Analysis of factors affecting the length of productive life in Croatian dairy cows
Keywords:
dairy cows, length of productive life, survival analysis, risk ratioAbstract
An analysis of factors affecting productive life of Croatian dairy cows was carried out using a proportional
hazard model based on Weibull distribution, using Survival Kit 6.0. The data consisted of 62,877 Simmental and
43,653 Holstein cows collected from October 1, 1992 until December 11, 2009. Cows alive at the end of the
study (20.1% Holstein, 19.4% Simmental) were considered as right censored. The probability of being culled
was assigned as a product of the baseline hazard function and the function of explanatory variables. The factors
that most affected the length of productive life were parity, year * season and milk production level within a herd,
all time dependent variables. The risk decreased with parity. Animals in first parity were most exposed to risk of
culling, that is about 21 times (Holstein) and about 14 times (Simmental) more than the animals in fourth parity,
which was assigned as a reference class. The year * season effect showed linear increase of risk to be culled,
with a peak year 2007. Relation between milk production level and the risk ratios indicated slightly non-linear
trend. Very low producing animals (less than 1.5 standard deviations below average) were at 1.311 (Holstein)
and 1.265 (Simmental) times higher risk to be culled than the average producing cows. At the same time, the
Simmental cows from small herds (<5 cows) were at even higher risks of being culled according to their milk
production. Results of the herd size effect indicated increase of culling risk in bigger herds under assumption of
higher selection intensity. The effect of age at first calving, as well as effect of region had lowest influence on
the length of productive life.
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