THE EFFECT OF WEED MANAGEMENT FOLLOWING GLYPHOSATE-RESISTANT ALFALFA STAND ESTABLISHMENT ON FORAGE PRODUCTIVITY AND STAND LONGEVITY. W. J. Everman*1, J. J. Kells1, R. H. Leep1, D. Min2, A. J. Chomas1, T. Dietz1; 1Michigan State University, East Lansing, MI, 2Michigan State University, Chatham, MI (281)


Glyphosate-resistant (GR) alfalfa offers growers new options for weed control in alfalfa. One potential benefit of using GR alfalfa, is increased longevity of an alfalfa stand under frequent harvests. It is hypothesized that GR alfalfa will have a greater longevity due to removal of weed interference with minimal crop injury. To study GR alfalfa yield, weed invasion, alfalfa stand persistence and relative forage quality (RFQ), a field trial with three weed control methods (no herbicide, glyphosate, and hexazinone) under two harvest frequencies (high and moderate) was established in August 2003 at the Michigan State University Agronomy Farm in East Lansing, Michigan, USA. Alfalfa yield was not affected by herbicide treatments in most years. There were no differences in weed biomass between alfalfa treated with glyphosate and hexazinone, except in 2007. Alfalfa stand density decreased approximately 90% (from 236 to 27 plant m-2 ) while GR alfalfa yield decreased approximately 30% (from 11.04 to 7.87 Mg ha-1). Stand density of GR alfalfa showed natural thinning over the seven year period regardless of harvest intensity or herbicide treatment. In more than half of the production years (four out of six years), relative forage quality of GR alfalfa was significantly higher under a high intensity harvesting system than a moderate intensity harvesting system. RFQ was not affected by weed removal with herbicides in most years. Weed removal and harvest intensity in established GR alfalfa  ha dno effect on stand persistence.