Integrated strategies to manage herbicide-resistant kochia and Palmer amaranth in Kansas cropping systems

Abstract

Kochia [Bassia scoparia (L.) A. J. Scott] and Palmer amaranth (Amaranthus palmeri S. Watson) are the two most troublesome summer annual weeds in the Central Great Plains (CGP). The widespread evolution of herbicide resistance in both species warrants alternate weed management strategies in the region. The major objectives of this dissertation were to 1) determine the response of kochia populations from Kansas, Oklahoma, and Texas to commonly used herbicides and quantify the level of resistance in putative multiple herbicide-resistant (MHR) kochia populations, 2) determine the interactions of 2,4-D, dichlorprop-p, dicamba, and halauxifen/fluroxypyr applied in two- or three-way combinations for controlling MHR kochia, 3) evaluate pyridate-based tank mixtures for controlling MHR kochia, 4) determine the effect of fall-planted cover crop (CC) and soil residual herbicides on emergence pattern and emergence periodicity of glyphosate-resistant Palmer amaranth, and 5) determine the effect of fall-planted CC in combination with soil residual herbicides on weed suppression and on grain yield and net returns in no-till dryland grain sorghum. Greenhouse screening experiments showed that 23% of tested kochia populations (82 total) from Kansas, Oklahoma, and Texas were potentially four-way resistant (≥20% survival) to chlorsulfuron, dicamba, fluroxypyr, and glyphosate. Dose-response studies further revealed resistance to dicamba (5- to 13-fold), fluroxypyr (3- to 6-fold), and glyphosate (3- to 5-fold) in kochia populations from Kansas and Oklahoma and resistance to dicamba (2- to 4-fold) and glyphosate (5-fold) in Texas populations compared to susceptible population. Three-way tank-mixtures of 2,4-D, dicamba, dichlorprop-p, and halauxifen/fluroxypyr in various combinations had synergistic interactions and provided 85 to 97% control of MHR kochia. Results from greenhouse experiments also showed synergistic interactions when pyridate was tank-mixed with atrazine, dicamba, dichlorprop-p, fluroxypyr, glyphosate, or halauxifen/fluroxypyr and resulted in >94% control and shoot dry biomass reduction of MHR kochia. In a separate field study, a synergistic interaction was observed when pyridate was tank-mixed with glyphosate or atrazine. Results from a 3-year field study showed that fall-planted CC after winter wheat harvest and terminated before grain sorghum planting with glyphosate plus acetochlor/atrazine required 105 to 1257 more cumulative growing degree days for 90% emergence of glyphosate-resistant Palmer amaranth and reduced the cumulative emergence by 42 to 56% compared to chemical fallow. Furthermore, the same treatment reduced total weed density by 34 to 81% and total weed biomass by 45 to 73% compared to chemical fallow. Average grain sorghum yield was 790 to 1430 kg ha⁻¹ and did not differ between chemical fallow and CC terminated with glyphosate plus acetochlor/atrazine. However, net returns were lower with both CC treatments (USD -$275 to $66 ha⁻¹) in all three years than chemical fallow (USD -$111 to $120 ha⁻¹). These results suggest that integrating a fall-planted CC after wheat harvest can help suppress glyphosate-resistant kochia and Palmer amaranth in the subsequent grain sorghum. This practice was not profitable most years but could be profitable during wet years with average yields (3800 to 5000 kg ha⁻¹) of the region. Overall, the results confirm the widespread presence of MHR kochia populations in the southcentral Great Plains. Growers should adopt alternative herbicide tank-mixtures and CC strategies to manage herbicide-resistant kochia and Palmer amaranth populations in the region.