Declining fertility in dairy cattle: changes in traditional and endocrine parameters of fertility

Reproductive performance of 714 Holstein Friesian dairy cows was monitored between October 1995 and June 1998 using thrice weekly milk progesterone de terminations. Defined endocrine parameters such as interval to post-partum commencement of luteal activity, inter-ovulatory interval and length of lut eal and inter-luteal intervals were used with a number of traditional measu res of reproductive performance to investigate the current status of fertil ity in a sample of United Kingdom dairy herds. A comparison of the results of the 1995 to 1998 trial with those of a previous (1975 to 1982) milk prog esterone database, which included 2503 lactations in British Friesian cows monitored using a similar milk sampling protocol, revealed a decline infert ility between these periods. Between 1975-1982 and 1995-1998, pregnancy rate to first service declined f rom 55.6% to 39.7% (P < 0.001), at a derived average rate approaching 1% pe r year This decline was associated with an increase (P < 0.001) in the prop ortion of animals with one or move atypical ovarian horn-lone patterns from 32% to 44%. There was a significant (P < 0.001) increase in the incidence of delayed luteolysis during the first cycle post partum (delayed luteolysi s type I; 7.3% to 18.2%) and during subsequent cycles (delayed luteolysis t ype II; 6.4% to 16.8%), although the incidence of prolonged anovulation pos t partum (delayed ovulation type I; 10.9% to 12.9%) and prolonged inter-lut eal intervals (delayed ovulation type II; 12.9% to 10.6%) did not alter sig nificantly. These changes resulted in an increase in mean luteal phase leng th from 12.9 (s.e. 0.09) to 14.8 (s.e. 0.17) days and an increase in inter- ovulatory interval from 20.2 (s.e. 0.1) to 22.3 (s.e. 0.2) days. The declin e in fertility was also reflected in traditional measures of fertility sinc e although interval to first service remained relatively unchanged (74.0 (s .e. 0.4) to 77.6 (s.e. 1.1) days) calving interval lengthened from 370 (s.e . 2.2) to 390 (s.e. 2.5) days. Collectively these changes may have contribu ted to the decline in pregnancy rates observed over the last 20 years.