A previous work reported the importance of a sire effect in the modeling of length 
of productive life of Mexican Holstein cows with a Weibull proportional hazards model. 
Another approach to this problem was taken in this paper which consisted of using a 
frailty model, assuming a distribution for the random frailty term, estimating the 
distribution parameters, estimating individual frailties through their expectations and 
.comparing these to the sire effects of the Weibull model. The results showed a good 
agreement between individual frailty terms and sire effects and further reinforces the 
possibility of genetic improvement of length of productive life in this population.

F. J Ruiz, B. Trejo-Valdivia

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 17. Genetics and breeding of dairy and beef cattle, swine and horses, , 101–104, 1994
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