Delayed Glyphosate Application Doesn't Reduce Plant Density of Glyphosate‐Tolerant Alfalfa

Commercial seed of glyphosate‐tolerant alfalfa contains small percentages of seeds, called nulls, that do not posses the herbicide trait. Differential competitiveness of glyphosate‐tolerant seedlings compared with null seedlings may be detrimental to alfalfa stand density if delayed glyphosate application allows the null and tolerant seedlings to compete with each other. Our objective was to determine if delayed glyphosate application affects plant density of glyphosate‐tolerant alfalfa stands. Commercially available glyphosate‐tolerant alfalfa was seeded at 17 and 29 kg/ha, plots were treated with glyphosate one month (3‐ to 6‐trifoliate leaf stage) or one‐year after planting and plant density was monitored throughout the study. Regardless of seedling rate an average of 6.5 and 5.5% of the alfalfa plants died when glyphosate was applied one month and one year after planting, respectively, but plant density remained unchanged during the same time periods when glyphosate was not applied. Planting alfalfa at 17 kg/ha resulted in lower seedling density, but also lower plant mortality during the seeding year than planting at 29 kg/ha. We conclude that delaying glyphosate application to glyphosate‐tolerant alfalfa until one year after planting does not alter the percent of glyphosate‐tolerant and non‐tolerant plants.