Twenty-seven Boran and 37 Boran x Friesian crossbred bulls were weaned at 6 months of age and randomly assigned to either a dry season supplementary feed containing 16% crude
protein or control groups. Data were collected on
body weight, wither height and scrotal circumference (SC) at monthly intervals. Bulls were also challenged with 100 mug of
gonadotropin releasing hormone (
GnRH) at 2-month intervals and plasma
testosterone responses were determined. Semen was collected at monthly intervals by electroejaculation. Post weaning average daily
weight gain was significantly (P<0.001) higher in supplemented than in control groups (302 vs
208 g/day). Scrotal circumference growth rates were also significantly (P<0.001) higher in supplemented than in control bulls (0.38 vs 0.31 mm/day). Bulls fed supplemented diets were younger (430 vs 473 days; P<0.001) and heavier (217 vs 203 kg; P<0.05) and had better semen quality at puberty than the control bulls. There was a trend for larger SC at puberty in supplemented than in control bulls (P=0.06). There were no significant differences in wither height or in
testosterone response to
GnRH at puberty between supplemented and control bulls. Body and SC growth rates to puberty for Boran and Boran x Friesian bulls were 189 and 321 g/day (P<0.001) and 0.27 and 0.42 mm/day (P<0.01), respectively. Boran bulls were older (513 vs 406 days; P<0.01) and lighter (201
vs 215 kg; P<0.05), and had smaller SC (23.9 vs 26 cm; P<0.01) at puberty than Boran x Friesian bulls.
Testosterone responses to
GnRH increased (P<0.001) with age and were significantly higher (P<0.05) in supplemented and in Boran x Friesian bulls than in control and Boran bulls. Wither height,
testosterone response to
GnRH and semen quality at puberty did not differ between breeds. Dry season supplementation improved growth rate and enhanced sexual development and maturity in both Boran and Boran x Friesian bulls, and would be a useful management strategy for enhancing fertility in bulls in the tropics.