Evaluation of contrasting weed management systems using multiple performance criteria

Widespread problems with herbicide resistance in weeds and environmental contamination by herbicides indicate the need for alternative weed management strategies. Cropping system diversification and integration of physical and chemical control tactics may be useful for reducing dependence on herbicides while providing effective weed suppression, desirable levels of profitability, and improved environmental performance. To test this hypothesis, we used data from a long-term, 9-ha field experiment in Boone Co., IA that included three rotation systems (2-yr: corn/soybean; 3-yr: corn/soybean/oat + red clover; 4-yr: corn/soybean/oat + alfalfa/alfalfa) managed with two herbicide regimes (conventional and low) applied to corn and soybean crops. Oat, red clover, and alfalfa were not treated with herbicides. Herbicide inputs in the 3-yr and 4-yr systems were 33% and 50% lower, respectively, than in the 2-yr system, and the low regime received 93% less herbicide than the conventional regime. Weed biomass measured in corn and soybean during 2008–2014 differed among rotation system–herbicide regime combinations (p=0.003), but mean values were <60 kg ha^-1 in all treatments. Weed biomass in corn and soybean was greatest for the 3-yr rotation under the low herbicide regime; it was equivalent between the 2-yr–conventional and 4-yr–low input treatments. Net returns to land and management during 2008­­–2014 were unaffected by rotation system–herbicide regime combinations (p=0.74), with a mean value over years and treatments of $971 ha^-1 yr^-1. Potential freshwater toxicity of herbicide active ingredients was 3.6 times greater and potential soil toxicity was >100-fold greater in the conventional herbicide regime compared with the low herbicide regime; both toxicity potentials decreased 33–50% with increasing length of rotation. Results of this study indicate that cropping system diversification and integration of physical and chemical weed control tactics can engender large reductions in herbicide use while maintaining profitability and improving environmental safety.