Sumario: |
Data on the reproductive traits of dairy heifers
were analyzed using event-time techniques. Traits
analyzed were age at first calving (n = 4631), days to
first breeding, and days open (n = 1992) during the
first 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 model for days open and days to first breeding
included two additional fixed effects of herd and
heifer milk yield at 100 d. A significant effect of heifer
weight category on age at first calving was found. The
chance of calving was consistently higher for herds
and heifers with higher body weight at 390 d and
decreased linearly from the top to the lowest quar-
tiles. The effects of herd weight category on days to
first breeding and days open were significant. Heifers
in herds with a higher average body weight were less
likely to be bred, and heifers in herds with lower
average body weight were less likely to get pregnant.
The effect of heifer weight category on days to first
breeding or days open was not significant. The effect
of herd milk yield on days to first breeding was sig-
nificant. Heifers in herds with lower yield were more
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 was found for the
estimates of the hazard ratios. The chance of a heifer
being bred and becoming pregnant was similar among
the first three quartiles and was lower for heifers in
the lowest quartile. The probability of a heifer reach-
ing a first calving can be improved by increasing the
body weight at 390 d. Body weight at 390 d did not
appear to have a large effect on reproductive perfor-
mance 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.
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