Estimation of the Effects of Mutations Causing Complex Vertebral Malformation and Brachyspina on Milk Production, Milk Composition and Fertility Traits in Holstein Friesian Dairy Cattle

The frequencies of mutations with lethal effects are of significant economic importance in cattle production. The elimination from or at least management of such mutations in the national breeding herd is a desirable objective, however, estimation of the potential pleiotropic effects on other traits of economic importance would ascertain if strategic matings of carrier animals would be advantageous. Therefore, the objective of the current study was to estimate the effects of two such lethal recessives, Complex Vertebral Malformation (CVM) and Brachyspina (BY) on milk and fertility traits in Holstein-Friesian dairy cattle. CVM and BY SNP genotypes and phenotypes (expressed as predicted transmitting abilities (PTA)) on 10,707 dairy cows were obtained through the Irish Cattle Breeding Federation (ICBF). The association between each SNP and deregressed PTA was analysed in ASREML using a weighted mixed animal model. Only cows with an adjusted reliability of >10% were included in the analysis and included n=6876 for milk yield and composition traits and n = 1193, 264, 4566, 8564, 152 and 2380 cows for calving interval, survival, calving difficulty, gestation length, calf mortality and maternal calving difficulty, respectively. CVM (MAF 1.7 %) was associated with both increased milk protein (0.019, s.e. 0.006, p<0.01) and increased milk fat concentration (0.039, s.e. 0.0132, p<0.01) whereas no associations (p>0.05) were observed between CVM and any of the other milk traits (milk yield, milk fat yield, milk protein yield) or fertility traits (calving interval, survival, calving difficulty, gestation length, calf mortality and maternal calving difficulty). Significant associations were observed between BY (<1 %) and decreased milk protein concentration (0.024, s.e. 0.008, p<0.01) and increased milk yield (73.21 kg, s.e. 30.12, p<0.05). No associations (p>0.05) were observed between BY and any of the fertility traits considered. These results provide additional evidence that carriers of these recessive mutations exhibit effects on milk production and/or composition in Holstein Friesian cattle, however, with no evidence of effects on the fertility traits examined. Cognisance and monitoring of the potential pleiotrophic effects of lethal recessives such as examined in this study will aid livestock breeders when considering elimination of carriers to minimise reduction of the genetic merit of farm enterprises and inform the benefits of strategic matings in controlling these mutations in the population while also sustaining productivity.