Assessment of the influence of genetic and non-genetic factors on the gestation length in dairy cows (using the TTE analysis)

The primary objective of this study was to evaluate the impact of both genetic and non-genetic factors on the gestation length in dairy cows, employing the Time-to-Event analysis methodologies. The study utilised the data set on 237 primiparous Holstein cows maintained at PJSC “Stepnoy”, situated in Kamiansko-Dniprovskyi district, Zaporizhzhia region, during the period 2014–2016. The dependent variable, 'the gestation length', was calculated as the difference between the date of calving and the date of conception (in days) for each primiparous cow. The Time-to-Event analysis was employed to estimate median (and 95 % CI) for the gestation length for each subgroup of primiparous cows based on the levels of genetic and non-genetic factors. In addition, the null hypothesis of no effect of certain factor was tested based on the Hazard Ratio estimate and its 95 % confidence interval for the reference subgroup and each of the remaining subgroups. The results showed that for a total of 237 primiparous Holstein cows the gestation length ranged from 260 to 296 days with a mean of 277.8 ± 0.3 days. The survival curves for the gestation length and the median of this trait were characterised by some features for primiparous cows of different sire-bull origins (Likelihood Ratio test: χ2 = 23.545; df = 13; P = 0.036). The Hazard Ratio analysis revealed that the subgroup of primiparous cows sired by Vaillant’s line bulls exhibited a significantly different pattern compared to animals sired by Elevation’s and Chief’s line bulls. The influence of the calving year on the gestation length in primiparous cows of the experimental group was not significant. However, the Hazard Ratio analysis demonstrated a significant difference between the subgroups calving in winter versus summer with regard to the calving season. Finally, increasing age at calving resulted in an increased to gestation length in dairy cows.