Abstract
A total of 2216 lactations, initiated between July 1973 and June 1976, of 1382 Holstein cows in 36 herds from the Nicolet, Rock Forest and St-Hyacinthe regions of Quebec were studied. All cows were bred exclusively by AI. Mean services per conception, percent conception on first service, days from calving to first service and days open were 1.97 services, 47%, 88 days and 121 days, respectively. Based on maximum likelihood estimates of cow and error variance components, repeatabilities of services per conception, days from calving to first service and days open were 0.04, 0.09 and 0.07, respectively. Cows calving in spring had better reproductive performance than those calving during other times of the year. Reproductive efficiency was lower among both 2-yr-old and mature cows compared to intermediate aged cows. Lower reproductive efficiency was also associated with cows at high milk production levels. As herd size increased, conception rates declined. Herds feeding at high levels of net energy had higher conception rates than herds with low and medium net energy intake levels. Dairymen who managed their labor efficiently for milk production had herds with high reproductive efficiency.
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