The effect of an injection of GnRH during insemination on days open and conception rate in Iranian Holstein dairy cows

Abstract

Getting cows pregnant is the major goal in any dairy farm’s reproductive program. There has been alot of research done to determine the cost of number of days open after a cow’s voluntary waitingperiod following calving. The range varies from $3.00- $6.00 per day open per cow. With theaverage cow needing to be inseminated about 3 times to conceive, the sooner she can be exposed tothe chance to becoming pregnant the better. Gonadotropin releasing hormone (GnRH) is a decapetidethat regulates both lutenising hormone (LH) and follicle stimulating hormone release in farm animals.Gonadotropin releasing hormone has been used to induce the release of LH in lactating dairy cows.The postpartum period in cattle is characterised by a low frequency of pulsatile LH. This pulsatilerelease of LH is relatively low immediately after parturition (~0.5 ng/mL) and its concentrationincreases near the time of first ovulation. Exogenous GnRH can induce LH release in cycling cows,and an injection of exogenous GnRH can be used to promote ovarian activity.This study was carried out in the Dasht dairy farm, which is located in the northeast of Iran. Totalmixed rations for milking cows were calculated according to NRC (1989) and cows were milked 3times a day at regular intervals. We determined the effect of an injection of GnRH after inseminationon days open (DO) and conception rate (CR), and investigated the reasons for the prevention ofovulation in Iranian Holstein dairy cows (especially high producing cows).Ninety cows were divided into 2 similar groups. Cows that did not show heat until 38 ± 4 dayspostpartum were selected, and 2.5 mL PGF2α were injected before they were allocated randomly to 1of 2 treatments. They also received a PGF2α injection again 14 days later if they didn’t show heat. Intreatment 1, during artificial insemination, cows received 5 mL of GnRH, while treatment 2 was thecontrol. Pregnancy tests were undertaken 40-45 days after insemination by rectal palpation. Parity,adjusted 305 day milk production, conception rate (CR) and days open (DO) were measured for eachcow and analysed by general linear procedures and ANOVA to evaluate differences amongexperimental groups. Means were compared using Duncan test.