Weed Control for Stand Duration Perennial Ryegrass Seed Production

Weed control was a major concern of Oregon grass seed growers during their transition from open‐field burning to mechanical methods of managing postharvest residues. This study aimed to determine (i) efficacy of herbicides on volunteer perennial ryegrass ( Lolium perenne L.) seedlings, (ii) whether treatments required for weed‐free seed production could be reapplied annually without damaging stands, (iii) whether interactions between residue management and herbicides occurred, and (iv) whether weeds would increase over time. Twelve herbicide treatments and two residue management systems were compared for 3 yr in three commercial perennial ryegrass fields sown in 1991. Eight preemergence followed by postemergence herbicide treatment sequences provided good control (avg. 4% ground cover) of volunteer perennial ryegrass seedlings in the 1992–1993 growing season at all three sites, and in 1994–1995 at one of two sites. For the other sites and years, control ranged from good (avg. 7% ground cover) for 2.2 kg a.i. ha −1 rake‐incorporated preemergence pendimethalin [ N ‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitrobenzenamine] followed by postemergence 0.14 kg a.i. ha −1 oxyfluorfen [2‐chloro‐1‐(3‐ethoxy‐4‐nitrophenoxy)‐4‐(trifluoromethyl)benzene] plus 1.3 kg a.i. ha −1 diuron [ N ′‐(3,4‐dichlorophenyl)‐ N, N ‐dimethylurea] to poor (avg. 55% ground cover) for postemergence‐only 0.28 kg a.i. ha −1 oxyfluorfen plus 1.8 kg a.i. ha −1 diuron. Herbicide treatments affected perennial ryegrass seed yield, with an average yield loss of 3.2 kg ha −1 for each 1% increase in seedling ground cover. Vacuum sweep residue removal improved weed control over bale–flail chop–rake treatment in all tests, but increased yield only once. Annual bluegrass ( Poa annua L.) occurred only in plots that had not received pendimethalin.