Selectivity, Movement, and Persistence of Soil-Incorporated Herbicides in Carrot Plantings

Abstract

The comparative performances of several herbicides incorporated 2.5 cm deep and unincorporated in furrow-irrigated soil were studied in four field experiments. Among eight herbicides evaluated,a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) was the most effective herbicide and controlled redroot pigweed (Amaranthus retroflexusL.), Palmer amaranth (Amaranthus palmeriS. Wats.), common purslane (Portulaca oleraceaL.), and barnyardgrass (Echinochloa crusgalli(L.) Beauv.) when incorporated, without reduction of yield in carrots (Daucus carotaL., var.saliva, cv. Long Imperator). Trifluralin andO,O-diisopropyl phosphorodithioateSester withN-(2-mercaptoethyl)benzenesulfonamide (bensulide) gave consistently good results in all years although soil temperatures and evaporative losses of water were variable. Bioassays indicated and gas-liquid chromatographic (hereinafter referred to as GLC) assays confirmed that rainfall moved bensulide and trifluralin 2.5 cm downward in soil, and that dimethyl tetrachloroterephthalate (DCPA) was not moved below the original depth of incorporation in soil. GLC assays were most sensitive but generally agreed with bioassays of trifluralin-treated soils. Under high temperatures, infrequent rainfall, and furrow irrigation, the half-life of DCPA and trifluralin activities was 3 weeks, with no biological activity after 7 months. The half-life of bensulide activity was 6 months, with no biological activity after 22 months.