Evidence of genetic and maternal effects on secondary sex ratio in cattle

There is a paucity of estimates of genetic variation for secondary sex ratio (i.e., sex ratio at birth) in dairy cattle. The objective of this study was to estimate the direct and maternal genetic variance as well as maternal permanent environmental variance for offspring sex in dairy herds. The data consisted of 77,508 births from 61,963 dams and 2,859 sires in 1,369 Irish dairy herds across the years 2003 to 2008, inclusive. Mixed models were used to estimate all parameters. Significant genetic variation in sex ratio existed, with a heritability for secondary sex ratio estimated at 0.02; the genetic standard deviation was 0.07 percentage units. No maternal genetic effects on secondary sex ratio were identified but the proportion of phenotypic variance in secondary sex ratio attributable to maternal permanent environmental effects was similar to that attributable to the additive genetic variance (i.e., 0.02). These results, therefore, suggest that the paternal (genetic) influence on secondary sex ratio is just as large as the maternal (non-genetic) influence, both of which are biologically substantial. The results from this study will be useful in generating a sample population of divergent animals for inclusion in a controlled experiment to elucidate the physiological mechanism underpinning differences in secondary sex ratio.