Greenhouse and field experiments were conducted to investigate degradation rates and efficacy of EPTC (S-ethyl dipropyl carbamothioate), butylate [S-ethyl bis(2-methylpropyl)carbamothioate], vernolate (S-propyl dipropylcarbamothioate), and cycloate (S-ethyl cyclohexylethylcarbamothioate) in soils with and without prior history of thiocarbamate use. Herbicide persistence was measured by an oat (Avena sativa L.) bioassay, and giant foxtail (Setaria faberi Herrm. # SETFA) control was visually estimated in field studies. Both greenhouse and field studies indicated that enhanced thiocarbamate degradation occurred when the same herbicide (EPTC, butylate, or vernolate) was applied over successive years. Addition of dietholate (O,O-diethyl O-phenyl phosphorothioate) or fonofos (O-ethyl-S-phenylethylphosphonodithioate) increased thiocarbamate persistence and giant foxtail control in soils with prior thiocarbamate exposure. Use of dietholate or fonofos two or three consecutive years with EPTC or butylate again resulted in enhanced degradation. EPTC persistence and giant foxtail control were not affected by prior treatments or tank mixes with carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate). Rotation away from successive applications of the same thiocarbamate herbicide (EPTC, butylate, or vernolate) improved herbicide efficacy.

Field experiments were conducted at two locations in 1989 and 1990 to investigate possible interactions between POST applications of primisulfuron and selected soil-applied carbamate and organophosphate corn insecticides. Insecticides were applied in-furrow at planting, at recommended use rates. Primisulfuron was applied POST to corn in the three- or five-leaf stage at 0, 20, 40, or 80 g ai ha-1. Combinations of primisulfuron with disulfoton, fonofos, isozophos, or terbufos resulted in foliar and root injury, plant height reductions, and yield losses. No foliar or root injury, plant height reductions, or yield losses were observed from combinations of primisulfuron and carbaryl, carbofuran, chlorpyrifos, or diazinon. Differences among and within the experiments are attributed to edaphic and environmental conditions at the time of primisulfuron application.

Field experiments were conducted in 1991 and 1992 to evaluate corn tolerance to nicosulfuron following in-furrow or surface-band applications of seven soil-applied insecticides. The organophosphate insecticides evaluated included terbufos (15G and 20CR formulations), phorate, chlorpyrifos, fonofos, and DPX-43898, as well as the synthetic pyrethroid tefluthrin. Potential for a nicosulfuron/ insecticide interaction to injure corn was ranked as follows: terbufos 15G = phorate > terbufos 20CR > fonofos. Chlorpyrifos, DPX-43898, or tefluthrin followed by nicosulfuron caused no significant corn injury. Corn injury and yield reduction were greater in corn treated in-furrow compared with surface-band. Soil moisture at nicosulfuron application was more influential in causing corn injury and loss of yield than growth stage of corn at nicosulfuron application.

Experiments were conducted to investigate enhanced biodegradation of carbamothioates and to evaluate the effect of microbial inhibitors on the efficacy of butylate [S-ethyl bis(2-methylpropyl)carbamothioate], EPTC (S-ethyl dipropylcarbamothioate), and vernolate (S-propyl dipropylcarbamothioate) in soils that had received butylate treatment in previous years (butylate-history soils). Inhibitors used were fonofos (O-ethyl-S-phenylether phosphonodithioate) and R-33865 (O,O-diethyl-O-phenylphosphorothioate). R-33865 and fonofos significantly improved control of large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] with butylate and EPTC in corn (Zea mays L. ‘Coker 21’). Bioassay data reflected more residual phytotoxicity from butylate and EPTC when these herbicides were combined with the microbial inhibitors. In butylate-history soils planted to soybeans [Glycine max (L.) Merr. ‘Wright’], R-33865 did not improve the efficacy of vernolate. These results indicated some cross-adaptation of the butylate-adapted microorganisms for EPTC, but no cross-adaptation was detected for vernolate. Under laboratory conditions, 14C-butylate was degraded more rapidly to 14CO2 in butylate-history soils than in non-butylate-history soils.