Abstract:
Stillbirth is a trait of high economic importance in dairy cattle and is increasingly being
included in dairy cattle breeding objectives worldwide. In South Africa, however, there
is limited information on stillbirth that can be used to improve this trait genetically.
Currently, there are no estimated breeding values (EBVs) for any measures of calving
performance produced under the national genetic evaluation programme. The current
study was, therefore, conducted to assess the incidence of stillbirth and estimate the
genetic and environmental influences on maternal effects for stillbirth in South African
Holstein cattle, to enable estimation of breeding values for the trait. Data used in the
study comprised 13 143 calving records of 7 723 Holstein cows, from 41 herds,
participating in the National Dairy Animal Recording and Improvement Scheme during
the period 2014 to 2018. Incidence of stillbirth was determined using the PROC FREQ
procedure and environmental effects were tested by the General Linear Models (GLM)
procedure of Statistical Analysis System (SAS 9.4, 2016). Maternal heritability of
stillbirth was estimated by the Restricted Maximum Likelihood (REML) procedure,
using the ASReml software (Gilmour et al., 2018). The analyses were carried out using
a threshold animal model and a repeatability animal model, where the latter
considered stillbirth in different parities as repeated measures of the same trait.
Environmental effects significantly influencing stillbirth (p<0.05) were herd-year season of calving, dam parity and calf sex, and these were included in the model for
variance component estimation. Estimates of maternal heritability effects from the
threshold animal model were 0.12±0.04, 0.15±0.08 and 0.14±0.06 for parities 1 to 3,
respectively. The repeatability animal model gave a heritability estimate of 0.09±0.03
and a repeatability of 0.18±0.03. The moderate estimates of maternal heritability
indicate scope for reducing incidence of stillbirth by selectively breeding cows that are
less genetically predisposed to calving dead calves. Stillbirth in different parities should
not be considered as the same trait, as indicated by the low repeatability estimate.
Results of the current study estimate genetic parameters that are required to compute
accurate estimated breeding values (EBVs) for stillbirth, which will enable South
African Holstein farmers to select for reduced stillbirths, thus improving calving
performance
