Terminal fluorescence (FT) was monitored following root or foliar uptake of metribuzin in soybean, velvetleaf, and smooth pigweed seedlings grown in solution culture. These species are known to be tolerant, moderately susceptible, and susceptible, respectively, to metribuzin under field conditions. The indirect monitoring of herbicide metabolism by fluorescence is complicated by several factors (particularly photoinhibition) and is difficult to interpret when plants are kept in the light. Rapid declines in FT at metribuzin concentrations causing high photosystem II reaction center inhibition were interpreted, in conjunction with fresh weight and injury data, as resulting from photoinhibition in velvetleaf and smooth pigweed, and from a combination of photoinhibition and herbicide metabolism in soybeans. At lower concentrations, FT declines were apparently not affected by photoinhibition but were probably representative of herbicide metabolism. These results suggested a significant rate of metribuzin metabolism in soybeans, a minor degree of metabolism in smooth pigweed beginning after 12 h, and no detectable metabolism in velvetleaf. FT monitored over a 24-h dark period following foliar absorption showed declines in soybeans indicating a modest degree of metribuzin metabolism (at intermediate herbicide doses) but no FT declines attributable to herbicide metabolism in velvetleaf or smooth pigweed. Results indicate that the inhibition of a high percentage of photosystem II reaction centers is required before photoinhibition and photooxidative leaf damage can result from photosystem II inhibitor herbicides such as metribuzin.

Imazethapyr was applied at 0.14 kg ae/ha to soybean, peanut, sicklepod. Florida beggarweed, and redroot pigweed as either a soil, foliar, or soil plus foliar application. Soybean and peanut were the most tolerant species; redroot pigweed was the most sensitive, with sicklepod and Florida beggarweed being intermediate. Foliar or foliar plus soil applications were more effective in reducing sicklepod and Florida beggarweed fresh weights than soil application alone. Foliar absorption of 14C-imazethapyr 72 h after treatment was greater than 90% for soybean, peanut, sicklepod, and redroot pigweed, but only 77% in Florida beggarweed. For the species evaluated, the amount translocated from the treated leaf ranged from 5 to 16% after 72 h. Within this same time period, an average of 90% of the root-absorbed imazethapyr had been translocated to the shoot in all species except peanut. The half-life of imazethapyr was 6.6, 6.5, 14.4, 24.0, and 32.1 days in soybean, peanut, Florida beggarweed, sicklepod, and redroot pigweed, respectively. Tolerance was most closely associated with imazethapyr half-life within these species.

Absorption and translocation of the methyl ester of haloxyfop (haloxyfop-methyl) in sorghum were reduced by presence of bentazon in the treatment emulsion. After 6 h, 18% of applied haloxyfop-methyl was recovered from the leaf surface in the absence of bentazon, compared to 43% of that applied in combination with bentazon. An average of 18% of applied haloxyfop-methyl was recovered as haloxyfop + haloxyfop-methyl from leaf tissues outside the treated zone in the absence of bentazon, compared to 8% of that applied in combination with bentazon. Haloxyfop-methyl absorption was similar whether applied with or without formulations of imazaquin or chlorimuron. However, the addition of imazaquin or chlorimuron to treatment emulsions decreased the translocation of haloxyfop + haloxyfop-methyl as only 13 and 12%, respectively, of the total amount of haloxyfop-methyl applied was translocated out of the treated zone. Mixtures of all herbicides were chemically stable; formulations of bentazon, imazaquin, or chlorimuron did not alter the ester or free acid levels of haloxyfop in an aqueous mixture over an 8-h period.

Ten corn inbred lines were field evaluated for their tolerance to DPX-M6316 at 32 and 64 g ai/ha. Inbreds ‘A619′, ‘A641′, and ‘ND246’ were highly susceptible to DPX-M6316; inbreds ‘A671′, ‘A632′, and ‘B73’ were highly tolerant; and inbreds ‘A654′, ‘CM105′, ‘W153R’, and ‘M017’ were intermediate in their response. The basis for differential tolerance was studied by comparing the susceptibility of acetolactate synthase (ALS) to inhibition by DPX-M6316 in tolerant A671 and susceptible A619, and by examining the absorption, translocation, and metabolism of DPX-M6316 in both genotypes. I50 values for DPX-M6316 inhibition of ALS activity in extracts from etiolated shoots of A671 and A619 were similar, 15.6 and 17.4 nM, respectively. There was little difference in 14C-DPX-M6316 absorption by the two inbreds, but twice as much of the absorbed 14C was translocated out of the treated leaf of A619 (13%) compared to A671 (6%). Differences in translocation may have been due to much more rapid DPX-M6316 metabolism in A671 than in A619. Extracts from treated leaves of A671 had only 23% DPX-M6316 remaining 5.5 h after treatment (HAT) compared to 78% DPX-M6316 remaining in extracts from A619 leaves. Therefore, rate of metabolism was the major factor involved in the tolerance of A671 and the susceptibility of A619 to DPX-M6316.

Cotton was grown alone (one plant/pot), in intraspecific competition (two plants/pot), and in interspecific competition with spurred anoda and velvetleaf in controlled-environment chambers, subjected to water stress by withholding water, and allowed to recover before the final harvest. Minimum leaf water potential reached during the drought period ranged from −1.87 MPa in cotton grown alone to −2.63 MPa in velvetleaf grown in intraspecific competition. Water stress reduced plant height, total dry weight, and leaf area in all species, compared to well-watered controls. Reductions in cotton growth due to intra- or interspecific competition were evident by 11 days following the onset of competition. The competitive impact of both weeds tended to increase with time. Growth reductions in cotton due to competition were associated primarily with reductions in leaf area duration; during drought the net assimilation rate also was reduced. Relative yields of cotton confirmed that interspecific competition from the weeds was more severe than intraspecific competition. Imposed drought did not affect the relative competitive abilities of the three species or the impact of the weeds on cotton.

Research was conducted to determine the potential for allelopathy to occur in pastures infested with bitter sneezeweed. Aqueous extracts of bitter sneezeweed leaves reduced alfalfa and Italian ryegrass seedling growth as much as 50% at concentrations of 0.5% (w/v). Leaf extracts were more phytotoxic than either stem or root extracts and seedling growth was reduced more than seed germination. Bitter sneezeweed tissue mixed in potting soil at concentrations as low as 0.3% w/w reduced alfalfa seedling numbers by 43%, plant height by 26%, and foliage dry matter production by 54% compared to plants cultured in soil without bitter sneezeweed leaf tissue. The potential concentration of bitter sneezeweed leaf material in soil in the pasture ecosystem was determined to be 0.5% w/v in the liquid phase and 0.2% w/w in the solid phase. Alfalfa seedling growth was reduced by 70% when germinating seed were placed under a bell jar with a potted mature bitter sneezeweed plant compared to control seedlings. A potential exists for bitter sneezeweed interference with developing alfalfa and Italian ryegrass seedlings following fall interseeding into pastures infested with bitter sneezeweed.

Intraspecific interference of sicklepod and interspecific interference between sicklepod and soybeans were investigated in field experiments in 1982 and 1983. Sicklepod growth and reproductive parameters were reduced as intraspecific spacing between plants decreased from 90 to 2.5 cm. Sicklepod plants growing intraspecifically were competing at spacings below 90 cm. Soybean plant heights were affected only slightly by sicklepod interference. Soybeans grew little in height from 35 to 84 days after emergence, while sicklepod plants continued to grow until they were 30 to 45 cm taller than the soybeans. Sicklepod plants spaced 10 and 30 cm apart in soybeans reduced soybean yield (25 to 35%) more than sicklepod spaced 90 cm apart. Sicklepod produced 900 to 2000 seeds per plant even when growth was restricted by interference from soybeans.

Cotton and pitted morningglory emergence and dry weight decreased approximately 60 to 80% when these plants were grown under greenhouse conditions in the presence of increasing amounts (0.8 to 6.7 mg debris/g soil) of field-grown crimson clover or hairy vetch debris incorporated into the soil medium. Conversely, corn dry weight increased 20 to 75% when legume debris was placed on the soil surface; incorporated debris had very little effect on corn emergence or dry weight. Germination and seedling growth of corn, Italian ryegrass, cotton, pitted morningglory, and wild mustard decreased progressively, with species-dependent variation, when exposed to increasing concentrations (8.3 to 33.3 g debris/L) of aqueous crimson clover and hairy vetch extract. Mustard and ryegrass germination and growth were almost completely inhibited by full-strength extracts of both legumes. Bioassay species exhibited greater phytotoxic responses to hairy vetch than to crimson clover in the debris and extract studies. Emergence and growth of corn and cotton were not affected when planted into soil samples, containing root biomass and possible leaf and root exudates, collected from beneath field-grown hairy vetch and crimson clover plants. However, morningglory dry weight increased 35% in the presence of either legume root debris and accompanying soil.

Surgical experiments were conducted on cultured five-node apical rhizome segments of quackgrass. Removal of scale leaves promoted an initial burst of growth within the axillary buds but did not support the continued growth of buds as effectively as removal of the rhizome apex. Replacement of detached scale leaves over denuded buds temporarily repressed the promotive effect of scale leaf removal. Aqueous extracts of scale leaf material inhibited apical growth in rhizome segments but did not inhibit bud growth. Anatomical sections revealed that removal of scale leaves promoted development of buds: cells enlarged, vascular tissues differentiated, and new nodes began to form within 4 days of the removal of scale leaves. It is suggested that scale leaves contribute to apical dominance by inhibiting the initial development of axillary buds.

The effects of hogpotato interference on cotton and of the crop on the weed were measured under field conditions in four environments. Full-season interference from 105 ± 21 hogpotato plants/m2 reduced cotton plant height by 14 to 44%. Conversely, weed dry weight was reduced 54% through full-season interference from cotton. Lint yield reductions in cotton ranged from 31 to 98% following full-season weed interference. Interference during the first 7 weeks of crop growth reduced lint yield by approximately 40%; however, interference after 7 weeks of weed-free maintenance did not affect lint yield. Interference reduced boll size in 3 of 4 yr, lint percent in 2 of 4, and boll number in the only year it was measured. Cotton fiber length, uniformity index, and micronaire were reduced by full-season interference in 1 of 2 yr; however, fiber strength was not affected in either year. Significant use of soil water by hogpotato occurred at 120 cm and deeper in the soil while cotton used water primarily in the upper 75 cm.

Four postemergence herbicides were evaluated for control of horse purslane and barnyardgrass occurring alone and in combination in three separate field experiments at Haryana Agricultural University, Hisar (India). Haloxyfop, fluazifop, and sethoxydim controlled barnyardgrass. Fomesafen controlled horse purslane most effectively when applied at the two-branch stage of horse purslane in mung bean. Combinations of haloxyfop, fluazifop, or sethoxydim with fomesafen applied at the five-branch stage of horse purslane and two-leaf stage of barnyardgrass controlled both weeds. Mung bean treated with fomesafen at 250 g ae/ha plus haloxyfop at 190 or 250 g ae/ha, fluazifop at 750 or 1000 g ae/ha, or sethoxydim at 750 g or 1000 g ae/ha produced dry weights, pods/plant, and grain yield similar to weed-free mung bean.

Greenhouse studies were conducted to determine the basis for reduced johnsongrass control when glyphosate was applied in mixtures with 2,4-D or dicamba. Glyphosate was applied to johnsongrass at 0.28, 0.56, 0.84, and 1.12 kg/ha alone and in combination with 2,4-D or dicamba at 0.14, 0.28, 0.14, or 0.56 kg/ha. Johnsongrass shoot and root fresh weights measured 4 weeks after treatment were higher when glyphosate was applied with 2,4-D (0.28 kg/ha glyphosate) or dicamba (0.28 kg/ha or 0.56 kg/ha glyphosate) compared to glyphosate applied alone at these rates. The antagonism of johnsongrass control was not observed with combinations of some of the higher glyphosate rates with 2,4-D (0.56 or 0.84 kg/ha glyphosate) or dicamba (0.84 or 1.12 kg/ha glyphosate). The reduction of glyphosate activity on johnsongrass occurred when any of four forms of 2,4-D or two forms of dicamba were added to the glyphosate spray mixture. Glyphosate uptake into johnsongrass leaves and subsequent translocation to the roots was reduced by the presence of 2,4-D or dicamba. The reduced glyphosate uptake and translocation could account for the decreased toxicity of glyphosate to johnsongrass when applied with 2,4-D or dicamba.

Postemergence control of broadleaf weeds in canola with DPX A7881 was studied under controlled-environment and field conditions. DPX A7881 controlled weeds of the Cruciferae family such as wild mustard, field pennycress, and flixweed, which are not controlled with existing herbicides used for weed control in canola. Weed control was better when DPX A7881 was applied at the two-leaf than at the six-leaf stage of the weeds. At the two-leaf stage, control of flixweed was achieved with 15 g ai/ha of DPX A7881; 20 to 30 g ai/ha was required for control of wild mustard and field pennycress. Control of redroot pigweed required 30 g ai/ha. Common lambsquarters was not controlled in this study with DPX A7881 up to 60 g ai/ha. Control of weeds with DPX A7881 increased canola yields in this study without altering the important quality components: oil content, 1000-kernel weight, and green seed content. DPX A7881 provides a postemergence alternative to existing herbicides for broadleaf weed control in canola.

Adsorption of imazaquin and imazethapyr on soils, sediments, clays, and ion exchange resins was studied. Multivariate regression analysis of Freundlich K values and soil and sediment properties yielded a model for imazaquin adsorption which included soil pH, organic carbon, and clay as significant independent variables. Clay content and pH were significant independent variables for the imazethapyr adsorption model. Adsorption of imazaquin and imazethapyr was greater at pH below 6 than at a higher pH. Adsorption of both herbicides on Ca- and H/Al-kaolinite and Ca-montmorillonite was low, but increased on H/Al-montmorillonite. The results of adsorption studies with ion exchange resins indicated that binding of protonated herbicide at low pH could be due to physical forces, hydrogen bonding, or cationic binding. At soil pH levels above the pKa's of imazaquin and imazethapyr, ionic bonding of herbicide anions with positively charged soil components could also occur.

Scanning electron microscopy (SEM) was used to study alachlor microcapsule morphology and the effects of straw age and moisture on herbicide release. Microcapsule diameter in the formulation ranged from 2 to 15 μm. The polyurea encapsulating material was stable in water over time. Diffusion was suggested as the primary mode of alachlor release, with diffusion rate controlled by herbicide and salt concentration gradients between the microcapsule and the surrounding aqueous solution. Alachlor release was promoted by drying the microcapsules before addition to water. Microcapsule morphology was unchanged 48 h after application to dry straw, and microcapsules did not appear to adhere to the straw surface. Microcapsules applied to overwintered wet straw were shriveled at 48 h after treatment, indicating herbicide loss. Capsule walls were broken and appeared brittle after application to fresh wet straw. Organic constituents of the fresh straw cuticle may affect capsule wall integrity.

Research was conducted to determine the utility of a single, temperature-independent Weibull function for describing cumulative seed germination under several temperature regimes with 14 sets of weed and crop seed germination data. A modified cumulative Weibull function was derived to distribute germination times for individuals within the population and distributed the occurrence of germination given ample sample size and appropriate sample interval. The descriptive and predictive attributes of the stochastic model component are well suited for incorporation into seed germination models and are likely applicable to models to predict distribution of times for other developmental processes of plants.