Effect of varying imazethapyr application rate and timing on yield of seedling grass-alfalfa mixtures

Imazethapyr is registered for weed control in pure alfalfa and in established alfalfa-grass mixtures. Orchardgrass (Dactylis glomerata L.) and timothy (Phleum pratense L.) tolerance to imazethapyr during establishment was examined in the greenhouse and in mixtures with alfalfa (Medicago sativa L.) in the field. Imazethapyr was applied following grass emergence at rates ranging from 0.5 to 1.5 oz ai/acre at three different stages of forage grass growth. In the greenhouse, application of imazethapyr to seedling timothy was more injurious than to seedling orchardgrass. Imazethapyr application to timothy reduced grass growth 19 to 48% in the greenhouse, while orchardgrass growth was reduced no more than 26%. Imazethapyr reduced 2-leaf timothy yield more than 4- or 7-leaf grass in the greenhouse study, while both 2- and 4-leaf orchardgrass yield was less than 7-leaf grass. In the field, injury from imazethapyr to timothy was greater than to orchardgrass when the herbicide was applied following the first harvest, especially during a drought year. Imazethapyr reduced both orchardgrass and timothy growth in the field by as much as 34% on average regardless of application rate. Weed control was best at the early postemergence application timing and less effective at the mid-post timing due to excessive weed size at application. Weed control after first harvest was not needed. Application of imazethapyr to mixed forages the year of establishment can reduce the grass component of the forage and could reduce total forage yield.