Interaction of an herbicide containing aminopyralid and metsulfuron and nitrogen fertilizer in tall fescue pastures grazed by stocker cattle

Our objective was to evaluate the effects of an herbicide containing aminopyralid and metsulfuron (Chapparal; Corteva Agriscience, Indianapolis, IN) follow- ing spring-applied nitrogen fertilizer on stocker cattle per- formance, forage species composition, seedhead density, and ergovaline concentration. Each year (n = 3), 96 steers (266 ± 19.3 kg) were randomly assigned to sixteen 1.8-ha pastures. Pastures were randomly assigned 1 of 4 treat- ments: no herbicide or nitrogen (NEGCON) or herbi- cide (140 g/ha) with 0 kg/ha N (CHAP0N), 67 kg/ha N (CHAP67N), or 134 kg/ha N (CHAP134N). Ammonium nitrate was applied March 7 ±3.6 d, and herbicide was applied April 14 ±1.5 d. Steers began grazing 72 h after herbicide application. Steer weight, standing forage height, forage nutritive value, and alkaloid concentrations in tall fescue tillers were measured monthly. Tall fescue seedhead frequency and forage species frequency were col- lected June 14 ±3.1 d. Tall fescue seedheads were reduced by 68% within pastures receiving herbicide. Her- bicide application did not affect ergovaline concentration, but ergovaline concentration increased across months. Ni- trogen fertilizer tended to quadratically increase forage mass. Steer ADG was greater in pastures treated with fertilizer and herbicide than in those not treated. Under the condi- tions of this experiment, application of herbicide contain- ing aminopyralid and metsulfuron to tall fescue pastures decreased seedhead concentration but did not affect ergov- aline. Nitrogen fertilizer increased forage mass on pastures treated with herbicide, and the combination improved steer gain under a short-duration continuously grazed sys- tem.