Response of Walnuts to Simulated Drift Rates of Bispyribac-Sodium, Bensulfuron-Methyl, and Propanil

A field study was established to study symptoms, growth and yield of 2-year-old walnut trees exposure to simulated drift of several herbicides commonly used in rice production. Bispyribac-sodium, bensulfuron-methyl, and propanil were applied at four rates representing 0.5%, 1%, 3% and 10% of the normal use rate in rice (45, 70, and 6725 g ai ha(-1), for the three herbicides respectively). Symptoms started to appear approximately 7 days after application (DAT) and peaked 28 DAT. At that time, bispyribac-sodium caused greater injury at low drift rates (6% and 15% visual injury for 0.5% and 1% rate, respectively) compared to bensulfuron-methyl and propanil. Bispyribac-sodium also appeared to slow walnut shoot elongation compared to nontreated trees; however, no yield reductions were observed either in the year after drift exposure. The effect of bispyribac-sodium simulated drift on the yield and nut quality in the year of drift exposure was evaluated in a separate study on 3-year-old walnut trees. While no yield or nut quality reductions were observed, a linear correlation between rate of bispyribac-sodium and color, an important quality factor, was found: higher herbicide rates tended to be associated with darker kernel color. Bispyribac-sodium may damage nearby walnut orchards if drifted at significant amounts. However, it is unlikely that in a field situation bispyribac-sodium would drift at high enough levels to cause the symptoms observed from the 10% use rates in this study.