Articles | Volume 59, issue 3
Original study
13 Sep 2016
Original study |  | 13 Sep 2016

Genetic analysis of productive life length in Holstein dairy cows using Weibull proportional risk model

Hamed Amirpour Najafabadi, Saeid Ansari Mahyari, Mohammad Ali Edriss, and Eva Strapakova

Abstract. In recent decades, there has been a downward trend in length of productive life (LPL) in Holstein cows across industrial dairy herds. This study examined the factors that might influence LPL and estimated the genetic parameters of LPL in Holstein dairy herds in Isfahan province, Iran. LPL is defined as the number of days between the first calving and the end of recording. Data consisted of 35 137 records of productive life from registered cows that started first calving between 1991 and 2012. Cows that remained alive at the end of the study were considered right-censored. The average lifetime for culled and censored cows was 938 and 1003 days, respectively. A survival analysis was applied using a proportional risk model with a Weibull distribution. Milk production was divided into five groups, where the culling risk of cows with a milk yield of less than 1.5 standard deviations (SD) of the mean was 3.5 times greater than the culling risk of high-producing cows producing more than 1.5 SD above the mean. Results showed that culling risk increased almost linearly beyond the milk production groups. Furthermore, somatic cell count and age at first calving significantly increased the culling risk across the herds. The results for the combined effect of parity  ×  stage of lactation showed a decrease in culling risk during the first calving, and an increase during the further parities. Moreover, a higher age at first calving was observed, reflecting a lower risk of culling. Estimated heritability were 0.074 and 0.18 based on a logarithmic scale and original scale, respectively. According to the results, use of Weibull models showed that the proportional culling risk was higher in low-production cows, but a higher risk ratio was revealed in high-milk-production cows. However, there were some fluctuations in genetic trends, but an overall increase was observed in LPL which will lead to a longer LPL of Holstein cows in Isfahan province.

Short summary
This study aimed to use survival analysis and examine the factors affecting functional herd life (FHL) of cows in Isfahan using the Weibull proportional risk model in Isfahan to estimate of breeding values for functional herd life. The SC, milk production, age at first calving and milk period showed significant effects on LPL (P < 0.001) according to the likelihood ratio test. Although there were some fluctuations in genetic trends, an overall increase was observed which leads to a longer FHL