RESIDUE REMOVAL METHOD AND HERBICIDES FOR TALL FESCUE SEED PRODUCTION.2. CROP TOLERANCE

Increased reliance on herbicides for controlling weeds is a significan t component of the ongoing transition from open-field burning (FB) to nonburn systems for grass seed production in the Pacific Northwest. Vo lunteer crop seedlings are the most abundant weeds present in many cer tified grass seed fields, but herbicide treatments to control them als o have the potential to injure the established crop Fourteen herbicide treatment sequences plus an untreated check were examined in five res idue removal systems at two held sites during two consecutive growing seasons to determine their effect on tall fescue (Festuca arundinacea Schreb.) seed yield and harvest index. Herbicide treatments were appli ed preemergence (PRE) to seedling grasses in mid-October, postemergenc e (POST) in early December, or at both times. Tall fescue seed yield w as not affected over a two-year period by competition from volunteer s eedlings. Seed yield was reduced by herbicide injury, particularly in the second year. PRE applications of oxyfluorfen -ethoxy-4-nitrophenox y)-4-(trifluoromethyl)benzene reduced seed yield by 87 and 151 kg ha(- 1) in 1990 and 1991, respectively, compared with the average for three other PRE herbicides, pendimethalin [N-(1-ethylpropyl)-3,4- dimethyl- 2,6-dinitrobenzenamine],metolachlor -ethyl-6-methylphenyl)-N-(2-methox y-1-methylethyl) acetamide], and trifluralin tro-N,N-dipropyl-4-(trifl uoromethyl)benzenamaine]. The untreated check yielded 122 kg ha(-1) mo re seed than the average of all 14 herbicide treatments in 1991. The l east injurious herbicide treatment was PRE pendimethalin applied witho ut subsequent POST herbicide, When applied following PRE herbicides, P OST applications of 2.7 kg a.i. ha(-1) diuron [N'-(3,4-dichlorophenyl) -N,N-dimethylurea] or 0.14 kg a.i. ha(-1) oxyfluorfen plus 1.8 kg ha(- 1) diuron did not differ in yield or harvest index. An aggressive syst em for mechanical removal of straw, seed, and postharvest regrowth, va cuum sweeping CVS), reduced tal fescue seed yield and harvest index in 1991 compared with other, less aggressive, mechanical methods, and co mpared with two methods of burning. Some herbicide treatments applied to control volunteer crop seedlings had a greater effect on tall fescu e seed yield than the adoption of nonburn methods of residue removal.