Effect of Varying Imazethapyr Application Rate and Timing on Yield of Seedling Grass-Alfalfa Mixtures

Abstract

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. Research Question Much of the hay and pasture in the northern USA is planted to legume‐grass mixtures. However, in the northeastern USA, no herbicides are labeled for alfalfa‐grass mixtures in the establishment year. Imazethapyr (Pursuit®), an imida‐zolinone herbicide, is labeled for control of broadleaf weeds and grasses in pure alfalfa seedings, established alfalfa, and established alfalfa‐grass mixtures. If imazethapyr could be safely used to control weeds in mixed seedings during the seedling year, it would greatly improve weed management options. The objective of this research was to evaluate the effect of imazethapyr rate and application timing on orchardgrass and timothy tolerance in the greenhouse and on spring‐seeded alfalfa‐orchardgrass and alfalfa‐timothy mixtures in the field. Study Description In the greenhouse, imazethapyr was applied at 0.5, 0.75, and 1 oz/acre to 2, 4, and 7‐leaf orchardgrass and timothy. In the field, imazethapyr was applied at 0.75, 1, and 1.5 oz/acre to alfalfa‐orchardgrass and alfalfa‐timothy mixtures when the grass component had 2 or 4 leaves and after the first forage cutting. No‐herbicide (control) treatments were included in both experiments. In the greenhouse study, forage grass dry weight was determined, while in the field, forage and weed component dry weights were measured independently up to two times during the establishment year and three to four times the year following establishment. In the greenhouse and field, data were subjected to analysis of varia...

Target site resistance to acetolactate synthase inhibitors in a fall panicum (Panicum dichotomiflorum Michx.) accession from Wisconsin and its response to alternative herbicides

“…The imazethapyr dose-response was conducted to confirm whether fall panicum also had cross-resistance to ALS-inhibiting imidazolinones; however, given the low efficacy of imazethapyr to control fall panicum, as previously reported in the literature (Curran et al 1999), poor control (<70.0%) and biomass reduction levels (<55.0%) were observed even with the highest rate tested (560 g ha −1 ; data not shown) for both the resistant and susceptible accessions. Moreover, despite slight differences in visual control between the two accessions, with a lower response from the resistant accession, it did not translate into significant differences in ED 50 for control (P = 0.885) and biomass reduction (P = 0.192) (data not shown).…”

Winter Annual Weed Control with Herbicides in Alfalfa-Orchardgrass Mixtures

“…Imazethapyr is registered for use in alfalfa and nongrazed Conservation Reserve Program lands containing legumes and forage grasses (Anonymous 2006b). Imazethapyr reduced growth of orchardgrass when applied at the seedling stage and at first cutting (Curran et al 1999), but studies were not found documenting imazethapyr safety on established orchardgrass when applied in early spring.…”