Effect of PG F2 alpha treatment on conception rates of dairy cows treated with a GnRH agonist 12 to 14 days after Artificial Insemination

The effect of a GnRH agonist (10 ug Buserelin) on conception rate was determined when injected into dairy cows 12 to 14 days after Artificial Insemination (AI). The following factors were taken into account: previous treatment prior to AI with Prostaglandin F2 alpha, clinical history recorded prior to AI since last calving, parity and milk yield. A number of 118 cows, from one large dairy herd, were involved in this study. A total of 210 AI's were performed, followed by 140 GnRH treatments and 70 saline injections. Reproductive events were then recorded. The rank of AI was equally distributed among the two groups. 153 AI's were preceded by a Prostaglandin F2 alpha (Dinoprost - 25 mg) treatment among which 103 were subsequently GnRH-treated and 50 were controls. Plasma progesterone concentrations were determined daily for 34 days after AI in 13 treated and 13 control cows. An Early Pregnoncy Diagnosis (EPD) from milk progesterone concentration was performed on Day 21 after AI in all cases. Post-AI GnRH agonist treatment resulted in a significant enhancement of the conception rate: 60% vs 44%; p < 0.01, respectively for treated and control animals. PG F2 alpha treatment prior to GnRH injection had a major influence on the conception rate (62% vs 40%; p < 0.01). No effect was seen (54 % vs 55 %) in non-PG F2 alpha treated females which were subsequently injected respectively with GnRH or saline after AI. Previous clinical reproductive disorders, parity and milk yield had no significant effect. In non-pregnant treated animals, GnRH agonist treatment resulted in an increased rate of heat detection on days 20 - 25 after AI (91 % vs 74 %) and a higher fertility rate on the following AI was seen (59 % vs 44 %; p < 0.05). In conclusion, GnRH agonist treatment 12 to 14 days after AI only enhanced the conception rate of females which had previously been treated with PG F2 alpha. In non-pregnant cows, this treatment had also a benficial effect on heat detection and improved the conception rate at the subsequent AI.