Glyphosate-resistant alfalfa can exhibit injury after glyphosate application in the Intermountain West

Authors: LOVELAND, CHET - Utah State University; ORLOFF, STEVE - University Of California; YOST, MATTHEW - Utah State University; BOHLE, MYLEN - Oregon State University; GALDI, GIULIANO - University Of California; GETTS, THOMAS - University Of California; PUTNAM, DANIEL - University Of California; RANSON, COREY - Utah State University; Samac, Deborah; WILSON, ROBERT - University Of California; CREECH, EARL - University Of California

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2023
Publication Date: 5/25/2023
Citation: Loveland, C., Orloff, S., Yost, M., Bohle, M., Galdi, G., Getts, T., Putnam, D., Ranson, C., Samac, D.A., Wilson, R., Creech, E. 2023. Glyphosate-resistant alfalfa can exhibit injury after glyphosate application in the Intermountain West. Agronomy Journal. 115(4):1827-1841. https://doi.org/10.1002/agj2.21352.
DOI: https://doi.org/10.1002/agj2.21352

Interpretive Summary: Weed control in perennial crops such as alfalfa can be difficult. Glyphosate-resistant alfalfa is widely planted in the United States and enables farmers to control weeds at all stages of plant growth. However, damage to the crop can occur under certain environmental conditions. This study documented and characterized injury at 24 sites over five years to determine best management practices for avoiding injury to glyphosate-resistant alfalfa in the Intermountain West. Spring applications reduced crop height at 76% of the sites and biomass yield at 62% of the sites. Alfalfa treated in the late spring, when plants were 15-20 cm tall, had the greatest amount of damage compared with untreated alfalfa. Summer glyphosate applications did not injure alfalfa. To mitigate the risk of injury, spring applications using a low glyphosate rate should occur before alfalfa is 10 cm tall. The results of this study will help farmers continue to utilize spring herbicide applications for control of winter weeds with minimal risk to crop yield.

Technical Abstract: Glyphosate-resistant (GR) alfalfa (Medicago sativa L.) has been widely adopted in the United States, where alfalfa plays an important role in agriculture. Exceptional tolerance to glyphosate application has been a reported strength of this technology; however, growers have recently reported potential crop injury under certain environmental conditions. The purpose of this study was to document and characterize the injury and determine best management practices for avoiding injury to GR alfalfa in the Intermountain West. The effects of glyphosate rate and application timing during various seasons were investigated at 24 sites over five years, measuring the impact on alfalfa crop height and yield. Summer glyphosate applications did not injure alfalfa. Spring applications reduced crop height at 76% of the sites and biomass yield at 62% of the sites. At responsive sites, low (869 g ha-1 a.e.) and high (1739 g ha-1 a.e.) rates reduced yield by 0.53 and 1.06 Mg ha-1, respectively. Alfalfa treated with a high rate in the late spring, when 15-20 cm tall, had mean yield reductions of 16-17% compared with untreated alfalfa. These results suggest that glyphosate applications made at tall crop heights or high rates on GR alfalfa are more likely to reduce crop height and yield in the Intermountain West. To mitigate the risk of injury, we recommend that spring applications using a low glyphosate rate occur before alfalfa is 10 cm tall. If a high glyphosate rate is necessary, then application should be made at or before alfalfa is 5 cm tall.