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
