Integrating cover crops (CCs) 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 to 2023 to determine the effect of fall-planted CCs on weed suppression in grain sorghum [Sorghum bicolor (L.) Moench], crop yield, and net returns in no-till dryland winter wheat (Triticum aestivum L.)–grain sorghum–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 [winter triticale (×Triticosecale Wittm. ex A. Camus [Secale × Triticum])–winter peas (Pisum sativum L.)–canola (Brassica napus L.)–radish (Raphanus sativus L.)] 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 3 yr, CC terminated with GLY+ACR/ATZ reduced total weed density by 34% to 81% and total weed biomass by 45% to 73% compared with chemical fallow during the sorghum growing season. Average grain sorghum yield was 786 to 1,432 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 (−US$275 to US$66) in all 3 yr compared with chemical fallow (−US$111 to US$120). These results suggest that fallow replacement with fall-planted CCs 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 CCs exceeded the benefits of improved weed control, and lower net returns were recorded in all 3 yr compared with chemical fallow.

Kochia [Bassia scoparia (L.) A.J. Scott] is an invasive tumbleweed in the North American Great Plains that is difficult to manage in croplands and ruderal areas due to widespread resistance to up to four herbicide sites of action, including auxin mimics (Herbicide Resistance Action Committee [HRAC] Group 4) and inhibitors of acetolactate synthase (HRAC Group 2), photosystem II (HRAC Group 5), and 5-enolpyruvylshikimate-3-phosphate synthase (HRAC Group 9). Poor B. scoparia control with protoporphyrinogen oxidase (PPO)-inhibiting (HRAC Group 14) herbicides was noted in a brown mustard [Brassica juncea (L.) Czern.] field near Kindersley, SK, Canada, in 2021. Similar observations were made in a sunflower (Helianthus annuus L.) field near Mandan, ND, USA, and in research plots near Minot, ND, USA, in 2022. Whole-plant dose–response experiments were conducted to determine whether these B. scoparia accessions were resistant to the PPO-inhibiting herbicides saflufenacil and carfentrazone and the level of resistance observed. All three B. scoparia accessions were highly resistant to foliar-applied saflufenacil and carfentrazone compared with two locally relevant susceptible accessions. The Kindersley accession exhibited 57- to 87-fold resistance to saflufenacil and 97- to 121-fold resistance to carfentrazone based on biomass dry weight at 21 d after treatment (DAT). Similarly, the Mandan accession exhibited 204- to 321-fold resistance to saflufenacil and 111- to 330-fold resistance to carfentrazone, while the Minot accession exhibited 45- to 71-fold resistance to saflufenacil and 88- to 264-fold resistance to carfentrazone. Substantial differences in visible control at 7 and 21/28 DAT were also observed between the putative-resistant and susceptible accessions. This study represents the first confirmations of PPO inhibitor–resistant B. scoparia globally and the fifth herbicide site of action to which B. scoparia has evolved resistance. It also documents this issue present at three locations in the Northern Great Plains region that occur up to 790 km apart and on both sides of the Canada/U.S. border.