bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Growth Stage and Environment on Foliar Absorption, Translocation, Metabolism, and Activity of Nicosulfuron in Quackgrass (Elytrigia repens)
Authors: Bruce Joseph A. , Carey J. Boyd , Penner Donald , Kells James J.
Published: May 2019

Experiments were conducted to examine the influence of adjuvant, growth stage, air temperature, and soil moisture on nicosulfuron activity, absorption, and distribution in quackgrass. Foliar absorption of14C-nicosulfuron by quackgrass was greater in one-leaf than five-leaf plants. Total translocation of14C-nicosulfuron was similar regardless of growth stage. However, nicosulfuron was more phytotoxic to three- and five-leaf than one-leaf quackgrass. Of the absorbed14C, at least 71 % remained as14C-nicosulfuron 168 h after application in three-leaf plants.14C-Nicosulfuron absorption, translocation, and accumulation increased with increasing temperatures and was independent of soil moisture. Under adequate soil moisture conditions (-0.03 MPa), nicosulfuron activity was not influenced by temperature. Nicosulfuron efficacy was not influenced by soil moisture at a cool (11/6 C) temperature. As plant moisture stress increased, due to low soil moisture and increasing air temperature, nicosulfuron efficacy declined. Differences in nicosulfuron efficacy due to growth stage or environmental conditions could not be explained by differential14C recovery, absorption, translocation, or accumulation. The addition of 28% urea-ammonium nitrate liquid fertilizer (UAN) to14C-nico-sulfuron plus petroleum oil adjuvant (POA) provided greater14C absorption than POA alone. Translocation of14C with POA alone 168 h after treatment (HAT) was similar to POA plus UAN 24 HAT. However, 168 HAT there was no difference in the total amount of radiolabelled material translocated among the adjuvants.
Root/Rhizome Exudation of Nicosulfuron from Treated Johnsongrass (Sorghum halepense) and Possible Implications for Corn (Zea mays)
Authors: Gubbiga Nagabhushana G. , Worsham A. Douglas , Corbin Frederick T.
Published: May 2019

Experiments were conducted to evaluate the occurrence and significance of release of herbicide through subterranean parts of nicosulfuron-treated johnsongrass. In a bioassay, the rooting medium of johnsongrass treated foliarly with 50 or 100 μg nicosulfuron plant−1was inhibitory to the radicle elongation of sorghum and corn indicating the increased toxicity of the rooting medium of nicosulfuron-treated johnsongrass. The study with14C-nicosulfuron indicated a basipetal translocation of foliarly applied nicosulfuron in johnsongrass to its roots/rhizomes and also into the rooting medium. By 30 DAT, around 23% of the14C-label absorbed by johnsongrass was found exuded into the rooting medium. Radiochromatogram scans of thin layer chromatography plates of rooting medium indicated unmetabolized nicosulfuron as the major14C-labeled compound (56%). The study also revealed a subsequent uptake of exuded14C by corn roots sharing the medium. On the whole, the amount of14C-label recovered from untreated corn amounted to 4.3% of the total applied to johnsongrass. In another experiment, the presence of nicosulfuron in the rooting medium was detrimental to corn growth. Reductions in corn growth occurred at concentrations of 10−8M nicosulfuron or greater in the rooting medium. The sensitivity of corn to root uptake was attributed to greater accumulation of nicosulfuron at a faster rate in the growing parts.
Antagonism of Diclofop Control of Wild Oat (Avena fatua) by Tribenuron
Authors: Baerg Roger J. , Gronwald John W. , Eberlein Charlotte V. , Stucker Robert E.
Published: May 2019

Tribenuron antagonized diclofop control of wild oat in greenhouse studies. Wild oat shoot fresh weight was reduced 79% when plants were treated with diclofop at 1.12 kg ha−1, but only 46% when treated with diclofop at 1.12 kg ha−1plus tribenuron at 18 g ha−1. Increasing diclofop rate increased control of wild oat but did not overcome the antagonism. Separating the applications of diclofop and tribenuron by as little as 12 s reduced the antagonism by 40%, indicating the importance of close proximity of the two herbicides on the leaf surface for the antagonistic response. The inhibitory effect of diclofop at its target site (acetylCoA carboxylase) was not reduced by tribenuron. Metabolism of14C-diclofop by wild oat leaves was not altered quantitatively or qualitatively by tribenuron. Tribenuron had no effect on spray retention or absorption of14C-diclofop by wild oat leaves, and only slightly decreased the total amount of radiolabel translocated out of the treated zone on the leaf. However, tribenuron decreased basipetal translocation of diclofop from the treated zone by approximately 20%. The ability of tribenuron to reduce basipetal translocation of diclofop to meristematic regions of wild oat may be a factor in antagonism.
A New Approach To Determine When to Control Weeds
Authors: Berti Antonio , Dunan Claudio , Sattin Maurizio , Zanin Giuseppe , Westra Philip
Published: May 2019

A methodological approach to determine the optimum time to control weeds that integrates aspects of weed biology, weed-crop competition, and economics is presented. The approach is based on the concept of Time Density Equivalent: this is defined as the density of weed plants that germinate with the crop and compete until harvest that causes the same yield loss caused by a group of weeds with a given density, time of emergence, and time of removal. A model was developed that accounts for pattern of weed emergence and permits determination of timing of weed control that minimizes economic loss due to weeds emerging both before and after treatments. The outcomes of the model are presented with two examples: corn in competition with velvetleaf and soybean in competition withAmaranthus cruentus.For both crops, six different weed control strategies involving preemergence, chemical, and mechanical postemergence treatments are considered. The results obtained with the model are compared with the calculation of net margin based on assumptions of simultaneous emergence of crop and weeds and no effect of different times of control. Different control strategies are compared considering not only maximum net margin but also its dependence on time of control, because a strategy with a lower value of maximum net margin, but a flatter net margin curve, allows more flexibility of time of control.
Postdispersal Predation of Velvetleaf (Abutilon theophrasti) Seeds
Authors: Cardina John , Norquay Heather M. , Stinner Benjamin R. , McCartney David A.
Published: May 2019

Studies were conducted from 1989 to 1993 in continuous no-tillage and moldboard plow corn fields to describe rates of velvetleaf seed predation with time and with seed density, and to identify principal seed predators. Rates of seed loss from the soil surface averaged 1 to 57% day−1and were equivalent in the two tillage systems. Predator populations were the same in no-tillage and moldboard plow fields. The predation rate was generally low in winter months, increased in mid-summer, and declined in late summer. In 2 of the 4 yr, predation increased in October and November. The predation rate was described by an exponential decay function of seed density, with high rates of seed loss at low densities and leveling off to a nearly constant level at densities above 600 seeds m−2. Predation was highest where seed access was not restricted, and exclosures of 6.5 and 1.6 cm2reduced predation up to 15 and 52%, respectively. Mice were important predators in the field. In laboratory feeding studies, the carabid beetleAmara cupreolata, the slugsArion subfuscusandDeroceras reticulatum, and cutworms (Agrotis ipsilon) consumed imbibed velvetleaf seeds.Amara cupreolataand A.subfuscuswere the only predators to damage unimbibed velvetleaf seeds.
Existence of Ecotypes Among Populations of Entireleaf Morningglory (Ipomoea hederaceavar.integriuscula)
Authors: Klingaman Tracy E. , Oliver Lawrence R.
Published: May 2019

Field experiments were conducted using entireleaf morningglory seed collected from areas of adaptation throughout the United States to determine whether biotypes or ecotypes exist and whether differences in susceptibility to acifluorfen exist. Initiation of first flower for the entireleaf morningglory populations ranged from 63 to 81 d after emergence. The interval between emergence and initiation of first flower decreased 2.8 d for each increase in degree of latitude from which the seed was collected. Plants originating from southern latitudes remained in the vegetative phase longer and tended to produce more total dry-weight biomass than plants originating from northern latitudes. Thus, ecotypes do exist for entireleaf morningglory because of adaptation to a specific environment. The adaptation allows ecotypes to utilize the length of the growing season associated with the area of origin. Trichome density on the adaxial leaf surface ranged from 147 to 206 cm−2across the ecotypes and was not correlated with latitude or differences in acifluorfen susceptibility. Ecotypes differed in susceptibility to acifluorfen, but resistance among ecotypes was not evident.
Total Nonstructural Carbohydrates and Regrowth in Honey Mesquite (Prosopis glandulosa) Following Hand Defoliation or Clopyralid Treatment
Authors: Cralle Harry T. , Bovey Rodney W.
Published: May 2019

Greenhouse experiments were conducted to assess the storage of nonstructural carbohydrates in the stems and roots of honey mesquite and their mobilization and utilization following defoliation by hand or clopyralid. All leaves were removed by hand from one group of plants while leaving stems intact. Another group received a foliar spray of clopyralid at a sublethal acid equivalent rate of 0.07 kg ha−1. Samplings at treatment on day 0 and at 14, 28, 42, and 58 d after treatment measured leaf area; dry weight of leaves, stems, and roots; and grams and percent total nonstructural carbohydrates in stems and roots. Controls accumulated dry weight and grams total nonstructural carbohydrates in stems and roots, while maintaining a constant percent total nonstructural carbohydrates throughout the experiment. Percent and grams total nonstructural carbohydrates in stems and roots of hand defoliated plants decreased by about one-half from day 0 to 14 as new leaf growth occurred, but increased and returned to pretreatment levels on day 28. A similiar, though slower, pattern of decline and recovery of total nonstructural carbohydrates in stems and roots occurred in response to defoliation by clopyralid. These results showed that the nonstructural carbohydrates in stems and roots of honey mesquite were important sources of carbohydrates to support new leaf regrowth following defoliation by hand or herbicide.
Allelopathic Influence of Germinating Seeds and Seedlings of Cover Crops on Weed Species
Authors: Hoffman Melinda L. , Weston Leslie A. , Snyder John C. , Regnier Emilie E.
Published: May 2019

Bioassays using binary mixtures that included a cover crop with known allelopathic potential and a weed species were employed to determine the importance of allelopathy compared to resource competition as interference mechanisms. Responses of weed species germinated with cover crops in a petri dish were measured. Interference between weed and cover crop seedlings was determined in a greenhouse experiment using the additive design, which included partitions to reduce above- and below-ground competition and used capillary mat subirrigation to control moisture and fertilizer availability. Germinating sorghum reduced radicle length of weeds, whereas germinating rye tended to increase weed radicle length. Methods limited above-ground competition, so likely interference mechanisms were below-ground competition and allelopathy. Germination with a cover crop had little effect on germination and shoot length of weeds. Increased density of rye but not of sorghum reduced growth of barnyardgrass seedlings. Reduced number of barnyardgrass leaves in the presence of rye was likely due to allelopathy. Suppression of barnyardgrass dry weight attributed to allelopathic interference by rye was successfully separated and compared to the combined effects of competition and allelopathy.
Growth and Competitiveness of Common Lambsquarters (Chenopodium album) after Foliar Application ofAscochyta caulinaas a Mycoherbicide
Authors: Kempenaar Corné , Horsten Petra J. F. M. , Scheepens Piet C.
Published: May 2019

Control of common lambsquarters by the use ofAscochyta caldinaas a postemergence mycoherbicide was studied in corn and sugar beet, in 1992 or 1993. The weed was planted at determined positions in the crops. Plots were treated with suspensions ofA. caulinaspores, and wetness duration's were varied to create different levels of disease development. Application ofA. caulinaresulted in necrosis development on, and mortality of common lambsquarters. Average severities of leaf necrosis 1 wk after treatment ranged from 0.01 to 0.75. Average proportions of dead plants 3 wk after treatment ranged from 0.00 to 0.65. Necrosis development and mortality were affected by wetness duration in two experiments. Sublethally diseased plants showed reduced growth. Maximum dry matter was affected by crop and by necrosis development. Numbers of fruits per plant showed a positive, almost linear relationship with plant dry matter weight. Seed weight was less affected by necrosis than number of fruits per plant. Competitiveness of common lambsquarters was reduced after infection byA. caulina.Crop dry matter weight showed a positive relationship with the level of common lambsquarters control. In corn, yield reduction by competition was prevented by application of A.caulina, but not in sugar beet.
Leafy Spurge (Euphorbia esula) Genotype Affects Gall Midge (Spurgia esulae) Establishment
Authors: Lym Rodney G. , Nissen Scott J. , Rowe Martha L. , Lee Donald J. , Masters Robert A.
Published: May 2019

Greenhouse cage studies were conducted to determine the influence of shoot morphology and genetic variation on establishment ofSpurgia esulaegall midge on seven leafy spurge genotypes. The genotypes were collected from South Dakota, North Dakota, Nebraska, Montana, Wyoming, Manitoba, and Austria. Genotypes from South Dakota and Nebraska were most susceptible to gall formation and had the highest larvae survival, while the genotypes from Montana and Manitoba were most resistant. Morphological characteristics of the leafy spurge stem tips, such as stem diameter, leaf length, width, and area did not correlate with gall formation or larvae survival. Chloroplast DNA restriction fragment length polymorphism analysis of the genotypes identified six chloroplast types among the seven leafy spurge genotypes. The two genotypes most resistant to galling byS. esulae, Manitoba and Montana, had the same chloroplast genotype, but also were closely related to the two most susceptible genotypes. Because eggs were laid on all genotypes, it appears that adult females were not preferentially selecting appropriate host genotypes, but that egg and larvae survival was strongly influenced by genotype.
Interaction Between Wheat (Triticum aestivum) and Diclofop to Reduce the Cost of Annual Ryegrass (Lolium rigidum) Control
Authors: Lemerle Deirdre , Verbeek Birgitte , Coombes Neil E.
Published: May 2019

The influence of wheat variety on the dose-response of annual ryegrass to diclofop-methyl (POST) was examined in the field in 1992 and 1993 in southern New South Wales, Australia. The aim was to determine if planting a strongly competitive variety of wheat improved control of annual ryegrass at reduced doses of diclofop-methyl. Suppression of ryegrass was dependent on herbicide dose, season, and wheat variety. In the absence of herbicide, dry matter (DM) production of annual ryegrass at 300 plants m−2at anthesis was 500 g ha−1with Dollarbird and Katunga compared to 1000 g ha−1with Rosella or Shrike in 1992. In 1993, DM was approximately 150 g ha−1with Dollarbird or Katunga, and 350 g ha−1with Shrike or Rosella. Ryegrass DM was reduced by diclofop-methyl to a greater extent, relative to the weedy unsprayed controls, with less competitive varieties Rosella and Shrike than with the more competitive Dollarbird or Katunga. Diclofop-methyl at 0.28 kg a.i. ha−1reduced DM of ryegrass growing with Katunga to less than 100 g m−2in 1992, compared to more than 200 g m2with the other varieties. In 1993, diclofop-methyl was more effective on ryegrass, and the same dose reduced ryegrass DM to almost zero in all varieties. Grain yields in unsprayed weedy controls of Dollarbird and Katunga were reduced approximately 20% by annual ryegrass compared with yields achieved with herbicides in both years. Yields of Rosella and Shrike in the unsprayed controls were reduced about 40% in 1992 and 60% in 1993. Only small increases in grain yields of all varieties occurred from diclofop-methyl doses above 0.13 kg a.i. ha−1. Poorly competitive varieties were dependent on herbicides to achieve grain yield potential and had a greater risk of weed survival when herbicide efficacy was reduced. In contrast, strongly competitive varieties, likely to retard build-up of weed seed in the soil, are less dependent on herbicides to achieve grain yield potential, and therefore result in reduced weed control cost.
Investigations into the Growth Suppressing Effect of Nicosulfuron-Treated Johnsongrass (Sorghum halepense) on Corn (Zea mays)
Authors: Gubbiga Nagabhushana G. , Worsham A. Douglas , Corbin Frederick T.
Published: May 2019

Greenhouse and growth chamber experiments were conducted to determine the reasons for stunted growth and yield suppression of corn noticed sometimes in nicosulfuron-treated corn fields infested with heavy population of johnsongrass. Results indicated that in the absence of johnsongrass, nicosulfuron applied broadcast POST at 35 g ai ha−1had no effect on corn. However, growth reduction of corn occurred when nicosulfuron-treated johnsongrass and corn were allowed to share the same rooting medium with their root systems intermingled. The reduction in growth was even greater when corn foliage or the soil surface were also treated with johnsongrass. The extent of growth reduction of corn growing with nicosulfuron-killed johnsongrass depended on weed density and herbicide application rate. Greater growth reductions occurred at four johnsongrass plants per pot compared to two and at a higher application rate of 100 μg nicosulfuron per plant. In general, johnsongrass killed by nicosulfuron appeared to be more phytotoxic to corn than plants killed by paraquat. Nicosulfuron provided excellent control of johnsongrass and improved corn growth by two to three times that of not controlling johnsongrass, but it could not elevate corn growth to the level obtained when johnsongrass was controlled by paraquat or in the absence of interference from johnsongrass.
Biological Control of Kudzu (Pueraria lobata) with the Plant PathogenPseudomonas syringaepv.phaseolicola
Authors: Zidack Nina K. , Backman Paul A.
Published: May 2019

Pseudomonas syringaepv.phaseolicolaformulated with an organosilicone surfactant was tested as a potential bioherbicide for kudzu. Greenhouse studies were performed to determine the effect of kudzu plant age and leaf age on disease severity and growth of the bacteriumin planta.Eight-week-old plants were more diseased 4 wk after spraying and had a higher potential for regrowth 7 wk after spraying than 12-wk-old plants. Young leaves developed water-soaked lesions earlier than older leaves and supported higher populations of bacteria. Data from field experiments at two separate locations indicated that single applications in late spring were as effective as multiple applications in early spring and were enhanced by repeat application. Three months after the beginning of the experiment, extensive regrowth occurred for all bioherbicide treatments and no differences were noted between treatments and the nontreated control.
Multi-Year Validation of a Decision Aid for Integrated Weed Management in Row Crops
Authors: Forcella Frank , King Robert P. , Swinton Scott M. , Buhler Douglas D. , Gunsolus Jeffrey L.
Published: May 2019

WEEDSIMis a bioeconomic decision aid for management of annual weeds in corn and soybean. It was field-tested for 4 yr in Minnesota. The decision aid has two categories of management recommendations: soil-applied plus postemergence (PRE+), based on estimated weed seedbank composition and density; and postemergence (POST), based upon observed weed seedling composition and density. Weed densities, weed control, herbicide use, environmental impact of herbicide use, weed management costs, crop yields, and economic returns that resulted from PRE+ and POST recommendations were compared to those associated with herbicide management systems (HERB) that were standard for the region. After 4 yr of applyingWEEDSIMrecommendations to the same plots, there were no increases in annual weed densities (seedbanks, seedlings, established plants, or seed production) or decreases in weed control or crop (soybean, rotation corn, and continuous corn) yields, compared to HERB.WEEDSIMrecommendations resulted in average annual herbicide applications of 1.1 kg ai ha−1for PRE+ and 1.0 kg ai ha−1for POST, compared to 3.5 kg ai ha−1for HERB. Environmental impact indices associated with PRE+, POST, and HERB were 0.75, 0.71, and 0.54, with the lowest value indicating greater environmental risk than the two higher values. Similarly, average weed management costs were $24, $33, and $77 ha−1for PRE+, POST, and HERB, respectively. Based on crop prices of $94 Mg−1for corn and $220 Mg-1for soybean, the average gross margins over weed control costs were higher for PRE+ ($509 ha−1) and POST ($522 ha−1) than for HERB ($455 ha−1). In general,WEEDSIMappeared to make management recommendations that adequately controlled weeds, maintained crop yields, reduced herbicide use, decreased environmental risk, lowered weed management costs, and increased gross margins over weed control costs compared to the use of herbicides standard for the region.
Impact of Vegetative Filter Strips on Herbicide Loss in Runoff from Soybean (Glycine max)
Authors: Webster Eric P. , Shaw David R.
Published: May 2019

Metolachlor and metribuzin loss in runoff was determined in three soybean tillage systems with and without a 4 by 2 m tall fescue vegetative filter strip. Soil erosion plots were 4 by 22 m with 3% slope. Regression analysis was used to describe herbicide concentration in runoff, and to determine if vegetative filter strips reduced herbicide concentration. Analysis of covariance indicated no difference in concentration of metolachlor or metribuzin in runoff from the three tillage systems within any vegetative filter strip treatment. Metolachlor loss in 1991 was highest from the no-till monocrop without a vegetative filter strip, and it was 65g ha−1 or approximately 2% of the amount applied. In 1992 and 1993, the no-till doublecrop had a total loss of 120 and 147 g ha−1, respectively, approximately 4% of the amount applied. Similar results were noted with metribuzin, but total loss was as high as 46 g ha−1 or 11 % of the amount applied in 1993 from a no-till doublecrop system without a vegetative filter strip. When a vegetative filter strip was present, losses of metribuzin and metolachlor were reduced over 85% in 1993, and totaled 1.2 and 0.5%, respectively, of the amount applied. The vegetative filter strip reduced herbicide and suspended solids from runoff produced by a conventional-till production system to levels equal to or lower than a no-till doublecrop system.
Comparison of Solid Phase Extraction Techniques for Herbicides
Authors: Riley Melissa B. , Keese Renee J.
Published: May 2019

Solid phase extraction (SPE) procedures were used to determine the recoveries of herbicides typically used in containerized ornamental plant production from water samples. Recoveries from C18cartridges and disks were compared for each of 12 herbicides with variations in elution solvent and volume of elution solvent tested. Recoveries for nine of the herbicides from the cartridges and disks using acetone as an elution solvent were not affected by SPE matrix. Fluazifop recovery was greater with the disks, while napropamide and oxadiazon recoveries were greater with cartridges. Both cartridges and disks yielded low recoveries (23 to 47%) of benefin and prodiamine. Changing the elution solvent from acetone to acetonitrile resulted in 10% improvement for the recovery of benefin and a three- to four-fold increase in recovery of prodiamine. Acetonitrile decreased recoveries of napropamide, oryzalin, oxadiazon, oxyfluorfen, and pendimethalin from cartridges. For the disks, oxyfluorfen, prodiamine, and trifluralin had increased recovery, while fluazifop, oxadiazon, and simazine had decreased recovery with acetonitrile as the elution solvent. Increasing the amount of acetone eluting solvent increased the recovery of prodiamine and oxyfluorfen while decreasing the recovery of fluazifop, pendimethalin, simazine, and trifluralin. Binding capacities of oryzalin on cartridges and disks averaged 13.2 and 7.8 mg, respectively. The advantage of the disk lies in the greater volume of water that can be processed, while the higher cost and greater variability are disadvantages. Cartridge extraction yielded good recoveries with lower standard deviations, and used less organic solvent. Selection of an SPE extraction method depends upon the herbicides under evaluation, expected levels, and the water volume being processed. Both SPE techniques offer advantages over traditional liquid-liquid extraction methods such as reduced requirements for organic solvent and sample preparation.
Effect of Thiazopyr on Soybean Growth and Dissipation in Soil
Authors: Mueller Thomas C. , Hayes Robert M.
Published: May 2019

Field studies indicated thiazopyr at 220 g ha−1injured soybeans planted in no-till, and 340 g ha−1injured soybean in 1 of 2 yr planted into tilled soil. Thiazopyr half-lives in three field studies ranged from 12 to 14 d, indicating minimal potential for carryover injury to sensitive rotational crops. Thiazopyr dissipation under controlled conditions was slower than under field conditions. Half-lives at 15 and 30 C were 133 and 43 d, respectively. This indicates that microbial degradation is an important mechanism in thiazopyr dissipation, but that other loss mechanisms also play a role. A method using methanol extraction of moist soil samples followed by gas chromatography using a nitrogen-phosphorous detector had > 90% recovery of thiazopyr.
Spatial and Temporal Analysis of Weed Seedling Populations Using Geostatistics
Authors: Johnson Gregg A. , Mortensen David A. , Gotway Carol A.
Published: May 2019

An intensive field survey of an eastern Nebraska corn and soybean field was conducted to characterize the spatial structure and temporal stability of broadleaf weed seedling populations over two growing seasons. Anisotropy, the effect of direction on the relationship between observations, is present in the semivariogram for the velvetleaf and common sunflower populations in 1992 and 1993. The directional trends in aggregation are visible in kriged maps as elliptical shapes oriented east to west across the study area. In addition, there are two distinct zones of aggregation from north to south. These two distinct areas of aggregation are reflected as a ‘plateau’ in the north-south semivariogram. The distance over which this plateau extends indicates that the shape or size of the patch is contracting in the north-south direction (perpendicular to the crop row). The slope of the semivariogram in the east-west direction (aligned with the crop row) remains consistent from 1992 to 1993 suggesting that the shape of the patch is not changing. For sunflower populations, the slope of the north-south empirical semivariogram changes at 20 m, similar to the velvetleaf population semivariograms. This change, however, is reflected as a downward trend in the empirical semivariogram. The distance over which this trend occurs increases from 1992 to 1993 suggesting that seedling patch size was smaller in 1993 compared to 1992. Weed seedling establishment resulting from seed dispersal, differential seed and seedling mortality, or emergence may have resulted in the observed patch dynamics.
Computer Assisted Molecular Design: Overview of cModern Techniques in Quantitative Structure Activity Relationships (QSAR)
Authors: Vaz Roy
Published: May 2019

This paper gives an overview of computer assisted molecule design using quantitative structure activity relationships (QSAR). In deriving a QSAR for a set of compounds, one should be aware of possible properties one can use to correlate with the activity of this set of compounds. The paper describes some of the traditional and also more recently used properties that can be calculated. The traditionally used properties have been those calculated for the substituent groups that were varied on a particular scaffold, whereas the more recently used properties are those calculated on the molecule as a whole. One of the most recently used properties for comparative purposes are fields around the molecules. The technique is termed comparative molecular field analysis (CoMFA). The statistical techniques used to correlate the properties with the activity are also described and, finally, a case study is described. The case study shows the application of traditional and 3D QSAR (CoMFA) techniques on some photosystem II inhibitors.
Biochemical Approaches to Herbicide Discovery: Advances in Enzyme Target Identification and Inhibitor Design
Authors: Abell Lynn M.
Published: May 2019

This paper focuses primarily on the means by which biochemical information can be used to identify enzymes which, upon inhibition, produce lethal phenotypes and the enzyme inhibitor design strategies that have the highest probability of not only inhibiting the enzyme but also translating that inhibition into herbicidal efficacy. The identification of an exquisitely lethal target site is the key initial component to this approach and has often been one of the most difficult steps because the attributes of a lethal site have, at best, been ill-defined. An examination of the characteristics of known targets provides some insight as to the definition of a lethal target. Recently, antisense RNA suppression of enzyme translation has been used to determine the extent of inhibition required for toxicity and offers potential as a strategy for identifying lethal target sites. After identification of a lethal target, detailed knowledge of the enzyme's chemical and kinetic mechanism as well as the protein's structure may be used to design potent inhibitors. Various types of inhibitors may be designed for a given enzyme. The advantages and disadvantages of a given type with respect toin vivoefficacy as well as the probability of herbicide resistance development will be discussed.
Quantifying the Inhibitor-Target Site Interactions of Photosystem II Herbicides
Authors: Huppatz John L.
Published: May 2019

A convergence of research effort in a number of scientific disciplines in the early 1980s resulted in a rapid expansion of knowledge of the structure and function of the photosynthetic reaction center in bacteria and higher plants. The structure of the reaction center from photosynthetic bacteria was determined by X-ray analysis. The herbicide binding protein (the D1 protein) was identified by photoaffinity labelling and found to be an integral part of the photosynthetic reaction center complex in higher plants. Studies using herbicide-resistant mutants enabled the location of the herbicide binding niche on D1 to be determined. Quantitative Structure Activity Relationships (QSAR) of families of inhibitors and their effect on photosynthetic electron transport helped elucidate the nature of the interaction between inhibitors and receptor. Binding appeared to be predominantly hydrophobic with hydrogen bonding also having an important role. Studies with a series of highly potent inhibitors, the 2-cyanoacrylates, identified certain steric constraints in the interaction of these molecules with the binding site. The activity of these inhibitors was particularly sensitive to minor structural change and they proved to be useful probes of receptor topography. The results of structure-activity studies of the 2-cyanoacrylates combined with a refined knowledge of the three-dimensional structure of the inhibitor binding site has enabled computer-based molecular modelling of interactions of these inhibitors with the receptor. The spatial arrangement of the inhibitor functional groups within the binding domain was shown to be a critical factor in determining binding affinity.
Translation ofIn VitroActivity toIn VivoActivity: Lessons from the Triazolopyrimidine Sulfonanilides
Authors: Gerwick B. Clifford , Cseke Csaba T. , Deboer Gerry , Kleschick William A. , Schmitzer Paul R.
Published: May 2019

Eight triazolopyrimidine sulfonanilides were tested for metabolic stability in a number of crop and weed species. These data, along within vitrodeterminations of activity (I50) against acetolactate synthase, successfully described thein vivoactivity of these compounds in a two-parameter model. Whole plant activity increased with increasing compound stability and decreasing I50(r2=.78, N = 36). The difficulty in obtaining metabolic stability data during a structure optimization program prompted a study with substituent parameters in models ofin vivoactivity. Models describing whole plant activity in jimsonweed were developed using a series of 5-methyl triazolopyrimidine sulfonanilides that differed only in ortho and meta substituents on the aniline ring. The I50term and clogP were most important to jimsonweed activity. Hence,in vitroactivity (I50) may be a useful component of whole plant structure activity models to aid in identification of barriers toin vivoperformance.
The Basis of Imazethapyr Tolerance in Cowpea (Vigna sinensis)
Authors: Baerg Roger J. , Barrett Michael
Published: May 2019

Greenhouse and laboratory experiments were conducted to determine the basis of imazethapyr tolerance in cowpea. Cowpea required more than 700 g ha−1imazethapyr to reduce shoot growth 50% while corn required 70 g ha−1to reduce growth 50% in greenhouse experiments. Extractable acetohydroxy acid synthase activity from cowpea 24 h after foliar imazethapyr treatment was only 20% of that of untreated cowpea but increased to control levels 72 h after treatment (HAT). The unifoliate leaves of cowpea absorbed 25% of the14C-imazethapyr applied 72 HAT. Movement of14C-imazethapyr out of the treated zone increased gradually to approximately 47% of the absorbed14C 168 HAT. Parent imazethapyr decreased from 41 to 27% of the recovered14C between 4 and 24 HAT, and continued to decrease to 14% by 168 HAT. Polar metabolites increased from 27 to 55% of the recovered14C from 24 to 168 HAT. The polar metabolites of imazethapyr included hydroxyimazethapyr and conjugates of hydroxyimazethapyr. Tolerance of cowpea to imazethapyr is based on the plants ability to metabolize the herbicide to nontoxic metabolites.
Mechanisms of Resistance to Diclofop of Two Wild Oat (Avena fatua) Biotypes from the Willamette Valley of Oregon
Authors: Seefeldt Steven S. , Fuerst E. Patrick , Gealy David R. , Shukla Amit , Irzyk Gerard P. , Devine Malcolm D.
Published: May 2019

Laboratory experiments were conducted to determine the mechanism of resistance to diclofop in two wild oat biotypes (designated ‘B’ and ‘C’ biotypes) from the Willamette Valley of Oregon. Resistance could not be attributed to differential absorption, translocation, or metabolism of diclofop. Resistance was not correlated with membrane plasmalemma repolarization following diclofop acid treatment. Compared to a susceptible (’S') wild oat biotype, acetyl CoA carboxylase from the B and C biotypes showed a 10.3 and 4.5 fold increase in the level of resistance, respectively, to diclofop acid. Cross-resistance to fenoxaprop acid was 5.5 and 7.3 times higher in the B and C biotypes, respectively than the S biotype. Correlation between resistance at the whole plant level and at the ACCase level was good for diclofop and fenoxaprop in the B biotype. For the C biotype, this correlation was not as good. Possible reasons for the discrepancy are given.
Postemergence Activity of Sulfentrazone: Effects of Surfactants and Leaf Surfaces
Authors: Dayan Franck E. , Green Hannah M. , Weete John D. , Hancock H. Gary
Published: May 2019

Sulfentrazone was foliar applied at 34 and 56 g ai ha−1alone or in combination with surfactants to soybean cultivars Hutcheson and Centennial and to sicklepod, coffee senna, smallflower morningglory, velvetleaf, and yellow nutsedge. The most sensitive weeds, including coffee senna, smallflower morningglory, and velvetleaf, were severely injured by the lowest rate when sulfentrazone was applied with surfactants. Sulfentrazone provided the highest control of yellow nutsedge with X-77. Soybeans were not severely injured by sulfentrazone applied alone, but 55% foliar injury occurred when the herbicide was applied with X-77. However, the seedlings were not killed. Sicklepod was the most tolerant of the weeds tested. In the absence of surfactants, the order of radiolabeled sulfentrazone absorption by the foliage was Centennial (5.8%) = Hutcheson (8.5%) = coffee senna (10.4%) < yellow nutsedge (17.0%) < velvetleaf (22.3%) = smallflower morningglory (24%). Sicklepod leaves did not retain droplets containing sulfentrazone when no surfactant was used. Species with the highest foliar absorption also showed the greatest phytotoxic response to the herbicide. Addition of surfactants to the spray mixture enhanced the foliar absorption and overall phytotoxicity of sulfentrazone in the weeds. An inverse relationship was detected between the foliar absorption of sulfentrazone without surfactants and the amount of cuticular wax present on the leaves. No such correlation was observed when surfactants were used. Thus, surfactants overcame the barrier to absorption imposed by the cuticular wax and, under these conditions, selectivity apparently became dependent upon species-specific cellular tolerance to sulfentrazone.
Morphological and Phenological Variation in Barnyardgrass (Echinochloa crus-galli) in California
Authors: Norris Robert F.
Published: May 2019

Individual barnyardgrass plants were grown in the absence of competition in a common garden environment. Cohorts were initiated in mid-March, late April, early June, mid-July, mid-August, and mid-September. Plants within a cohort varied from prostrate to upright. Early and late cohorts required slightly longer to achieve flowering and seed shatter than those initiated in late spring and early summer. The onset and development of senescence varied by as much as 5 wk between individuals within a cohort, and senescence progressed more rapidly for plants in later cohorts. Longest tiller length per plant averaged 150 cm and did not differ by cohort, but within cohort variation was approximately 33%. The number of tillers per plant declined with increasing delay in cohort initiation; within cohort variation exceeded 40%. Leaf numbers decreased from more than 10,000 per plant to less than 400 per plant with increasing delay in cohort initiation date. Individuals within a cohort had more than two-fold variation in leaf numbers. The number of inflorescences decreased from more than 4500 per plant to less than 100 with increasing delay in cohort initiation. Mean inflorescence length and frequency of different inflorescence lengths per plant varied between plants within a cohort. Vegetative biomass exceeded 3000 g for many plants in the early cohorts, and decreased to less than 25 g per plant for some individuals in the September cohort. More than two-fold variation in biomass occurred between plants within a cohort. Plasticity in morphology and phenology may contribute to the success of barnyardgrass as a weed.
Interference and Water Use of Biotypes Differing in Sensitivity to MSMA
Authors: Akanda M. Raisuddin , Walker Robert H. , Wehtje Glenn
Published: May 2019

Biotypes of common cocklebur were identified, which were assumed to differ only with respect to sensitivity to MSMA. Water use of these biotypes when grown as isolated plants in a growth chamber was determined with stem flow gauges. The MSMA-tolerant biotype exhibited less whole-plant sap flow at media moisture levels above 17%. Furthermore, only in the tolerant biotype did sap flow differ between the upper and lower canopy; the upper canopy had greater sap flow. Field experiments were conducted to compare growth parameters, water use and competitive ability toward cotton of the MSMA-sensitive and -tolerant biotypes. Sensitive biotype plants were taller and had greater leaf area than those of the tolerant biotype; dry weight was equivalent. The tolerant biotype used less water and reduced seed cotton yield more than the sensitive biotype.
Characterization of Wild Oat (Avena fatuaL.) Populations and an Inbred Line with Multiple Herbicide Resistance
Authors: Kern Anthony J. , Colliver Corey T. , Maxwell Bruce D. , Fay Peter K. , Dyer William E.
Published: May 2019

Repeated use of the preemergence herbicide triallate has selected for wild oat populations that are resistant (R) to field use rates. Field collections and an inbred R line were shown in greenhouse and petri dish dose response experiments to be 6- to 20-fold more tolerant to triallate than susceptible (S) lines. R populations and the inbred line were also resistant (8-fold) to the related thiocarbamate herbicide diallate, as well as to the chemically unrelated postemergence herbicide difenzoquat (60-fold).14C-triallate uptake and translocation patterns were similar between R and S lines for the first 24 h after application. However, translocation of radioactivity was more rapid in S lines than R lines from 24 through 60 h after application.14C-difenzoquat uptake was the same in R and S lines 12 h after application, but was 10 to 20% higher in R lines than S lines by 24 through 96 h after application. Similarly, translocation of radioactivity after14C-difenzoquat application was 7 to 14% greater in R than S lines after 12 h, although translocated radioactivity amounts were not significantly different between R and S lines. The relatively minor differences in triallate and difenzoquat uptake and translocation patterns between R and S lines are most likely not of sufficient magnitude to explain the observed resistance levels.
Influence of Density on Johnsongrass (Sorghum halepense) Interference in Field Corn (Zea mays)
Authors: Ghosheh Hani Z. , Holshouser David L. , Chandler James M.
Published: May 2019

Experiments were conducted from 1990 to 1994 to investigate the density-dependent effects of rhizome and seedling johnsongrass on the magnitude of full-season interference in field corn. Corn grain yield was decreased as rhizome johnsongrass density increased. A maximum density of 3 rhizome johnsongrass plants 9.8 m−1of row was considered critical to avoid yield losses above 5% of full-season weed-free corn. Interference of johnsongrass plants originating from seed and growing at densities up to 128 plants 9.8 m−1of row did not affect grain yield. Seedling johnsongrass plants growing at a constant density of 9.8 plants m−1of row did not affect yield from corn planted at five densities.
Coleorhizal Hairs in Germinating Medusahead (Taeniatherum caput-medusae)
Authors: Northam Francis E. , Callihan Robert H. , Puchalski Michelle M.
Published: May 2019

This study documented an undescribed aspect of medusahead germination behavior. Fibrous hairs were consistently observed growing from the coleorhizal surface of germinating medusahead seeds. Three medusahead accessions were investigated to determine temperature effects on coleorhizal hair growth and germination. Coleorhizal hair emergence at 8 C was first noted at 72 h. By 24 h at 18 C, 15 to 74% of seeds had hairs, and at 28 C, hairs were visible on 75 to 94% of seeds. Germination at 18 and 28 C was 80% at 96 h; germination at 8 C did not exceed 80% until 168 h. Less than 4% of germinated seeds were without coleorhizal hairs. Appearance of coleorhizal hairs confirmed that germinating medusahead embryos were active at least 72 h before germination was complete. These data affirm coleorhizal hairs are a normal part of medusahead germination biology. Coleorhizal hairs probably aid early phases of medusahead seedling establishment.
Influence of Weed Control on Weed Population in Ridge-till Corn (Zea mays)
Authors: Wicks Gail A. , Wilson Robert G. , Mahnken Garold W. , Hanson Gordon E.
Published: May 2019

Field studies were conducted to determine the influence of annual herbicide treatments plus cultivation on weed populations and corn yields in ridge-till corn during a 3-yr period at Mitchell, NE, and a 7-yr period at North Platte, NE. When the experiment was initiated at North Platte, no weeds were present before corn planting. It took 4 yr before triazine-resistant kochia became a problem before corn planting in plots treated with atrazine, but these were controlled by other operations prior to corn harvest. In the cultivated check, green foxtail densities before harvest increased from 0 in 1985 to 32 plants 100 m−2in 1991. Annual applications of dicamba plus 2,4-D 10 d early preplant followed by cultivation controlled triazine-resistant kochia and velvetleaf, but common lambsquarters, nightshade species, and green foxtail increased. Volunteer corn was controlled with cultivation. After 3 yr at Mitchell, the annual weed population increased 10-fold in the cultivated check. Thus, corn yields were reduced 64% with two cultivations compared with an annual early preplant application of dicamba plus 2,4-D followed by alachlor plus cyanazine PRE and two cultivations. With two cultivations under low annual weed populations at North Platte, grain yield from the cultivated check treatment was not different from annual treatments of herbicides after 7 yr. Metolachlor plus atrazine occasionally caused a reduction in corn grain yields.
Infection by the Fungal PathogenColletotrichum coccodesAffects Velvetleaf (Abutilon theophrasti)-Soybean Competition in the Field
Authors: Ditommaso Antonio , Watson Alan K. , Hallett Steve G.
Published: May 2019

Field research was conducted from 1990 through 1992 to evaluate the effect of the fungal pathogen,Colletotrichum coccodes, on velvetleaf intra- and interspecific (with soybean) competition across a range of monoculture and 1:1 mixture densities. In pure stand, application of this velvetleaf foliar pathogen had little impact on seed yield of the weed. In these plots, velvetleaf intraspecific competition stimulated vertical growth and favored the rapid replacement of diseased leaf tissue that had prematurely senesced. In mixtures, however,C. coccodesinoculation differentially influenced the yield of both species. In two of three years,C. coccodesinoculation reduced velvetleaf seed yields by, an average, 60% compared with yields for control (uninoculated) plants. Velvetleaf suffered greater yield losses from soybean interspecific competition in the presence ofC. coccodes, especially at the lower planting densities. The decline in velvetleaf yield was primarily attributed to the stunting effect of the pathogen, which allowed soybean plants to grow above the weed. Consequently, soybean yield losses within inoculated mixture plots were generally lower than for control plots, although significant increases (23%) in soybean yield were recorded only in 1992. The inoculation treatment had relatively little impact on the number of seeds produced per fruit or seed unit weight in both species regardless of whether plants were grown in monocultures or in mixtures. The finding thatC. coccodeshas only a limited effect on velvetleaf performance in pure stand, while having a significantly greater effect in a competitive environment with a soybean crop, has important ramifications as to the value and accuracy of initial efficacy testing that rates potential biocontrol agents based solely on their effect within pure stands of the target weed.