Does Timing Influence the Utility of Reduced Atrazine Rates for Proactive Resistance Management?

Atrazine is an important herbicide for broadleaf weed control in corn. Use rates have declined in many corn production systems due to environmental concerns and the availability of other effective herbicides, especially glyphosate in glyphosate-resistant hybrids. However, using multiple effective herbicide modes of action is ever more important because occurrence of herbicide-resistant weeds is increasing. An experiment to compare application timings of reduced rates of atrazine to benefit resistance management in broadleaf weeds while protecting corn yield was conducted in Wisconsin across four site-years in 2012 and 2013. Herbicide treatments consisted of five atrazine rate and timing combinations and three POST base herbicides: glyphosate, glufosinate, and tembotrione. Metolachlor was applied PRE at 2.1 kg ai ha⁻¹ for grass control in all treatments. A linear regression model estimated that atrazine rates ≥ 1.0 kg ai ha⁻¹ applied PRE would prevent exposure of common lambsquarters plants to POST herbicides, but giant ragweed and velvetleaf exposure was not influenced by timing. Corn yield was also not influenced by atrazine rate and timing combinations at the α = 0.05 level; however, at P = 0.06, corn yield was greater for atrazine applied PRE at 1.1 kg ha⁻¹ than for atrazine applied PRE at 0.5 kg ha⁻¹, POST at 1.1 kg ha⁻¹, or not at all. In summary, higher rates of atrazine applied PRE may improve yield, as reported by others, but this study concludes reduced rates of atrazine (i.e., ≤ 1.1 kg ha⁻¹) applied to corn in a POST tank mixture combination provided more consistent control of giant ragweed, velvetleaf, and common lambsquarters compared with atrazine applied PRE. This information should help direct atrazine application timing applied POST when applied at low rates to improve proactive herbicide resistance management.Nomenclature: Atrazine; glufosinate; glyphosate; metolachlor; tembotrione; common lambsquarters, Chenopodium album L. CHEAL; giant ragweed, Ambrosia trifida L. AMBTR; velvetleaf, Abutilon theophrasti Medik. ABUTH; corn, Zea mays L.