The effects of several crop rotations and herbicide programs on populations of goosegrass, stinkgrass, large crabgrass, smooth crabgrass, fall panicum, and yellow nutsedge were investigated at two sites from 1991 to 1994. Crop rotations were continuous corn, continuous soybean, corn–soybean, and corn–tomato–soybean. Herbicide programs were the split-plots and included continuous use of acetolactate synthase (ALS)-inhibitor herbicides, continuous use of non–ALS-inhibitor herbicides, annual rotations between ALS- and non–ALS-inhibitor herbicides, combinations of ALS- and non–ALS-inhibitor herbicides in the same year, and no herbicide. Grass and yellow nutsedge densities generally were affected by an interaction between crop rotations and herbicide programs by 1994. Goosegrass densities in 1994 were highest from the continuous use of ALS-inhibitor herbicides in the corn–tomato–soybean rotation and were generally high in 1994 in the continuous corn and corn–tomato–soybean rotations. Stinkgrass densities were highest by 1994 where imazethapyr was applied alone for four consecutive years. Stinkgrass densities were also high where imazethapyr was applied in combination with butylate for 4 yr and at Site 2 (designated Northampton) where imazaquin plus nicosulfuron was applied for 4 yr. Herbicide programs did not produce shifts in large crabgrass densities, except that densities were highest where butylate plus atrazine was applied for 4 yr. Smooth crabgrass was present in significant densities at Site 1 (designated Accomac) only where imazaquin plus nicosulfuron was used for 4 yr or at Northampton from continuous ALS-inhibitor programs. Fall panicum densities were highest by 1994 where the combination of butylate plus atrazine was applied continuously for 4 yr. Yellow nutsedge control was lowest and densities were highest at Northampton where the combination of fomesafen plus fluazifop-P plus fenoxaprop was applied continuously for 4 yr. Yellow nutsedge densities by 1994 at Northampton were also high where these herbicides were applied with imazaquin for 4 yr or where these herbicides were applied in rotation with imazaquin plus nicosulfuron or butylate plus atrazine.

Nomenclature: Atrazine; butylate; fenoxaprop; fluazifop-P; fomesafen; imazaquin; imazethapyr; nicosulfuron; fall panicum, Panicum dichotomiflorum Michx. PANDI; goosegrass, Eleucine indica (L.) Gaertn. ELEIN; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; smooth crabgrass, Digitaria ischaemum (Schreb. ex Schweig.) Schreb. ex Muhl DIGIS; stinkgrass, Eragrostis cilianensis (All.) E. Mosher ERACN; yellow nutsedge, Cyperus esculentus L. CYPES; corn, Zea mays L. ‘ICI 8532IT’ in 1991, 1992, and 1993 and ‘Pioneer 3245IR’ in 1994; soybean, Glycine max (L.) Merr. ‘W20-STS’ in 1991, ‘Asgrow STS-9122’ in 1992, ‘Asgrow 3200 STS’ in 1993, and ‘Asgrow 4045 STS’ in 1994; tomato, Lycopersicon esculentum L. ‘Floradade’.