Abstract: Diuron, simazine, and terbacil were applied together or separately in the field each May from 1981 through 1996. Weed control was over 90% in 1981 and 1982, but by 1984 weeds increased in plots treated with diuron and simazine. Weed abundance was relatively low from 1981 through 1996 in plots treated with terbacil. Broadleaf and grass species abundance was similar in most herbicide-treated plots from 1992 through 1996. Perennial species, particularly fescue (Festuca arundinacea) and ailanthus (Ailanthus altissima), dominated sites treated with diuron and simazine. The weed community changed within 3 yr of the implementation of the weed management program that relied solely on herbicides. A relatively stable weed community persisted from 1992 through 1996. Repeated use of the combined high rate of diuron and low rate of terbacil provided excellent weed control for 15 yr.
Nomenclature: Diuron, N′-(3,4-dichlorophenyl)-N,N-dimethylurea; simazine, 6-chloro-N,N′-diethyl-1,3,5-triazine-2,4-diamine; terbacil, 5-chloro-3-(1,1-dimethylethyl)-6-methyl-2,4(1H,3H)-pyrimidinedione; ailanthus, Ailanthus altissima (Mill.) Swingle. #3 AILAL; tall fescue, Festuca arundinacea Schreb. # FESAR.
Additional index words: Fruit orchards, selection, weed shifts, cheat, poison ivy, johnsongrass, yellow foxtail.