Clopyralid applied with sponge wiper for Cirsium arvense control in hops in the western United States

Abstract

Cirsium arvense L. (Canada thistle) is an important weed species commonly found in perennial crops. In hop production, the management of C. arvense is largely herbicide-based. Field studies were conducted in 2019 and 2020 in mature hopyards in Independence, OR, to compare C. arvense control in hops using a spray or targeted application of herbicides. Treatments included applying the herbicides immediately after training hops, four weeks later, or at both times. Treatments were applied as sprays directed to the base of hop plants with clopyralid or as target applications of clopyralid or glyphosate with a sponge wiper. Hop injury and weed control were evaluated 28 and 56 days after initial treatment. Herbicide residue in cones was quantified, and treatment costs were compared. Clopyralid did not injure hops, but wiper applications of glyphosate resulted in crop injury. The targeted application of clopyralid by wiper was as effective as the spray application. No differences were observed when clopyralid was applied after training or 28 days later, but two applications outperformed a single application. Clopyralid provided better control of C. arvense than glyphosate. Clopyralid hops cone levels were below the quantitation limit in both years (0.01 ppm). A partial cost analysis indicates that the targeted application of clopyralid with the wiper was 37% less costly than the spray application of clopyralid. The cost reduction was primarily driven by 84% reduced clopyralid use, and savings with the wiper application are likely to be exacerbated in a low infestation and patchy distribution of C. arvense.