Soil Science Society of America

Potential for delayed orchardgrass seeding during glyphosate-resistant alfalfa establishment

Few herbicides are labeled for weed control when establishing alfalfa (Medicago sativa L.)-grass mixtures. Varying the planting date of the grass in relation to alfalfa and adjusting herbicide application timing could improve weed control and forage grass safety. The research objective was to evaluate the use of glyphosate [N-(phosphonomethyl) glycine] for weed control in seedling glyphosate-resistant alfalfa mixed with orchardgrass (Dactylis glomerata L.). Orchardgrass was seeded with alfalfa or 4 and 5 wk after alfalfa; while glyphosate was applied 4 wk after planting and all treatments included an untreated check. First year harvests varied in composition and orchardgrass was slower to establish than alfalfa. At the first harvest, glyphosate treatments had 17 to 35% higher alfalfa content, reduced or similar orchardgrass content, and 75 to 90% lower weed content than treatments without glyphosate. By the third harvest of the establishment year, weed biomass in the 4 and 5 wk after planting treatments not treated with herbicide was at least 4.5 times greater and orchardgrass content was the same or less than the comparable glyphosate treatments. Forage species composition and weed control differences present during the establishment year dissipated during the second year of the study. By 2 yr after establishment, orchardgrass content was lower only in the first two cuttings when orchardgrass was seeded with the alfalfa and a glyphosate application followed, a practice not generally recommended. This study demonstrates the utility of glyphosate in glyphosate-resistant alfalfa and should provide some guidance for establishment of successful alfalfa-grass mixtures.

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