Genetic parameters for calf mortality and correlated cow and calf traits in tropically adapted beef breeds managed in extensive Australian production systems

The genetic associations between cow teat and udder traits with maternal contributions to calf mortality were studied in Brahman (BRAH) and Tropical Composite (TCOMP) cattle managed in extensive production systems of northern Australia. Data from 9286 purebred and crossbred calves, progeny of 2076 cows and 149 sires, were recorded from 2003 to 2011. Calf weights at birth (BWT) and weaning (WWT) were routinely recorded. The event of calf death before weaning (DWEAN) was analysed as a repeated-measure of the cow. Cows were also scored at each calving for front- and back-teat size and udder size (US) on an ascending five-point scale. Heritabilities for front-teat size, back-teat size and US were 0.38 +/- 0.05, 0.31 +/- 0.05 and 0.49 +/- 0.01, and estimates were the same for BRAH and TCOMP. The heritability of DWEAN was higher in BRAH (0.09 +/- 0.02) than in TCOMP (0.02 +/- 0.01). Variance ratios for maternal genetic effects contributing to variation in BWT and WWT were 0.13 +/- 0.02 and 0.18 +/- 0.05, and tended to be larger in TCOMP than in BRAH. Teat and udder scores were moderately correlated phenotypically (0.37 +/- 0.01) and genetically (0.53 +/- 0.04) with each other. Both traits were uncorrelated genetically with calf birthweight but positively correlated with WWT and DWEAN. The genetic correlation between average teat score at calving and DWEAN was larger (0.54 +/- 0.05) than that between US and DWEAN (0.33 +/- 0.06), whereas the genetic correlation between US and maternal effects for WWT was larger (0.60 +/- 0.08) than the corresponding value for average teat score with maternal WWT (0.37 +/- 0.13). Correlations between BWT and WWT were high for both direct (0.63 +/- 0.07) and maternal (0.50 +/- 0.09) genetic effects. Genetic correlations between maternal effects for BWT or WWT with DWEAN were both negative (-0.23 +/- 0.10 and -0.21 +/- 0.04), while the correlation between BWT and WWT for maternal effects was positive (0.54 +/- 0.09), showing that larger calves at birth are less likely to die before weaning and have heavier weaning weights from maternal genetic contributions to these traits. Selection on maternal components of BWT and WWT should be accompanied by recording for teat and udder characteristics to assist in preventing any undesired correlated response in teat or udder size, which can have detrimental outcomes for calf survival, despite expectations of higher milk yield.