Submitted to: Proceedings of the Northwest Alfalfa Seed Growers Association
Publication Type: Proceedings
Publication Acceptance Date: December 15, 2007
Publication Date: January 21, 2008
Citation: Boydston, R.A. 2008. Weed Research in Alfalfa Seed Production 2007. Proceedings 2008 Winter Alfalfa Seed School Conference. Reno, NV. Pp.47-60. Technical Abstract: Weed control in alfalfa seed production is important to produce high quality and high yield of alfalfa seed. Herbicides were tested on a commercial field of alfalfa seed in central Washington in 2007. Flumioxzin slightly injured alfalfa when applied at 0.125 and 0.25 lb ai/a. to dormant alfalfa in March. The alfalfa recovered quickly and injury was 5% or less by mid April. Fluroxypyr applied soon after alfalfa began to emerge injured alfalfa about 8% at 2 WAT. Alfalfa bloom was not delayed by flumioxazin or fluroxypyr treatments and seed yield was not reduced. Two sequential applications of carfentrazone at 0.016 or 0.032 burned back the majority of the alfalfa emerged and alfalfa bloom was delayed relative to other treatments in late May. Mayweed control was excellent with diuron, flumioxazin at 0.25 lb ai/a, and hexazinone. Alfalfa seed yield ranged from 1183 to 1928 lbs/acre and was statistically similar among most treatments, but was lower in plots treated with the low rate of hexazinone and carfentrazone treatments. In a second trial, alfalfa, var. ‘Perfect’ was planted on August 15, September 5, and September 26, 2006 in order to establish three distinct growth stages of alfalfa entering the winter. Flumioxazin was applied at 0.125 and 0.25 lb ai/a with 0.5 lb ai/a paraquat to dormant alfalfa February 4, 2007. In mid November, alfalfa height averaged 15.7, 4.6, and 2. 5 cm and root length averaged 26.1, 14.7, and 8.8 cm for the early, middle, and late planting dates, respectively. Flumioxazin plus paraquat injured the late planting of alfalfa the most (about 30%) in early April, but did not reduce stand counts. Alfalfa hay yield from the first cutting averaged 2.7, 1.8, and 1.6 ton dry/acre from the early, mid, and late planting dates, respectively. Averaged over the three planting dates, flumioxazin at 0.125 and 0.25 lb ai/a reduced first cutting hay yield 18 and 25%, respectively compared to nontreated checks. Flumioxazin had no effect on hay yield from the second cutting regardless of planting date. Asulam applied March 1, 2007 to alfalfa planted the previous August partially controlled prickly lettuce without injuring alfalfa. Hay yield of the first cutting was not different among treatments and averaged 2.8 ton (dry wt.)/acre. Greenhouse trials were conducted to evaluate effective doses of three herbicides; fluroxypyr, carfentrazone, and imazamox on catchweed bedstraw. Fluroxypyr controlled bedstraw well at 0.031 lb ae/a or greater when applied at the 2 leaf stage. Fluroxypyr was less effective when applied at the 4 leaf stage, but controlled bedstraw 95 to 96% at rates of 0.063 or 0.125 lb ae/a. Carfentrazone controlled catchweed bedstraw well at all rates tested when applied at the 2 leaf stage and controlled 98% or more of bedstraw in the 4 lf stage when applied at rates of 0.008 or 0.016 lb ai/a. Imazamox controlled bedstraw 91 and 97% at 0.0195 and 0.039 lb ai/a, respectively, when applied at the 2 leaf stage. Imazamox controlled 95% or more of bedstraw in the 4 lf stage at the highest rate of 0.039 lb ai/a.