Genetic evaluation of calving traits in the United Kingdom
The consequences of complications during calving are currently costing the UK dairy cattle industry approximately £110 for a calving of moderate difficulty and £400 for one that is severe. With incidences of difficult first calvings reaching 24% and 4% for a moderate and severe difficult calving respectively, reducing calving complications would be of great benefit to the UK dairy cattle industry. This PhD evaluates (i) the importance, (ii) the most optimal way, and (iii) the potential consequences of genetically selecting for two main traits associated with calving complications, calving ease (CE) and stillbirth (SB). For this, approximately 50.000 first parity and 300.000 later parity national calving data records were kindly provided by two major milk recording organisations in the UK. The work carried out shows that detrimental effects following a difficult first calving are long-lasting as subsequent performance of both the dam and the calf involved is worsened. Fertility of the dam is negatively affected by a difficult calving resulting in an increased calving interval and decreased ability to conceive. A reduction in milk production of a veterinary assisted dam, compared to a non-assisted dam, was detected in the first part of lactation. Veterinary assisted born calves showed a significantly lower milk yield, compared to farmer assisted calves, throughout their first lactation as adult heifers suggesting that the physiological effects, or causes underlying a difficult birth, are long lived. The study advises that genetic parameters of calving traits are to be estimated with an extended sire multi-trait model (accuracy vs. practicality). On average, direct and maternal heritabilities for calving traits are low. A highly heritable indicator trait such as gestation length (GL) can aid in the estimation of genetic parameters for CE and SB although genetic correlations of these traits with GL are moderate. A significant negative genetic direct-maternal correlation was found for CE in first parity recommending farmers to consider both direct and maternal genetic merit for CE when making a selection decision. CE and SB are highly positively correlated traits. GL is maternally correlated to SB in first parity and directly to CE in later parities, both correlations are positive. Estimated genetic correlations with other important traits in dairy cattle breeding show that CE and GL are both directly and maternally related to important selection traits which need to be taken into account if implementation into breeding indices occurs. Results advise limiting the use of GL to benefit the prediction of parameters for low heritable calving traits rather than selecting on GL itself. Genetic correlations suggest that individuals born easily are genetically prone to high milk yield and have reduced fertility in first lactation. Difficult calving heifers are likely to be associated with being wide and deep and high producing animals with a reduced ability to subsequently conceive. Individuals that are born relatively early are associated with good genetic merit for milk production. And finally, individuals carrying their young longer are genetically associated with being wide and large animals that were born relatively early themselves. Lastly, an extension of the current univariate genomic model to a bivariate model, which allows for a possible genetic direct-maternal covariance, shows that improvement in accuracy of genomic breeding values can be gained from use of a bivariate genomic model for maternal traits such as CE. Further development of the model is however recommended prior to the publication of genomic proofs for CE or any other maternal trait. Genetic selection can serve as a tool in the reduction of difficult calvings. The results of this thesis serve to let this happen in a controlled and sustainable manner.