Reducing Herbicide Reliance through Cropping System Diversification: A Case Study in the Central Corn Belt.

Increasing problems from weed resistance to herbicides and increasing concerns over environmental damage from herbicides have led to a search for sustainable and profitable weed management strategies for Midwestern U.S. crop production systems. Approaches that exploit diverse cropping systems to subject weeds to multiple stress and mortality factors may provide reliable weed management with reduced dependence on chemicals. We tested the this hypothesis in a field experiment conducted in Boone County, Iowa, USA, by comparing weed growth in three cropping systems (2-, 3- and 4-year rotation sequences) managed with two herbicide regimes (low and conventional input levels). Additionally, we assessed the relationship between soil sampling intensity and precision in measuring weed viable seedbank density. For a given rotation system, the low herbicide regime, i.e. reduced herbicide inputs and increased mechanical weed control, used 62% and 94% less herbicide active ingredient (kg a.i. ha-1) on corn and soybean, respectively, compared to the conventional regime. Averaged over herbicide regimes, the 3- and 4-year systems reduced herbicide use 33% and 50%, respectively, compared with the 2-year system. In 2014 and 2015, mean weed biomass was two-fold greater under the low herbicide regime (181 kg ha-1) than with the conventional regime (90 kg ha-1), and six- to seven-fold greater in the 3-year (143 kg ha-1) and 4-year (185 kg ha-1) rotations than in the 2-year rotation (25 kg ha-1). However, increases in weed biomass did not affect corn or soybean yield. Estimates of total weed seed density that were based on soil samples drawn from a total surface area of 87 cm2 per experimental unit to a depth of 20 cm had CVs <18%. Results of our work indicate that multitactic weed management strategies within diversified cropping systems offer opportunities to reduce reliance on herbicides.