Effect of fall-planted cover crops on weed suppression, grain sorghum yield, and profitability in the semi-arid Great Plains

Integrating cover crops (CC) in dryland crop rotations could help in controlling herbicide-resistant weeds. Field experiments were conducted at Kansas State University Agricultural Research Center near Hays, KS from 2020-2023 to determine the effect of fall-planted CC on weed suppression in grain sorghum, crop yield, and net returns in no-till dryland winter wheat (Triticum aestivum L.)-grain sorghum [Sorghum bicolor (L.) Moench]-fallow (W-S-F) rotation. The field site had a natural seedbank of glyphosate-resistant (GR) kochia [Bassia scoparia (L.) A. J. Scott] and Palmer amaranth (Amaranthus palmeri S. Watson). A CC mixture (triticale/winter peas/radish/rapeseed) was planted after wheat harvest and terminated at triticale heading stage before sorghum planting. Treatments included nontreated control, chemical fallow, CC terminated with glyphosate (GLY), and CC terminated with GLY+ acetochlor/atrazine (ACR/ATZ). Across three years, CC terminated with GLY+ACR/ATZ reduced total weed density by 34-81% and total weed biomass by 45-73% compared to chemical fallow during the sorghum growing season. Average grain sorghum yield was 786 to 1432 kg ha-1 and did not differ between chemical fallow and CC terminated with GLY+ACR/ATZ. However, net returns were lower with both CC treatments (USD -$275 to $66) in all three years compared to chemical fallow (USD -$111 to $120). These results suggest that fallow replacement with fall-planted CC in the W-S-F rotation can help suppress GR B. scoparia and A. palmeri in the subsequent grain sorghum. However, the cost of integrating CC exceeded the benefits of improved weed control and lower net returns were recorded in all three years compared to chemical fallow.