Evaluation of reproductive performance of Holstein bull daughters using statistical methods of survival analysis

The study was conducted to test a method for genetically evaluating sires based on the length of the daughters’ service period using a mixed Cox proportional hazards model. The work was performed on retrospective breeding data for 2002-2022. The sample included information on 22,087 lactations of daughters of 42 Holstein sires (at least 100 lactations per sire). The time variable was the length of the service period (days from calving to successful insemination). Observations were censored if an animal was discarded before pregnancy or if there was no pregnancy after 305 days post-calving (event rate of 0.71). The mixed Cox model included fixed effects (lactation number, year and month of calving, calving ease) and a random effect (sire). The between-group variance for the random factor was 0.0243 (p < 0.001), the 50% median service period for the sample was 138 ± 1 days. Paratypic factors had a significant impact: the median decreased from 177 days in 2012 to 126 days in 2021, and the use of obstetric assistance increased this indicator by 12 days, compared to normal calving. Based on the assessment of the chances of pregnancy, 14 bulls were classified as improving reproductive quality, 13 – as worsening. The daughters of the best sire (Fortune 12302930) had a 43% higher probability of becoming pregnant during lactation than the herd average (odds ratio = 1.43). The Spearman rank correlation coefficient between the obtained estimates and the Canadian Daughter Fertility Index (Lactanet) was p = 0.525 (p < 0.001). The use of a mixed Cox model ensures the calculation of accurate, unbiased estimates of bull breeding value based on fertility, taking into account paratypic factors. Integration of the proposed approach into the national evaluation system will improve the accuracy of animal selection based on reproductive traits.

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