Sunflower (Helianthus annuus L) tolerance to sublethal doses of imazethapyr

Field experiments were conducted from 1992 to 1995 at Morden, Manitoba to investigate the tolerance of sunflower to sublethal doses of imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid). Imazethapyr was applied at 0, 0.375, 0.75, 1.5, 3, and 6 g a.i. ha−1 plus a nonionic surfactant at 0.25% vol/vol when sunflowers were in the six-leaf stage. Dosages higher than 1.5 g ha−1 severely injured sunflowers in some years. Injury symptoms consisted of chlorosis, stunting and seed head deformities. At the dosages tested, imazethapyr reduced sunflower yields in only 1 of 4 yr. In 1994, dosages greater than 1.5 g ha−1 reduced achene yields by 19–38%. Seed oil content was reduced in 2 of 4 yr. At 6 g ha−1 of imazethapyr seed oil content was reduced by as much as 3.2 percentage points compared with the untreated check. While low dosages of imazethapyr may not reduce yields in most years, the potential for severe yield losses and reduced oil contents exist and caution should be used when applying imazethapyr in fields next to sunflowers. Key words: Crop injury, herbicide drift, yield, oil content