bluuebunny/crossref_metadata_2025 huggingface dataset:
Allelopathic Effect of Lantana (Lantana camara) and Siam Weed (Chromolaena odorata) on Selected Crops
Authors: Sahid Ismail B. , Sugau John B.
Published: May 2019

Laboratory, greenhouse, and field studies were conducted to determine the allelopathic potential of lantana and Siam weed on the germination and growth of five crops. Emergence and dry weight of bioassay species were affected when lantana or Siam weed debris was present on the soil surface or incorporated into the soil. However, presence of Siam weed on the soil surface did not affect dry weight of rape. Emergence and dry weight of crops (except emergence of spinach) were not affected when crops were grown in soil samples collected from beneath field-grown lantana. Dry weight of Chinese cabbage and chilli was reduced when these crops were grown in Siam weed-contaminated soil. Germination of Chinese cabbage, chilli, and rape decreased progressively when exposed to increasing concentrations of aqueous lantana extract. However, the lantana extract at full strength (66.7 g L−1) did not reduce germination of spinach and cucumber seed. Siam weed extract, when applied at full strength to seed of spinach, Chinese cabbage, rape, and chilli, reduced germination by 10, 12, 21, and 19% of control, respectively. Full-strength extract of lantana and Siam weed decreased seedling length and fresh weight of all crops.
Effect of Corn-Induced Shading and Temperature on Rate of Leaf Appearance in Redroot Pigweed (Amaranthus retroflexusL.)
Authors: Mclachlan Stephane M. , Swanton Clarence J. , Weise Stephan F. , Tollenaar Matthijs
Published: May 2019

Leaf development and expansion are important factors in determining the outcome of crop-weed interference. The comparative effects of temperature and corn canopy-induced shading on the rate of leaf appearance (RLA) of redroot pigweed were quantified in this study. Growth cabinet results indicated a linear increase in RLA with increased temperature. Weed RLA was predicted utilizing both this function and field temperature data. The ratio of observed to predicted RLA of redroot pigweed grown in field experiments decreased in 1990 and 1991 as shading increased with increased corn density and delayed weed planting date. Results indicated that RLA is substantially affected by canopy-induced shading in addition to temperature.
Carbon Balance, Transpiration, and Biomass Partitioning of Glyphosate-Treated Wheat (Triticum aestivum) Plants
Authors: Fernandez Carlos J. , McInnes Kevin J. , Cothren J. Tom
Published: May 2019

Whole plant studies were conducted to examine the effects of glyphosate on components of carbon balance, transpiration, and biomass partitioning of wheat plants grown in Olton sandy clay loam soil and in a well-aerated fritted clay medium under controlled environmental conditions. Well-irrigated plants were transferred from a nursery room into a test chamber about 48 d after planting. Two to five days later, 12 to 42 ml of a glyphosate solution with a concentration of 480 mg ai L–1were sprayed until full coverage of the foliage. Environmental conditions in the chamber were air temperature 25 C, dew point 18 C, windspeed 1.1 m s–1, and PPFD 1500 mmol m–2s–1(at the top of the foliage) for 12 h daily. Glyphosate treatment resulted in destruction of the root system, as determined at the end of the tests, and at the start of tests using companion plants. Plants grown in soil lost 0.53 kg kg–1of the initial root mass, while this loss was 0.38 kg kg–1in plants grown in fritted clay. Glyphosate treatment rapidly inhibited daily rates of gross carbon uptake and transpiration of wheat plants grown in both media. Effects occurred more than twice as rapidly in plants grown in soil as in fritted day. Similarity in the patterns of inhibition of gross carbon uptake and transpiration suggests that glyphosate may also affect leaf stomata. After applying glyphosate, daily rates of carbon loss increased for 3 d in soil-grown plants but remained almost constant for 10 d in plants grown in fritted clay; thereafter, the rates of carbon loss declined. The early increase or the constancy of carbon loss observed after applying glyphosate was related to catabolic processes occurring in roots.
Palmer Amaranth (Amaranthus palmeri) Interference in Soybeans (Glycine max)
Authors: Klingaman Tracy E. , Oliver Lawrence R.
Published: May 2019

A 2-yr field study was conducted at Fayetteville, AR, to determine the effect of Palmer amaranth interference on soybean growth and yield. Palmer amaranth density had little effect on soybean height, but soybean canopy width ranged from 77 cm in the weed-free check to 35 cm in plots with 10 plants m–1of row 12 wk after emergence. Soybean yield reduction was highly correlated to Palmer amaranth biomass at 8 wk after emergence and maturity, soybean biomass at 8 wk after emergence, and Palmer amaranth density. Soybean yield reduction was 17, 27, 32, 48, 64, and 68%, respectively, for Palmer amaranth densities of 033, 0.66, 1, 2, 333, and 10 plants m–1of row. Soybean yield reduction and Palmer amaranth biomass were linear to approximately 2 Palmer amaranth m–1of row, suggesting intraspecific interference between adjacent Palmer amaranth is initiated at Palmer amaranth densities between 2 and 3.33 plants m–1of row.
Dissipation of Alachlor in Four Soils as Influenced by Degradation and Sorption Processes
Authors: Yen Pau Y. , Koskinen William C. , Schweizer Edward E.
Published: May 2019

Laboratory studies were conducted to determine the influence of degradation and sorption processes on the dissipation of alachlor in one Colorado soil (Kim clay loam) and three Minnesota soils (Port Byron silt loam, Webster silty clay loam, and Estherville sandy loam) as a function of soil depth. Persistence and movement of alachlor in an irrigated corn production system also were determined on the Kim soil. Laboratory degradation data fit first-order kinetics, and rate constants ranged from 0.0094 to 0.0251 d-1and varied with soil type and depth. For instance, in 60- to 75-cm-depth Kim soil, alachlor degraded at a slower rate (k = 0.011 d-1) than in surface soil samples (k = 0.022 d-1). Alachlor sorption to the four soils was moderate (Kf= 0.7 to 7.4; Kf,oc= 71 to 470) and concentration dependent (1/n < 1.0). Significant hysteretic desorption of alachlor from soils also was observed (1/n desorption < 1/n sorption). The combined effect of degradation and sorption processes has been used to classify a chemical's potential to leach to groundwater. Based on Kf,ocand dissipation half-life, alachlor would be classified as a “leacher” in Kim, Port Byron, and Estherville soils and classified as transitional between “leacher” and “nonleacher” in the Webster soil. The dissipation first-order rate constant (k) of alachlor in Kim soil in the field was 0.036 α 0.012 d-1. Dissipation was apparently not due to leaching since bromide applied at the same time remained in the top 15 cm during the first 28 d. It appears that laboratory-derived leaching indices may overestimate actual leaching and should be used with caution for predictive or regulatory purposes.
Evidence Against a Direct Membrane Effect in the Mechanism of Action of Graminicides
Authors: Di Tomaso Joseph M.
Published: May 2019

The aryloxyphenoxypropionate and cyclohexanedione herbicides, which inhibit acetyl-coenzyme A carboxylase (EC 6.4.1.2), have also been hypothesized to act at specific sites on the plasmalemma. An impermeant sulfhydryl binding agent was reported to block the diclofop acid-induced depolarization of the membrane potential (Em) in rigid ryegrass. A correlation between the antagonistic interaction with auxin herbicides both in the field and in the Emresponse, and the repolarization of Emin herbicide-resistant rigid ryegrass following removal of diclofop acid also provide support for this hypothesis. However, similar membrane responses in resistant grasses and broadleaf species suggest that the membrane response may not be important in the phytotoxic activity of the postemergence graminicides under field conditions. In addition, an antagonistic interaction was not observed in roots of susceptible grasses exposed to combinations of diclofop-methyl and 2,4-D. Furthermore, the repolarization of the Emin diclofop-resistant rigid ryegrass was correlated to differential acidification of the external solution and an increase in the protonated form of diclofop acid, rather than a site-specific interaction at the plasmalemma. Although the membrane response is probably not involved in herbicide phytotoxicity in agricultural systems, a higher extracellular pH in the resistant biotypes of rigid ryegrass may inhibit the movement of these weak-acid herbicides across the plasmalemma, and possibly contribute to increased herbicide tolerance.
Fertilizer Shank Placement Impact on Atrazine Movement in a Ridge Tillage System
Authors: Clay Sharon A. , Scholes Kimberly A. , Clay David E.
Published: May 2019

Surface microrelief and agrichemical placement influence agrichemical movement through soil. This study determined if anhydrous ammonia shank placement influenced atrazine movement in a ridge tillage system. Atrazine and Br−were broadcast over the plot or banded on the ridge while N fertilizer was Injected into the ridge or valley. Rainfall (17 cm) was applied and leachate was collected incrementally from grid lysimeters placed 75 cm below the soil surface. Amounts of atrazine and Br−in leachate and postrainfall soil atrazine distribution were determined. The injection slot created by fertilizer application influenced chemical movement. When atrazine and the fertilizer injection slot overlapped, atrazine movement through the soil was enhanced. These data suggest that herbicide applications and fertilizer injection slots that are not covered should not overlap due to increased potential for movement.
Inheritance of Trifluralin Resistance in Green Foxtail (Setaria viridis)
Authors: Jasieniuk Marie , Brûlé-Babel Anita L. , Morrison Ian N.
Published: May 2019

In F2progeny, derived from F1hybrids, shoot growth of seedlings, measured 4 d after germination, distinguished susceptible (S) and resistant (R) phenotypes. Chisquare values indicated that the F2data fit a 3:1 (S:R) ratio for both populations and all trifluralin concentrations in which S and R phenotypes could be differentiated. Results indicate that trifluralin resistance in these green foxtail populations is controlled by a single, nuclear recessive gene. This study is the first to demonstrate recessive gene control of herbicide resistance in a weed species. The highly selfed nature and prolific seed production of green foxtail may have facilitated evolution of the recessive trait.
Interaction of glyphosate with postemergence soybean (Glycine max) herbicides
Authors: Lich Julie M. , Renner Karen A. , Penner Donald
Published: May 2019

Greenhouse and field experiments were conducted to evaluate the potential for antagonistic or synergistic interactions from tank mixtures of glyphosate plus a selective herbicide applied postemergence. In the greenhouse, glyphosate at 420 g ae ha−1plus 28% liquid urea-ammonium nitrate (28% UAN) provided at least 89% control of common lambsquarters and common ragweed. Glyphosate at 1,680 g ha−1plus 28% UAN provided less than 81% control of velvetleaf and less than 75% control of ivyleaf morningglory. Tank mixing bentazon at 1,120 g ai ha−1with glyphosate at 420 g ha−1synergistically increased control of velvetleaf. Tank mixtures of glyphosate plus a selective herbicide were predominately additive in control of common lambsquarters, common ragweed, and velvetleaf. Several tank mix combinations of chlorimuron or imazethapyr plus glyphosate plus 28% UAN were antagonistic in control of ivyleaf morningglory. In the field, glyphosate at 840 g ha−1plus 28% UAN provided at least 88% control of common lambsquarters and velvetleaf in 1994. However, glyphosate at 840 g ha−1plus 28% UAN provided only 60% control of velvetleaf in 1995. Tank-mixing bentazon or CGA-248757 with glyphosate at 420 g ha−1increased velvetleaf and common lambsquarters control in 1995. In general, adding chlorimuron, imazethapyr, or thifensulfuron to glyphosate plus 28% UAN did not increase control of common lambsquarters or velvetleaf. Tank mixing imazethapyr with glyphosate plus 28% UAN antagonized velvetleaf control in 1994 and in 1995. The tank mixture of thifensulfuron at 2 g ha−1plus glyphosate at 420 g ha−1plus 28% UAN increased soybean injury in the field in 1994. However, tank mixing chlorimuron, imazethapyr, or thifensulfuron with glyphosate plus 28% UAN did not increase soybean injury in the greenhouse or in the field in 1995.
Physiological basis for nicosulfuron and primisulfuron selectivity in five plant species
Authors: Carey J. Boyd , Penner Donald , Kells James J.
Published: May 2019

Greenhouse and laboratory studies were conducted to determine the physiological basis for selectivity of nicosulfuron and primisulfuron in 5 plant species. Differential sensitivity of the species was quantified by determining GR50values (herbicide rate required to reduce plant growth 50%) for each species/herbicide combination. GR50data indicated the following levels of sensitivity: corn—tolerant to both herbicides; seedling johnsongrass—sensitive to both herbicides; barnyardgrass—sensitive to nicosulfuron and tolerant to primisulfuron; giant foxtail—sensitive to nicosulfuron and tolerant to primisulfuron; and eastern black nightshade—tolerant to nicosulfuron and sensitive to primisulfuron. Studies using14C-radiolabeled herbicides were conducted to determine whether differential herbicide absorption, translocation, or metabolism contributed to whole plant responses. Nicosulfuron and primisulfuron selectivity in corn, johnsongrass, barnyardgrass, and giant foxtail was primarily due to differential herbicide metabolism rate. Tolerant species metabolized the herbicide more rapidly and extensively than sensitive species. Differential herbicide absorption, translocation, or metabolism did not explain differential sensitivity of eastern black nightshade to the herbicides. Further studies indicated that the tolerance of eastern black nightshade to nicosulfuron and its sensitivity to primisulfuron was directly related to lower sensitivity of the acetolactate synthase (ALS) to nicosulfuron than to primisulfuron. Eastern black nightshade translocated very little (3%) of the nicosulfuron applied. The ALS sensitivity of johnsongrass and eastern black nightshade was similar in the presence of nicosulfuron. A combination of a higher ALS level and less herbicide translocation contributes to tolerance of eastern black nightshade and to sensitivity of johnsongrass to nicosulfuron.
Establishment of common milkweed (Asclepias syriaca) in corn, soybean, and wheat
Authors: Yenish Joseph P. , Fry Thomas A. , Durgan Beverly R. , Wyse Donald L.
Published: May 2019

The establishment of common milkweed in corn, soybean, and wheat and rotations of corn-soybean and soybean-wheat was determined in field studies at Rosemount and Morris, MN. Common milkweed seedling density 115 d after initial seedling emergence was 500, 15,000, and 31,300 seedlings ha−1in corn, soybean, and wheat, respectively, in 1990; 500, 7,300, and 15,600 in 1991 at Rosemount and 2,900, 26,500, and 37,000 in 1990; and 2,200, 6,700, and 8,100 in 1991 at Morris in the year of seedling establishment. Common milkweed seedling defoliation by mon arch butterfly larva was 1,31, and 0% at Rosemount and 20, 78, and 0% at Morris for corn, soybean, and wheat, respectively. In the 2nd and 3rd rotational years, corn-soybean rotations had fewer shoots ha−1115 d after shoot emergence than soybean wheat rotations. Seedling densities 115 d after initial seedling emergence were greater in corn-soybean rotations where soybean rather than corn was the rotational crop and in soybean-wheat rotations where wheat rather than soybean was the rotational crop immediately following common milkweed seeding. Conventional tillage treatments had lower common milkweed shoot density than conservation tillage in all years of all rotations except in the initial count, when soybean was the first rotational crop in which shoot density was greater in conventional than in conservation tillage treatments. Cyanazine, imazethapyr, or bromoxynil applications in corn, soybean, or wheat rotational crops, respectively, reduced common milkweed shoot density.
Seed bank characterization and emergence of a weed community in a moldboard plow system
Authors: Mulugeta Dawit , Stoltenberg David E.
Published: May 2019

Field experiments were conducted in 1992 and 1993 to characterize the weed seed bank, to determine the influence of moldboard plowing and secondary soil disturbance on the emergence pattern of weeds, and to measure weed seed bank depletion by emergence in a long-term moldboard plow corn cropping system. Viable seeds of common lambsquarters, redroot pigweed, and each of 10 other species accounted for about 85, about 9, and less than 1%, respectively, of the total weed species in the seed bank. A negative binomial distribution described the spatial distribution of viable seeds of 10 species, but not of common lambsquarters or of redroot pigweed. Decreased density of seeds among species was associated with increased aggregation. Secondary soil disturbance increased the rate and magnitude of common lambs quarters emergence in 1992 but did not influence emergence in 1993. Secondary soil disturbance did not influence the magnitude and rate of emergence of redroot pigweed or velvetleaf. Whereas cumulative growing degree days from April through July were similar between years, the amount of rainfall was about 50% less in 1992 than in 1993. Secondary soil disturbance may have increased common lambsquarters emergence by increasing the availability of soil moisture and improving conditions for seed germination during the dry year. Even though seed bank depletion by seedling emergence was relatively low for all species, secondary soil disturbance in creased seed bank depletion of common lambsquarters and redroot pigweed about 7- and 3-fold, respectively, in 1992. Seasonal variation in the amount of rainfall may have influenced the effect of soil disturbance on emergence and seed bank depletion of common lambsquarters, which is the most abundant species in the weed community.
Temporal changes in velvetleaf (Abutilon theophrasti) seed dormancy
Authors: Cardina John , Sparrow Denise H.
Published: May 2019

Primary physical dormancy caused by seed coat impermeability to water is a major reason for the persistence of velvetleaf in soil seedbanks. Understanding temporal trends in seed dormancy status will help predict potential emergence in the spring. Experiments were begun in 1992 and 1993 to determine the effects of velvetleaf seed maturation time, storage environment, and storage duration on changes in seed dormancy and germination over 20 mo. Seeds buried 1 and 10 cm deep exhibited a 30 to 70% decline in physical dormancy from maturity until winter, little change in dormancy from winter through the following summer, and a further decline the next autumn. The loss of physical dormancy was more rapid for early than for late maturing seeds and more rapid in 1992 than in 1993. Physical dormancy of seeds held at 4 C declined steadily, at a rate of approximately 0.8% per day, over the course of the study. Germination of seeds buried 1 cm averaged 23 to 37% in the first spring after harvest, which was equivalent to 68 to 100% of seeds that had lost physical dormancy over autumn and winter. The percentage of seeds with enforced dormancy reflected the loss of physical dormancy during autumn and the loss of seeds to germination during spring and summer. Additional information on how autumn temperature and moisture conditions influence the pattern of dormancy decline could aid in explaining the variation in velvetleaf infestations over time.
Characterizing spatial stability of weed populations using interpolated maps
Authors: Gerhards Roland , Wyse-Pester Dawn Y. , Mortensen David , Johnson Gregg A.
Published: May 2019

Intensive surveys were conducted in 2 fields in eastern Nebraska to determine the spatial stability of common sunflower, velvetleaf, green and yellow foxtail, and hemp dogbane over 4 yr (1992 to 1995). The 1st field was planted to soybean in 1992 and corn in 1993, 1994, and 1995. The 2nd field was planted to corn in 1992 and 1994 and soybean in 1993 and 1995. Weed density was sampled prior to postemergence herbicide application at approximately 800 locations per year in each field on a regular 7 m grid. The same locations were sampled every year. Weed density at locations between the sample sites was determined by linear triangulation interpolation. Weed seedling distribution was significantly aggregated, with large weed-free areas in both fields. Common sunflower, velvetleaf, and hemp dogbane patches were very persistent in diameter in the east-west and north-south directions and in location and area over 4 yr in the 1st field. Foxtail distribution and density continuously increased in each of the 4 yr in the first field and decreased in the 2nd field. A geographic information system was used to overlay maps from each year for a species. This showed that 36% of the sampled area was continuously free of common sunflower, 62.5% was free of hemp dogbane, and 11.5% was free of velvetleaf in the 1st field, but only 1% was free of velvetleaf in the 2nd field. The persistence of broadleaf weed patches suggests that weed seedling distributions mapped in one year are good predictors of future seedling distributions. Improved and more efficient sampling methods are needed.
Effect of tillage and corn on pigweed (Amaranthusspp.) seedling emergence and density
Authors: Oryokot Joseph O. E. , Murphy Stephen D. , Swanton Clarence J.
Published: May 2019

We studied the effect of no-till, chisel, and moldboard plow and the presence or absence of corn on soil temperature, moisture and, subsequently, the emergence phenology and density of pigweed seedlings at 2 sites from 1993 to 1995 inclusively. Tillage significantly affected the phenology of pigweed seedling emergence only during a June drought at one site in 1994. Soil temperature and moisture, measured at 2.5-cm depths, also were unaffected by tillage. Weed phenology is usually earlier in no-till because more seeds are located closer to the surface (< 5 cm deep) in no-till, thereby reducing the delay in penetrating through the soil, and because soil temperatures and moisture are nearer the germination and emergence optima. However, pigweed seedlings are already physiologically restricted to germination depths of less than 2.5 cm regardless of tillage; therefore, this prior constraint eliminated any potential differences in emergence phenologies caused by tillage. The presence or absence of corn also did not affect soil temperatures, soil moisture, or pigweed seedling emergence phenologies. Pigweed seedling density was significantly higher in no-till; this may have been caused by increased numbers of seeds near the soil surface in no-till. The presence or absence of corn did not affect pigweed seedling density; the lack of a significant effect probably reflects high variances in density. Although necessary for most weed species, tillage may be a less important factor to consider in predicting pigweed population dynamics and subsequent management recommendations.
Nicosulfuron and Primisulfuron Selectivity in Corn (Zea mays) and Two Annual Grass Weeds
Authors: Hinz John R. R. , Owen Micheal D. K.
Published: May 2019

Research was conducted to elucidate mechanism(s) of nicosulfuron and primisulfuron selectivity in corn, woolly cupgrass, and shattercane. Corn absorbed less than one half the nicosulfuron and primisulfuron that woolly cupgrass and shattercane absorbed. Acetolactate synthase (ALS) isolated from three species responded similarly to nicosulfuron and primisulfuron. Values for 50% (I50) inhibition of ALS for nicosulfuron were 36.9, 37.2 and 29.5 nM for corn, woolly cupgrass and shattercane, respectively. Primisulfuron I50values were 13.8, 11.5 and 10.0 nM for corn, woolly cupgrass and shattercane, respectively. Shattercane's I50was different from corn's but not from that of woolly cupgrass. Corn rapidly metabolized nicosulfuron and primisulfuron, with a half-life of less than 4 h. Shattercane metabolized the herbicides more slowly, with a half-life greater than 72 h for nicosulfuron and 36 h for primisulfuron. Nicosulfuron and primisulfuron half-lives were greater than 72 h and less than 4 h, respectively, in woolly cupgrass. Therefore, selectivity with nicosulfuron and primisulfuron is likely based on metabolism to nonphytotoxic compounds. Corn tolerance to nicosulfuron and primisulfuron was also attributed to reduced herbicide penetration and translocation below the treated leaf.
Smallseed Dodder (Cuscuta planiflora) Phototropism toward Far-red When in White Light
Authors: Orr Gregory L. , Haidar Mustapha A. , Orr Deborah A.
Published: May 2019

White light-grown seedlings of smallseed dodder were (a) provided with unilateral far-red (700 to 800 nm) at photon irradiances ranging from 20 to 110 μmol m−2s−1against a background of cool white light (400 to 700 nm) from above at 77 μmol m−2s−1, or (b) transferred to darkness and provided with unilateral white light at 20 μmol m−2s−1, unilateral blue light (400 to 500 nm) at 10 μmol m−2s−1, unilateral red light (600 to 700 nm) at 10 μmol m−2s−1, unilateral far-red at 50 μmol m−2s−1, or (c) in experiments utilizing bilateral irradiations, provided with unilateral far-red perpendicular to unilateral white light. Positive phototropic curvature was induced by unilateral white light and by unilateral blue light in otherwise darkness and by unilateral far-red in a background of cool white light. Seedling vines were also phototropic toward unilateral far-red when provided with unilateral white light perpendicular to unilateral far-red. Phototropism to unilateral white light was inhibited in seedlings treated with 200 μM norflurazon and 50 mM potassium iodide. Norflurazon- and potassium iodide-treated seedlings remained phototropic toward unilateral far-red when provided with unilateral white light perpendicular to unilateral far-red. Seedling vines were not phototropic to unilateral red or to unilateral far-red in otherwise darkness, and seedlings in cool white light were neither skototropic (i.e., tropic toward unilateral darkness) nor tropic to or from infra-red (radiation with wavelengths greater than 900 nm). Phototropism toward regions of lowered red:far-red may aid smallseed dodder in chlorophyllous host location and attachment.
Response of Common Ragweed (Ambrosia artemisiifolia) and Giant Ragweed (Ambrosia trifida) to Postemergence Imazethapyr
Authors: Ballard Thomas O. , Foley Michael E. , Bauman Thomas T.
Published: May 2019

A study was conducted to evaluate the response of common and giant ragweed to postemergence applications of imazethapyr using relative growth rate parameters. The relative growth rate was the same for untreated common and giant ragweed through the 21 d harvest interval. Relative growth rates of treated common and giant ragweed were 50% lower than the relative growth rates of untreated ragweeds 21 d after treatment. Between 21 and 56 d after treatment, the relative growth rate of common ragweed declined an additional 13%, while the relative growth rate of giant ragweed declined an additional 38%. The sharp continued decline in the relative growth rate of giant ragweed indicated plant death. The moderation and slight increase in the relative growth rate of common ragweed between 21 and 56 d corresponded with the initiation of lateral axillary buds and the regeneration of plant growth. Relative growth rate parameters identified differences in the response of common and giant ragweed to imazethapyr as early as 21 d after treatment. Relative growth rate demonstrated utility by objectively measuring differences in the growth response of these two weeds that are moderately susceptible to imazethapyr under laboratory conditions.
Estimation of Critical Period of Weed Control
Authors: Singh M. , Saxena M. C. , Abu-Irmaileh B. E. , Al-Thahabi S. A. , Haddad N. I.
Published: May 2019

An estimation of the critical period of weed control is helpful in formulating appropriate weed-control strategies. A regression approach is presented to estimate the thresholds of critical period of weed control and time of equal interference (or time of onset of competition). In this approach, yields were either a linear or logistic function of the duration of weed-free and weed-infested periods. Confidence intervals of the thresholds of critical period and time of equal interference were determined for the linear model. An approximation to the standard error of critical period and associated confidence interval were given for any general form of the model. The method was applied to estimate the critical period of weed control in rainfed lentil using data from four field experiments conducted in Jordan. The relationship of yield with the duration of weed-free period was described by a linear function, whereas the relationship with the duration of weed-infested period showed a better fit with a logistic function. To maintain 90% of maximum seed yield, the maximum time allowed to let weeds grow after the crop emergence varied over locations from 4.8 to 5.8 wk. The same level could be achieved if the crop is kept free of weeds from its emergence until 12.1 to 14.1 wk; while the time when the same amount of yield would be achieved under both approaches varied from 7.7 to 9.3 wk after crop emergence. For straw yield, the time to get 90% of the maximum yield could vary over location from a maximum of 4.5 to 8.0 wk under weed-infestation and from at least 11.5 to 13.5 wk when weed-free. The time to achieve the same amount of straw under two systems of competition varied from 6.5 to 9.9 wk after crop emergence. One of the four experiments showed a longer critical period than the others for seed and straw yields.
Effects of Environmental Factors on Dormancy and Germination of Crenate Broomrape (Orobanche crenata)
Authors: López-Granados Francisca , García-Torres Luis
Published: May 2019

The effects of acid scarification, prechilling, summer irrigation, and root extract stimulant from faba bean plants grown during different seasons on crenate broomrape seed dormancy and germination were determined. Immersion of 6- to 12-mo old seed in 0.5 to 5 N H2SO4solution from 1 to 20 min increased germination by 20%. Similarly, seed pretreatment at temperatures between −4 and 6 C increased germination, regardless of the pretreatment duration. Results suggest that the hard seed coat is an important dormancy mechanism in newly formed crenate broomrape seeds. Summer irrigation did not substantially change the germination capacity of crenate broomrape seeds. The germination stimulant activity of the root extract from faba bean plants grown during different seasons varied considerably, decreasing in this order: autumn > winter > spring > summer. This suggests an ecological adaptation of crenate broomrape to its main host crop.
Investigations into Alternative Methods to Predict the Competitive Effects of Weeds on Crop Yields
Authors: Lutman Peter J.W. , Risiott Ruth , Ostermann H. Peter
Published: May 2019

Sixteen experiments have investigated alternative methods of predicting the competitive effects of a simulated weed (oats) on the yields of spring barley, spring oilseed rape (canola), peas, spring field (faba) beans and flax. The experiments were designed to discover whether early postemergence assessments of crop and weed vigor would achieve more reliable prediction of yield loss than weed density. Weed density (plants m−2) was a very variable predictor of yield loss. The standardized ranges (range/mean) of values over 3 to 4 years of data for the five crops, in the densities causing 5% yield loss, were between 1.14 and 2.59. Predictions based on the relative dry weight of crop and oats (oat dwt/(oat dwt + crop dwt)), assessed while the plants were still small, achieved more reliable predictions, as the standardized ranges were between 0.10 and 1.86. In three of the experiments, predictions based on relative dry weights were compared to similarly timed predictions based on measurements of relative leaf area and of ground cover, assessed subjectively (by eye) and photographically. Subjective and objective (photographic) assessments of cover achieved similar predictions of yield loss, indicating that visual assessments could be a viable tool to assess the potential competitive effects of weeds.
Spatial Relationships Between Seedbank and Seedling Populations of Common Lambsquarters (Chenopodium album) and Annual Grasses
Authors: CardiNa John , Sparrow Denise H. , McCoy Edward L.
Published: May 2019

Predictions of weed seedling populations from seedbank data should characterize the spatial distribution as well as the composition and abundance of weeds. The spatial distribution of seedbank and seedling populations of common lambsquarters and annual grasses (giant foxtail, large crabgrass, and fall panicum) were described in moldboard plow and no-tillage soybean fields from 1990 to 1993. Spearman rank correlations between seedbank and seedling densities were significant for common lambsquarters in both tillages and all years, but for annual grasses correlations were significant only in no-tillage. Semivariograms showed spatial autocorrelation in seedbank and seedling populations of common lambsquarters in all years in no-till, but less often in the moldboard plow field. Annual grass seed and seedling populations were autocorrelated in the no-till field every year except 1993, and in the moldboard plow field in 1992 and 1993 only. Cross-semivariograms showed spatial continuity between seedbank and seedling population densities in 3 of 4 yr in no-till for common lambsquarters, and in all years of no-till and 1 yr of moldboard plow for annual grasses. Grey-scale field maps of common lambsquarters seedbanks corresponded visually to maps of seedling populations and could have been used to target control efforts, but visual correspondence between annual grass seedbank and seedling maps was poor. Seedbank and seedling mapping may be useful for site-specific management, but additional information is needed to understand the variation in the relationships between these two populations over time and space.
Stability of Corn (Zea mays)-Velvetleaf (Abutilon theophrasti) Interference Relationships
Authors: Lindquist John L. , Mortensen David A. , Clay Sharon A. , Schmenk Richard , Kells James J. , Howatt Kirk , Westra Philip
Published: May 2019

The crop-weed interference relationship is a critical component of bioeconomic weed management models. Multi-year field experiments were conducted at five locations to determine the stability of corn-velvetleaf interference relationships across years and locations. Two coefficients (I and A) of a hyperbolic equation were estimated for each data set using nonlinear regression procedures. The I and A coefficients represent percent corn yield loss as velvetleaf density approaches zero, and maximum percent corn yield loss, respectively. The coefficient I was stable across years at two locations, but varied across years at one location. The coefficient A did not vary across years within locations. Both coefficients, however, varied among locations. Results do not support the use of common coefficient estimates for all locations within a region.
Spatial Growth of Kikuyugrass (Pennisetum clandestinum)
Authors: Wilen Cheryl A. , Holt Jodie S.
Published: May 2019

Photographic techniques and the point-frame transect method were used to study areal growth of kikuyugrass grown in the field without competition in 1990 and 1991. The effect of cutting height on plant extension was also examined in 1991. There were no differences in sward area between eight selections planted in 1990, except on the first measurement date, but selections differed in height and number of primary and secondary stolons. Expansion was irregular in all but one selection, and the patch shape of all irregular selections was defined by the direction and number of the earliest buds to sprout from the stolons. Only two of the eight selections were planted in 1991. Cutting height affected extension from the plot center, linear plant frequency, and plant biomass. The lowest height of cut, 2.5 cm, had the least extension and frequency, the higher cut, 5 cm, was intermediate, and the unmowed plots had the highest extension and frequency. There were differences between selections in number of nodes per unit area, which resulted in differences in response to mowing. Results from a greenhouse clipping experiment showed that although root weight of stolon sections used as propagules was reduced by defoliation, starch content was not affected. The spreading ability of kikuyugrass appears to be based on its ability to branch from its stolons presumably due to reallocation of resources. Based on these results, close mowing could slow the process of kikuyugrass invasion into other turf species but is inadequate to control it.
Johnsongrass (Sorghum halepense) Control with Imazethapyr and Haloxyfop in Conventional and Vertical-tilled Soybean (Glycine max)
Authors: Tassara Hector J. , Santoro Jose , De Seiler Mircza C. , Bojanich Esteban , Rubione Claudio , Pavon Ricardo , Satorre Emilio H. , Benech-Arnold Roberto L.
Published: May 2019

Field studies in conventional and vertical-tilled soybeans were conducted in 1992/93 and 1993/94 at four locations in Argentina to compare the efficacy of imazethapyr and haloxyfop for johnsongrass control. Control was evaluated by measuring frequency of johnsongrass-infested areas at the beginning of the 1992/93 and 1993/94 growing seasons. Relative johnsongrass leaf area and number of rhizome phytomers within infested areas also was evaluated. Herbicide application reduced frequency of infested areas in the 1992/93 growing season. Frequency of infested areas increased in check plots. No differences were found among herbicide treatments, suggesting that the effect of imazethapyr was as high as that observed for specific graminicides. Reduction in frequency of infested areas caused by herbicide treatments was accompanied to some extent by a decrease in intensity of infestation within infested patches assessed by measuring johnsongrass relative leaf area. Phytomer population levels, on the other hand, were reduced only after two consecutive years of herbicide application. No differences were found among herbicide treatments in terms of impact on both measures of weed abundance. Johnsongrass responded similarly to all treatments regardless of tillage system.
Clopyralid Use in Strawberries (Fragaria×ananassaDuch.) Grown on Plastic Mulch
Authors: McMurray G. L. , Monks D.W. , Leidy R. B.
Published: May 2019

Experiments were conducted in North Carolina in winter/spring 1992, 1993, and 1994 to determine crop tolerance, weed response, and clopyralid residue levels in fruit of ‘Chandler’ strawberry plants treated with clopyralid. Clopyralid at 0.07, 0.14, 0.20, or 0.28 kg ai ha−1applied POST over strawberry plants and vetch resulted in 100% control of vetch species, 49 to 83% control of black medic, and less than 6% crop injury. As a comparison, 2,4-D at 0.84 kg ai ha−1applied POST at 5 to 10% strawberry bloom resulted in 50% control of black medic and 48 to 73% crop injury, while 2,4-D at 0.84 kg ai ha−1applied POST to 7- to 9- and 9- to 10-leaf strawberries resulted in 5 to 22% crop injury and no adverse affect on strawberry yield. Single applications of clopyralid at 0.07, 0.14, or 0.28 kg ai ha−1applied POST to weed-free strawberries at the 5- to 6-, 6- to 7-, 9- to 10-, or 12- to 14-leaf stage caused less than 6% injury and did not adversely affect strawberry yield. In 1993 with preharvest intervals (PHI) of 39, 66, and 101 d after treatment all clopyralid residue levels in strawberry fruit were below the detectable level of 0.3 parts per billion (ppb). In 1994, with PHI of 30, 59, and 87 d after treatment trace clopyralid residues were found in strawberry fruit with a range from 0.25 to 1.9 ppb, with a level of detection of 0.2 ppb.
Weed Control and Sweet Corn (Zea maysvar.rugosa) Response in a No-till System with Cover Crops
Authors: Burgos Nilda R. , Talbert Ronald E.
Published: May 2019

Studies were conducted at the Main Agricultural Experiment Station in Fayetteville and the Vegetable Substation in Kibler, Arkansas, in 1992 and 1993 on the same plots to evaluate weed suppression by winter cover crops alone or in combination with reduced herbicide rates in no-till sweet corn and to evaluate cover crop effects on growth and yield of sweet corn. Plots seeded to rye plus hairy vetch, rye, or wheat had at least 50% fewer early season weeds than hairy vetch alone or no cover crop. None of the cover crops reduced population of yellow nutsedge. Without herbicides, hairy vetch did not suppress weeds 8 wk after cover crop desiccation. Half rates of atrazine and metolachlor (1.1 + 1.1 kg ai ha−1) reduced total weed density more effectively in no cover crop than in hairy vetch. Half rates of atrazine and metolachlor controlled redroot pigweed, Palmer amaranth, and goosegrass regardless of cover crop. Full rates of atrazine and metolachlor (2.2 + 2.2 kg ai ha−1) were needed to control large crabgrass in hairy vetch. Control of yellow nutsedge in hairy vetch was marginal even with full herbicide rates. Yellow nutsedge population increased and control with herbicides declined the second year, particularly with half rates of atrazine and metolachlor. All cover crops except hairy vetch alone reduced emergence, height, and yield of sweet corn. Sweet corn yields from half rates of atrazine and metolachlor equalled the full rates regardless of cover crops.
Effect of Wetting/Drying Cycles on Dissipation Patterns of Bioavailable Imazaquin
Authors: Baughman Todd A. , Shaw David R.
Published: May 2019

Greenhouse experiments determined differences in imazaquin bioavailability over time under various soil moisture regimes. All soils were initially fortified with 63 ppb (w/w) of commercially formulated imazaquin. Treatments consisted of maintaining at field capacity for 15 wk (FC), maintaining air-dry for 15 wk (AD), soil that was maintained air-dry for 2 wk and then wet to field capacity at 2 wk intervals (2WAD), and soil that was wet to field capacity at the initiation of the experiment and at 2 wk intervals (2WFC). Little dissipation of imazaquin occurred in the AD soil over the 100 d of this study. Rapid dissipation over the first 35 d occurred for the other three treatments. Imazaquin half-life ranged from 17 to 18 d for the FC, 2WAD, and 2WFC soils. Imazaquin concentration increased or only slightly decreased in samples taken after the second watering in both the 2WAD and the 2WFC soils. Due to differences in adsorption/desorption of imazaquin with changing soil moisture levels, bioassays may not determine the dissipation of the actual concentration of imazaquin contained in soil.
Allelopathic Potential ofPluchea lanceolata:Comparative Studies of Cultivated Fields
Authors: Inderjit  , Dakshini K. M. M.
Published: May 2019

The study was undertaken to compare the level of total phenolics and the allelopathic potential ofPluchea lanceolatain soils cultivated once or twice a year. Thirty-five cultivated fields were sampled from seven sites in three different seasons. The leaf area, leaf weight, height, and density ofP. lanceolataplants were measured. Leaves ofP. lanceolatawere analyzed for nine chemical characteristics: leaf ash, total phenolics, phosphate, Cu, Zn, Na, K, Mg, and Ca. Weed-infested topsoil and subsoil were analyzed for pH, electrical conductivity, organic carbon, chloride, bicarbonate, total carbonate, phosphate, total phenolics, Zn, Na, K, Mg, and Ca. Fields cultivated twice a year were designated as Group I, while those cultivated once a year were designated as Group II. Biotic and chemical characteristics of plants of Group I and II were not different. Topsoil and subsoil of Group II had higher values for Ca, while topsoil of Group I had higher values for total phenolics. The higher phenolic content of Group I topsoil was probably due to greater incorporation of weed plant parts into the soils as compared to Group II fields. Seedling growth of crop plants such as radish, carrot, maize, mustard, tomato, turnip, and wheat was affected more by soil fromP. lanceolata-infested fields that were cultivated twice a year than those cultivated only once. A correlation between higher total phenolic content ofP. lanceolata-infested topsoil with two cultivations, and growth response of crop plants was recorded.
Development of Stale Seedbed Weed Control Programs for Southern Row Crops
Authors: Shaw David R.
Published: May 2019

Planting row crops into a stale seedbed has gained popularity in recent years. This concept uses tillage either after harvest in the fall or early in the spring to smooth the seedbed and eliminate ruts and excess residue on the soil surface. Tillage is not used immediately before planting, thus conserving moisture, eliminating costly and time-consuming tillage trips at planting, and allowing more timely planting. While stale seedbed planting minimizes tillage to establish the crop, conservation is not the primary objective of this concept; rather, timely stand establishment following adverse weather conditions has been the driving force behind the popularity of stale seedbed programs. The stale seedbed approach has been used with success on a variety of soils in the southern U.S., but is best suited on the poorly-drained clay soils in the Mississippi River Delta. The system has been adapted for use in soybean, cotton, corn, and rice. A key component of a well-prepared stale seedbed is the absence of weeds at planting. Producers must keep in mind that weeds present and not controlled at planting have the potential to significantly impair stand establishment and crop yields. Although these weeds are often beginning to senesce by the time soybean planting occurs, they are still using soil moisture and shading newly emerging crop plants early in the season.
Integrated Weed Management Systems for Irrigated Corn (Zea mays) Production in Colorado—A Case Study
Authors: Vangessel Mark J. , Schweizer Edward E. , Lybecker Donald W. , Westra Philip
Published: May 2019

Cooperative research was initiated by a multidisciplinary team of weed scientists and agricultural economists in 1975 that laid the foundation for integrated weed management research in Colorado. Colorado's integrated weed management research has four phases: weed seed bank decline, systems approach to weed management, bioeconomic modeling, and multiple weed management tactics. Each phase considered weed seed bank, weed control efficacy, weed control costs, crop yield, and gross margin. All four phases emphasize the need for a weed management system that minimizes seed production in situations where a high soil weed seed bank exists. However, once the weed seed bank is reduced, a weed management program with less than 100% weed control can be employed without increasing the seed bank. Intensive weed management systems are seldom justified when economics, weed control, and weed seed bank are considered. Integrated weed management approaches require integrating crop and weed biology with management tactics. Integrated weed management requires advanced managerial skills and often more time than traditional approaches.