Improved management of Italian ryegrass (Lolium perenne spp. multiflorum) with mixtures of tiafenacil and ACCase or glutamine synthetase inhibitors

Herbicide-resistant Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] is a significant problem in multiple cropping systems because of its rapid growth, open pollination, prolific seed production, and multiple cases of resistance worldwide, except to protoporphyrinogen oxidase (PPO) inhibitors. This research evaluated tiafenacil, a new PPO inhibitor, in mixtures with glutamine synthetase- or acetyl-CoA carboxylase (ACCase)-inhibiting herbicides to manage resistant L. perenne ssp. multiflorum populations. Tiafenacil efficacy against L. perenne ssp. multiflorum was growth stage dependent, with increased efficacy at earlier stages in greenhouse studies. The LD₉₀ was 41.06 g ai ha⁻¹ at BBCH 23 and increased to 9.0-fold at BBCH 33. Field studies indicated that changes in carrier volume did not affect tiafenacil’s efficacy; the highest tested rate of tiafenacil (75 g ai ha⁻¹) reduced L. perenne ssp. multiflorum inflorescence weight by 50% to 90%. Mixtures of tiafenacil and glufosinate (1,150 g ai ha⁻¹) improved L. perenne ssp. multiflorum control (+24% to 43%) and reduced inflorescence weight (+15% to 34%), particularly at the highest tested rates (50 and 75 g ai ha⁻¹), suggesting synergistic effects based on Colby’s test. Tiafenacil with ACCase inhibitors improved L. perenne ssp. multiflorum control (+19% to 49%) and inflorescence weight reduction (+8% to 13%). These mixtures exhibited an additive effect when combined with fluazifop and a synergistic effect with clethodim. Herbicide mixtures and application strategies are critical to effective L. perenne ssp. multiflorum management. Tiafenacil, especially when used with glufosinate or ACCase inhibitors, offers an effective solution to L. perenne ssp. multiflorum management and is a strategic tool against herbicide resistance, as resistance to PPO inhibitors has not evolved. Further research should assess practices to ensure the long-term viability of these mixtures for resistance management.