Trifluralin-resistant Palmer amaranth populations were confirmed at eight locations within two of the ten-county cotton growing region of South Carolina. Different levels of resistance to six dinitroaniline herbicides were observed, but no resistance was observed to herbicides from nine other chemical groups. Five to six times more trifluralin was required to control a resistant than a susceptible biotype under field conditions.

Studies were conducted to measure the foliar activity of sethoxydim (BAS 9052 OH) {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl-3-hydroxy-2-cyclohexen-1-one} and RO-13-8895 {acetone-O-[D-2-[p-[(α,α,α-trifluoro-p-tolyl)-oxy] phenoxy] propionyl] oxime} as affected by adjuvants, soil moisture, and growth stage of grasses, and to compare the responses of three grass species to these herbicides. The activity of both herbicides was increased by adjuvants. The activity of sethoxydim was not significantly affected by soil moisture. Goosegrass [Eleusine indica(L.) Gaertn. ♯3ELEIN] and broadleaf signalgrass [Brachiaria platyphylla(Griseb. ♯ BRAPP) Nash] control was greater with RO-13-8895 at the high than at the low soil moisture level. Soil moisture did not influence large crabgrass [Digitaria sanguinalis(L.) Scop. ♯ DIGSA] control, since it was relatively tolerant to RO-13-8895. Both herbicides gave greater control at early than at late growth stages. When averaged over all variables, sethoxydim gave greater large crabgrass control than RO-13-8895, but RO-13-8895 gave greater goosegrass and broadleaf signalgrass control than sethoxydim.

Two kinds of dinitroaniline-resistant biotypes of goosegrass have been found in counties in South Carolina where trifluralin is used for weed control. One biotype, termed the resistant (R) biotype, resists trifluralin about 1000- to 10 000-fold. The other biotype is intermediate (I) in resistance between the R and susceptible (S) biotypes, resisting trifluralin about 50-fold. Growth studies and root tip squashes were used to determine the level of resistance to other dinitroaniline herbicides. The R biotype is cross resistant to all dinitroaniline herbicides, and the I biotype is highly resistant to some and is resistant to others only at a low level. The cross resistance of the I biotype to other dinitroaniline herbicides does not correlate with structural or solubility characteristics of the herbicides. Electron microscopic investigations indicate that cell plate formation (controlled by phragmoplast microtubule arrays) is affected at lower herbicide concentrations than spindle microtubule formations. The level of herbicide resistance, the pattern of cross resistance, and the ultrastructural effects induced by the herbicide indicate that the I biotype has a unique mechanism of resistance from the R biotype.

The response of crop and weed species to herbicides within the same family may vary considerably. The detrimental effect of late seeding of soybeans [Glycine max (L.) Merr.] is expressed as a reduction in individual plant yield components. Any temporary suppression in plant growth due to herbicides may cause additional yield reductions. Field studies were conducted to determine the response of 15 weed species and soybeans to acifluorfen, fomesafen, and lactofen, and the effect of these herbicides on foliar injury and seed yields of determinate soybeans when seeded at post‐optimal dates. Herbicides were applied to weeds and soybeans at 3 wk after seeding and to soybeans in the V4 to V5 stage when grown under weed‐free conditions. Weed control was similar with all three herbicides. Differences observed in weed control among herbicides appeared to be of little practical importance. Soybeans exhibited differential herbicide tolerance (fomesafen > acifluorfen > lactofen). Soybean injury increased as rates of acifluorfen and lactofen increased, but was not affected by increasing rates of fomesafen. Soybean seed yields were not reduced by herbicides. Determinate soybeans seeded at post‐optimal dates appear to recover from foliar injury caused by these herbicides and seed yields are not affected.

A comparative study of the growth and development of several populations of dinitroaniline-susceptible (DS) and -resistant (DR) goosegrass [Eleusine indica(L.) Gaertn. # ELEIN] biotypes was conducted under noncompetitive field conditions. Plant height and width, number of tillers, aboveground dry weight, relative growth rate (RGR), number of inflorescences/plant, total inflorescence dry weight, individual inflorescence dry weight, number of spicate branches/inflorescence, and flag leaf length and width were recorded periodically during the growing season. There were no significant differences between biotypes in most characteristics with the exception of total inflorescence dry weight. The DS biotype produced more total inflorescence dry weight than the DR biotype at 8 and 13 weeks after transplanting. Significant variation among DS and DR populations occurred in 37 and 33 of 56 evaluations, respectively. When variation existed within a biotype, populations exhibited broad ranges of response for the various growth parameters. With the exception of total inflorescence dry weight, DS and DR biotypes generally exhibited similar ranges of variability.