Effect of herbicide and plant growth regulator mixtures on timothy seed crops

Abstract Timothy is the primary forage grass seed crop in Norway, where effective control of dicot weeds and lodging management are critical to maximizing seed yield and meeting seed purity standards. Optimal timing of gibberellic acid–inhibiting plant growth regulators (PGRs), such as chlormequat chloride (CCC) and trinexapac-ethyl (TE), often coincides with spring herbicide applications, raising concerns about crop safety and herbicide efficacy when products are tank-mixed. Field experiments were conducted in 2022 and 2023 at two locations in Norway to evaluate the effect of tank-mixing PGRs with commonly used herbicides on seed yield and quality, weed control, and lodging in timothy. Plant growth regulators (CCC (1500 g ai ha⁻¹) and TE (150 g ai ha⁻¹) were applied either as split applications (herbicide at BBCH 25; PGR at BBCH 35) or tank-mixed (BBCH 30–35) with one of three herbicide combinations: fluroxypyr + florasulam, florasulam + diflufenican, or aminopyralid + 2,4-D + florasulam. Seed yield was affected by PGR (P ≤ 0.05) but not by herbicide treatments. TE reduced plant height and lodging more than CCC; however, relative yield responses varied with stand age. Tank-mixing did not reduce seed yield and herbicide efficacy. Crop injury from tank-mixtures was minimal and temporary, and no reductions in final seed germination were observed. Tank-mixing CCC and TE PGRs with herbicides provided effective weed control and maintained optimal seed yield and quality, offering operational flexibility for timothy seed producers.