EVENT-TIME ANALYSIS OF REPRODUCTIVE TRAITS OF DAIRY HEIFERS

Data on the reproductive traits of dairy heifers were analyzed using e vent-time techniques. Traits analyzed were age at first calving (n = 4 631), days to first breeding, and days open (n = 1992) during the firs t lactation. A proportional hazard model was used that included fixed effects of herd-year, year-season, breed type, herd weight, and heifer weight. Body weights were recorded at 390 d of age, on average. The m odel for days open and days to first breeding included two additional fixed effects of herd and heifer milk yield at 100 d. A significant ef fect of heifer weight category on age at first calving was found. The chance of calving was consistently higher for herds and heifers with h igher body weight at 390 d and decreased linearly from the top to the lowest quartiles, The effects of herd weight category on days to first breeding and days open were significant. Heifers in herds with a high er average body weight were less likely to be bred, and heifers in her ds with lower average body weight were less likely to get pregnant. Th e effect of heifer weight category on days to first breeding or days o pen was not significant. The effect of herd milk yield on days to firs t breeding was significant. Heifers in herds with lower yield were mor e likely to be bred. The effect of heifer milk yield category on days to first breeding and days open was significant, but no linear trend w as found for the estimates of the hazard ratios. The chance of a heife r being bred and becoming pregnant was similar among the first three q uartiles and was lower for heifers in the lowest quartile. The probabi lity of a heifer reaching a first calving can be improved by increasin g the body weight at 390 d. Body weight at 390 d did not appear to hav e a large effect on reproductive performance after first calving. High milk yield appears not to have a large negative effect on days open, at least for the milk yield levels analyzed in this study.