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 FriesianxJersey 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 (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 1-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.