Effects of 0 and 94% Endophyte-Infected Tall Fescue Hay on Growth Rate, Diet Digestibility, and Serum Prolactin Levels in Early Yearling Horses

Twelve pairs of yearling horses were fed a diet consisting of a concentrate and either noninfected (E-) or 94% infected tall fescue hay (E+) for 75 d. While consuming the 50% concentrate:50% tall fescue ration, the yearlings were weighed and measured for withers height, length from knee to fetlock, length from hock to fetlock, and circumference of fore and rear cannons at 25-d intervals. Serum was collected three times/wk during the first 2 wk of the study and at the end of each 25-d interval. A digestibility study on the E– and E+ based diets was conducted at the end of the growth study utilizing 12 yearlings (6/treatment). During the growth trial, yearlings grew at approximately the same rate for all measurements taken. Pairs of yearlings averaged .62 and .53 ± .07 kg/d in weight gain for the E– and E+ based diets, respectively. Withers height for each pair of yearlings changed an average 4.18 and 4.74 ± .32 cm for the E– and E+ treatments, respectively, over the 75-d period. Serum prolactin was not different between treatments. The digestion study revealed no difference in digestibility in crude protein and energy of the E– and E+ based diets, averaging 49.4 and 53.75%, respectively. NDF digestibility was greater (P<.05) for the E+ diet but equal between treatments for the ADF component. Since the highly infected tall fescue hay and concentrate diet did not suppress weight gain, skeletal growth, serum prolactin, or digestibility of major nutrient components, it was concluded that highly Infected tall fescue hay is a safe feed for early yearling horses. Tall fescue hay-based diets supported moderate growth rates greater than .2 kg/d when an adequate amount of concentrate was consumed with the tall fescue hay to meet the NRC recommendations for yearling horses.