CORN-WEED INTERACTIONS WITH LONG-TERM CONSERVATION TILLAGE MANAGEMENT

The use of monoculture production systems such as continuous corn (Zea mays L.) in conservation-tillage systems has created a requirement for information concerning crop and weed responses to annual herbicide inputs. The objective of this 5-yr investigation (1981-1985) was to evaluate weed problems and delineate weed and crop interactions in continuous conservation-tillage corn as affected by annual herbicide applications. Preemergence treatments contained selected combinations of triazines or a triazine plus alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide]. Preemergence or pre-plant incorporated herbicide mixtures were applied at common usage rates and compared for their effects on weed cover, weed species stability, and grain production. All plots were essentially weed-free the first year. Weed cover in most treatments increased significantly by the second year with fall panicum (Panicum dichotomiflorum Michx.) dominating the flora in Years 2 and 3, and smooth pigweed Amaranthus hybridus L.) dominating in Years 4 and 5. Smooth pigweed reached densities of 85% in several treatments. Fall panicum was the dominant weed in pre-plant incorporated treatments that contained atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] plus a carbamothioate herbicide. Grain yields were negatively correlated with fall panicum densities in the second year in all herbicide treatments. Smooth pigweed densities were negatively correlated with grain yields in the fifth year in all preemergence treatments. Smooth pigweed densities were lower in triazine plus alachlor treatments than in double triazine treatments for the last 2 yr of the study.