STEER PERFORMANCE IN FESCUE-CLOVER PASTURES WITH DIFFERENT LEVELS OF ENDOPHYTE INFESTATION

A 3-yr study was conducted at Ames Plantation, Grand Junction, TN to determine the effects of fungal endophyte (Acremonium conenophialum Morgan-Jones & Gams) infestation (E+) levels in 'Kentucky 31' tall fescue (Festuca arundinacea Schreb.) pastures on steer (Bos taurus L.) performance. Two replications were used of 1.2-ha pastures overseeded yearly with ladino white clover (Trifolium repens L.) on a fine-silty, mixed, thermic Typic Hapludalf soil. Yearling tester steers (three per pasture) and additional steers of similar size maintained available forage at heights of 2.5 to 7.5 cm. Botanical composition, available forage, average daily gain (ADG), grazing days, and gain per hectare were determined every 21 d; serum prolactin, haircoat roughness, rectal temperatures, and E+ levels were measured at longer intervals. Results were computed using a generalized linear mixed models procedure. Generalized least squares means and standard errors for the broad inference space were estimated (BLUE) and differences for specific linear contrasts were predicted (BLUP). Steer ADG was depressed from over 800 to about 450 g in spring and from about 700 to 400 g during the entire grazing season as A. coenophialum increased from 3 to 81%. Animal grazing days were not different among treatments. Gain was about 65 kg ha-1 more with 22% E+ (379 kg ha-1) than at higher endophyte levels, but it was almost 500 kg ha-1 at 3% E+. Serum prolactin (125 ng mL-1 in steers grazing 3% E+) was halved with 22% E+ and was depressed further at higher E+ levels (41 to 18 ng mL-1). Results suggested that the effect of the endophyte on animal performance in tall fescue-clover pastures is not linear, with greater depression per unit infestation at lower levels than at higher levels.