting these data, as

indicated by the following examples.

The USDA-ARMS data indicate that field scouting was

carried out for weeds on 89% of the corn hectares planted in

2005, 91% of the soybean hectares planted in 2006, and 92%

of the cotton hectares planted in 2007 (Table 1). Although

these appear to be high levels of scouting in all three crops, it

should not be assumed they represent a widespread adoption of

BMP 5. First, details on whether the scouting was carried out

before, after, or both before and after herbicide applications

were not recorded. As noted previously, field scouting has little

utility in detecting emerging resistance problems unless it is

carried out following the herbicide use. Second, the high levels

of scouting may simply reflect a move from preventive weed

management to a curative approach, one that greatly depends

on POST herbicide treatments. An increase in POST herbicide

use promoted by a parallel increase in the adoption of HR

cultivars is shown for soybean and cotton in Table 1. That

trend in adoption of HR cultivars was less apparent for corn

(Table 1), where HR cultivars constituted only 31% of total

corn hectares in 2005, much lower than it was for soybean or

cotton (97 and 90%, respectively) in that year. USDA-ARMS

data are only available up to 2005, but HR seed in corn is now

estimated at 70% for 2010 (USDA 2011).

Use of a burndown herbicide indicates that producers are

attempting to plant into a weed-free field, although burndown

treatments are employed in the Mississippi Delta preceding

spring tillage as well (BMP 3). This practice is used extensively

in reduced tillage systems where the burndown herbicide

replaces preplant tillage to control existing weeds. Burndown

herbicides were used on 31% of the soybean hectares planted in

2006, 41% of the cotton hectares planted in 2007, and 18% of

the corn hectares planted in 2005 (Table 1). Concurrent with

the expansion of conservation tillage in cotton, use of

burndown treatments increased dramatically, expanding from

6 to 41%, reflecting the adoption of transgenic HR varieties

and reduced tillage systems in this crop. There was little change

in the use of burndown herbicides in soybean and corn over the

reported period. Unfortunately, burndown herbicide use is not

an effective component of resistance management if the same

herbicide is used for both burndown and later POST

applications, which is likely to be the case in GR crops because

glyphosate has been the preferred choice for both of these

applications. In GR crops, growers typically make more than

one application of glyphosate and, in some areas, make more

than three applications (Norsworthy 2003; Norsworthy et al.

2007d; Wilson et al. 2011). Repeated use of the same herbicide

in the same season is not consistent with weed management

diversification (BMP 2) or the use of multiple herbicide MOAs

(BMP 6).

Cultivation for weed control adds diversity to a herbicidebased weed management system (BMP 2). Cultivation was

practiced on 38% of U.S. cotton hectares in 2007 and on

15% of the corn hectares in 2005 (Table 1). The last reliable

estimate of cultivation for weed control in soybean was 13%

in 2002. Weed management through cultivation has steadily

decreased with the adoption of transgenic HR varieties of

soybean, cotton, and corn and is now less than half the levels

reported in the late 1990s. This reduction reflects the efficacy

of current herbicide-based weed control systems and the

increased adoption of reduced-tillage systems that this efficacy

has allowed.

Table 1. National trends in weed management for soybean, cotton, and corn.a,b

Practice

Soybean Cotton Corn

1996 1998 2000 2002 2006 1997 1999 2000 2003 2007 1996 1998 2000 2001 2005

-----------------------------------------------------------------------------------------------------% of ha with practice used (RSEc

) ----------------------------------------------------------------------------------------------------

GM HR seed 7 (13) 44 (4) 59 (4) 81 (6) 97 (5) NA 40 (6) 58 (10) 67 (3) 90 (2) NAd 11 (12) 11 (7) 9 (45) 31 (6)

Field scouted for weeds 79 (1) 85 (1) 85 (2) 82 (12) 91 (1) 71 (3) 80 (3) 82 (3) 85 (2) 92 (1) 81 (2) 85 (1) 83 (2) 50 (19) 89 (1)

Burndown herbicide used 33 (8) 27 (11) 27 (9) 27 (96) 31 (3) 6 (27) 10 (15) 23 (14) 56 (16) 41 (6) 9 (8) 9 (15) 12 (14) 15 (21) 18 (10)

PRE weed control 67 (3) 58 (5) 46 (4) 16 (24) 28 (4) 90 (2) 89 (2) 79 (1) 71 (3) 73 (2) 78 (2) 78 (2) 71 (4) 54 (27) 61 (4)

POST weed control 78 (2) 79 (2) 87 (2) 91 (4) 95 (1) 62 (5) 70 (3) 76 (3) 79 (3) 89 (2) 59 (5) 62 (2) 63 (2) 58 (6) 66 (3)

Cultivated for weed control 29 (4) 26 (8) 17 (10) 13 (25) NAd 89 (2) 77 (2) 63 (8) 54 (5) 38 (6) 33 (16) 42 (5) 38 (6) 37 (21) 15 (8)

a Abbreviations: RSE, relative standard error; GM, genetically modified; HR, herbicide resistant; USDA, U.S. Department of Agriculture.

b Source: Agricultural Resource Management Survey, USDA; http://www.ers.usda.gov/Data/BiotechCrops and http://www.ers.usda.gov/Data/ARMS/app/default.

aspx?survey5CROP#startForm. c The RSE is the standard error of the estimate expressed as a percentage of the estimate; the larger the RSE, the less precise the estimate.

d Estimate does not comply with the USDA–Economic Research Service disclosure limitation practices, is not available, or is not applicable.

50 N Weed Science 12, Special Issue 2012
The data from Table 1 indicate that a combination of all five

weed management practices (scouting, burndown, PRE and

POST herbicide applications, and tillage) were used on less

than 10% of the hectares in any of these crops. Weed control on

an estimated 25% of the soybean hectares in 2006, 25% of the

cotton hectares in 2007, and 8% of the corn hectares in 2005

combined scouting, PRE herbicide use, and cultivation. Use of

PRE and POST herbicides on the same land occurred on

approximately 28% of the soybean, 65% of the cotton, and

40% of the corn hectares at the last reporting period for each

crop. Based on those estimates, it appears that most of the U.S.

hectares planted to those three crops were not under a

diversified weed management program (BMP 2) that integrates

multiple herbicide MOAs (BMP 6). Both of those BMPs are

foundations for herbicide-resistance management.

A closer examination of weed management trends in major

soybean- and cotton-producing regions (Table 2) shows a

dramatic decrease in the use of PRE herbicides and cultivation

with the post-1996 advent of GR crops. For example, in the

Lake States in 1998, 55% of the soybean hectares were treated

with a PRE herbicide and 46% were cultivated. By 2006, the

hectares had dropped to 11 and 0% (acreage too low to

estimate accurately), respectively; however, there has been a

recent upward trend in the use of PRE herbicides in areas were

GR weeds are prominent. In 1998, 93% of cotton hectares in

the U.S. Delta region were treated with a PRE herbicide and

90% were cultivated to control weeds. These dropped to 62

and 11%, respectively, by 2007. The adoption of HR seed in

soybean and cotton was accompanied by widespread

discontinuation of cultivation and PRE herbicide use and a

shift to predominantly POST herbicide-based weed management programs. Such programs initially provided high levels

of control, and their efficacy—including the decreased need

for cultivation—allowed the adoption of conservation tillage

in areas where weed problems had previously prevented its

use. However, that shift has greatly reduced the diversity of

weed management practices. As previously noted, the current

(2010) estimate of HR seed use in corn (70% of national

hectares) is equal to that of cotton and 70% of that in soybean

(USDA 2011). As described below, recent grower surveys

show that weed management practices in HR corn are

becoming less diverse, mirroring the trend already seen in

soybean and cotton. A variety of practices are still being used

in the Pacific and Southern Plains regions, and the occurrence

of herbicide resistance appears lower in those regions.

An alternative approach to assessing the diversity of weed

management programs is to examine the range of herbicides

being used in a crop. Table 3 lists the major herbicide families

and their MOAs used in 1996 and 2006 in soybean, in 1996

and 2007 in cotton, and in 1996 and 2005 in corn. For all

three crops, there was a reduction in MOA diversity between

the earlier and later dates. In soybean and cotton, there was a

dramatic shift to dominance of glyphosate over that period,

whereas the use of several other herbicide families with

different MOAs effectively ceased. Glyphosate (WSSA group

9) was used on 19% of the treated corn acres in 2005, less

than soybean or cotton. Between 1996 and 2005, the use of

several other herbicide families on corn, notably the amides,

benzoics, and sulfonylureas, declined sharply, whereas the use

of triazine herbicides increased from 38 to 48% of the

herbicide-treated corn acres. Overall, with the exception of

triazine use in corn, there has been a marked trend toward

reduced diversity of herbicide MOAs being used in soybean,

corn, and cotton. That contravenes BMP 6 and greatly

increases the selection pressure for resistance.

Surveys of grower practices provide another vehicle for

examining levels of adoption of BMPs for herbicide-resistance

management. Shaw et al. (2009) and Givens et al. (2009)

described the results of a 2005 survey of 1,050 producers from

Illinois, Indiana, Iowa, Mississippi, Nebraska, and North

Carolina. Survey participants were selected from a list of corn,

soybean, and cotton growers who had signed agreements to

use Roundup Ready (Monsanto Company, St. Louis, MO

63167) technology incorporating transgenic GR crop varieties

into their cropping systems. When producers were questioned

about changes in herbicide use on their farms, it was found

that nonglyphosate-based weed management programs were

being replaced in corn or had already been replaced in

soybean and cotton by glyphosate as the foundational, if not

the only, herbicide used to manage weeds. Furthermore, in a

single season, glyphosate was applied three times by between

30 and 40% of the GR cotton growers and two or additional

times by 66 to 74% of the GR soybean producers (Givens et

al. 2009). Follow-up surveys in 2006 and 2007 found that

GR cotton growers were using glyphosate on fewer hectares

(13%), whereas 83% of GR cotton land was treated with

residual herbicides either PRE or POST. In contrast, weed

management was dependent on glyphosate alone on 62% of

the hectares in a GR corn–GR soybean rotation, 55% of the

hectares in continuous GR soybean, and 49% of the hectares

in continuous GR corn (Shaw et al. 2011; Wilson et al.

2011). Together, these data indicate failure to follow either

BMP 2 or BMP 6 in those crops by a substantial proportion

of growers.

Table 2. Regional trends in weed management for soybean and cotton.a–c

Practice

Soybean Cotton

Northern

Plains Lake States Corn Belt Delta Appalachian Pacific

Southern

Plains Delta Appalachian Southeast

1998 2006 1998 2006 1998 2006 1998 2006 1998 2006 1998 2007 1998 2007 1998 2007 1998 2007 1998 2007

--------------------------------------------------------------------------------------------------------------------------------% of ha -------------------------------------------------------------------------------------------------------------------------------

GM HR seed 52 99 30 96 43 97 47 97 64 97 NAd 97 34 92 24 90 63 84 33 87

PRE weed control 56 23 55 11 59 33 52 35 70 40 96 57 90 79 93 62 83 68 85 81

Cultivated for weed control 24 NA 46 NA 24 NA 28 NA 3 NA 100 95 67 61 90 11 45 6 66 6

a Abbreviations: GM, genetically modified; HR, herbicide resistant; USDA, U.S. Department of Agriculture.

b Source: Agricultural Resource Management Survey, USDA. c Regions: Northern Plains—North Dakota, South Dakota, Kansas, Nebraska; Lake States—Minnesota, Michigan, Wisconsin; Corn Belt—Ohio, Iowa, Missouri,

Indiana, Illinois; Delta—Louisiana, Arkansas, Mississippi; Appalachian—West Virginia, Tennessee, North Carolina, Virginia, Kentucky; Pacific—Oregon, California,

Washington; Southern Plains—Texas, Oklahoma; Southeast—South Carolina, Alabama, Georgia, Florida. d Estimate does not comply with the USDA–Economic Research Service disclosure limitation practices, is not available, or is not applicable.

Norsworthy et al.: Best practices for reducing herbicide resistance N 51
A follow-up survey to Shaw et al. (2009) was conducted in

2010 to measure changes in attitudes, perceptions, and

practices between 2005 and 2010 (Prince et al. 2012a).

Growers were more likely to see glyphosate resistance as a

problem in the 2010 survey (Prince et al. 2012b), particularly

in the southern United States. Growers were also able to

identify specific GR weeds in their state or county (Prince et

al. 2012c). As a result, more growers reported using MOAs

other than glyphosate in the 2010 survey (Prince et al.

2012d). Growers also had a greater recognition that tillage

and residual herbicides were effective herbicide-resistance

management tools (Prince et al. 2012e).

Frisvold et al. (2009) described a 2007 survey of more

than 1,000 corn, cotton, and soybean producers who were

asked about their use of the following 10 BMPs for HR

weeds: (1) scouting fields before applying herbicides; (2)

scouting fields after applying herbicides; (3) starting with a

clean field, accomplished either by a burndown herbicide or

tillage; (4) controlling weeds when they were relatively small;

(5) controlling weed escapes and preventing weeds from

setting seed; (6) cleaning equipment before moving it to a

new field to minimize weed seed spread; (7) planting new

seed that is as weed free as possible; (8) using multiple

herbicides with different MOAs; (9) supplementing herbicides with tillage; and (10) using the label-recommended

herbicide rate. Most growers reported always or often

practicing six of the BMPs (scouting before or after herbicide

application or both, planting into a clean field, controlling

weeds early and preventing escapes, planting new weed-free

seed, and applying herbicide at the labeled rate) (Table 4).

However, a significant proportion of participating growers

reported rarely or never cleaning equipment before moving

between fields (54%), rarely or never using multiple

herbicides with different MOAs (28%), and rarely or never

using supplemental tillage (53%).

Adoption patterns were similar across producer groups: the

same seven most widely adopted BMPs were practiced by

71% or more of corn, cotton, or soybean producers (Frisvold

et al. 2009). Use of multiple herbicides with different MOAs,

cleaned equipment, and supplemental tillage were used

infrequently by the three producer groups: fewer than half

of any of those producers practiced these three BMPs often or

Table 3. Herbicides, by herbicide family, applied to soybean, cotton, and corn by percentage of total treated area.a

Herbicide family

Herbicide group

based on mechanism

of actionb

Soybean Cotton Corn

1996 2006 1996 2007 1996 2005

---------------------------------------------------------------------------% of ha with practice used (RSEc

) --------------------------------------------------------------------------

Amides 15 4 (18) 2 (12) NAd NA 27 (2) 4 (10)

Aryloxyphenoxy propionic acid 1 7 (7) NAc NA NA NA NA

Benzoic 4 NA NA NA NA 10 (6) 2 (18)

Benzothiadiazole 6 4 (24) NA 3 (12) 1 (20) NA NA

Dinitroaniline 3 20 (8) 3 (9) 26 (7) 14 (4) NA NA

Diphenyl ether 14 8 (9) 1 (24) NA NA NA NA

Imidazolinone 2 21 (5) 2 (12) NA NA NA NA

Organic arsenical 9 NA NA 12 (9) 1 (18) NA NA

Oxime 1 7 (11) 1 (17) NA NA NA NA

Phenoxy 4 5 (14) 5 (9) NA NA 5 (7) 3 (14)

Phosphinic acid 9 10 (8) 77 (1) 3 (10) 60 (2) 2 (21) 19 (5)

Pyridine 12 NA NA NA NA 0 (37) 6 (9)

Sulfonylurea 2 9 (6) NA NA NA 11 (7) 5 (18)

Triazine 5 4 (18) 1 (17) 13 (8) 2 (15) 38 (2) 48 (3)

Urea 7 NA NA 20 (5) 6 (8) NA NA

Other herbicides 2 (13) 6 (6) 23 (7) 17 (4) 8 (6) 9 (8)

a Source: Agricultural Resource Management Survey, U.S. Department of Agriculture; http://www.ers.usda.gov/Data/ARMS/app/default.aspx?survey5CROP#

startFor. b The number is the Weed Science Society of America classification (Herbicide Handbook 2007). c The relative standard error (RSE) is the standard error of the estimate expressed as a percentage of the estimate; the larger the RSE, the less precise the estimate. d Estimate does not comply with the U.S. Department of Agriculture–Economic Research Service disclosure limitation practices, is not available, or is not applicable.

Table 4. Frequency of herbicide-resistance best management practice implementation as percentage of all respondents from a 2007 survey of 1,000 U.S. corn, cotton,

and soybean growers.a,b

BMP Often or always Sometimes Rarely or never

----------------------------------------------------------------------------------------------------------% of respondents ---------------------------------------------------------------------------------------------------------

Scout before herbicide application 1 11 5

Scout after herbicide application 81 15 4

Plant into clean field 75 13 12

Control weeds early 89 9 2

Control escapes 79 15 6

Clean equipment 25 20 54

Plant new weed-free seed 94 3 2

Use different herbicide MOAs 39 33 28

Supplemental tillage 21 26 53

Apply herbicide at labeled dose 93 4 1

a Abbreviation: BMP, best management practice; MOA, mechanism of action.

b Adapted from Frisvold et al. 2009.

52 N Weed Science 12, Special Issue 2012
always. More corn producers used multiple herbicides with

different MOAs often or always (49%) than did either cotton

(38%) or soybean (28%) growers.

In summary, most corn, soybean, and cotton growers in the

United States are using at least some herbicide-resistance

BMPs. However, a significant fraction of growers are not

practicing adequate, proactive herbicide-resistance management. The fields of these growers are likely to be the places

where HR weeds are evolving, and that lack of stewardship

may discourage other growers from being more diligent about

resistance management. In particular, diversifying weed

management practices (BMP 2) and using multiple herbicide

MOAs (BMP 6) are key resistance-management recommendations that need to be more widely implemented. Recent

survey data suggest a shift from sole reliance on glyphosate to

the reintroduction of residual herbicides with more-diverse

MOAs into weed management programs among U.S. cotton

growers. Unfortunately, that may reflect a reactive response to

the emergence of hard-to-control GR weeds in cotton fields,

rather than a proactive attempt to prevent resistance

evolution. Whether this pattern will be repeated in corn,

soybean, and other crops remains to be seen.

There is also a clear need for more-extensive and moresystematic surveys of grower adoption of herbicide-resistance

BMPs; those data will be vital for determining the

effectiveness of educational and other measures encouraging

their use.

Recommendations for Adoption of BMPs

Recommendation One: Reduce the Weed Seedbank

through Diversified Programs that Minimize Weed Seed

Production. Sustainable herbicide-resistance management

requires a much longer-term perspective than that of a single

season. Maintaining low weed-seed numbers in the soil

seedbank reduces the number of plants that will be exposed to

future herbicide treatment and can substantially reduce

resistance risk (Neve et al. 2011a). Thus, there are advantages

to controlling weed seed production even if such management

might not lead to direct economic benefits in the current

season.

For herbicide-resistance management, preventing viable

weed seed production is necessary. Occasionally, there will be

failures, and weeds capable of producing seed may not be

controlled. In situations in which initial weed management

efforts are unsuccessful, producers must take appropriate

action to prevent weed seed production or vegetative

propagation, including postharvest weed control.

Although model data show that weed seed added to the soil

seedbank increases the risk of resistant weeds, research is

needed on more weed species, locales, and production

scenarios to confidently incorporate vital weed-seed reduction

strategies into weed resistance BMPs. Additionally, preventing

seed production, including after the harvest, must be strongly

emphasized by extension educators, local distributors, and

others communicating these BMPs to producers and

encouraging their use.

Recommendation Two: Implement a Herbicide-MOA

Labeling System for All Herbicide Products, and Conduct

an Awareness Campaign. Given the number of different

herbicide MOAs, the unfamiliarity of MOA specification to

nonspecialists, the frequent use of mixed MOA herbicide

products, frequent changes in herbicide products in the

market, and the complexity of herbicide chemistry and MOA

biochemistry, there is a need for the introduction of clear,

standard herbicide-MOA labeling on herbicide use instructions (labels) and containers. To reduce overreliance on

particular herbicide MOAs and to enable diversity in

herbicide use, there must be an easy and consistent

identification of product MOAs. All herbicide products

should be clearly identified by standard MOA designations.

The inclusion of MOAs on labels is essential for basic

communication and will facilitate programs to better inform

producers and their advisors of the importance of herbicide

diversity. This recommendation is supported by the positive

experience of establishing MOA labeling in Australia and

Canada, where MOA labeling has facilitated awareness and

adoption of herbicide-MOA rotation as a resistance management strategy (Beckie 2006; Shaner et al. 1999). Currently,

some herbicide manufacturers in the United States voluntarily

provide MOA labeling on their products.

To be successful, MOA labeling must be implemented and

supported by manufacturers. However, given the diversity of

manufacturers, suppliers, products, and product names, some

form of industry-wide coordination and compliance will be

needed to ensure comprehensive implementation. In countries

that have adopted the practice, the herbicide industry selfregulated to ensure MOA labeling appeared on all herbicide

products and associated materials (Beckie 2006; Shaner et al.

1999). Should self-regulation not be implemented or not be

effective, regulatory authorities could mandate MOA labeling.

Ideally, the herbicide industry and the scientific community

would work together with regulators to achieve MOA

labeling. In Australia, mandatory herbicide MOA labeling

was cooperatively achieved 15 yr ago. Establishing a similar

requirement, whether by agreement among companies or

through a regulatory agency, is essential to the implementation of herbicide-resistance management in the United States.

Both the Weed Science Society of America (WSSA) and the

Herbicide Resistance Action Committee have developed

simple, clear MOA designations. Because only the WSSA

MOA designations are approved in the United States and

Canada for use on herbicide labels, the WSSA system should

be used for this application.

We propose MOA labeling with a unified program of

education jointly developed, supported, and espoused by the

WSSA, pesticide manufacturers and their trade association, and

the purveyors of HR cultivars. We also propose that herbicide

MOAs for each recommended herbicide be included and easily

referenced in all state recommendation guides and websites.

Recommendation Three: Communicate that Discovery of

New, Highly Effective Herbicide MOAs Is Rare and that

the Existing Herbicide Resource Is Exhaustible. The

international herbicide-discovery industry has been spectacularly successful in producing new herbicides in the second half

of the 20th century. Producers are aware of this success and,

consequently, have high expectations that new herbicides

capable of overcoming resistance issues will continue to enter

the market (Llewellyn et al. 2002; Wilson et al. 2008).

However, the reality is that herbicide discovery has slowed

dramatically in recent years, as is evident in the reduction in

the number of herbicide patents issued (Ruegg et al. 2007).

Norsworthy et al.: Best practices for reducing herbicide resistance N 53
Therefore, to encourage crop producers to achieve herbicide

sustainability, current optimistic expectations about new

herbicide introductions will need to be addressed. Dampening

high expectations will be difficult because of the successful

record of herbicide technology, the confidential nature of the

development pipeline, and the unpredictability of new

herbicide discovery. Achieving consensus on this matter in a

highly competitive herbicide industry may also be difficult.

However, focusing on important and unique herbicide

examples, such as glyphosate, may be an effective way to

illustrate the potential for losing valuable herbicide resources

and to communicate the need to use products in a sustainable

manner so as to retain their utility. Should there be a

widespread reduction in the weed control spectrum of a

dominant herbicide, such as glyphosate, it will be important

to stress the sustainability of strategies and technologies that

would replace it.

Recommendation Four: Demonstrate the Benefits and

Costs of Proactive, Diversified Weed Management Systems

for the Mitigation of HR Weeds. Because of their relative

advantages in cost efficiency, reliability, and ease of use,

herbicides will remain central to weed control strategies. Such

reliance means that the case for reducing dependence on a

preferred herbicide option must be strong and clearly

communicated. Where the sustainability of a key herbicide

is threatened, practices to sustain its use must be emphasized.

The most effective means of conserving herbicide MOAs is

diversifying weed management programs by combining

herbicides with different MOAs and incorporating nonchemical control tools. Diversification reduces direct selection

pressure by reducing the number of applications of a single

herbicide, especially the number of consecutive applications of

a single MOA. Application of two MOAs in sequence tends to

conserve the efficacy of both, but particularly that of the first

applied because the second application may control weeds that

escaped the first by reason of incipient resistance. Growers may

be reluctant to replace simple weed management programs that

rely on a single or very few herbicides with more diverse weed

management programs because the complexity of such

programs is a deterrent (Jordan et al. 2003; Liebman and

Gallandt 1997; Swanton et al. 2008).

Making alternative weed management programs as simple

and as cost effective as possible will be required if preemptive

herbicide-resistance management is to be widely adopted. The

rapid and extensive adoption of GR crops illustrates that

simplicity and convenience in weed control are highly valued

by modern crop producers (Dill et al. 2008; Marra et al.

2004; Piggott and Marra 2008; Shaw et al. 2009).

Complexity consumes management time, attention, and labor

(Kingwell 2010; Pardo et al. 2010), and those costs must be

clearly addressed when promoting herbicide sustainability

through adoption of diversity in weed management strategies.

A persuasive argument for adoption of diverse resistancemanagement practices may be that costs of control after

herbicide resistance has evolved are often higher than the cost

of a program for reducing the risk of resistance in the first

place (Mueller et al. 2005; Orson 1999). A successful example

of preemptive adoption of a resistance risk-averting practice is

the campaign to encourage glyphosate sustainability in

Australia using a ‘‘double knock’’ treatment. The doubleknock treatment incorporates an application of a second

herbicide MOA, other than glyphosate, shortly after the

glyphosate preseeding application (Neve et al. 2003b;

Weersink et al. 2005).

Recommendation Five: Foster the Development of Incentives by Government Agencies and Industry that Conserve

Critical Herbicide MOAs as a Means to Encourage

Adoption of Best Practices. Herbicides are the most costeffective means of weed control in most cropping systems.

Growers may accept the risk of herbicide resistance and

overuse a single or a few herbicide MOAs when competitive

diversified programs are not readily apparent. Diversification

may occur in three ways:

1. An alternative herbicide program with more than one

MOA becomes available and is perceived by the grower to

be more effective or less costly than the former program

that had high risks of generating resistance. In such a

situation, little or no inducement is needed because

growers will use programs that are most effective and least

costly.

2. An alternative program is available that is as effective and

economical as one that has a high risk of resistance, but it is

underused. In this case, education by the cooperative

extension service, distributors, and manufacturers’ representatives can be effective in conveying to growers the

advantage of conserving valuable MOAs by using

diversified programs.

3. Alternative programs exist but are less effective or more

costly than the standard program. In this scenario, it will

be difficult to convince growers to use the less effective or

more costly approach because it is difficult to estimate

future economic consequences. Consequently, growers

tend to choose short-term advantages. In this scenario,

economic incentives may be needed for conservation of

products with broad utility for the public good.

We recommend the expansion of the USDA Natural

Resources Conservation Service (NRCS) program, which

supports producers by creating positions for technical service

providers to assist in the development and implementation

of sustainable herbicide-resistance management programs.

USDA-NRCS is also developing plans for cost-share incentives

for producers to help reduce the risks of resistance on farms, or

to contend with weeds that have already evolved resistance

(Robinson 2011). Support for soil-conservation farming

systems and improved environmental outcomes could be a

focus for conserving valuable herbicide MOAs (D’Emden et al.

2008; Givens et al. 2009; Gray et al. 1996; Marsh et al. 2006).

Recommendation Six: Promote the Application of Full,

Labeled Rates at the Appropriate Weed and Crop

Growth Stage. When tank mixtures are employed to control

the range of weeds present in a field, each product should be

used at the specified label rate appropriate for the weeds

present. Repeated use of rates below those recommended on

labels increases the probability of resistance (Neve and Powles

2005a,b; Norsworthy 2012). Below-labeled rates allow more

weeds to survive treatment, and those weeds produce seed in

the survivors that will lead to an accumulation of genes that

confer resistance via multigenic mechanisms. Although

producers can avoid knowingly applying a reduced herbicide

54 N Weed Science 12, Special Issue 2012
rate, they must also be aware that rates can be unintentionally

reduced by poor spray coverage or by applying a herbicide to

weeds larger than those recommended.

It is in the herbicide manufacturers’ interest to specify the

manner of use of a proprietary herbicide because of probable

adverse market consequences from misuse or complaints of

product failure. Such stipulations are standard in herbicide

labeling where specific off-label uses are not permitted, e.g.,

those that have a high likelihood of failure or of off-target

damage. All herbicide labels give quite specific information on

the application rates, weed sizes, conditions, and other factors

that may affect activity of that herbicide. We recommend the

crop-protection industry draft herbicide labels with mandatory

minimum-use rates so the state regulatory agencies can enforce

compliance. Although a pesticide label is a legal document, it

may contain both mandatory and advisory statements.

Minimum use rates can be made mandatory if correctly

written, and that would require users to comply with those

specifications to avoid increasing the risks of resistance

evolution.

Recommendation Seven: Identify and Promote Individual

BMPs that Fit Specific Farming Segments with the

Greatest Potential Impact. We recognize that there are

substantial gaps in research knowledge of herbicide-resistance

BMPs. We also recognize that a BMP for a specific weed in

one locale may not be the best practice in another locale or

under different crop management practices. For example,

tillage may offer a viable management strategy in some areas,

whereas in areas susceptible to wind or water erosion, tillage

would be a poor option. Likewise, herbicide use varies among

locales because of differences in soils, weather, and general

production practices. Weed scientists, including extension

specialists, industry personnel, and producers must work

cooperatively to establish and verify BMPs for problem weeds

in various regions. In some cases, computer modeling can be

used to predict outcomes without the need for numerous

long-term field studies (Neve et al. 2011a). However, field

studies are needed to collect data to parameterize models and

validate model predictions. General BMPs, whether developed by model simulation or field experience, should be used

as a base recommendation, allowing local weed scientists and

producers to refine programs to fit individual management

needs. IWM to reduce the risk of resistance combines

multiple BMPs, and the path to reducing resistance risk is

likely to be achieved in incremental steps from simple to more

complex practices as producers gain confidence about

adopting new strategies (Byerlee and Polanco 1986).

Information flow to producers occurs through private crop

consultants, herbicide retailers, popular media, university

research and extension systems, and other producers (Givens

et al. 2009; Norsworthy et al. 2007d). Therefore, the first step

in encouraging producers to adopt BMPs for herbicide

sustainability in a particular state or region is to understand

and access those same information pathways, using educational materials that include practices suitable for local

situations and farming practices. A wide range of factors

affects producer decisions regarding IWM and herbicide

resistance (Shaw et al. 2009; Wilson et al. 2008, 2009).

Identifying those factors at a local level allows targeted

information delivery to improve decision-making (Llewellyn

2007). Many aspects of herbicide management, including

managing for sustainability, require region-specific decisionmaking and, therefore, regional research.

Many of the BMPs offer benefits to the farming system

beyond weed control, and those benefits will give producers

extra incentive to adopt the practices (Llewellyn et al. 2004,

2005). Relative cost–benefit predictions for various BMPs

should be presented in educational materials and programs

and should be based on local farming economics. Practices

that offer the greatest relative advantage in the short term are

more likely to be adopted preemptively. At present, adoption

of complex and costly practices is more likely with increased

producer awareness of herbicide resistance, presence, or risk

on their farms. However, area-wide promotion of preemptive

BMPs developed to fit local production problems may

encourage producers to realize the importance of these

practices and increase their confidence that resistance

management can be successful for their operation.

Recommendation Eight: Engage the Public and Private

Sectors in the Promotion of BMPs, Including Those

Concerning Appropriate Herbicide Use. The weed resistance crisis provides an excellent opportunity for partnerships

between public- and private-sector consultants and scientists.

The potential to reach growers and their advisors can be

greatly increased when a science-based recommendation gains

the support of the communication resources of a manufacturer. The double-knock concept in Australia (Walsh and

Powles 2007) is one example that led to high levels of

preemptive adoption of a practice.

Herbicide manufacturers should not constrain sales in any

way as a part of a cooperative effort with the public sector to

promote herbicide diversity in resistance management

programs. However, given the realities of resistance evolution,

we recommend that herbicide manufacturers unequivocally

state and promote on the label and in all advertising material

that the herbicide be used in a manner that embraces the

principles of sustainable weed management. In this way,

herbicide manufacturers are promoting a message that is

consistent with public- and private-sector educational efforts.

Recommendation Nine: Direct Federal, State, and Industry Funding to Research Addressing the Substantial

Knowledge Gaps in BMPs for Herbicide Resistance and

Support Cooperative Extension Services as Vital Agents in

Education for Resistance Management. Our current knowledge of best practices for managing herbicide resistance is

incomplete. Much of the research on developing herbicide-based

management strategies in cropping systems is funded by

herbicide manufacturers, who are primarily interested in

approaches that benefit their bottom line. Federal and state

funds are, therefore, needed to meet additional research needs.

Weed management is local and requires specific solutions that do

not rely on one technique or one herbicide. Agriculture needs a

diversity of weed management tools and strategies for every crop,

including resistance management programs that use more than

one herbicide or management technique in each cropping

scenario. Although industry support will likely continue for some

research, federal and state funding must be available and be

substantial enough to support resistance-management projects

for which private funding is not available or insufficient.

A vital aspect of developing and implementing successful

resistance-management BMPs is information transfer to

Norsworthy et al.: Best practices for reducing herbicide resistance N 55
consumers of those practices. A great deal of this information

is articulated through state and local extension specialists.

These professionals, in addition to private crop consultants,

are at the forefront of introducing producers to new

information and strategies through one-on-one communication, grower and consultant meetings, cooperation in field

demonstrations at research station field days, popular press

news releases, and technical brochures and websites. In some

states, weed resistance has compelled extraordinary cooperation among academia, extension specialists, consultants,

industry personnel, and producers to develop solutions to

urgent problems. However, it is vital that resistancemanagement research and education be supported by both

private and public sectors. The need for public funding is

necessary and justified given the dependence of our society on

the agricultural community and its crop production.

Acknowledgments

Improvements made to this document by Marilyn McClelland,

Muthukumar Bagavathiannan, James Anderson, and two anonymous reviewers are gratefully appreciated. Support for preparation of

this document was provided to the WSSA by the USDA Animal and

Plant Health Inspection Service. This publication was supported in

part by the United States Department of Agriculture’s (USDA)

Animal and Plant Health Inspection Service Agreement Number 09-

2000-0050-CA.

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Received September 16, 2011, and approved January 24, 2012.

62 N Weed Science 12, Special Issue 2012