Effect of plane of nutrition and season on body and testicular growth and on semen characteristics in Boran and Boran X Friesian bulls in Ethiopia

Ten Boran and 12 Boran X Friesian bulls with a mean (+standard error of the mean) age and body weight of 20.2+0.5 months and 225.5+2.5 kg were used over a 1 year period to determine the effect of plane of nutrition and season on body and testicular growth and on semen traits. Within genotype, bulls were allocated to either a high (HP) or a low (LP) plane of nutrition. Data on body weight, scrotal circumference (SC) and semen characteristics were collected every 2 weeks. Body weight and SC increased (P<0.001) in bulls on HP than on LP nutrition (760+12.6 vs 236+12.6 g day-1) and in Boran than in Boran X Friesian bulls (620+12.8 vs. 375+12.3 g day-1). Mean SC and changes in SC were higher in bulls on HP than on LP nutrition (0.23+0.01 vs. 0.04+0.01 mm day-1). Season affected changes in SC in both Boran and Boran X Friesian bulls and was highest during the short rainy season and lowest during the dry season. Data from 528 ejaculates showed that the two genotypes reacted differently to season with respect to semen volume, percentage motility and sperm concentrations. Although these semen characteristics were highest during the dry season and lowest during the short rainy season in both genotypes, Boran bulls and better semen traits than Boran X Friesian bulls during the dry season and vice versa during the long rainy season, while genotype differences were minimal during the short rainy season. The percentage of normal sperm cells was higher in bulls on LP than on HP nutrition (P<0.01), in Boran than in Boran X Friesian bulls (P<0.05), and was highest (P<0.001) during the dry season. It was concluded that genotype differences in semen characteristics in response to season and plane of nutrition have to be taken into account in the management of zebu and crossbred bulls in tropical environments.