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 recor ds over a period of 2 to 8 years from eight dairy herds, were used to assess the interrelationship between different calving to first servic e (C-S) intervals and levels of production. Lactating cows were placed into low and high yielding groups with due consideration to parity. F ertility parameters such as first service to conception (S-C) interval , calving to conception (C-C) interval and serves per conception for c ows conceiving (S/C) were significantly lower (P < 0.001) for low yiel ding cows than high yielding animals. Low yielding cows had higher fir st service conception rate (FSCR) and overall conception rate (OCR). O verall, the high yielding cows conceived 19 days later after calving t han low yielding herd-mates and required 0.27 extra inseminations per conception. The differences were even greater for those cows inseminat ed within 80 days of calving, i.e. approximately 25 days longer from c alving to conception with 0.42 extra inseminations needed for the high yielding cows. With the increase of C-S interval, the FSCR of low yie lding cows increased steadily while there was a sudden step-wise incre ase in high yielding animals; the differences were highly significant (P < 0.001) for the cows served within 60 days of calving but were mor e consistent for the matings after 100 days. Within high and low yield ing groups, the probability of conceiving to each successive service d id not differ significantly. To conclude, the inherent genetic potenti al of milk production of individual cows may lower their fertility.