The effects of glyphosate-tolerant technology on reduced alfalfa seeding rates

Higher costs associated with glyphosate tolerant (GT) compared to nonGT alfalfa (Medicago sativa L.) seed stimulates questions about reduced seeding rates in combination with the GT technology. Our objective was to determine if glyphosate herbicide in combination with GT alfalfa could improve the persistence, productivity, or forage quality when seeding alfalfa at a reduced rate. Glyphosate-tolerant alfalfa was seeded at seven locations into conventionally tilled seedbeds at rates of 6.7, 11.2, 15.7, and 20.2 kg ha^-1 pure live seed (PLS) in the spring of 2006. Stand density, botanical composition, yield, and forage quality were determined for each seeding rate under three herbicide treatments: (i) glyphosate [N-(phosphonomethyl)glycine], (ii) a nonglyphosate herbicide, and (iii) no herbicide. Level of weed infestation was different among locations, but there were no weed infestation × seeding rate or herbicide × seeding rate interaction. Lower seeding rates had lower plant mortality than higher seeding rates. Seeding rate had no affect on forage quality or weed content at any harvest. At only the first harvest in the seeding year did the 6.7 kg ha^-1 seeding rate produce less alfalfa forage (about 250 kg ha^-1) than other seeding rates. In both the seeding year and year after seeding, using herbicides resulted in less weed and greater alfalfa yield than when no herbicide was used. Regardless of weed control treatment, seeding rates of GT alfalfa greater than 6.7 kg ha^-1 did not improve weed control, alfalfa yield, total herbage (alfalfa + weeds) yield, or forage quality.