Effects of Nitrogen with Trifluralin and Vernolate on Soybeans

Rates of trifluralin (α,α,α,-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and vernolate (S-propyl dipropylthiocarbamate) were compared with and without 45 kg/ha of nitrogen for 2 yr on soybeans [Glycine max(L.) Merr.]. Nodulation was the only factor affected significantly by nitrogen. Nodule weight was 65% greater where nitrogen was not applied. Nodulation was suppressed by trifluralin, but not by vernolate. Delays in emergence, stunting, plant injury, and yield reduction were associated with preplant incorporated treatments of trifluralin at 1.12 kg/ha and with incorporated vernolate at 2.24 and 4.48 kg/ha. Trifluralin incorporated at 0.56 kg/ha and vernolate injected at 2.24 and 4.48 kg/ha had essentially no effect on plant injury and did not reduce yields significantly. Soybeans were injured more by incorporated vernolate than by trifluralin. Average yields of weed-free soybeans for 2 yr were reduced 10.5, 8.6, and 14.0%, respectively, by (a) trifluralin at 1.12 kg/ha and (b) incorporated vernolate at 2.24 and 4.48 kg/ha. However, injected vernolate (compared to other treatments) significantly increased soybean emergence, reduced plant injury, and increased yields.