Effects of milk yield, and calving to first service interval, in determining herd fertility in dairy cows

Fertility and 305-day milk yield data, comprising 3527 lactation records over a period of 2 to 8 years from eight dairy herds, were used to assess the interrelationship between different calving to first service (C-S) intervals and levels of production. Lactating cows were placed into low and high yielding groups with due consideration to parity. Fertility parameters such as first service to conception (S-C) interval, calving to conception (C-C) interval and serves per conception for cows conceiving (S/C) were significantly lower (P < 0.001) for low yielding cows than high yielding animals. Low yielding cows had higher first service conception rate (FSCR) and overall conception rate (OCR). Overall, the high yielding cows conceived 19 days later after calving than low yielding herd-mates and required 0.27 extra inseminations per conception. The differences were even greater for those cows inseminated within 80 days of calving, i.e. approximately 25 days longer from calving to conception with 0.42 extra inseminations needed for the high yielding cows. With the increase of C-S interval, the FSCR of low yielding cows increased steadily while there was a sudden step-wise increase in high yielding animals; the differences were highly significant (P < 0.001) for the cows served within 60 days of calving but were more consistent for the matings after 100 days. Within high and low yielding groups, the probability of conceiving to each successive service did not differ significantly. To conclude, the inherent genetic potential of milk production of individual cows may lower their fertility.