Date of Award:

8-2020

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Plants, Soils, and Climate

Committee

J. Earl Creech

Committee

Matt Yost

Committee

Deborah Samac

Abstract

Glyphosate-resistant (GR) alfalfa (Medicago sativa L.) has been widely adopted in the Intermountain West 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, identify local conditions that may have contributed to crop injury, and determine best management practices for avoiding injury to GR alfalfa in the Intermountain West. The effects of glyphosate rate and application timing were investigated at 24 sites over five years, measuring the impact on alfalfa crop height and yield. Glyphosate applications were made during various seasons. 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 when 15-20 cm tall had mean yield reductions of 16-17% compared with untreated alfalfa. Three variables were significant predictors of glyphosate injury: soil pH, glyphosate rate, and the number of days with sub-zero temperatures post-dormancy before glyphosate application. Predicted yield reduction from a one-unit increase in soil pH was 0.60 Mg ha-1. Each extra day of crop exposure to sub-zero temperatures before glyphosate application increased the odds that glyphosate injury would occur by 13%. The results of these studies suggest that high rate glyphosate applications on GR alfalfa have a high probability of reducing crop height and yield in regions with high soil pH and cold spring temperatures, such as the Intermountain West. As glyphosate rate or crop height at application increased, so did the likelihood of alfalfa height and yield reductions. To mitigate the risk of injury, we recommend that spring glyphosate applications are made using low rate of glyphosate 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.

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