The antagonistic effect of Na-bentazon on sethoxydim absorption and herbicidal activity has been documented. The addition of ammonium sulfate (AMS) with a surfactant overcomes the observed loss of sethoxydim activity. Nuclear Magnetic Resonance (NMR) was used to study the chemical effects of commercially formulated Na-bentazon, NaCl, NaHCO3, Na-bentazon plus AMS, and NaHCO3 plus AMS on commercially formulated sethoxydim. Technical grade Li-sethoxydim and sethoxydim were analyzed and 1H-NMR spectra were used as comparative standards. Data indicate an association of Na+ from Na-bentazon, NaHCO3, and NaCl with the sethoxydim molecule. NH4+ from AMS appears to associate spatially with sethoxydim but does not exert the same electronic effect on the sethoxydim ring protons as observed with Na+ or Li+. The addition of AMS to sethoxydim plus Na-bentazon or NaHCO3 treatments prevents the complexation of Na+ with the sethoxydim molecule. The data support the hypothesis that the observed Na-bentazon antagonism and ammonium sulfate reversal of antagonism are chemically based.

Field studies have shown primisulfuron to be more injurious to sugarbeet than nicosulfuron 1 and 2 yr after herbicide application. Experiments were initiated to determine if primisulfuron is more injurious to sugarbeet grown in a nutrient culture and if the difference in sugarbeet response is a result of greater uptake, translocation, or acetolactate synthase (ALS) site sensitivity with primisulfuron. Concentrations of primisulfuron and nicosulfuron that reduced sugarbeet growth by 50% were 1.9 and 8.9 μg ai L−1, respectively, at pH 6.5. The pH of the nutrient solution did not influence sugarbeet response to either herbicide. Uptake of primisulfuron was greater (3%) than that of nicosulfuron (1%). Translocation (expressed as a percent of uptake) of nicosulfuron was more rapid than primisulfuron. Fifty-seven percent of the absorbed nicosulfuron translocated out of the root during the 12-h pulse period, while an equal concentration of primisulfuron was not translocated out of the root until 48 h after pulsing. The total nicosulfuron translocated after 144 h was half that of primisulfuron. The nutrient solution taken up by sugarbeet in the 12-h pulse period was reduced by 41% in the presence of either herbicide compared to the untreated control. The ALS enzyme was a minimum of 15 times more sensitive to primisulfuron compared to nicosulfuron which may account for greater sugarbeet response to primisulfuron.

The effect of photoperiod and growth stage on translocation of 14C-glyphosate was compared in Canada thistle plants at the bud and rosette stage of growth. Canada thistle plants grown under controlled environment conditions with a 10 h photoperiod remained as low growing rosettes and developed a mature root system. When the photoperiod for half of these plants was increased to 16 h, stem elongation occurred and flowering was initiated. Growth stage at the time of application affected the distribution of 14C-glyphosate within the elongated shoot and between the shoot and root. The shoot tissue of the bud stage plants contained 25.9% of the 14C-glyphosate recovered, while the rosette plants contained only 3.6%; a seven-fold difference. 14C-glyphosate was applied to leaves 19 and 20, which corresponded to the mid-point of the elongated stem of the bud-stage plants. In the bud-stage plants, 14C-glyphosate moved preferentially into the apical portion of the stem, with three to four times as much in the apical portion of the elongated stem as in the basal portion. In the roots, the effect of growth stage on distribution of 14C-glyphosate was reversed, application at the rosette stage resulted in a four-fold increase in the amount of 14C-glyphosate in the root. When applied in the rosette stage, 19.1% of the 14C-glyphosate detected was in the root compared to only 4.9% when applied at the bud stage. Although the root of the rosette plants was larger than in bud-stage plants, the concentration of 14C-glyphosate in the root tissue of the rosette plants was three times greater. Photoperiod indirectly affected the distribution of 14C-glyphosate in Canada thistle by its effect on growth.

Field and greenhouse research was conducted to quantify the level of resistance to atrazine in Wisconsin (WRB1) and Maryland (MRB) velvetleaf biotypes and to determine cross-resistance of the WRB1 and MRB biotypes to other selected herbicides as compared to a Wisconsin atrazine-susceptible velvetleaf accession (WSA1). In field studies, the WRB1 and MRB biotypes survived atrazine applied POST at dosages as high as 4.5 kg ha−1. In contrast, the WSA1 accession had 50% survival following a 1.1 kg ha−1 POST atrazine application. The WRB1 biotype demonstrated neither cross-resistance nor negative cross-resistance to alachlor, bentazon, bromoxynil, cyanazine, dicamba, linuron, metribuzin, or thifensulfuron. The MRB biotype demonstrated neither cross-resistance nor negative cross-resistance to alachlor, bentazon, dicamba, metribuzin, or thifensulfuron; slight negative cross-resistance was demonstrated to bromoxynil, cyanazine, and linuron. In greenhouse studies, the WRB1 and MRB biotypes were approximately 100-fold more resistant to atrazine than the WSA1 accession; the WRB1 and MRB biotypes demonstrated neither cross-resistance nor negative cross-resistance to alachlor, ametryn, bentazon, bromoxynil, cyanazine, dicamba, linuron, metribuzin, pendimethalin, terbacil, or thifensulfuron. Absence of cross-resistance to cyanazine, ametryn, metribuzin, and terbacil in the WRB1 and MRB biotypes of velvetleaf is in contrast to most other atrazine-resistant weed biotypes.

The joint action of metribuzin and tridiphane was investigated in metribuzin-tolerant (T) and metribuzin-susceptible (S) soybean and tomato cultivars within species, respectively, under growth room studies. Maple Arrow (T) and Maple Amber (S) exhibited similar tolerance to tridiphane applied at soybean emergence. Vision (T) tomato was more sensitive to tridiphane than was Springset (S) tomato, the reverse of the relative tolerance to metribuzin. A phytotoxic interaction was demonstrated following application of tridiphane and metribuzin at the respective rates (kg ai ha−1) of 0.1 and 1.1 in Maple Arrow (T) soybeans, 0.05 and 0.25 in Maple Amber (S) soybeans, and 0.25 and 0.2 in Springset (S) tomato. Tridiphane applied 1 or 4 h before metribuzin caused the greatest phytotoxicity in Maple Amber (S) soybeans. Soybean field results generally supported those of growth-room studies. Foliar spray pretreatment with tridiphane increased total radioactivity in Springset (S), decreased the total root radioactivity and increased total shoot radioactivity in both Vision (T) and Springset (S) and decreased metabolism of metribuzin to water soluble conjugates in Springset (S) roots over 24 h following 14C-metribuzin application to roots of intact tomato seedlings. The increased uptake, translocation to the shoots, and decreased root metabolism of metribuzin in Springset when pretreated with tridiphane could explain the phytotoxic interaction (which was unexpected, based only on the glucose detoxification pathway of metribuzin in tomato).

Terbacil tolerance was investigated in field-selected alfalfa strain ‘OK 182.’ Growth response studies indicated a 78% increase in tolerance over the unselected parental cultivar ‘Cimarron.’ Photosynthetic electron transport was inhibited similarly in both cultivars when isolated thylakoids were exposed to a wide range of terbacil concentration indicating that tolerance was not due to reduced sensitivity at the site-of-action. ‘OK 182’ took up an average of 22% less 14C-terbacil over the 6-d labeling period. A similar proportion of total radiolabel was translocated in both strains indicating that differential translocation was not a factor in the tolerance mechanism. Once in the leaves, terbacil was rapidly metabolized to a glucosidic conjugate by both strains. Terbacil and total concentration of metabolites in leaf tissues were 17% and 33% lower in ‘OK 182’ than in ‘Cimarron,’ respectively. Enhanced tolerance to terbacil in ‘OK 182’ was attributed to decreased terbacil uptake.

Field experiments were conducted during the winters of 1987–88 and 1988–89 at Haryana Agricultural University, Hisar, India to evaluate the response of winter wild oat to sowing date and isoproturon application time in wheat. In another experiment, the effect of sowing time on emergence and growth of winter wild oat and wheat sown separately was studied. The treatments that provided more than 75% control of winter wild oat were: isoproturon at 0.75 kg ha−1 applied at the 2-leaf stage of winter wild oat; isoproturon 1 kg ha−1 applied at the 4-leaf stage of winter wild oat in the November 30 planting; and isoproturon 0.50 kg ha−1 applied at the 1- to 2-leaf stage of winter wild oat in the December 20 sowing. Isoproturon did not provide more than 50% control of winter wild oat in the November 10 sowing. Mortality of winter wild oat increased from 38 to 72 to 87% in November 10, November 30, and December 20 sowings, respectively. Mortality of winter wild oat was similar in November 30 and December 20 sowings but higher grain yield was recorded in November 30 sowing. Due to vigorous growth of winter wild oat in the November 10 sowing, isoproturon did not provide good control. December 20 sowing favored wheat growth due to lower density and late emergence of winter wild oat but reduction in wheat grain yield was greater. When averaged over isoproturon treatments, the grain yield of wheat was 4607, 5297, and 4457 kg ha−1 in the November 10, November 30, and December 20 sowings, respectively.

Common ragweed is a major problem in white bean production systems in Ontario. The influence of time of emergence and density of common ragweed on white bean growth and seed yield was examined in Ontario at Elora in 1990, and at Woodstock and Staffa in 1991 and 1992. Ragweed emerged with white bean seedlings (VE) and at the second trifoliate stage of white bean (V3). Time of ragweed emergence and weed density affected white bean yield at all locations. When 1.5 ragweed seedlings m−1 of row emerged at the VE stage of crop growth 10 to 22% seed yield loss occurred. Yield losses of 4 to 9% occurred when 1.5 ragweed seedlings m−1 of row emerged at the V3 crop stage. Yield loss parameter estimates, i.e., the predicted weed-free crop yield (YWF) and the maximum yield loss (A), varied among locations and with time of ragweed emergence, whereas the parameter for yield loss at low weed density (I) was more consistent across all locations and times of weed emergence. Although I values were relatively consistent across locations and times of ragweed emergence, the standard errors associated with each estimate were large. White bean leaf area index, above-ground biomass and pod number m−2 were affected most by ragweed interference. White bean density, number of seeds per pod, and seed weight per plant were not affected by ragweed interference. Ragweed emerging at VE and V3 produced a maximum of 6000 and 1000 seeds m−2, respectively. Time of ragweed emergence may be more important than weed density when evaluating weed control options.

The competitive ability of annual round-leaved mallow was determined in spring wheat and lentil at Indian Head and Regina, Saskatchewan, in 1985 and 1986 using paired quadrats. Significant biomass and seed yield loss occurred in three of four tests in lentil and two of three tests in spring wheat. Differences in numbers of wheat tillers produced between weedy and weed-free plots were found in three of four tests. A two-variable model comprised of early season crop density loss and round-leaved mallow biomass best accounted for the majority of variation in crop yield loss for both lentil and wheat, and tiller density loss in wheat. In 1985 at Indian Head, where no yield loss occurred for either wheat or lentil, round-leaved mallow had been seeded immediately before the crop. Greater yield losses occurred at Regina, in the presence of an older, well-established infestation. In the years and locations with the greatest crop yield losses, round-leaved mallow emerged before the crop causing poor crop emergence. At Regina in 1986, crop yield losses were more than 60% in wheat and 90 to 100% in lentil because of large differences in crop density between weed-free and weedy subplots. Round-leaved mallow exhibited great variability in growth, producing more biomass per plant, more capsules per plant, and more capsules per gram of biomass in the less competitive crop, lentil, than in wheat. Density and biomass of round-leaved mallow were not correlated; with a density of 200 plants m−2, round-leaved mallow biomass in wheat ranged from 100 to 500 g m−2; while in lentil, from 200 to as high as 1000 g m−2, approximately double the range found in wheat. The type of round-leaved mallow infestation (newly seeded or well-established) and environmental conditions (mainly early season precipitation) account for differences between sites and between years.

Trials were carried out to determine the degree of light transmittance through soil and its role in photo-induced germination of buried weed seeds. Transmittance was found to be strongly dependent on soil type and decreased with decreasing particle size and with an increase in darker components. In each soil examined, light penetration fell below 0.01% at a depth of no more than 4 mm. It was also noted that with increasing soil depth, light permeability was proportional to wavelength, leading to progressive decline in the red-far red ratio. Low light penetration played a crucial role in germination of jimsonweed seeds, which had became light-sensitive after prolonged burial. Although this effect is a function of soil type, it may represent an important ecological factor influencing seed emergence in the first few mm of upper soil layers.

When a range of weed densities is needed for competition experiments, one method of reducing existing populations is to remove all weeds in strips of specified length down the crop row. This technique also can be used to create different sizes of weed dusters. Two methods of sampling weed densities after such thinning were compared: unrestricted sampling (under which quadrats are placed randomly within the row) and restricted sampling (under which quadrats are randomly placed within a row under the restriction that they coincide with the beginning of an infested strip). The bias of estimators of weed density under each of these two approaches was derived and bias-corrected estimators of weed density from the two methods were compared on the basis of their variance. The variance under restricted sampling is less than or equal to the variance under unrestricted sampling so that, by a minimum variance criterion, restricted sampling using a bias correction is the better technique.

Field experiments were conducted to evaluate soybean interference on common cocklebur and entireleaf morningglory under irrigated and nonirrigated conditions. Interference from the mixture of soybean and entireleaf morningglory, and soybean alone reduced the total leaf area index (LAI), LAI within the common cocklebur canopy, and growth rate of common cocklebur more than did entireleaf morningglory; more reduction occurred without irrigation than with irrigation. Irrigated common cocklebur produced 687 burs per plant, while nonirrigated produced 359. Irrigated and nonirrigated soybeans and the mixture of soybeans and entireleaf morningglory reduced common cocklebur bur production 43 to 47%. Irrigated entireleaf morningglory reduced common cocklebur bur production 42%, while nonirrigated reduced bur production 28%. Interference from the mixture of irrigated soybeans and common cocklebur reduced the total leaf area index (LAI), LAI within the entireleaf morningglory canopy, and growth rate of entireleaf morningglory more than did soybean or common cocklebur alone. Irrigated entireleaf morningglory produced 542 and 1107 seeds per plant in 1982 and 1983, respectively. The mixture of irrigated soybeans and common cocklebur reduced entireleaf morningglory seed production 84 to 90%. Nonirrigated entireleaf morningglory was not competitive in any treatment combination either year due to lack of sufficient LAI.

Above-ground seedling development was characterized for five annual grass weeds: downy brome, bulbous bluegrass, jointed goatgrass, Italian ryegrass, wild oat; and three cereals: winter wheat, winter barley, and winter triticale in field experiments over two years. The rate of leaf production on the main stem of each species was linearly related to cumulative growing degree days (GDD) since planting. Leaf production rates were faster for bulbous bluegrass, downy brome, Italian ryegrass, wild oat, and barley than for wheat, triticale, and jointed goatgrass. The main stem development stage when individual tillers appeared was similar in all species except under poor seedbed conditions in 1991, in which case lower-node tillers were delayed in the cereals and jointed goatgrass, but not in most of the weed species. Bulbous bluegrass, downy brome, and barley had the same percentage of plants produce the first four primary tillers on the main stem in both years; the other species showed more year-to-year variation. Seedling heights at full emergence were generally greater for large-seeded species. Small-seeded species compared to large-seeded species tended to have greater relative increases in plant height over time. Knowledge of comparative development rates between these weeds and cereals could provide information for development of growth models for each of the species and could also improve understanding of the competitive relationships between grass weeds and cereal grains.

Field and laboratory studies were conducted to determine if differences existed in pinto bean varietal tolerance to postemergence application of imazethapyr under field conditions; if differences in tolerance were due to differential acetolactate synthase enzyme sensitivity or differences in 14C-imazethapyr absorption, translocation, and metabolism; and the heritability of imazethapyr tolerance in pinto bean. All rates of imazethapyr injured Olathe, Sierra, UI-114, P89405, Aztec, and P90570 pinto bean varieties 7 d after treatment in 1991 and 1992, except 53 g ai ha−1 of imazethapyr applied to Sierra pinto bean in 1991. Olathe was injured more than other varieties in 1991, and physiological maturity of Olathe was delayed more than Sierra in 1991 and 1992. Seed yields of all varieties were not reduced in 1991, and only P90570 had reduced seed yields from 53 g ha−1 of imazethapyr in 1992. Differential sensitivity of the acetolactate synthase enzyme to imazethapyr was not the mechanism of differential varietal response. Olathe pinto bean absorbed and translocated 1.4 and 1.3 times more 14C-imazethapyr, respectively, than Sierra pinto bean 24 h after application. No differences in 14C-imazethapyr metabolism were detected between Olathe and Sierra pinto bean. Broad heritability of imazethapyr tolerance in pinto bean was calculated to be 0.85. The number of genes controlling the inheritance of imazethapyr tolerance in pinto bean was greater than one.

The efficacy of Alternaria helianthi on imazaquin-susceptible and -resistant biotypes of common cocklebur was evaluated with and without imazaquin. A. helianthi caused severe damage to both biotypes when applied at 80,000 conidia ml−1 at the five- to six-leaf stage. Symptoms included necrotic lesions, stunting, and mortality. Mortality from imazaquin at 140 g ai ha−1 was 0 and 100% and mortality from A. helianthi at 80,000 conidia ml−1 was 33 to 75% and 67 to 100% for resistant and susceptible biotypes, respectively, and surviving plants showed reduction in height, dry weight, and number of new leaves. The combination of imazaquin at 18 g ai ha−1 and A. helianthi at 40,000 conidia ml−1 was not synergistic or antagonistic in killing common cocklebur.

Tolerance of strawberry to Colletotrichum gloeosporioides f. sp. malvae, (C. g. malvae) a mycoherbicide for control of round-leaved mallow, was tested in the growth chamber and field in an area infested with round-leaved mallow. No visual infections of C. g. malvae were observed on inoculated strawberry plants in the growth chamber or field. However, C. g. malvae was reisolated from inoculated leaves and petioles from both situations, but no adverse effects were observed on strawberry plant development in the growth chamber or on berry yield in the field. Round-leaved mallow competition was detrimental to strawberry establishment, and berry yield was reduced by half compared to weed-free plots. In round-leaved mallow infested plots, treatment with C. g. malvae resulted in more vigorous strawberry plants, with berry production almost to the level of weed-free. Therefore, use of this mycoherbicide for control of round-leaved mallow in strawberries is a viable option that fits well in an integrated pest management system.

The effect of herbicide and urea-ammonium nitrate (UAN) combinations on winter wheat injury in absence of noncompetitive weeds and weed control during a winter wheat-fallow and a winter wheat-sorghum-fallow rotation were investigated. Winter wheat was planted at different dates to obtain different growth stages for spraying in the spring. Winter wheat produced greater grain yields when planted Sept. 15 or Sept. 25, 1987, 1988, and 1989 vs. Sept. 1 at North Platte, NE, while at Sidney, NE, grain yield was higher in wheat planted on Sept. 10 or Sept. 20, 1988, compared to Aug. 26. Spring-applied UAN increased grain yield on wheat planted Sept. 10 compared to no UAN in 1988–89 at Sidney, but not in 1987–88, while at North Platte, grain yields were not affected by UAN. At Sidney 2,4-D ester at 0.6 kg ae ha−1, 2,4-D amine plus dicamba at 0.3 plus 0.1 kg ae ha−1, metsulfuron at 0.007 kg ai ha−1 plus 0.25% nonionic surfactant (NIS), and metsulfuron plus 2,4-D ester at 0.007 plus 0.3 kg ha−1 plus NIS decreased grain yields compared to one handweeding. At North Platte in 1988–89, when UAN was applied with 2,4-D ester, 2,4-D amine plus dicamba, or metsulfuron plus 2,4-D plus NIS grain yields were reduced compared to the handweeded check on wheat planted Sept. 15. Occasionally, metsulfuron plus 2,4-D ester plus NIS treated wheat yielded less grain than metsulfuron plus NIS treated wheat. One or more herbicide treatments reduced wheat grain yields 4 of 15 application dates. Crop injury was related to growth stage and health of winter wheat when treatments were applied. Wheat under stress was more susceptible to herbicide damage than healthy wheat. Metsulfuron and metsulfuron plus 2,4-D controlled kochia, tumble thistle, and redroot pigweed better after wheat harvest than 2,4-D or 2,4-D plus dicamba at North Platte, but allowed summer annual grass weeds to grow. Yields of grain sorghum planted after a 10-mo fallow period were higher following winter wheat treated with three of four herbicides than the handweeded treatment.

The effects of formulation on clomazone volatilization and transport through soil were studied. After 22 days of leaching under unsaturated flow in 49-cm long intact soil cores, greater clomazone movement was observed in Plainfield sand than in Cisne silt loam or Drummer silty clay loam soils. Soil clomazone concentrations resulting in injury to oats occurred throughout Plainfield soil cores but were restricted to the upper 14 cm of Cisne and Drummer soils. In addition, clomazone was detected in the leachate from Plainfield soil only. In a similar study with Plainfield sand cores, clomazone was less mobile than atrazine; encapsulation of the herbicides in starch granules did not affect clomazone movement but greatly decreased atrazine movement from the soil surface. Similarly, starch encapsulation did not affect bioavailability of clomazone but did reduce bioavailability of atrazine. In a laboratory study with continual air flow, volatilization of clomazone applied to the soil surface was reduced by encapsulation in starch and starch/clay granules. Clomazone volatilization was not affected by soil water content within a range of 33 to 1500 kPa water tension. Following soil saturation with water, clomazone volatilization from both liquid and granular formulations increased. Granule size appeared to have a greater impact than granule composition on clomazone volatilization.

The duration of imazaquin soil activity on corn, grain sorghum, rice, and cotton was studied from 1988 to 1990. Imazaquin applied at 0.035, 0.07, 0.14 (1× rate), and 0.28 kg ai ha−1 was incorporated into the soil, and rotational crops were planted at various times after application. Initial crop visual injury was directly related to imazaquin application rate. Cotton was most susceptible, followed by corn, grain sorghum, and rice. The carryover of imazaquin from soybeans to cotton, grain sorghum, and wheat was studied in separate experiments in Arkansas in 1987 to 1989. Wheat planted in the fall following soybean harvest was not affected by imazaquin residues. Grain sorghum planted the following spring was injured (16–35%), but there was no yield reduction. Cotton recovered from early injury (< 35%) in one year and did not recover from severe injury (> 50%) in the other, resulting in yield reduction.

The adsorption of atrazine and alachlor was studied on samples of three horizons from soils with different textures and organic carbon contents. Soils were equilibrated with five concentrations of atrazine and alachlor using batch techniques. Adsorption affinity for atrazine and alachlor was approximated by the Freundlich constant (Kf), distribution coefficient (Kd), and the normalized Kd based on organic carbon (Koc). Adsorption was not significantly correlated with soil depth, clay content, or organic carbon. Atrazine adsorption was a linear function of equilibrium concentration for nearly all soil horizons but was nonlinear in most horizons for alachlor. The extent of atrazine adsorption was greater in all horizons of the fine-textured soils (Kd = 1.5 to 5.5) compared to coarse-textured soils (Kd = 0.40 to 0.87). The same general trends with texture were not apparent for alachlor. Conversion of Kd to Koc failed to reduce the variability in the linear adsorption coefficient for atrazine and alachlor in the different soils of this study.