Effect of season of birth on milk, fat, and protein production of Israeli Holsteins

The effects of birth month on production of milk, fat, and protein and percentages of fat and protein were analyzed based on production records of 101,653 first parity, 77,541 second parity, and 51,856 third parity Israeli Holstein cows. Each parity was analyzed separately. The analysis model also included the effects of herd-year, DIM, calving age, and calving month. First parity Type III sums of squares for birth month were nearly as large as those for calving month but decreased for later parities. Similar results were obtained using multiplicative models in which the dependent variables were the logarithms of the production traits. The effects of calving month and birth month were not similar, but birth month had similar effects for milk, fat, and protein production. Production was lowest by cows born in the early spring and highest by cows born in the fall. Analyses of the log-transformed traits showed that the F values for calving month were greater than, and the F values for birth month were nearly identical to, the F values for the untransformed trait analyses. The physiological basis for these trends was not clear.