Published online by Cambridge University Press: 12 June 2017
The initial and residual toxicities, after 13, 22, and 31 weeks, of dimethyl 2,3,5,6-tetrachloroterephthalate (DCPA), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), 3-(hexahydro-4,7-methanoindan-5-yl)-1,1-dimethylurea (norea), 2,4-bis(isopropylamino)-6-methylmercapto-s-triazine (prometryne), and α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) were determined in the greenhouse in four soil types by an oats bioassay. DCPA caused the least oat injury initially, and also after 13 weeks. None of the herbicides persisted in amounts toxic to oats for as long as 22 weeks when applied at concentrations approximately equivalent to field rates necessary for weed control. The residual toxicities of these same herbicides, after 22 weeks or longer, were also determined in Bosket silt loam collected from two depths in two preplanting subsurface and two preemergence surface field experiments by using oats, cotton and / or soybeans as bioassay indicators. Results from field applications at rates recommended for weed control in cotton confirmed that neither DCPA, diuron, norea, nor trifluralin persisted in amounts toxic to oats, soybeans, or cotton; however, prometryne residues were moderately toxic to oats and soybeans. Prometyrne residues from a 4 lb/A non-incorporated subsurface treatment were nearly twice as toxic to oats 25 weeks after application, as were residues from the same amount applied as a surface preemergence treatment.