Growth Performance and Serum Prolactin Concentrations of Stocker Steers Implanted with Trenbolone Acetate While Grazing Endophyte-Infected Fescue in the Spring

A 64-d grazing study was conducted with a 2 × 2 factorial arrangement of treatments to evaluate the impact of implant treatment on growth performance, hair score, and serum prolactin levels of steers grazing tall fescue with high (HE) or low (LE) levels of infection with the endophytic fungus Neotyphodium coenophialum. Mixed-breed steers (n = 130; 246 ± 3.5 kg initial BW) were allocated randomly to one of three HE or one of four LE pastures beginning April 13. Within each replication, one-half of the steers were implanted (IMP) with trenbolone acetate (40 mg) and estradiol (8 mg), and one-half were not implanted (NI). No implant treatment × endophyte level interactions were detected (P>0.10). Steers grazing HE had lower (P<0.01) BW gain, inferior (P<0.05) hair scores, and lower (P<0.01) serum prolactin concentrations on d 64 than those steers grazing LE. Total BW gains were greater (P=0.01) from IMP steers than from NI steers, but serum prolactin concentrations and hair scores did not differ (P>0.10) between IMP and NI steers on either d 36 or 64. When tabulated across forage and implant treatments, correlations were negative between overall BW gains and hair scores measured on d 64 (r = 0.28; P<0.01), and positive between overall BW gains and serum prolactin levels measured on d 36 and 64 (r = 0.33 and 0.43, respectively; P<0.001). Therefore, fescue toxicity symptoms were manifested in HE steers, and implanting trenbolene acetate and estradiol improved grazing BW gain, but no endophyte level × implant interactions were detected.