The role of preplant glyphosate applications and residual herbicides in the efficacy of glyphosate for weed management in double-crop no-till glyphosate-resistant soybean (GRS) was investigated in the coastal plains of Mid-Atlantic United States. The experiment had a two- by two- by five-factorial treatment structure laid out in three or four randomized complete blocks at research centers in Delaware and New Jersey. The factors investigated were preplant weed management: preplant or no preplant glyphosate applications; postemergence (POST) herbicide treatments: 0.8 kg ae/ha glyphosate alone or 0.8 kg/ha glyphosate tank-mixed with 0.6 kg ai/ha clomazone plus 0.07 kg ai/ha imazethapyr; and GRS growth stage at herbicide application which ranged from cracking, 5 to 8 d after planting, (DAP) to the V6 stage (35 DAP). Preplant glyphosate applications did not influence the efficacy of POST glyphosate applications alone or with the residual herbicides. Glyphosate alone or with clomazone plus imazethapyr provided excellent control of horseweed and fall panicum irrespective of the time of herbicide application from GRS at cracking to the V6 stage. With other weed species, residual herbicide influence varied with year, weed species, and GRS growth stage at herbicide application. Generally, glyphosate alone was most effective when applied at the V2 to V6 stages (16 to 35 DAP). A tank-mix of glyphosate with clomazone plus imazethapyr extended this window to include applications at GRS cracking and the V1 stage. Herbicide treatments were safe on GRS at all stages of application up to the V6 stage (35 DAP).
Nomenclature: Clomazone, glyphosate, imazethapyr, horseweed, Conyza (= Erigeron) canadensis L. #3 ERICA; fall panicum, Panicum dichotomiflorum Michx. # PANDI.
Additional index words: Preplant glyphosate applications, critical weed period, integrated weed management, Amaranthus hybridus, Ambrosia artemisiifolia, Chenopodium album, Ipomea hederacea, Xanthium strumarium, AMACH, AMBEL, CHEAL, IPOHE, SETFA, XANST.
Abbreviations: DAP, days after planting; GRS, glyphosate-resistant soybean; POST, postemergence; RAREC, Rutgers Agricultural Research and Extension Center; UD-REC, University of Delaware Research and Education Center.