Endocrine and ovarian responses and pregnancy rates in dairy cows following the administration of a gonadotrophin 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 (AI), or 12 days later and the effect on serum concentration of luteinizing hormone (LH) and progesterone, follicle populations, time of ovulation and pregnancy rate was measured. GnRH-A at AI induced a secondary LH 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 12 after AI induced a surge of LH, increased serum concentration of progesterone (P<0.05), decreased the number of large (≥ 10 mm diameter) follicles (P<0.05) and increased the inter-oestrus interval (P<0.06). Pregnancy rate in 1661 cows was not affected by administration of GnRH-A at AI, 12 days later or following AI at the first repeat. In conclusion, treatment of lactating dairy cows at pasture in a temperate climate with GnRH-A, either at AI or on Day 12 after AI, altered endocrine responses and ovarian follicle populations, but did not affect pregnancy rate.