عنوان مقاله [English]
Introduction One of the important breeding goals in dairy cattle is increasing length of productive life (LPL). In the recent decades, genetic evaluations of dairy cattle longevity have been a major concern for breeders. The trait LPL is defined as the number of days from the first calving to culling, death or censoring. Increasing LPL by reducing the costs of replacement of the heifers and increasing the number of high producing cows plays an important role in increasing the herd incomes and profitability.
Materials and Methods This study aimed to evaluate genetic variations for LPL based on the survival analysis models was used to evaluate the impact of environmental and genetic factors on the risk of culling and to estimate the genetic parameters for longevity in Holstein dairy herds. Data included 35,137 records of productive lifetime from the first calving during 1991 and 2012, collected from dairy herds in Isfahan province. Culled and un-culled animals were assigned as uncensored and censored cows, respectively. However, it may be of interest to distinguish between disposal mostly beyond the control of dairy managers such as the sale of profitable but sterile can (involuntary culling) and voluntary disposal of a healthy but not profitable cow. The number of observations was considered with at least 20 records per herd and at least 10 daughters per sire. The last lactation was considered for the animals whose culling date was missed. In this case, cow assigned as culled animal only if the time interval between end of the last lactation and date of recording exceeds 365 days. Three types of cows were excluded in this study: sold, without any records and transferred to other herds. The sires with one daughter in a herd were removed. Genetic parameters were estimated based on a sire model which was implemented in Weibull model in Survival Kit software Survival analysis using proportional hazard model was used to analyze data on LPL. The existence analysis models are the best for the genetic PL evaluation; these models are referred to as the Proportional Risk Models, which are categorized in two semi-parametric Cox and Weibull. Following the designed algorithm in this software, the records with known longevity and low FHL limit were used. Hence, the records were considered uncensored data if the cows were either culled or died for any reason. Therefore, censoring the records represented the cows were sold, exported or leased to other herds. Both Cox and Weibull models were implemented in Survival Kit, and they could be used for continuous and discontinuous (time-dependent) variables.
Results and Discussion The average lifetime in uncensored and censored cows were 937.8 and 1002.8 days, respectively. It is obvious that some cows are culled due to calving difficulties on day one, therefore LPL of One day is considered for them. Heritability could change based on the estimates of ρ and scale (λ). Estimates of heritability of LPL according to logarithmic scale and original scale were 0.074 and 0.18, respectively. In many studies on different populations, the heritability evaluated through survival analysis is higher than what is determined through linear models. Regression of phenotypic changes was -0.03±0.01, which showed that the reduction of relative culling risk has occurred slowly across the studied herds. The genetic trends of culling risk showed that regression coefficient was close to zero and therefore, it can be concluded that according to variance of the estimated breeding values in LPL, it would be possible to increase LPL by selecting the high ranked cows. The range of culling risk were calculated from 0.96 to 0.99. An attempt to estimate the genetic trend for sires was made by grouping sires according to their year of birth. Besides, negative phenotypic trends in this study for the proportional culling risk was achieved which demonstrated that LPL was phenotypically improved but based on the genetic trend, an increase in culling risk was observed that indicated a genetically decreasing in productive lifetime in studied dairy herds. More research is needed to analyze more data in other dairy farms in Iran.
Conclusion Based on the variation of the obtained breeding value, it is possible to increase the lifetime of cows via selecting the higher breeding value cows.