Effect of Seeding Rate and Weed Control on Glyphosate-Resistant Alfalfa Establishment

The introduction of glyphosate-resistant (GR) alfalfa offers a new weed management system for alfalfa establishment; however, alfalfa seeding rates are based on conventional cultivars. Determining optimum seeding rates allows forage producers to maximize yield, quality, and profitability with GR alfalfa. Field experiments were established in 2005 and 2006 to determine the effect of seeding rate and weed control on GR alfalfa yield, forage quality, and persistence up to 3 yr after establishment. Seeding rates of 4.5, 9.0, and 18 kg ha−1 were evaluated. Weed control methods during the seeding year included no herbicide, glyphosate applied once before the first harvest, and glyphosate applied once before the first harvest and then 7 to 10 d following subsequent harvests. Alfalfa yield was greater at higher seeding rates and when weeds were removed with glyphosate. Season forage yields were the greatest with the 18 kg ha−1 seeding rate and where no herbicide was applied. Weed biomass often was lower at the higher seeding rates and was 91 to 98% lower in the glyphosate treatments compared to the nontreated. Forage quality was not affected by seeding rate but varied by herbicide treatment depending on establishment year. Plant density increased with seeding rate and treatment effects persisted for three growing seasons. Herbicide treatment did not affect stand density as greatly as seeding rate and did not influence stand longevity. Nomenclature: Alfalfa, Medicago sativa L