The performance of Brahman-Shorthorn and Sahiwal-Shorthorn beef cattle in the dry tropics of northern Queensland. 5. Scrotal circumference, temperament, ectoparasite resistance, and the genetics of growth and other traits in bulls

Abstract

The genetics of growth to 24 months, scrotal circumference (SC) at 24 months and temperaments at 6, 12 and 18 months in 7 year groups of F2 et seq., 1/2 Brahman, 1/2 Sahiwal, 3/4 Brahman and 3/4 Sahiwal bulls were investigated in the dry tropics of northern Queensland. Cattle tick (Boophilus microplus) and buffalo fly (Haematobia irritans exigua) resistance were also assessed at 24 months. Scrotal circumference at 24 months was highest in 1/2 Sahiwal crosses (28 cm v. 26-27 cm; P<0.05) with pronounced variation between years (24-31 cm; P<0.05). Temperaments (TEM score; flight distance) were generally poorest in 3/4 Sahiwal crosses (P<0.05). Bulls of 1/2 Bos indicus content were twice as susceptible as 3/4 crosses to tick infestations (P<0.05). There were no genotype differences in natural buffalo fly infestations. Temperaments, tick resistance and buffalo fly resistance all had no consistent significant relationships with either each other or with weights or growth rates to 24 months. Paternal half-sib estimates of heritabilities across all genotypes were generally moderate for weight (0.20-0.36), growth rates (0.08-0.46), and SC (0.32), low for TEM score (0.08-0.14), and high for flight distance (0.32-0.70). The genetic correlations (rG) between growth rates in both dry and wet seasons after weaning were high (>0.9); however, the rG of weaning weight with postweaning seasonal growth rates to 24 months were consistently negative (-0.09 to -0.71; P>0.05). The rG of SC with post-weaning growth rates were negative (-0.4 to -0.8). The rG of SC with weights up to 18 months were not significantly different to zero, but tended to be negative with weight at 24 months.