Delayed Preemergence Applications of Pendimethalin in Dry Bulb Onion in the United States Southwest

Pendimethalin applied after onion germination but before onion emergence [i.e., “delayed preemergence application” (delayed PRE)] may be a useful method for controlling early-season weeds in dry bulb onion ( Allium cepa L.; herein referred to as onion) in New Mexico. However, the efficacy and safety of delayed PRE applications of pendimethalin in New Mexico onion are poorly understood. The objectives of this study were to compare delayed PRE pendimethalin applications with PRE applications of herbicides currently and formerly registered for New Mexico onion as well as to determine if delayed PRE pendimethalin causes onion stand loss, bulb yield loss, or pendimethalin residues on harvested bulbs that exceed the federal tolerance level in the United States. To address these objectives, autumn-sown and winter-sown onion were grown with subsurface drip irrigation on clay loam (run 1) and sandy loam (run 2) soil. Treatments included the following: delayed PRE applications of pendimethalin at 1.10 kg·ha −1 active ingredient (a.i.); delayed PRE applications of pendimethalin at 0.53 kg·ha −1 a.i. followed by applications of pendimethalin at 0.53 kg·ha −1 a.i. at the two-leaf to three-leaf stage of onion; PRE applications of bensulide at 6.70 kg·ha −1 a.i.; PRE applications of dimethyl tetrachloroterephthalate (DCPA) at 6.70 kg·ha −1 a.i.; PRE applications of a tank mix of bensulide (6.70 kg·ha −1 a.i.) and DCPA (6.70 kg·ha −1 a.i.); and nontreated control. Treatments with delayed PRE applications of pendimethalin reduced densities of annual weeds by 64% to 87% and hand weeding times by 45% to 49% compared with nontreated controls. Only treatments with delayed PRE pendimethalin reduced hand weeding time of both autumn-sown and winter-sown onion. Delayed PRE pendimethalin did not cause onion stand loss, reductions in yield, and bulbs with detectable residues of pendimethalin. These results suggest that delayed PRE applications of pendimethalin are promising options for controlling annual weeds in New Mexico onion.