INFLUENCE OF RESIDUE REMOVAL METHOD AND HERBICIDES ON PERENNIAL RYEGRASS SEED PRODUCTION .2. CROP TOLERANCE

Inherent in the ongoing transition from open-field burning to non-burn systems for grass seed production in the Pacific Northwest is an incr eased reliance on herbicides for weed control. However, herbicide trea tments able to control volunteer crop seedlings, the most abundant wee d in certified grass seed fields, may pose severe hazards to the estab lished crop itself. Fourteen herbicide treatment sequences plus an unt reated check were examined in five residue removal systems at two site s during two consecutive growing seasons to determine their impact on perennial ryegrass (Lolium perenne L.) seed yield. Seed yield was redu ced by weed competition in nonburned, untreated checks, and by herbici de injury in some of the 14 herbicide treatment sequences. When compar ed across 13 promising herbicide treatments, seed yield did not differ between burned and nonburned plots in the first year, although burnin g was slightly superior in the second. However, the cumulative yield a dvantage to burning was only 84 kg ha-1, or 2.6%, over the 2-yr period . Although differences in yield among four herbicides applied preemerg ence (PRE) to seedling grasses in mid-October were relatively small, p endimethalin -ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] and ox yfluorfen thoxy-4-nitrophenoxy)-4-(trifluoromethyl)-benzene] were supe rior to either metolachlor ethylphenyl)-N-(2-methoxy-1-methylethyl)ace tamide] or trifiuralin nitro-N,N-dipropyl-4-(trifluoromethyl)benzenami ne] in specific instances. When applied following PRE herbicides, earl y December post-emergence (POST) applications of 12 kg ha-1 diuron [N' -(3,4-dichlorophenyl)-N,N-dimethylurea] or 0.14 kg ha-1 oxyfluorfen pl us 1.3 kg ha-1 diuron generally did not differ in yield. While there w as usually little net effect on seed yield of applying POST herbicides following pendimethalin, they did depress yield 129 kg ha-1 the first year in burned plots, while increasing it an average of 79 kg ha-1 th e second year in all plots. With properly chosen herbicide treatments, the impact on perennial ryegrass seed yield of changing to nonburned methods of residue removal should be minimal. If such herbicide treatm ents are not available, however, the elimination of field burning may seriously disrupt perennial ryegrass seed production.