The Effect of Progesterone and Oestradiol Benzoate on Fertility of Artificially Inseminated Repeat-breeder Dairy Cows during Summer

An experiment was conducted to examine the effect of progesterone (P(4)) and oestradiol benzoate (ODB) on fertility of repeat-breeder lactating dairy cows during summer. One hundred repeat-breeder lactating dairy cows were randomly allocated to four groups (Tr1, Tr2, Tr3 and C) in a study conducted at a private dairy farm. All cows were injected with 2 mg ODB (day 0), which were at random stages of their oestrous cycles. Cows in Tr1, Tr2 and Tr3 were administered with intravaginal progesterone-releasing devices (controlled internal drug-releasing, CIDR) at the time of ODB injection for 7 days and those in group C were untreated and served as controls. Following CIDR removal, all cows were given an intramuscular injection of 25 mg Prostaglandin (PGF(2 alpha)). Twenty-four hours after the PGF(2 alpha) injection, cows in Tr1, Tr2 and C groups were injected with 1 mg ODB. Cows in Tr3 group were injected with 10 microg gonadotropin-releasing hormone (GnRH) agonist 48 h after CIDR removal. Artificial insemination was performed between 24 and 30 h following the second ODB injection for cows in Tr1 group and at the time of GnRH injection for cows in Tr3 group. Cows in Tr2 and C groups were inseminated at detected oestrus. Plasma P(4) and oestradiol 17beta (E(2)) concentrations were determined for all cows daily from day 0 to day 9. Plasma concentrations of P(4) and E(2) among cows of groups Tr1, Tr2 and Tr3 were increased and reached maximum values within 48 h following administration and were greater (p < 0.001) than those of group C cows. The proportion of cows detected in oestrus based on P4 concentration on day 9 was 88%, 72%, 88% and 60% in groups Tr1, Tr2, Tr3 and C, respectively. Oestrous detection rate differed (p < 0.01) significantly between time-inseminated groups (Tr1 and Tr3) and those inseminated at observed oestrous (Tr2 and C) groups. Pregnancy rates based on ultrasonography performed on day 28 were 52%, 56%, 60% and 40%, and those based on rectal palpation on day 45 were 32%, 44%, 36% and 28% for Tr1, Tr2, Tr3 and C cows (p > 0.1), respectively. Whereas pregnancy rates for cows with four or more previous services in all groups (54.55%) were higher (p < 0.03) than those for cows with three previous services (29.49%). In pregnant cows, mean days from calving to the day of insemination were higher (p < 0.01) among cows with four or more previous services (204 +/- 8.0 days) than those with three previous services (157 +/- 6.0 days). Results indicate that treatment with a combination of ODB and CIDR in repeat-breeder dairy cows causes elevation in plasma concentrations of E(2) and P(4). Oestrous detection rate was better in cows that were primed with P(4) than those without P(4) priming. Cows with four or more previous services had significantly higher pregnancy rates than those with three previous services.