Spring barley often is grown in rotation with winter wheat, and sometimes barley can overwinter in the subsequent winter wheat crop reducing grain yield and quality. Studies were established during 1996 and 1997 in winter wheat fields in southeastern Washington and near Moscow, ID, respectively, to evaluate control of ‘Steptoe’ volunteer barley with MON 37500, diclofop, and fenoxaprop/2,4-D/MCPA. Herbicides were applied to volunteer barley at two growth stages: two leaves to four tillers and more than four tillers with stems beginning to elongate. MON 37500 at 0.018, 0.026, and 0.035 kg ai/ha visibly controlled volunteer barley 83% or more at both application times. Diclofop at 1.12 kg ai/ha did not control volunteer barley, whereas fenoxaprop/2,4-D/MCPA at 0.66 kg ai/ha controlled volunteer barley 64 to 97% in 1996, but only 0 to 23% in 1997. In 1996 and 1997, volunteer barley density was reduced 80 to 99% in MON 37500-treated plots compared to the untreated control plots. Wheat grain grade was #1 for all MON 37500 treatments compared to grade #4 in 1996 and #3 in 1997 in the untreated plots. Grain price was reduced by dockage (barley kernels) for MON 37500-treated wheat $0 to $3.12/metric ton (MT), whereas price was reduced $23 to $26/MT for grain from untreated plots. In greenhouse studies, visible injury and height and biomass reduction varied among the 36 barley varieties treated with MON 37500.

Quinclorac is a systemic herbicide registered to control annual grass and broadleaf weeds in rice. Field research was conducted to determine the most effective quinclorac treatment for leafy spurge control. Leafy spurge control 12 mo after treatment (MAT) averaged 38 and 77% when quinclorac was applied at 0.56 and 1.7 kg ae/ha, respectively. Quinclorac applied with the crop-oil-based adjuvant Scoil provided similar leafy spurge control compared to quinclorac applied alone. Quinclorac plus Scoil applied with picloram provided better leafy spurge control than quinclorac plus Scoil alone. The optimum date for fall treatment with quinclorac is broad. Quinclorac at 1.1 kg/ha plus Scoil at 2.3 L/ha applied on September 1, September 15, October 1, and October 15 provided similar leafy spurge control and averaged 56% 12 MAT.

Experiments were conducted near Duke and Forgan, OK, on land enrolled in the Conservation Reserve program (CRP) that had been seeded to Old World bluestem (OWB) to evaluate tillage systems for returning CRP grassland to winter wheat production. Glyphosate controlled OWB 72% or less in no-till (NT) wheat. Disk tillage (DT) and moldboard plow tillage (MPT) for wheat seedbed preparation controlled OWB 87 and 99%, respectively, at Forgan and 96 and 100%, respectively, at Duke. At Forgan, OWB control in NT was higher when glyphosate was applied in July than when applied in May. Soil water content to a depth of 120 cm at planting was as high in DT and MPT without herbicide as in NT with 1,680 g ae/ha glyphosate. Within NT and MPT, glyphosate did not consistently increase soil water content compared to the respective nontreated checks. In DT, soil water content to a depth of 120 cm was greater following glyphosate at 1,680 g/ ha than without glyphosate. Wheat density was greater in DT and MPT than in NT. Compared to the NT no herbicide treatment, tillage tripled wheat yields. Wheat yields were often greater where glyphosate was applied before tillage. No-till wheat production immediately after CPR in Oklahoma without prior destruction of accumulated OWB residue does not appear feasible.

Field experiments were conducted to determine the effect of different rates and application times of MON 37500 on downy brome control in winter wheat and on rotational crop injury. In southeastern Washington, MON 37500 applied postemergence at 36 to 72 g ai/ha in the fall and spring controlled downy brome 90 to 97% and 45 to 71%, respectively. MON 37500 was applied postemergence to winter wheat at 18, 36, and 72 g/ha during fall 1995 and spring 1996 near Moscow, ID; Pendleton, OR; and Endicott, WA, to determine its effect on rotational crops planted during spring 1997. At Pendleton, OR, fall applications of MON 37500 at 18, 36, and 72 g/ha applied to actively growing wheat reduced seed yield of spring barley and pea 13 to 59% and 26 to 29%, respectively, when these crops followed winter wheat in the rotation. At this site, spring applications of 36 and 72 g/ha reduced seed yield of spring barley 17 to 34%; when applied in the fall or spring, MON 37500 reduced canola yield 31 to 73%. MON 37500 at 18 g/ha in the spring reduced pea biomass 32% at Endicott. Aboveground biomass and seed yield of barley, pea, lentil, and mustard at Moscow were not affected by MON 37500 carryover. However, using a grain sorghum bioassay, predicted final concentrations of MON 37500 in soil at all three locations 360 d after treatment (DAT) was similar, ranging from 0.5 to 2 ng ai/g. In growth chamber herbicide dose–response studies, grain sorghum shoot height was reduced more at lower doses (0.025 to 5 ng/g) of MON 37500 in Pendleton soil than in soils from the other field locations. Perhaps the greater rotational crop injury observed at the Pendleton location was partly related to greater soil activity of the herbicide.

The effect of soil residues of flumetsulam plus clopyralid on several vegetable crops grown in rotation with field corn was investigated over 2 yr. The effect of soil residues of flumetsulam plus clopyralid on sweet corn cultivars of varying sensitivity to acetolactate synthase (ALS)-inhibiting herbicide was also investigated. Flumetsulam plus clopyralid was applied preemergence (PRE) to field corn at 0, 50 + 135, and 100 + 270 g ai/ha, respectively, in 1993 and 1994. Cucumber, tomato, cabbage, potato, pepper, pea, and sweet corn were planted each spring, both 1 and 2 yr following the herbicide application. Cabbage was very sensitive and showed visible injury symptoms and yield reductions both years following flumetsulam plus clopyralid application. Pepper showed a yield reduction only in 1995. All other crops showed no injury or yield reductions. Flumetsulam plus clopyralid was applied PRE and postemergence (POST) to six sweet corn cultivars in 1995. These six sweet corn cultivars were replanted into the same site 1 yr later. Flumetsulam plus clopyralid severely injured three of six sweet corn cultivars in 1995, the year of application. Soil residues 1 yr later, however, did not injure or reduce yield of any sweet corn cultivars, including cultivars sensitive to ALS-inhibiting herbicides.

Chlorimuron, clomazone, imazaquin, imazethapyr, and pendimethalin were each applied at five rates to soils at 10 different sites each year for three years to determine which soil properties influenced their bioactivity. Six soils in the U.S. (NC) and four in Zimbabwe were characterized for their plow-layer contents of organic matter (OM), humic matter (HM), clay mineral (CM), and silt and for pH and cation exchange capacity (CEC). At each site, the rate of each herbicide yielding 80% weed control (I80) was determined by visually evaluating the treated plots. The I80 values were then regressed with the various soil properties to determine which of them contributed to this rate. In the U.S. soils, the I80 was highly correlated with % OM (r 2 = 0.64–0.72) and % HM (r 2 = 0.55–0.69) for chlorimuron, imazaquin, and pendimethalin but was less correlated with % OM (r 2 = 0.30) and pH (r 2 = 0.31) for imazethapyr. Equations relating the herbicide rate for 100% weed control to soil properties were derived for chlorimuron, imazaquin, imazethapyr, and pendimethalin then compared with registered recommended rates for each chemical from the label. Herbicide rate equations based on similar soil parameters were taken from the literature to compare with our rate equations and with registered rate recommendations for comparison purposes and to add validity to the use of soil parameters for making herbicide rate recommendations. The rate equations, based on selected soil parameters, produced rates comparable with registered rates for chlorimuron, pendimethalin, and metribuzin and lower rates of application than registered rates for imazaquin, imazethapyr, alachlor, and metolachlor for soils with OM levels below 3.5%.

Field trials were conducted at two sites in 1996 and 1997 to evaluate weed control and crop response to herbicide programs in no-till soybean that include flumioxazin or sulfentrazone alone and each herbicide tank-mixed with either chlorimuron or imazaquin. A prepackaged mixture of metribuzin plus chlorimuron also was included for comparison. Cool, wet conditions after herbicide application in both years were conducive to soybean injury up to 21% at 3 wk after planting (WAP). Flumioxazin and sulfentrazone alone provided ≥ 89% control of velvetleaf, Pennsylvania smartweed, and common cocklebur. Sulfentrazone alone provided greater control of common waterhemp, common cocklebur, and ivyleaf and pitted morningglories than flumioxazin. Flumioxazin provided greater control of common ragweed than sulfentrazone. The addition of chlorimuron to flumioxazin or sulfentrazone provided greater control of velvetleaf than flumioxazin or sulfentrazone alone. The addition of imazaquin or chlorimuron to flumioxazin provided greater control of common cocklebur than flumioxazin alone. The addition of imazaquin or chlorimuron to sulfentrazone provided greater control of common ragweed than sulfentrazone alone. The addition of chlorimuron or imazaquin to sulfentrazone or flumioxazin provided little or no additional control of pitted morningglory, common waterhemp, or Pennsylvania smartweed. Metribuzin plus chlorimuron provided control of velvetleaf, Pennsylvania smartweed, common ragweed, and common cocklebur that was equal to or better than the control provided by sulfentrazone or flumioxazin plus chlorimuron or imazaquin.

Pesticide leaching experiments using widely spaced sampling sites may not adequately characterize chemical leaching behavior such as nonuniform flow between sampling points. We conducted this study to determine the three-dimensional variability of atrazine and chloride movement within a small volume of soil (2,700 cm1) under field conditions. A 1-m2area of Williamson silt loam (coarse-silty, mixed, mesic, Typic Fragiochrept) was sprayed uniformly with atrazine (1.1 kg ai/ha) and chloride (80 kg/ha). We used the Leaching Estimation and Chemistry Model (LEACHM) to simulate chemical movement. After 6.5 cm of rainfall during a 29-d period, we sampled 36 squares (5 by 5 cm) in the central 30- by 30-cm portion of the treated area at six depth increments (0 to 2, 2 to 5, 5 to 10, 10 to 15, 15 to 21, and 21 to 30 cm) and determined atrazine and Cl−concentrations. We recovered 26% of the applied atrazine and 138% of the applied chloride. Low atrazine recovery may have been due to leaching beyond 30 cm and/or degradation while excess chloride recovery is attributed to high background concentrations. Coefficients of variation (CVs) for atrazine significantly increased with depth and ranged from 26 to 353%, while CVs for Cl−were independent of depth and ranged from 32 to 66%. Derived atrazine concentration isograms illustrated highly nonuniform herbicide transport. Although LEACHM overestimated atrazine movement in the upper 15 cm, it was fairly accurate in the lower 15 cm. The overall trend in Cl−flow was adequately predicted, even though the predicted Cl−concentrations were underestimated. LEACHM could not accurately predict nonuniform flow or the variability in solute concentrations between points. However, its prediction of the atrazine center of mass (about 4.7 cm) agreed well with the derived isograms. These findings demonstrate that localized nonideal solute transport may be missed in larger sampling schemes and in simulation models.

Proso millet is a short-season summer annual grass that is well adapted to the central Great Plains. Proso millet is commonly planted as a summer crop when winter wheat stands are lost due to adverse conditions. Sulfonylurea herbicides labeled for use in winter wheat prohibit planting proso millet for intervals up to 10 mo following application. A series of greenhouse and field studies determined proso millet tolerance to CGA-152005, metsulfuron, and triasulfuron soil residue. In the greenhouse, proso millet was not affected by soil-applied CGA-152005 at doses up to 160 g ai/ha, while metsulfuron and triasulfuron doses of 4 and 15 g ai/ha, respectively, inhibited proso millet biomass accumulation. In the field, metsulfuron and triasulfuron caused early season stunting and chlorosis at doses two to four times those recommended; however, grain yields were not affected. Organic matter and clay content were highly correlated with proso millet growth response to the herbicides under greenhouse conditions, but in the field, soil pH may have influenced herbicide bioavailability.

Potato crops often are grown in rotation with cereal crops such as wheat and barley. Imazamethabenz is a postemergence herbicide that selectively controls wild oat and certain other weeds in wheat and barley. Experiments were conducted at the University of Idaho Research and Extension Center, Aberdeen, ID, over a 3-yr period to evaluate the effect of irrigation management in an imazamethabenz-treated wheat crop on imazamethabenz persistence and injury in a subsequent potato crop. The experimental design was a split plot arrangement of a randomized complete block; main plots were seasonal irrigation amounts in wheat (20, 30, 40, or 50 cm), and subplots were imazamethabenz rates applied to the wheat crop (0, 0.26, 0.52, or 1.05 kg/ha). Soil analyses for imazamethabenz residues showed that herbicide degradation during the wheat growing season (i.e., 0 to 128 d after treatment [DAT]) followed first-order kinetics at all irrigation levels. Herbicide degradation rate increased with irrigation rate. Soil samples taken 338 or 351 DAT showed no detectable imazamethabenz residues (<10 ng/g) in most plots. However, some foliar injury to potato grown the year following imazamethabenz treatment was noted (<10% average injury), suggesting that potato is sensitive to imazamethabenz or imazamethabenz acid residues below the analytical detection limit. U.S. No. 1 and total potato tuber yield were not reduced by any treatment compared to the nontreated control. Therefore, under good growing conditions, potato can recover from mild imazamethabenz carryover injury without effects on tuber yield or quality.

Kochia (Kochia scoparia) control dropped from about 90% to less than 20% after three consecutive annual applications of acetolactate synthase (ALS)-inhibiting herbicides in several northeastern South Dakota fields. Field experiments were conducted to evaluate control of kochia resistant to ALS-inhibiting herbicides using combinations of herbicides applied postemergence (POST) in wheat (Triticum aestivum) and using herbicides applied preemergence (PRE), POST, and PRE plus POST in soybean (Glycine max). Thifensulfuron plus tribenuron did not control kochia in wheat (> 600 kochia plants/m2 at harvest) and wheat yield was similar to yield of the untreated control. Dicamba plus 2,4-D (in 1996) and bromoxynil plus MCPA (in 1996 and 1997) controlled kochia and increased yields by 15% or more. Kochia was sparse (less than 1 plant/m2) in soybean plots, but grass weeds were prevalent. PRE plus POST or POST herbicide combinations that included ALS-inhibiting herbicides controlled grasses, and soybean yields were as much as 20% greater than yields with combinations that did not contain ALS-inhibiting herbicides. Adding ALS-inhibiting herbicides to wheat herbicide combinations decreased profitability because treatment costs increased while yields did not. In contrast, adding ALS-inhibiting herbicides to soybean treatments increased profitability due to better grass control, resulting in increased yield and better return.

Effective methods are needed to eradicate tall fescue (Festuca arundinacea) and convert tall fescue fields into habitats more suitable to wildlife species. The objectives of this study were to determine the efficacy of (1) a single glyphosate application during the spring or fall and (2) AC 263,222, alone and in combination with glyphosate, applied during four different tall fescue growth stages for control of established tall fescue. Studies were conducted during 1996 to 1998 in fields dominated by tall fescue located in central Kentucky. Pre- and posttreatment plant communities were described to quantify differences in vegetative characteristics due to herbicide applications. Glyphosate at 2.2 kg ai/ha applied during the spring or fall was effective in reducing tall fescue to less than 12% cover. AC 263,222 at 0.2 kg ai/ha, AC 263,222 at 0.2 kg/ha plus glyphosate at 0.6 kg/ha, or AC 263,222 at 0.2 kg/ha plus glyphosate at 1.1 kg/ha applied during the spring growth, boot, summer dormancy, and fall growth stages were equally effective, reducing tall fescue cover to less than 3% at 2 to 7 mo after treatment. Glyphosate and AC 263,222 are effective tools for the initial removal of tall fescue.

At locations in Argentina and the United States, solaria (miniature, portable, plastic greenhouses or a plastic sheet approximately 1 m2) were placed on field soils in autumn or late winter in an attempt to predict summer annual weed densities. Initial emergence of summer annual weeds covered by solaria commenced weeks before that of weeds in exposed seedbeds. Cumulative emergence of many species in solaria reached asymptotes before crops were sown. At asymptotic cumulative emergence, densities of dominant weeds in solaria (common lambsquarters, green foxtail, and large crabgrass) were correlated with weed densities occurring 4 wk after sowing, the typical time for making postemergence weed control decisions. These results indicate that solaria may supplement seedbank-sampling techniques for predicting weed densities in crops.

Site-specific weed management can increase crop production efficiency by minimizing herbicide input costs without compromising crop yields. A reduction in herbicide inputs resulting from site-specific weed management may also decrease the probability level of nonpoint pollution compared with conventional herbicide applications. A 4.5-ha field was selected to compare site-specific and conventional weed management techniques in 1997 and 1998 at Knoxville, TN. Variable rate applications (VRAs) of atrazine preemergence (PRE) followed by dicamba postemergence (POST) were investigated for the reduction of herbicide inputs and their resulting impact on weed control and corn yield. VRAs of atrazine were on the basis of weed density data collected in 1996. VRAs of dicamba were according to common cocklebur density evaluations within the field. Compared with conventional applications, atrazine usage was decreased by 43 and 32% in the site-specific application treatments in 1997 and 1998, respectively. VRAs of dicamba reduced herbicide inputs by greater than 45% for 1997 and 1998. Corn yields were similar for the conventional and site-specific treatments in both years. On the basis of these data, site-specific herbicide applications have the greatest potential and least risk for managing weeds when POST or PRE + POST variable rate herbicide applications are used.

Field studies were conducted to determine if varying carrier volume proportionally with herbicide dosage, thus maintaining constant herbicide concentration in the carrier, would change the response of sweet corn to glyphosate and of cotton to 2,4-D when compared with using a constant carrier volume where herbicide concentration would vary and be more dilute. For all the parameters measured, more sweet corn injury occurred if the concentration of glyphosate was constant in all volumes of spray. The glyphosate no-effect level for sweet corn was determined to be 0.046 kg/ha when using the variable carrier volume but was over four times greater (0.185 kg/ha) when applied at the constant carrier volume of 281 L/ha. Cotton response to 2,4-D was similar, with the constant herbicide concentration in the carrier at the lower volumes causing greater injury. The response of seed cotton yield was not different when comparing constant to variable carrier volume. The highly sensitive growth stage of cotton at the time of application (bud formation before blooming) may explain this result. These studies demonstrate the need to use carrier volumes that are proportional to the herbicide dosage, thus maintaining constant herbicide concentration in the carrier, when conducting simulated herbicide drift research. Failure to do so could underestimate the potential for injury.

Model simulations predict that lowering herbicide efficacy by reducing the application rate would slow the rate of enrichment of herbicide-resistant individuals in a weed population, but the resulting increase in density of susceptible plants would reduce crop yield and increase the weed seed bank. A study was conducted at three sites in Saskatchewan, Canada, from 1997 to 2000 to examine the implication of reduced rates of acetyl-CoA carboxylase (ACCase) inhibitors in a diverse 4-yr crop rotation, in conjunction with variable crop seeding rates, on the enrichment of resistant wild oat in a mixed (resistant and susceptible) population. Main-plot treatments were crop (barley, canola, field pea, and spring wheat), subplot treatments were crop seeding rate (recommended and high), and sub-subplot treatments were ACCase inhibitor rate (0, 0.33, 0.67, and 1.0 times the recommended rate). Herbicide rate frequently interacted with seeding rate in affecting wild oat seedling density, seed return, the viable fraction of the weed seed bank, and crop seed yield. As simulation models predict, reduced herbicide efficacy decreased the proportion of resistant individuals in the population. The high crop seeding rate compensated for a one-third reduction in herbicide rate by limiting total wild oat seed return and by reducing the number of resistant seedlings recruited from the seed bank. The level of resistance in the seed bank can be reduced without increasing the total (resistant plus susceptible) seed bank population by manipulating agronomic practices to increase crop competitiveness against wild oat when ACCase inhibitor rates are reduced to a maximum of two-thirds of that recommended.

Field studies were conducted in 1998 and 1999 to evaluate the sensitivity of a common sunflower population to acetolactate synthase (ALS)-inhibiting herbicides and evaluate alternative control methods for ALS-resistant common sunflower in soybean. Imazaquin, imazethapyr, imazamox, flumetsulam, chlorimuron, cloransulam, and CGA277476 provided 35 to 73% control of common sunflower with labeled rates. Postemergence glyphosate and sequential applications of bentazon effectively controlled ALS-resistant common sunflower. Although pendimethalin + imazaquin + imazethapyr preemergence followed by glyphosate postemergence with and without cultivation provided 100% control of ALS-resistant common sunflower, sequential applications of glyphosate provided net incomes approximately $50 to $110/ha higher than the alternative control methods. Pendimethalin preemergence followed by acifluorfen + bentazon postemergence followed by bentazon 10 d later provided the lowest overall weed control and net income. In treatments with a soil-applied herbicide followed by glyphosate postemergence, cultivation generally did not improve weed control. Soybean yields were the same comparing glyphosate with a soil-applied herbicide followed by glyphosate.

Lima bean cultivar ‘Improved Kingston’ was evaluated for sensitivity to herbicide treatments in a field experiment, conducted from 1999 to 2001 in Ontario. Crop was evaluated for visual injury at 7, 14, and 28 d after emergence following preemergence (PRE) applications of metolachlor (1,600 and 3,200 g ai/ha) and imazethapyr (75 and 150 g ai/ha). Crop visual injury to postemergence (POST) applications of imazamox + fomesafen (25 + 200 g ai/ha and 50 + 400 g ai/ha) and quizalopfop-P (72 and 144 g ai/ha) was evaluated at 7, 14, and 28 d after treatment. Plant height and crop yield were also assessed. The imazamox + fomesafen mixture caused significant visual injury and tended to decrease lima bean height and yield. Despite some initial injury observed in the metolachlor, imazethapyr, and quizalofop-P treatments, yield was not significantly decreased. Because of their margin of crop safety, metolachlor applied PRE at 1,600 g/ha, imazethapyr applied PRE at 75 g/ha, and quizalofop-P applied POST at 72 g/ha have excellent potential as weed management tools in Ontario lima bean production.

Bahiagrass is used for roadsides, pastures, and lawns in the southeastern United States mainly because of drought and nematode tolerance. Metsulfuron is a sulfonylurea herbicide, which selectively controls bahiagrass in bermudagrass. Certain cultivars of bahiagrass were observed to be tolerant to recommended rates of metsulfuron. Therefore, research was conducted to investigate the susceptibility of five major bahiagrass cultivars to metsulfuron applied at increasing rates to 42 g ai/ha. Five bahiagrass cultivars were evaluated: ‘Pensacola’, ‘Tifton-9’, ‘Argentine’, ‘Common’, and ‘Paraguayan’. Argentine, Common, and Paraguayan cultivars showed a four- to fivefold increased tolerance to metsulfuron compared with Pensacola. Because of yearly inconsistencies, results for Tifton-9 were inconclusive.

Excessive persistence of imazethapyr has been responsible for injury to corn grown after soybean. Factors implicated in corn injury reported from certain parts of the main corn-producing region in South Africa were: leaching of herbicide to deep soil layers during the season of application, followed in the next season by capillary movement to the root zone of corn, and increased bioactivity of herbicide residues following liming of fields. Bioassays were employed to determine to what extent imazethapyr leached in a soil that typically contains less than 10% total clay and 0.1% organic C in the 0- to 300-mm zone and to assess the role of pH and liming in the bioactivity of the herbicide. Undisturbed soil columns were collected in polyvinyl chloride pipe for the leaching experiment. In the greenhouse, the equivalent of 40 g ai/ha imazethapyr was applied on the column surface, followed by leaching with simulated rain of 25 or 50 mm. Leaf area measurements of the test species rapeseed showed that the herbicide was readily leached to at least 30 cm at both water regimes and that it subsequently moved upward in the soil, with evaporation as the driving force. In the other bioassay, the soil was ameliorated with different amounts of Ca(OH)2 or CaCO3 to adjust pH levels to between 5.7 and 7.1 These soil samples were each treated with imazethapyr at rates equivalent to 1.8, 3.75, 7.5, 15, and 30 g ai/ha. The growth response of the test species indicated that where Ca(OH)2 was applied, the bioactivity of imazethapyr in most instances was significantly greater than where CaCO3 was used. At all the imazethapyr rates, the activity of the herbicide increased significantly with an increase in pH from 5.6 to 6.5 where Ca(OH)2 was used, but with CaCO3, activity was significant only at 15 and 30 g ai/ha. Changes in imazethapyr adsorption and in the organic matter in the soil were not monitored, but it is suggested that the increase in herbicide activity caused by Ca(OH)2 may be due to the degradation of organic matter in the soil or to desorption of the herbicide, which would render the herbicide more available for uptake. These effects, if they do occur, are likely to be of significance for herbicide adsorption only in soil with very low organic matter content. Results indicate that imazethapyr could leach easily in coarse-textured soils low in clay and organic matter content and that the type of lime used on those soils may influence the bioactivity of the herbicide.