The aim of the present study is to determine whether supplementing with exogenous P4 or the administration of gonadotrophin-releasing hormone (GnRH) or human chorionic gonadotrophin (hCG) during early luteal phase could increase conception rate (CR) in RB cows. 103 Holstein repeat breeder (RB) cows with a history of three to seven unsuccessful artificial inseminations (AI) and normal inter-estrus intervals, in a large dairy herd, were selected for the study. They were less than ten years of age and had no history of recent reproductive diseases. All of the selected cows were paired according to their parity, milk yield, number of Al services and body condition scores. Cows were divided into four groups: Group I cows (n=26) received 20g of the GnRH agonist, buserelin (Receptal; Intervet International, Holland) intramuscularly (TM) on day five to six post AI. Cows in group II (n=25) received an TM injection of 1500 IU hCG (Chorulon; Intervet International, Holland) 5-6 days post Al. In group III (n=25) a controlled internal drug release device (CIDR; Inter Ag, New Zealand) previously used for seven days was applied to each cow on day five to six after Al and removed ten days later. Group IV cows (n=27) received five ml of sterile normal saline solution IM as placebo the same time period after Al. Milk samples (15-20 ml) were collected five to six days after Al, just before treatment was administered for measuring P4 concentrations by an ELISA method. All cows were palpated per rectum for pregnancy diagnosis 45 days after AI. CR on day 45 was significantly lower in cows in groups one and four compared to groups II and III. There was no significant difference in CR between groups one and four. There was also no significant difference between groups II and III. There was no significant difference in milk fat P4 concentrations among four groups. Milk fat P4 concentrations in pregnant cows and non-pregnant cows did not differ significantly between the four groups. Milk fat P4 concentrations between pregnant and non-pregnant cows in each group were compared separately. It was in the second group (hCG) were P4 concentrations was significantly lower in pregnant cows Results of this study also showed that RB cows treated with hCG on five to six after Al had significantly higher CR. Human chorionic gonadotrophin induces the formation of accessory CLs and increases the concentrations of P4 during the mid-luteal phase. It also promotes three follicular wave oestrous cycles that may delay the occurrence of pre-ovulatory follicles around the time of maternal recognition of pregnancy. In the present study, GnRH administration on day five to six after insemination did not increase CR on Day 45. It seems that the timing of administration and the potency of GnRH, or both, may affect the ability of GnRH to alter luteal composition and subsequent P4 concentrations. In conclusion, treating RB cows with hCG or a used CIDR improved CR in this study.

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