Genetic analysis of some maternal reproductive traits in the Belgian Blue cattle breed.
The reproductive traits analysed from female calves born in the period 1978-81 were as follows: body measurements (chest girth and height at withers) at 1, 2, 3 and 4 yr of age; percentage culled for infertility; proportion of calvings resulting from a single insemination; number of inseminations per gestation; ages at 1st and 2nd calving; calving interval; milk yield score. For these traits, the differences between the two biological types (double-muscled and conventional) were estimated. The beneficial effects of the double muscling on carcass composition and commercial value represented several phenotypic standard deviations, whilst the detrimental effects on the fertility traits were only a fraction of the phenotypic standard deviation of these traits. The h2s at 1, 2 and 3 yr of age were 0.042±0.014, 0.111±0.030 and 0.133±0.038 resp. for chest girth, and 0.066±0.019, 0.118±0.032 and 0.238±0.060 for withers height. The values for ages at 1st and 2nd calving were 0.030±0.013 and 0.044±0.021 resp., for calving interval 0.031±0.019, and for milk yield score in the 1st and 2nd lactations 0.078±0.025 and 0.105±0.035. The genetic correlations between successive body measurements were large and positive. Those between the ages at calving and body measurements or milk yield score were negative, and thus also favourable.