Concentrations of clomazone in nutrient solution causing 50% reduction in shoot fresh weights were 325 and 2 μM for soybean and velvetleaf, respectively. Root uptake of 14C–clomazone from nutrient solution and translocation to shoots were similar for the two species. Soybean and velvetleaf contained similar amounts of 14C per plant; however, because velvetleaf had lower tissue weights, concentration of 14C was higher in velvetleaf. Clomazone did not accumulate in roots but was rapidly translocated to shoots of both species. No more than 30% of absorbed clomazone remained in roots at any time in the experiment. Soybean and velvetleaf leaves contained 4.2 and 1.7 nmol clomazone g−1 fresh weight, respectively, and 3.7 and 6.1 nmol clomazone metabolites g−1 fresh weight, respectively. Thus, velvetleaf metabolized clomazone more rapidly than soybean did. Treatment of clomazone metabolites with β-glucosidase demonstrated that metabolism occurred by oxidative cleavage and conjugation of the benzyl moiety in both plant species. One aglycone cochromatographed with 2–chlorobenzylalcohol. The results suggest that reduced absorption or translocation or increased detoxication of clomazone by soybean seedlings is probably not responsible for their tolerance to this herbicide.

Twenty accessions within 11 perennial Glycine species plus soybean were tested for tolerance to 2,4-D. Soybean was severely injured by 2,4-D, but 13 of the Glycine accessions had 15% or less injury 4 weeks after 2,4-D application. Greatest 2,4-D tolerance occurred with accessions of G. latifolia and G. microphylla. There was no difference among tolerant accessions of G. latifolia and G. microphylla and susceptible accessions of G. canescens in recovery or absorption of 14C-2,4-D 1, 3, 7, or 14 days after treatment (DAT). Distribution of 14C from 14C-2,4-D in various plant parts was similar among accessions. Metabolism of 2,4-D in the tolerant accessions (81 to 89% 1 DAT) was higher and more rapid than in susceptible accessions (approximately 50%, 1 DAT). The same five metabolites plus parent 2,4-D were extracted from the treated leaf of all accessions at all sampling dates. However, relative distribution between metabolites differed between tolerant and susceptible accessions. More rapid metabolism of 2,4-D in treated leaves of tolerant Glycine accessions can explain differential 2,4-D responses.

Individual herbicides and all double combinations of imazaquin, imazethapyr, and imazapyr were applied alone and in combination to the foliage of pitted morningglory plants in order to evaluate potential influences on translocation. Unlabeled imazethapyr at 5.62 μg plant−1 stimulated basipetal translocation of both 14C-imazaquin and 14C–imazapyr. Unlabeled imazaquin at 7.49 μg plant−1 also increased basipetal translocation of 14C–imazethapyr but did not influence 14C–imazapyr translocation. No combination affected the amount of 14C herbicide remaining in treated leaves or moving into foliage above treated leaves. Unlabeled imazapyr at 0.11 μg plant−1 did not affect movement of either imazaquin or imazethapyr.

Research was conducted to determine foliar absorption of 14C-clethodim and its polar degradation products with five adjuvants and to compare the relative herbicidal activity of clethodim and its polar degradation products. Differential 14C-clethodim and 14C-polar degradation products absorption occurred among the five adjuvants. Adjuvants that mediated rapid foliar absorption of clethodim did not always mediate rapid foliar uptake of polar degradation products. Herbicidal activity of clethodim exceeded activity of polar degradation products regardless of time of day of application or whether adjuvant was used. Inclusion of adjuvant was required for herbicidal activity of polar degradation products. Clethodim activity declined when applied at 0800 h EDT compared to applications at 2000 h EDT. To maximize herbicidal activity of clethodim, adjuvants should be selected that mediate rapid foliar absorption of both clethodim and the polar degradation products of clethodim and that minimize photodegradation of clethodim.

Measurements of net photosynthesis, chlorophyll a fluorescence and solute leakage were evaluated as techniques for rapid detection of grass tolerance to fenoxaprop-ethyl and sethoxydim. Net photosynthesis measured as net CO2 uptake of the youngest fully expanded leaf of Calamagrostis arundinacea ‘Karl Foerster’ (Feather Reed Grass) detected tolerance to fenoxaprop-ethyl and susceptibility to sethoxydim 4 days after treatment and prior to the observation of visual injury. Sethoxydim reduced photosynthesis compared to untreated controls but fenoxaprop-ethyl did not. Measurements of older leaf (second most recently fully expanded leaf) photosynthesis were less consistent than those of the younger leaf. Chlorophyll a fluorescence of Calamagrostis leaf segments following treatment with fenoxaprop-ethyl and sethoxydim was ineffective for detecting grass tolerance. Solute leakage from leaf discs treated with 50 to 500 μg ml−1 fenoxaprop–ethyl and 10 to 100 μg ml−1 sethoxydim indicated differential tolerance of Calamagrostis and johnsongrass to fenoxaprop–ethyl and centipedegrass and johnsongrass to sethoxydim. Fenoxaprop–ethyl increased solute leakage from susceptible johnsongrass at 100 μg ml−1 while the 500 μg ml−1 concentration was required to cause substantial solute leakage from tolerant Calamagrostis. Sethoxydim caused greater leakage from johnsongrass than from centipedegrass at 50 and 100 μg ml−1. The best potential rapid screening technique was solute leakage measurements.

Seven corn inbred lines previously shown to differ in response to soil residues of chlorsulfuron were characterized as to the target-enzyme acetolactate synthase (ALS) specific activity and to its susceptibility to the herbicide. ALS from plantlets at the five-leaf stage of growth was similarly susceptible to chlorsulfuron in all lines and its specific activity in the shoots was not significantly correlated with in vivo tolerance to the herbicide. By contrast, differences in ALS specific activity in roots of plants both at the five- and three-leaf stages of growth were significantly correlated (r = 0.96∗∗ and r = 0.93∗∗, respectively) with in vivo tolerance. Correlation was also noted in extracts from cultured excised root tips (r = 0.94∗∗). Callus tissue of a chlorsulfuron-tolerant line was less affected by the herbicide and had a significantly higher ALS specific activity than callus from a chlorsulfuron susceptible line, whereas inhibition of ALS by the herbicide was similar in both lines. These results indicate that the naturally occurring differences in ALS levels in the roots of the investigated inbred lines contribute largely to the differential in vivo response observed to chlorsulfuron.

Replacement series and growth analysis experiments under greenhouse and field conditions quantified and explained the competitive ability of wild oats and barley. Barley was a stronger competitor than wild oats under greenhouse and field conditions. The reciprocal yield approach showed that barley's intraspecific competition was 7.3 times greater than its interspecific competition with wild oats when calculated on a dry weight per plant basis. When leaf area per plant was the yield variable, barley's intraspecific competition was only 2.4 times greater than its interspecific competition. The difference was explained by wild oats' higher leaf area ratio. Barley had a greater leaf area, root and shoot biomass, absolute growth rate, and shoot-root ratio than wild oats, but wild oats' leaf area ratio was always higher. No differences were detected in relative growth rate and net assimilation rate.

Field studies were conducted in 1986, 1987, 1988, and 1989 to determine the stability of crop loss functions across site by year environments. Environment was a significant source of variation for the soybean crop loss function as influenced by velvetleaf, but not as influenced by tall waterhemp and common sunflower. Weed density was a highly significant source of variation for all weed species studied. Regressions between percent soybean seed yield reductions and weed populations were linear. The velvetleaf interference regression equations were divided into two groups, those with high soybean-yielding intercepts and those with low-yielding intercepts, to explain the variance observed across environments. The regression equation for the high-yielding intercept group was Ŷ = 4.24X while the low-yielding group was Ŷ = 2.14X, where Y is percent soybean yield reduction and X is weed density per 10.7 m of soybean row. Tall waterhemp and common sunflower regression equations were determined to be Ŷ = 1.37X and Ŷ = 6.52X, respectively. Confidence intervals were used to account for the variance associated with the mean regression equation for each model and to develop economic threshold models that include risk aversion principles.

Several weed-free and bermudagrass competition periods were established and maintained in cotton to evaluate their influence on cotton yields and reproduction of bermudagrass. Cotton sprayed with fluazifop for bermudagrass control 4 or 8 weeks after emergence yielded 93 and 90% as much as weed-free cotton. This compared to losses of 16 and 26% when bermudagrass was permitted to compete with cotton for 12 and 25 weeks, respectively. Cotton handweeded for 8 to 12 weeks yielded the same as weed-free cotton and 9% more than cotton weed free for only 4 weeks. Bermudagrass seed production was prevented and rhizome production was negligible when weed competition did not exceed 8 weeks and weed-free periods exceeded 4 weeks. Bermudagrass produced significant amounts of seeds (45 to 88% of weedy checks) and rhizomes (9 to 31% of weedy checks) in plots weed free for 4 weeks and in plots where control was delayed for 12 weeks. Cotton grades were reduced because the cotton lint was contaminated with grass in plots weed free for only 4 weeks, and in plots where bermudagrass competed for 25 weeks.

Growth characteristics and competitive relationships of cotton and three perennial weeds (johnsongrass, purple nutsedge, and yellow nutsedge) were investigated at the USDA Cotton Research Station in Shafter, CA. In growth analysis experiments, yellow nutsedge and johnsongrass had highest values for height, biomass, leaf area production, growth rate, and photosynthetic efficiency. The weeds exhibited greater overall resource use and production efficiency than cotton, while cotton attained greater leafiness and canopy closure than weeds over the 10-wk experiment. Linear correlation indicated that most growth variables were significantly correlated with aggressivity (competitiveness); however, all correlation coefficients were less than 0.80. Stepwise multiple regression, using aggressivity as the dependent variable, defined 4 growth variables out of 12 that best described competitiveness. These were unit leaf rate (ULR), height, relative growth rate (RGR), and initial propagule weight (PWT). Thus, parameters of light utilization (ULR, height) and early establishment (RGR, PWT) were best predictors of competitive success in this system.

Experiments with similar experimental design, soil, and planting date were conducted between 1975 and 1979 to evaluate the interference of jimsonweed on soybeans and cotton in Alabama (AL), Arkansas (AR), and Mississippi (MS). Jimsonweed densities were 0, 4, 8, 16, 32, and 64 plants 12 m−1 of row. Because of variation in rainfall patterns, geographic region influenced jimsonweed interference in cotton but not in soybeans. Even though jimsonweed interference was reduced by lack of rainfall, jimsonweed was more competitive in the less competitive cotton than in soybeans. Jimsonweed at 64 plants 12 m−1 of row was more competitive with cotton than soybeans, causing a 56 and 16% yield reduction, respectively. Position of jimsonweed to the soybean row did not influence competitiveness. Interspecific interference with soybeans caused one-third more reduction in jimsonweed fresh weight and capsules per plant than interspecific interference with cotton. The potential for jimsonweed to become a problem in southern soybeans is less than its potential in cotton because of the competitive ability of soybeans to reduce the growth and reproductive potential of jimsonweed.

Greenhouse and field experiments were conducted to determine effects of light, water, and chlorsulfuron on growth of Canada thistle. In the greenhouse, shoot and root dry weight, leaf area, and number of inflorescences decreased as light and water levels decreased. In the field, shoot and root dry weight, leaf area, and number of Canada thistle inflorescences were positively correlated with light intensity from 1832 to 30 μmol m−2 s−1 photosynthetic photon flux density (PPFD). The greatest effect of light was on inflorescence production which was eliminated at 30 μmol m−2 s−1 (PPFD). The combined effect of water stress and chlorsulfuron decreased root and shoot growth but did not eliminate it.

Emergence periodicity of common purslane, hemp sesbania, horse purslane, prickly sida, spurred anoda, and velvetleaf was determined from March to October during 5 yr at Stoneville, MS, in the absence of reseeding. In early March of each year prior to emergence determinations, plots were not tilled or tilled to depths of 0, 5, 10, or 15 cm. Yearly emergence periodicity of the six weed species was not affected by tillage. In each year, weeds of all species emerged in identifiable flushes and very few emerged at other times during the growing season. Velvetleaf emerged mainly during the early growing season from March through May. Peak periods of emergence for the other species varied slightly over the 5 yr, but conformed generally to the early or the midseason peak period (June, July, and August) or both. In most years, common purslane, horse purslane, hemp sesbania, prickly sida, and spurred anoda emerged during the midseason period. Annual patterns of temperature fluctuations were similar during the 5 yr, but relatively warm soil temperatures during the early growing season of 1977 may have caused earlier emergence of velvetleaf, prickly sida, and hemp sesbania. Weekly water balance (rainfall minus open-pan evaporation) varied during March through May of each year but was negative during each June, July, and August. Apparently the main effect of a positive water balance during most of the year was to cause merger of the early and midseason peak emergence periods in 1979. Early and midseason emergence peaks were possibly due to different populations of seeds able to germinate in cooler soil temperatures of the early season and warmer soil temperatures of the midseason, respectively.

Field violet germination occurred during May to early July and again during September to November in Nova Scotia (N.S.), Canada. Germination occurred from October until early April in the North Carolina (NC) piedmont. Germination was stimulated by tillage. Seed collected in N.S. and in NC germinated optimally in the dark under a temperature regime of 15/5 C, on an 8/16 h thermoperiod. Germination at constant temperatures decreased with increasing temperatures, from 5 to 25 C. Seed collected in NC was capable of germinating at a higher constant temperature of 25 C than seed collected in N.S. Germination of seed collected in NC was partially inhibited by constant irradiance with a photosynthetic photon flux density of 100 μM m−2 s−1 (400 to 700 nm). Seed did not germinate when exposed to constant red or far-red irradiance.

A 6-yr field study was conducted to determine the effect of herbicide rotations on enhanced biodegradation of EPTC applied with and without dietholate, and on long-term control of wild-proso millet in sweet corn. Alachlor plus pendimethalin plus cyanazine or cycloate plus cyanazine for 1 yr following previous EPTC plus cyanazine applications reduced enhanced EPTC biodegradation in 3 of 4 and 2 of 3 yr, respectively. Enhanced biodegradation of EPTC applied with dietholate was reduced in 1 of 2 yr when EPTC plus dietholate plus cyanazine treatments were followed for 1 yr by alachlor plus cyanazine, pendimethalin plus cyanazine, or cycloate plus cyanazine, but was not reduced in 2 of 3 yr when EPTC plus dietholate plus cyanazine treatments were followed for 1 yr by alachlor plus pendimethalin plus cyanazine. Dietholate biodegradation was enhanced in soils treated 1, 2, or 3 yr previously with one application of EPTC plus dietholate plus cyanazine. After 3 yr of repeated use, cycloate biodegradation was also enhanced. Continuous use of EPTC plus cyanazine and EPTC plus dietholate plus cyanazine over 6 yr provided an average of only 46 and 64% wild-proso millet control, respectively. When applied once every second, third, or fourth year in rotation with alachlor plus pendimethalin plus cyanazine or cycloate plus cyanazine, EPTC plus cyanazine provided 83, 86, and 95% wild-proso millet control, respectively. EPTC plus dietholate plus cyanazine applied every second, third, or fourth year in rotation with cycloate plus cyanazine, alachlor plus cyanazine, pendimethalin plus cyanazine, or alachlor plus by pendimethalin plus cyanazine provided 88, 91, and 95% wild-proso millet control, respectively.

In the greenhouse, naptalam at 1.1 to 4.5 kg ai ha−1 antagonized activity of paraquat at 0.04 to 0.14 kg ai ha−1 in 14 of 16 rate combinations when applied to sicklepod. Sicklepod shoot fresh weight reduction was less compared to that obtained with paraquat alone when paraquat at 0.04 kg ha−1 was mixed with bentazon at 0.42 to 0.84 kg ai ha−1 or monocarbamide dihydrogensulfate at 14 to 58 kg ai ha−1. Paraquat activity on Florida beggarweed was antagonized by mixtures of paraquat at 0.04 or 0.07 kg ha−1 with bentazon at 0.63 or 0.84 kg ha−1 or paraquat at 0.04 to 0.14 kg ha−1 with naptalam at 3.4 or 4.5 kg ha−1. Mixtures of lactofen at 0.06 to 0.22 kg ai ha−1 with monocarbamide dihydrogensulfate at 14 kg ha−1 or lactofen at 0.06 or 0.16 kg ha−1 with alachlor at 1.4 to 2.8 kg ai ha−1 synergistically increased tall morningglory shoot fresh weight reduction. In the field, excellent sicklepod shoot fresh weight reduction by paraquat applied alone masked most antagonistic effects of paraquat mixtures. Addition of alachlor to monocarbamide dihydrogensulfate at 29 kg ha−1 improved activity on sicklepod compared to monocarbamide dihydrogensulfate applied alone.

Glyphosate is often applied with diammonium sulfate to increase weed control. However, many other salts in the spray carrier have antagonized glyphosate phytotoxicity. Research was conducted with wheat as a bioassay species to further determine the influence of various salts on glyphosate phytotoxicity. Cation antagonism of glyphosate occurred with iron > zinc > calcium ≥ magnesium > sodium > potassium. Ammonium cation with hydroxide or most other anions was not antagonistic. Anions of ammonium compounds were of primary importance in overcoming glyphosate antagonistic salts, while the ammonium cation was neutral or slightly stimulatory with certain anions. Sulfate, phosphate, citrate, and acetate anions were not antagonistic, but nitrate and chloride anions were slightly antagonistic when applied as ammonium salts or acids. Antagonism of glyphosate action by sodium bicarbonate and calcium chloride was overcome by phosphoric, sulfuric, and citric acid and phosphate, sulfate, and citrate ammonium salts. Acid and ammonium salts of nitrate and chloride were more effective in overcoming sodium bicarbonate than calcium chloride antagonists of glyphosate. Ferric sulfate antagonism was overcome only by citric, partly by phosphoric and sulfuric but not by nitric and hydrochloric acids or their ammonium salts. Acetic acid, ammonium acetate, and ammonium hydroxide did not overcome any salt antagonism of glyphosate. Glyphosate response to salts was independent of spray carrier pH.

Experiments to determine effects of chlorimuron, clomazone, and imazaquin applied the year before planting corn, cotton, and sorghum were conducted at three locations in Georgia. Corn injury was <20% in 1987 and 1988 and yield was not affected. In 1988, imazaquin applied at a high rate or sequentially the previous year to late-planted soybeans injured corn more than applications to the early-planted soybeans. Cotton was the most sensitive crop to herbicides applied the previous year. However, seed cotton yield was not decreased due to herbicides applied the previous year. Sorghum growth and yield 8 weeks after planting was not affected by soybean herbicide application the previous year. Sicklepod, corn, and wheat bioassays did not detect any of the herbicides in soil samples obtained the season after treatment. Bioassays accurately predicted potential injury in corn and sorghum but not cotton.

Greenhouse and field studies demonstrated that naptalam reduced paraquat activity by as much as 30% on sicklepod, smallflower morningglory, Florida beggarweed, and peanut Sequential application experiments, i.e. naptalam applied 2 or 24 h prior to an application of paraquat, as well as absorption studies utilizing 14C-paraquat, indicated that the antagonism was due largely to reduced paraquat absorption. Scanning electron microscopy revealed that application of naptalam, as well as naptalam applied with paraquat, resulted in amorphous deposits on the leaf surface which may account for the antagonism.

A field trial was conducted in 1988 and 1989 to confirm the efficacy of naphthalic anhydride (NA) for providing full-season protection of corn from imazethapyr applied at various times and rates. NA applied as a 1% by weight dust to corn seed caused 6% injury 2 weeks after treatment (WAT) in 1989; however, plants recovered within 6 WAT and grain yield was not affected. Imazethapyr applied to corn untreated with NA resulted in greater than 30% injury for all treatments in both years except PRE in 1988. NA reduced phytotoxicity from all imazethapyr applications in 1989 and from PPI, early POST, and 4- to 5-leaf stage (mid-POST) applications in 1988. Despite the safening effect, corn injury was still observed 6 WAT in NA-treated corn for all imazethapyr applications in both years except early POST in 1988. Higher yields were produced from NA-treated than untreated corn with imazethapyr applied PPI in both years, PRE in 1989, early POST in both years, and mid-POST in 1988. Safened corn treated PPI and early POST yielded the same as the safened corn in the control in both years. Imazethapyr applied at the 8- to 10-leaf stage caused total yield loss regardless of NA treatment. Drought conditions may have caused the lower corn injury and yield loss from imazethapyr observed in 1988 compared to 1989.