Occurrence Dynamics of Weeds, Yield Losses, and Herbicide Screening for Barnyardgrass (Echinochloa crus-galli) Control in Direct-Seeded Early Rice in Hunan Province, China

This study has investigated the occurrence characteristics and population damage of weeds in double-cropping direct-seeded rice fields in Hunan, and has identified efficient and safe pre- and post-emergence herbicides to enhance resistance management. Field trials were conducted at two representative sites (Yiyang and Changsha) in Hunan in 2024~2025. Weed community composition and emergence patterns were systematically monitored. The inhibitory effects of weed infestations on rice growth and yield were quantified. The biological activity and field efficacy of various herbicide classes against barnyardgrass (Echinochloa crus-galli) were evaluated via greenhouse bioassays and field trials. Weed emergence lasted 3–48 days after sowing (DAS) with three distinct peaks. Grasses emerged earliest and dominated the community, with barnyardgrass peaking at 13–17 DAS (≈50% of total weeds), followed by broadleaves at 20 DAS (≈40%) and sedges at 25 DAS (<20%). Weed infestation drastically suppressed rice height (max 19% reduction) and tillering (max 50% reduction), with mixed-weed and grass-dominated plots causing the severest yield losses (92.0% and 90.5%, respectively), versus only 18.0% in broadleaf-dominated plots. Greenhouse bioassays showed that oxaziclomefone had the highest intrinsic activity against barnyardgrass (GR90 = 17.70 g ai ha−1). In pre-emergence applications in field trials, pretilachlor (900 g ai ha−1) and mefenacet (147.6 g ai ha−1) provided >96.8% control at 20 and 40 days after treatment (DAT), while oxaziclomefone (66 g ai ha−1) achieved 88.2% control at 20 DAT. For post-emergence herbicides, Profoxydim showed the highest intrinsic activity (GR90 = 33.01 g ai ha−1), followed by feproxydim (GR90 = 33.45 g ai ha−1) and flusulfinam (GR90 = 64.55 g ai ha−1). In field trials, flusulfinam provided 100% control with superior crop safety at 20 and 40 DAT, while Florpyrauxifen-benzyl, feproxydim, and metamifop reached >93% efficacy. In conclusion, weed emergence in Hunan direct-seeded rice follows a three-peak pattern, with barnyardgrass being the most destructive species. An integrated strategy combining pretilachlor (pre-emergence) and flusulfinam (post-emergence), rotated with florpyrauxifen-benzyl and feproxydim, is recommended for effective barnyardgrass management and resistance mitigation.