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. 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 variance to determine the effect of imazethapyr rate and timing on forage yield. The effectiveness of imazethapyr on weed control was also examined in the field experiment. Applied Questions What effect did imazethapyr have on orchardgrass and timothy? In the greenhouse study, application of imazethapyr was more injurious to timothy than orchardgrass. Grass biomass for both orchardgrass and timothy decreased with increasing imazethapyr rates only when applied at the 2‐leaf stage. Imazethapyr applied at the 2‐leaf stage reduced timothy and orchardgrass biomass more than at the other stages. However, imazethapyr did not kill either orchardgrass or timothy regardless of rate or application timing, and the grasses resumed growth within 7 to 10 d after imazethapyr application, particularly as application was delayed. In the field studies, imazethapyr reduced grass yields during the establishment year and following years in only one of two studies, with timothy yields being reduced more than orchardgrass yields. In the establishment year of one study, grass yields decreased as imazethapyr rate increased. In the other study, grass yields were reduced most the year after establishment when imazethapyr had been applied after‐harvest in the seeding year. What effect did the imazethapyr applications have on alfalfa yield? In general, alfalfa yield was not affected by imazethapyr regardless of rate or timing. This response is consistent with previous research. How effective was imazethapyr for managing weeds? Imazethapyr applied when the forage grasses were at the 2‐leaf stage effectively controlled smooth pigweed and velvetleaf. Application at the 4‐leaf stage of forage growth did not control weeds primarily because of excessive weed size. The after‐harvest treatment was not necessary, since cutting the forage prior to herbicide application removed most summer annual weeds. Fall panicum was not controlled by imazethapyr regardless of the application timing or rate. Can imazethapyr be safely used for managing weeds in seedling alfalfa‐orchardgrass or alfalfa‐timothy? For effective control, imazethapyr must be applied when weeds are small. However, applying imazethapyr at this time will reduce orchardgrass and timothy growth and yield. Imazethapyr is currently labeled for alfalfa‐grass forage that has been established for at least one year. Because of the risk for reduced grass yield in mixed forages, imazethapyr should not be used the year of establishment.