Effect of Energy Intake on Reproductive Performance of Senepol and Angus Bulls in Florida

To determine the effect of energy intake on semen characteristics and libido, pubertal Angus (n = 10) and Senepol (n = 10) bulls (approximately 16 months of age and 816 lb) were stratified by age and weight between two treatments. Treatments were diets that provided 75% (LOW) or 150% (HIGH) of maintenance metabolizable energy requirements. Diets were group fed for 112 days (June through September) and provided equal amounts of crude protein. Measurements collected every 4 weeks included: body weight, plasma concentration of urea N and glucose, rectal temperature, back fat thickness and libido score. Twice during each 4 week interval scrotal circumference was measured, each bull was electroejaculated and semen and sperm quality evaluated. Bulls fed the LOW diet lost 43 lb and backfat thickness remained relatively unchanged at .10 to .12 in; in contrast, HIGH fed bulls gained 137 lb and backfat increased from .13 to .18 in. Plasma concentration of urea N was greater (P<.05) for LOW (19.7 mg/dl) compared to HIGH (5.6 mg/dl) fed bulls, while the concentration of glucose was lower (P<.05; LOW 70.7 vs HIGH 74.0 mg/dl). Senepol bulls had a higher (P<.01) concentration of urea N (14.3 vs 10.6 mg/dl), but lower (P < .05) glucose (70.4 vs 74.3 mg/dl) compared to Angus bulls. Angus bulls had a higher (P<.01) rectal temperature than Senepol bulls (39.6 vs 39.2°C). A significant diet by breed interaction was observed for libido. Libido scores were similar for LOW (6.9) and HIGH (7.1) fed Angus, but were lower for LOW (3.2) compared to HIGH (5.6) Senepol. Individual motility and velocity of spermatozoa were not influenced (P > .10) by diet or breed. Significant diet by breed interactions were observed for gross motility of the semen and the percentage of primary abnormalities of spermatozoa. Gross motility of semen was lowest for LOW fed Senepol (2.5), compared to HIGH-Senepol (3.4), LOW-Angus (3.3) or HIGH-Angus (3.2). Primary abnormalities were greater in LOW-Senepol (27.4%) compared to HIGH-Senepol (13.4%), however, the opposite was true for LOW (7.9%) and HIGH (17.3%) Angus. These data indicate an advantage for Senepol bulls in that they regulate body temperature, but under nutritional stress, reproductive characteristics of Senepol bulls appear not to be superior to Angus bulls.