Combined effects of spring-planted cover crops and residual herbicide on weed suppression and subsequent wheat yield in semiarid Central Great Plains

Cover crop (CC) is a component of the integrated weed management (IWM) strategies for controlling herbicide-resistant weed biotypes. A field study was conducted at Kansas State University Agricultural Research Center near Hays, KS from 2021-2024 to determine the effect of spring-planted CC in combination with residual herbicide on weed suppression during fallow phase and subsequent wheat yield in no-till winter wheat (Triticum aestivum L.)-grain sorghum [Sorghum bicolor (L.) Moench]-fallow rotation in the semiarid Central Great Plains (CGP). The study site had a natural seedbank of glyphosate-resistant (GR) kochia [Bassia scoparia (L.) A. J. Scott] and Palmer amaranth (Amaranthus palmeri S. Watson). A mixture of CC [oats (Avena sativa L.)–barley (Hordeum vulgare L.)–spring peas (Pisum sativum L.)] was spring-planted in no-till sorghum stubbles and terminated at oats heading stage. Four treatments, including (1) weedy fallow (no CC and no herbicide), (2) chemical fallow (no CC but glyphosate + flumioxazin/pyroxasulfone + dicamba), (3) CC terminated with glyphosate, and (4) CC terminated with glyphosate + flumioxazin/pyroxasulfone were tested. Across 3 yrs, CC at termination reduced total weed density by 78 to 99% and total weed biomass by 93 to 99% as compared to weedy fallow. Weed suppression by the CC terminated with glyphosate plus flumioxazin/pyroxasulfone continued for at least 90 days with reduced total weed density of 52 to 80% and total weed biomass reduction by 70% compared to weedy fallow across 3 yrs. No differences in subsequent wheat grain yield between CC treatments and chemical fallow were recorded in 2021-22 and 2022-23; however, in 2023-24, chemical fallow and CC terminated with glyphosate + flumioxazin/pyroxasulfone had greater wheat yield than CC terminated with glyphosate only. These results suggest that integration of spring-planted CC with residual herbicide may help suppress GR B. scoparia and A. palmeri in the CGP.