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Herbicide Efficacy for Various Application Times in Doublecrop Wheat and Soybean

P.O. Box 916, Hays, KS 67601
Agronomy and Soils Dep., Auburn Univ., Auburn, AL 36849
Exp. Statistics Unit, Clemson Univ., Clemson, SC 29634

^* Corresponding author.

Effective weed management is essential in reduced tillage doublecrop wheat (Triticum aestivum L.) and soybean [Glycine max Merr.] production. Field experiments were conducted on a Hartsells fine sandy loam soil (fine loamy, silaceous, thermic Typic Hapludalfs) near Crossville, AL, from 1981 to 1983 to determine the influence of application time on herbicidal efficacy of oryzalin [4-(dipropylamino)-3,5-dinitrobenzenesulfonamide], pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], and oryzalin + 2,4-D [(2,4dich1orophenoxy)acetic acid] for weed control. Herbicides were applied early preplant (EP) to fully tillered wheat, and preemergence (PRE) at soybean planting. Paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) was applied separately at 0.3 kg ha^–1 at the time of PRE applications to ail plots that received other herbicide applications (EP or PRE) and alone as a check treatment. Early preplant treatments did not reduce wheat seed yields in 2 of 3 yr compared to the untreated control. Fall panicum (Panicum dichotomiflorum Michx.) and prickly sida (Sida spinosa L.) control were similar with EP and PRE applications of all herbicides. Fall panicum control ranged from 63 to 100%, and prickly sida control from 20 to 87%. Early preplant applications provided greater Pennsylvania smartweed (Polygonum pensylvanicum L.) control than PRE applications. Pennsylvania smartweed control was 5 to 70% greater with EP applications compared to PRE applications. Preemergence applications of oryzalin + 2,4-D produced moderate (20%) soybean injury in 1982 and 1983. Soybean seed yields were similar with EP and PRE applications in 1981 and 1982; however, seed yields were from 26 to 103% greater with EP applications than PRE applications in 1983.

Key Words: No-till • Weed control • 2,4-D • Oryzalin • Paraquat • Pendimethalin • Triticum aestivum L. • Gycline max (L.) Merr.

Received for publication September 1, 1986.