163 Scrotal circumference in young Brahman and Nellore bulls

Scrotal circumference (SC) is a well-known variable associated with semen quality and fertility. Besides, it has been stated its high heritability and relationship with the offspring reproductive performance. Early selection by this characteristic is possible and reliable and accelerates the genetic progress in cattle herds, especially in tropical areas. However, it is still necessary to establish the mean SC values according to age and breed to predict the sire’s reproductive prospect. This paper aims to provide SC means in Brahman and Nellore bulls raised under tropical conditions and to study factors related to SC dynamics of growing from 6 to 24 months of age. The SC and bodyweight (BW) were assessed in 615 Bos indicus bulls (Brahman n = 492, Nellore n = 123) distributed in 11 farms in Costa Rica. Both variables were recorded monthly from 6 (weaning) to 12 months of age and then, bimonthly up to 24 months of age. Bulls were required to have testicular symmetry and be free of any other clinical disease. The bulls were grass-fed and supplemented with minerals ad libitum. Scrotal circumference (LSM) disclosed by breed was calculated according to age adjusted by BW using a GLM model for repeated-measurements (SAS), with farm and bull as random effects. Two sample t-tests were used to compare the SC for both breeds during the early post-weaning phase (period 1) (6–11 months of age) and from 12–24 months of age (period 2). Also, Spearman correlation coefficients were calculated between SC versus BW and age. The results showed that SC increased from 15.6 ± 0.3 to 31.8 ± 0.1 cm and from 14.0 ± 0.4 to 31.0 ± 0.2 cm in Brahman and Nellore bulls, respectively (Table 1). The mean SC of Brahman was larger during periods 1 (P < 0.001) and 2 (P < 0.01) compared with Nellore. These findings support breed differences at least during early growing for SC. Overall SC had a high positive correlation with BW (r = 0.92; P < 0.0001) and age (r = 0.85; P < 0.0001). A larger correlation of SC with BW compared with age has also been reported in older bulls and other ruminant species. The SC monthly growing rate was lower in period 1 (0.6 and 0.7 cm) than in period 2 (1.0 and 1.04 cm) for Brahman and Nellore, respectively. These outputs highlight that different physiological testis events occur during the immediate post-weaning period compared with older ages (>12 months). In conclusion, (1) SC means provided disclosed by breed and age can be used as standards for early reproductive selection. (2) Differences in SC values were observed between both breeds from 6 to 24 months of age. (3) Dissimilarities in SC growing rate at early post-weaning versus older ages should be considered by breeders to control those factors that can influence the young testicular development.