Weed control for stand duration perennial ryegrass seed production. II. Residue retained

Novel alternatives to open-field burning recently adopted by Oregon grass seed growers included retaining postharvest residues chopped into mulch. Because herbicide performance had never been tested under such conditions, research objectives included determining (i) efficacy of preemergence and postemergence herbicides on volunteer perennial ryegrass seedlings, (ii) whether treatments could be reapplied annually without damaging stands, (iii) whether common weeds would increase over time, and (iv) effects of volunteer seedlings on crop yield. Techniques were developed to incorporate pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] through crop residue. Tests were conducted in three commercial perennial ryegrass (Lolium perenne L.) fields sown in 1991. Preemergence-incorporated pendimethalin at 2.2 kg a.i. ha(-1) improved control of volunteer perennial ryegrass seedlings over postemergence treatments applied alone in 28 out of 46 comparisons, and 12 out of 18 failures to improve control occurred when postemergence treatments themselves adequately controlled weeds (avg. 5.5% ground cover by volunteer perennial ryegrass seedlings, or 93% control). Weed control by postemergence treatments varied greatly among years and sites. Early postemergence oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4- (trifluoromethyl)benzene] plus metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N- (2-methoxy-1-methylethyl)acetamide] followed by late postemergence diuron [N'-(3,4-dichlorophenyl)-N,N-dimethylurea] was the most effective postemergence treatment (avg. 92% control), but damaged the crop in several instances. Uncontrolled volunteer seedlings reduced seed yield in no-postemergence checks compared with treatments including postemergence applications in 19 out of 30 instances the first year, 24 out of 30 instances the second year, but only 3 out of 24 instances the third year. Susceptibility of perennial ryegrass to injury by herbicides increased with stand age but varied among sites.