ENDOCRINE AND OVARIAN RESPONSES AND PREGNANCY RATES IN DAIRY-COWS FOLLOWING THE ADMINISTRATION OF A GONADOTROPIN-RELEASING-HORMONE ANALOG AT THE TIME OF ARTIFICIAL-INSEMINATION OR AT MID-CYCLE POST INSEMINATION

Gonadotrophin releasing hormone analogue (GnRH-A) was administered to lactating dairy cows at the time of first artificial insemination (Al) , or 12 days later and the effect on serum concentration of luteinizin g hormone (LH) and progesterone, follicle populations, time of ovulati on and pregnancy rate was measured. GnRH-A at AI induced a secondary L H surge, suppressed serum concentrations of progesterone (P< 0.05) on Days 3-5 and did not affect the interval to ovulation. GnRH-A on Day 1 2 after AI induced a surge of LH, increased serum concentration of pro gesterone (P< 0.05), decreased the number of large (greater than or eq ual to 10 mm diameter) follicles (P< 0.05) and increased the inter-oes trus interval (P< 0.06). Pregnancy rate in 1661 cows was not affected by administration of GnRK-A at AI, 12 days later or following Al at th e first repeat. In conclusion, treatment of lactating dairy cows at pa sture in a temperate climate with GnRH-A, either at Al or on Day 12 af ter Al, altered endocrine responses and ovarian follicle populations, but did not affect pregnancy rate.