EFFECT OF DIETARY ENERGY ON GROWTH AND REPRODUCTIVE CHARACTERISTICS OF ANGUS AND SENEPOL BULLS DURING SUMMER IN FLORIDA

Pubertal Angus bulls (n=10, 503 days of age and weighing 366 kg) and S enepol bulls (n=10, 457 days of age and weighing 381 kg) were stratifi ed by age and weight into 2 dietary treatments formulated to provide e qual amounts of crude protein and 75% (below) or 150% (above) of the m aintenance requirements for metabolizable energy. Measurements to asse ss body growth and libido were collected at 28-day intervals for 112 d ays (June through September). Twice during each 28-day interval, the b ulls were subjected to breeding soundness examinations. At the end of the experiment, gonadotropin releasing hormone (GnRH)-induced secretio n of luteinizing hormone (LH) and testosterone (T) in the serum were d etermined. At the end of the experiment, bulls fed the above maintenan ce diet (P<0.0001) were 91 kg heavier, had 1.7 mm more backfat thickne ss and 12.6 cm2 larger ribeye area than bulls on a below maintenance d iet. Diet affected (P<0.003) the average daily change in scrotal circu mference, but not the libido score (P>0.1), or semen quality. In gener al, Angus bulls had superior initial semen quality (P<0.06); however, during summer, semen quality tended to decrease in Angus but not in Se nepol bulls. The final rectal temperature was 0.5-degrees-C lower (P<0 .003) in Senepol than in Angus bulls. Basal T concentrations and area under the GnRH-induced T curve were greater (P<0.07) for bulls fed the above rather than the below maintenance diet. Angus bulls had a highe r (P<0.03) maximal LH response to GnRH and larger area under the GnRH- induced LH curve than Senepol bulls.