A 2-yr field study was conducted at Fayetteville, AR, to determine the effect of Palmer amaranth interference on soybean growth and yield. Palmer amaranth density had little effect on soybean height, but soybean canopy width ranged from 77 cm in the weed-free check to 35 cm in plots with 10 plants m–1of row 12 wk after emergence. Soybean yield reduction was highly correlated to Palmer amaranth biomass at 8 wk after emergence and maturity, soybean biomass at 8 wk after emergence, and Palmer amaranth density. Soybean yield reduction was 17, 27, 32, 48, 64, and 68%, respectively, for Palmer amaranth densities of 033, 0.66, 1, 2, 333, and 10 plants m–1of row. Soybean yield reduction and Palmer amaranth biomass were linear to approximately 2 Palmer amaranth m–1of row, suggesting intraspecific interference between adjacent Palmer amaranth is initiated at Palmer amaranth densities between 2 and 3.33 plants m–1of row.

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−1provided 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−1at 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 experiments were conducted using entireleaf morningglory seed collected from areas of adaptation throughout the United States to determine whether biotypes or ecotypes exist and whether differences in susceptibility to acifluorfen exist. Initiation of first flower for the entireleaf morningglory populations ranged from 63 to 81 d after emergence. The interval between emergence and initiation of first flower decreased 2.8 d for each increase in degree of latitude from which the seed was collected. Plants originating from southern latitudes remained in the vegetative phase longer and tended to produce more total dry-weight biomass than plants originating from northern latitudes. Thus, ecotypes do exist for entireleaf morningglory because of adaptation to a specific environment. The adaptation allows ecotypes to utilize the length of the growing season associated with the area of origin. Trichome density on the adaxial leaf surface ranged from 147 to 206 cm−2across the ecotypes and was not correlated with latitude or differences in acifluorfen susceptibility. Ecotypes differed in susceptibility to acifluorfen, but resistance among ecotypes was not evident.

Separate field experiments were conducted for cotton and soybean in 1990 and 1991 to determine the influence of planting date on yield loss due to interspecific interference from entireleaf morningglory and sicklepod and to determine the relative competitiveness of each weed species. Percent soybean yield loss due to weed interference increased as planting date was delayed from early May to early June. Averaged over weed species, yield losses from 1.7 weeds m−1row were 10, 18, and 20% for soybeans planted in early May, mid-May, and early June, respectively. Yield loss from 6.7 weeds m−1row were 17, 31, and 35% at the early May, mid-May, and early June planting dates, respectively. Percent seed cotton yield losses averaged over weed species in 1990 were 33 and 28% for the early May and early June planting dates, respectively, at 1.7 weeds m−1and 50% for both planting dates at weed densities of 6.7 plants m−1. The only experimental factor that significantly affected seed cotton yield in 1991 was weed density. Unlike soybeans, planting date had little effect on weed interference in cotton. Entireleaf morningglory was more competitive than sicklepod in both crops. Results suggest that selection of optimum soy bean planting dates may be a viable means of reducing losses due to weed interference.

Field experiments were conducted using a stale seedbed production system to determine the effect of herbicide application time on preplant, preplant incorporated (PPI), and at-planting treatments on weed control and soybean yield. Herbicides were applied on the surface preplant (PPL) or PPI at 6 to 7, 4 to 5, and 2 to 3 wk before planting and just prior to planting. The differences in weed control and soybean yield among years were due to rainfall patterns 2 wk after herbicide application and during the growing season. Preplant treatments applied 2 to 5 wk before planting generally controlled common cocklebur and pitted morningglory better than preplant treatments applied 6 to 7 wk before planting due to persistence of herbicide activity or treatments at planting due to a greater chance of obtaining adequate rainfall for herbicide activation, more uniform seedbed at planting, and larger weeds at application. Metribuzin plus chlorimuron was less suited than imazaquin as a preplant treatment when applied more than 2 weeks before planting.