Impact of Late‐Season Herbicide Applications on Winter Canola Yield and Seed Quality

Abstract

Core Ideas The application of glyphosate during early reproductive growth resulted in significant declines in yield. Decreased yields with late‐season herbicides were associated with an increased number of aborted pods; however, this may not be the only factor contributing to the yield decline. Seed oil content followed the same general trend as grain yield, with decreased oil yields when glyphosate was applied during early reproductive growth. In canola (Brassica napus L.), control of grassy weeds can typically be achieved in the fall or spring with timely herbicide applications. However, environmental conditions can delay these applications into canola reproductive growth stages, which have an unknown effect on canola productivity. Therefore, a study was conducted to determine the impact of late‐season herbicide applications on canola grain yields and seed quality. The impact of three herbicides (glyphosate [N‐(phosphonomethyl)glycine], clethodim [(E)‐2‐[1‐[[3‐chloro‐2‐propenyl)oxy]imino]propyl]5=[2‐(ethylthio)propyl]‐2‐hydroxy‐2‐cyclohexen‐1‐one], and quizalofop‐P‐ethyl [Ethyl(R)‐2‐[4‐(6‐chloroquinoxalin‐2‐yloxy)‐phenoxy]propionate]) and three applications timings (bolting, early flower, and mid‐flower) were investigated on winter canola from 2015 through 2018 in Perkins and Stillwater, OK. Grain yields and pod abortion were collected at maturity. Subsamples of harvested seed were collected from each plot and used for oil analysis. When averaged across years, the application of glyphosate at bolting and early flower resulted in yield decreases of 531 and 488 kg ha–1 compared with the nontreated control. Yield following the application of clethodim at early flower resulted in a 185 kg ha–1 (2017) and 280 (2018) kg ha–1 decrease in yields compared with the control. Although yield decreases were observed following a few treatments, a significant increase in pod abortion was noted following many herbicide treatments, indicating that pod abortion was not the only factor contributing to yield declines. Decreases in oil content were only found following glyphosate applications at bolting and early flower. These results indicate that late‐season herbicide applications, especially glyphosate, should be avoided in winter canola.