Phenotypic relationships between the interval from calving to first luteal activity and fertility in a seasonal dairy production system

In seasonal dairy production systems a high level of reproductive performance is required to maintain a 365-day calving interval. Compared with many countries New Zealand dairy cattle are fertile, with an average calving interval of 368 days and first service conception rates of around 55%. However, fertility has been demonstrated to have declined over the past 15-years. An extended interval from calving to first luteal activity (CLA) has previously been identified as a major cause of infertility in New Zealand dairy cows. This paper investigates the phenotypic relationship between CLA and reproductive performance in a population of second cross Friesian x Jersey dairy cattle. The data consists of 1780 lactations, from 801 individuals that were managed in a seasonal, pasture-based dairy production system. Overall, reproductive performance in the herd was high, with first service conception rates of over 60% and more than 80% of the herd pregnant within 42-days of the start of the breeding period. Animals were divided into six groups of at least 270 animals per group based on CLA (<= 16, 17-20, 21-26, 27-34, 35-50, >50 days). First service conception rates and the interval from the start of mating to conception were not statistically different between the CLA groups (p > 0.05), despite the number of oestrous cycles prior to first mating being inversely proportional to CLA. However, only 47% of animals with a CLA of >70 days (n = 88) were pregnant within 21-day of the start of mating, compared with 65% of animals with CLA of <70 days. Animals that had not ovulated by I-week prior to the start of mating also had reduced reproductive performance (45% versus 67% pregnant within 21-days of the start of mating, p < 0.001). The poor reproductive performance of animals with CLA > 70 days is likely to be due to a lower proportion of animals having ovulated by the start of mating (0.28 versus 0.95, p < 0.001). Contrary to previous findings in non-seasonal production systems the length of CLA was not related to fertility, except when CLA was greater than 70 days. (c) 2006 Elsevier B.V. All rights reserved.