Pregnancy rate, estrous cycle lenght, serum progesterone and luteinizing hormone (LH) concentrations were determined in intramuscularly GnRH (10.5 μg synthetic gonadotrophin releasing hormone agonist, receptal) administered cows on day 12 post-mating (n=9) compared to control cows (n=8). Their oestrous cycles were synchronised by intramuscular administration of prostaglandin F₂ alpha (its analog, cloprostenol) twice at 11 dayS intervals. Estrous exhibited cows were mated naturally. Blood samples were collected every two days from all animals. Serum progesterone and LH concentrations were measured by ELISA method. GnRH administration significantly increased serum LH concentration which reached peak levels 2-3 h after treatment. However, serum progesterone concentration was not affected. There were no differences in mean progesterone concentrations on days 12 to 24 post-mating between GnRH administrated and control pregnant cows. However, in non pregnant animals, progesterone concentrations on days 16 in the treated group were lower than control group (P<0.01). Pregnancy diagnosis in animals made by B-mode ultrasonography between the 30^th and 35^th day showed that 77.7% of treated cows were pregnant compared to 50% in control group. Duration of the estrous cycle in the non-pregnant animals was not affected by the treatment (control, 21.3 ± 0.8 days; treated, 22.5 ± 0.5 days). In conclusion, this study supports the use of GnRH on day 12 post-mating as a method for enhancing pregnancy rates in lactating dairy cattle.

Key words: Twinning rate, year and season, age at first calving, heritability, Holstein.