The weed control spectrum of clomazone, the influence of clomazone on the performance of other herbicides, and the performance of tank mixtures containing clomazone were evaluated from 1985 through 1987. Clomazone applied preemergence at 280 g ai/ha controlled 90 to 100% of barnyardgrass, giant foxtail, goosegrass, large crabgrass, seedling johnsongrass, prickly sida, and velvetleaf. Clomazone at reduced rates was more effective applied preemergence than preplant-incorporated for 10 of 23 weed species. Clomazone enhanced imazaquin activity on entireleaf morningglory and sicklepod in 1986. A tank mixture of clomazone, imazaquin, and metribuzin at half the labeled rates reduced soybean injury, increased soybean yield, and controlled weeds as well as the mixture at labeled rates.

Field experiments were established to evaluate weed control with AC 263,222 alone and in combination with imazaquin or imazethapyr. Addition of AC 263,222 to either imazaquin or imazethapyr resulted in additive or synergistic increases in control of johnsongrass, pitted morningglory, and entireleaf morningglory. Reducing the rate of imazaquin from 140 to 18 g ai ha−1 did not reduce control of johnsongrass or pitted morningglory when AC 263,222 was added at 18 g ai ha−1 and above, compared to 140 g ha−1 imazaquin alone, through 8 weeks after treatment (WAT). When 9 g ha−1 or more AC 263,222 was added to imazethapyr, reducing imazethapyr rate from 70 to 9 g ai ha−1 did not reduce control of johnsongrass or pitted morningglory through 8 WAT. Adding 9 g ha−1 or more AC 263,222 to various rates of imazaquin or imazethapyr 2 and 8 WAT additively or synergistically increased control of johnsongrass, pitted morningglory, entireleaf morningglory, and tall waterhemp compared to those herbicides applied alone. Soybean injury increased 2 WAT when 35 or 70 g ha−1 AC 263,222 was added to most rates of imazaquin or imazethapyr, but was less than 15% in all treatments by 8 WAT. Visual soybean injury did not translate into a yield loss, and an increase in weed control often resulted in increased yield.

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.

Field experiments were conducted in 1986, 1987, and 1988 to evaluate imazethapyr rate and time of application on postemergence control of 24 weed species. Contour graphs were developed that predicted imazethapyr rates required for various levels of weed control based upon weed leaf number at application. Rates below the labeled rate (70 g ha−1) provided 90% or greater control of common cocklebur, smallflower morningglory, and smooth pigweed if applied to 3 true-leaf or smaller weeds and of barnyardgrass, seedling johnsongrass, and Palmer amaranth if applied while weeds were in the cotyledon or 1 true-leaf stage. A rate of 70 g ha−1 provided 90% control of large crabgrass in the 1 true-leaf stage. Entireleaf morningglory, red rice, pitted morningglory, and velvetleaf are not susceptible enough to imazethapyr for 90% or greater control to be obtained with rates lower than 70 g ha−1 at the 1 true-leaf growth stage. These data demonstrate how control data can be used for developing effective reduced-rate herbicide recommendations based on weed leaf number.

Field and laboratory investigations were conducted from 1983 through 1985 to evaluate morphological and environmental influences on the performance of imazaquin on 26 weed species common to the Mississippi Delta. Imazaquin applied preplant incorporated or preemergence at 140 g ai ha–1 controlled 16 species but controlled only five species at 140 g ha–1 foliar applied. Contour graphs accurately predicted imazaquin postemergence rates necessary to control a given weed species at a given time after emergence. Imazaquin applied preemergence with a sequential postemergence application controlled entireleaf morningglory and sicklepod better than a single soil application. Imazaquin at 140 g ha–1 applied preemergence required at least 50 mm of supplemental water following application to control sicklepod and was more phytotoxic to sicklepod grown at 25/10 C day/night temperatures than those grown at 32/22 C.