Variances and covariances between productive and adaptive traits and temperament in a composite breed of tropical beef cattle

Abstract

Variances and covariances for productive and adaptive traits and temperament were estimated in a tropically adapted composite breed of cattle grazed at pasture in the tropics. Measurements included weights and period weight gains between birth and 18 months, mature cow weights, repeated tick, worm egg and buffalo fly counts, rectal temperatures (measures of heat resistance), flight speed scores (measures of temperament), scrotal circumference at weaning, 12 and 18 months and repeated measures of pregnancy rate and days to calving. Additive heritabilities ( h 2 a) of all weights except weaning weight were moderate to high ( h 2 a=0.31–0.84), but low to moderate for weaning weight and period weight gains ( h 2 a=0.14–0.21). Maternal heritabilities ( h 2 m) for weights to 18 months and preweaning gain were significant, particularly for weaning weight (0.34) and preweaning gain (0.30). Genetic and phenotypic correlations between weights at all ages were positive, but ranged from low ( r g=0.07 between yearling weight and mature cow weight; r p=0.08 between birth weight and mature cow weight) to high ( r g=0.95; r p=0.87 between weights at 12 and 18 months), with genetic correlations generally being higher than phenotypic correlations. Heritability of tick and worm counts were moderate to high (0.35–0.44). Heritability (0.18) and repeatability (0.27) of rectal temperature was low. Flight speed scores were moderately to highly heritable (0.40–0.44). Heritability of scrotal size at all ages was moderate to high (0.35–0.46) and maternal genetic effects were also important. Pregnancy rate and days to calving were lowly heritable (0.04 and 0.07, respectively). Genetic correlations between tick and worm counts ( r g=0.30) and tick count and rectal temperature ( r g=0.22) were both favourable, though the genetic relationship between worm count and rectal temperature was close to zero. Relationships between tick and worm counts and growth, male and female fertility and flight speed scores were all close to zero, indicating that tick and worm resistance were largely independent traits, except for relationships amongst other resistance traits. Genetic relationships between rectal temperatures and most weights and period weight gains were favourable ( r g=−0.20 to −0.49) though genetic relationships between rectal temperatures and birth weight, mature cow weight and dry season gain were lower (−0.08 to −0.12). Low to moderate, favourable genetic relationships existed between rectal temperature and pregnancy rate (−0.16) and days to calving (0.16), although phenotypic relationships were close to zero. To maximise genetic progress, it is concluded that homogeneous rather than heterogeneous variance should be assumed for males and females to estimate breeding values in zebu derived cattle grazed at pasture in the tropics. For beef breeding programs based in the tropics, where there are more sources of variation for economically important traits, careful consideration needs to be given to use of appropriate analytical models, to ensure that effects such as sire×year or contemporary group interactions are correctly accounted for when population genetic statistics are estimated.