The Effects of Seeding Rate on Older Stands of Glyphosate-Tolerant Alfalfa

Increased seed cost of glyphosate tolerant (GT) compared to non-GT alfalfa (Medicago sativa L.) raises questions about reducing seeding rates. The objective of this study was to determine if alfalfa stand density, yield, and forage quality are compromised in the later years of the stand when reduced seeding rates, in combination with glyphosate application, were used during establishment. At seven locations in the United States, GT alfalfa was seeded into conventionally tilled seedbeds at 6.7, 11.2, 15.7, and 20.2 kg ha(-1) pure live seed (PLS) in the spring of 2006. Stand density, yield, and forage quality were determined for each seeding rate under three herbicide treatments: (i) glyphosate [N-(phosphonomethyl)glycine], (ii) non-glyphosate herbicide, and (iii) no herbicide. For the third through fifth years of the alfalfa stand, there was no herbicide treatment x seeding rate interaction. Higher seeding rates resulted in higher plant densities. The 11.2 and 15.7 kg ha(-1) seeding rates had greater alfalfa and total forage (alfalfa + weeds) yields than the 6.7 kg ha(-1) seeding rate. Herbicide treatments had no effect on alfalfa plant density but produced cumulative (over the duration of the study) alfalfa yield in the following order: glyphosate > non-glyphosate > no herbicide. Cumulative total forage yield was less when no herbicide was used compared to using a herbicide. While forage fiber content was generally unaffected by seeding rate or herbicide treatments, using glyphosate to control weeds during alfalfa establishment increased forage crude protein content of older alfalfa stands compared to no weed control.