Factors associated with pregnancy and growth rates in New Zealand replacement dairy heifers

AIMS: To investigate factors associated with pregnancy rate in a number of herds of contract rearing replacement dairy heifers. These factors included the average daily gain of a heifer mob, the average bodyweight of the mob at the start of grazing, the number of heifers in the mob, the length of the grazing period, the breed of heifers, the grazing season and the region from where the heifers originated. METHODS: The data included 21,061 heifers from 111 farms in the five North Island regions of Hawke’s Bay, Manawatu, Taranaki, Waikato and Wanganui. The heifers were part of a contract rearing programme where they were raised for a period of time on farms different to their farm of origin with a goal of obtaining high pregnancy rate and a maximum age at first calving of 24 months. Heifers were weighed at arrival post-weaning. A multivariate logistic regression model was fitted to predict mob-level pregnancy rates. RESULTS: A significant association was observed between the odds of pregnancy and average daily gain and heifer breed. Pregnancy in Holstein-Jersey crossbreed was 2.1 (95% CI 1.9 – 3.2) times that of Ayrshire, 1.2 (95% CI 1.1 – 1.6) times that of Holstein and 1.3 (95% CI 1.1 – 1.5) times that of Jersey heifers. An increase of one standard deviation in the average daily weight gain of a replacement heifer mob increased average pregnancy rates by 3.3% equivalent to an OR of 2.5 (95% CI 1.3– 4.9). CONCLUSIONS AND CLINICAL RELEVANCE: Post-weaning growth rate represents a major influence on the onset of puberty which in turn determines the suitability of heifers for mating when breeding starts. Improving the post-weaning average daily gain of replacement herds, especially Holstein-Jersey crossbred heifers, is therefore expected to improve pregnancy rate. Factors (e.g., management, environmental, etc.) associated with different grazing seasons should be investigated to determine potential associations with pregnancy rate.