Effects of a 28% N solution containing urea and ammonium nitrate (28% UAN) and a nonionic surfactant on efficacy, uptake, and translocation of chlorimuron ethyl ester in soybean and broadleaf weeds were investigated. Chlorimuron applied postemergence at 8.8 and 13 g ai/ha with either nonionic surfactant at 0.25% (v/v), 28% UAN at 9.4 L/ha, or nonionic surfactant plus 28% UAN controlled weeds better than without additives. Chlorimuron with nonionic surfactant and with 28% UAN controlled weeds similarly but with 28% UAN injured soybeans least. Chlorimuron with nonionic surfactant plus 28% UAN controlled velvetleaf best but injured soybeans most. Less than 2% of applied 14C-chlorimuron was absorbed in velvetleaf 84 h after treatment (HAT) when no additive was present, but adding nonionic surfactant, 28% UAN, and nonionic surfactant plus 28% UAN resulted in 21, 9, and 32% uptake, respectively, 84 HAT. Nonionic surfactant plus 28% UAN also increased translocation (primarily acropetal) of the 14C. Adding 28% UAN increased 14C-chlorimuron uptake in velvetleaf grown at low fertility but had no effect under high fertility.

Spraying with tank mix combinations of the fungus Colletotrichum coccodes and the plant growth regulator thidiazuron increased mortality, decreased biomass and height of velvetleaf, and increased soybean yield in field experiments. The velvetleaf isolate of C. coccodes was compatible with thidiazuron, and tank mix combinations consistently improved control over either component alone. In the moist 1986 season the fungus and the growth regulator interacted synergistically to increase velvetleaf mortality. In the drier 1987 season no significant interaction was detected between the two components.

Experiments were conducted in a growth chamber to determine the effects of planting depth on the developmental sequence of velvetleaf seedlings and the effect of cyanazine placement relative to velvetleaf planting depth on cyanazine toxicity. Velvetleaf seedling emergence was delayed when seeds were planted 6 cm deep compared to the 2- and 4-cm planting depths. Lateral roots did not appear until after the cotyledons had emerged and expanded. This was followed by secondary root emergence from the primary root which occurred before adventitious roots appeared from the hypocotyl for the 2- and 4-cm planting depths. The first true leaf did not unfold until after the secondary root system was well developed. The quantity of adventitious roots on the hypocotyl increased with increasing planting depth. Velvetleaf adventitious roots appeared to be involved in cyanazine toxicity when cyanazine was placed above the seed. Increasing planting depth increased the proportion of seedling absorptive tissue above compared to below the seed. This resulted in increased cyanazine exposure for the deeper planted velvetleaf seeds when herbicide was placed above the seed.

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.

Field experiments were conducted to evaluate the critical period for velvetleaf interference with cotton and to assess the reliability of using weed growth variables as predictors of cotton lint yield losses. An inverse linear relationship existed between velvetleaf dry weight and cotton lint yield. The relationship between the number of velvetleaf main-stem nodes or velvetleaf height with cotton lint yield was best described by quadratic regression equations. Weed dry weight appeared to be the most accurate predictor followed by weed height and by number of velvetleaf main-stem nodes. A nonlinear equation best described percent lint yield loss as a function of critical-period interference intervals.