THE APPLICATION OF EVENT-TIME REGRESSION TECHNIQUES TO THE STUDY OF DAIRY-COW INTERVAL-TO-CONCEPTION

This paper explains the use of event-time techniques, also known as su rvival analysis, to assess a relationship involving dairy cows: the ef fect of early lactation milk yield on the interval from calving to sub sequent conception. Possible confounding factors, such as disease hist ory, parity, calving season, and herd production level, were controlle d by their inclusion in the model equations. Both Cox proportional haz ards regression models and Weibull regression models were employed, wi th similar results, indicating that a Weibull distribution may be a go od approximation of the distribution of conception times in dairy catt le.