Effects of selected endophyte and tall fescue cultivar combinations on steer grazing performance, indicators of fescue toxicosis, feedlot performance, and carcass traits.

Five tall fescue [Lolium arundinaceum (Schreb.)] pastures [wild-type 'Kentucky-31' with 78.0% of plants infected with ergot alkaloid-producing endophyte Neotyphodium coenophialum (KY31 E+), 'Jesup' AR542 endophyte-infected contaminated with 30.3% tall fescue containing ergot alkaloid producing-endophyte (Jesup AR542 E+); 'GA-186' AR584 endophyte-infected contaminated with 11.8% tall fescue containing ergot alkaloid producing-endophyte (AGRFA 140); 'PDF' AR584 endophyte-infected contaminated with 5.5% tall fescue containing ergot alkaloid producing-endophyte (AGRFA 144); and 'KYFA 9301' AR584 endophyte-infected contaminated with 10.0% tall fescue containing ergot alkaloid producing-endophyte (AGRFA 150)] were compared for steer growth performance, toxicity, feedlot performance, and carcass traits. Steers (mean initial BW=322 kg) grazed pastures for 84 d in spring and 56 d in autumn for 2 yr. Steers were shipped after grazing in Prairie, MS, to Macedonia, IA, for finishing. Mean herbage mass was not different (P=0.15) among pastures. Posttreatment (d 28+) serum prolactin concentrations were depressed (P=0.013) on KY31 E+. Steers grazing KY31 E+ had greater (P<0.01) posttreatment rectal temperatures during spring. Spring hair coat scores were greatest (P<0.01) on KY31 E+ at d 56 and 84. Steer ADG was least (P<0.01) on KY31 E+ in spring and depressed (P=0.014) on KY31 E+ and Jesup AR542 E+ in autumn. Spring grazing ADG was greater (P=0.049) on AGRFA 150 than Jesup AR542 E+ and AGRFA 140. No BW differences (P=0.09) among pastures were seen at reimplant during feedlot finishing. Pasture had no effect on ADG after reimplant (P=0.68), days on feed (P=0.56), or final BW (P=0.55). Exposure to fescue toxicosis did not affect (P≥0.19) carcass traits. Hair coat price discounts applied for spring-grazed steers on KY-31 E+ affected (P<0.01) initial steer monetary values. There were no pasture differences for finishing costs (P≥0.61) or final carcass value (P=0.59). Elite tall fescue cultivar and novel endophyte combinations improve growth performance of grazing calves over KY31 E+. Producers whose calves graze KY-31 E+ tall fescue should consider retaining ownership of these cattle through feedlot finishing to avoid market discounts and capture value from compensatory BW gains during finishing.