Conception rate in dairy cattle treated with progesterone five days after insemination

Temporary increase in progesterone serum levels between days five and nine after insemination enhances embryo development and interferon-tau secretion. In this experiment, it was tested whether an injection of progesterone on d 5 after insemination increases conception rate (CR) in dairy cattle. Five hundred and sixty one (561) first-and second-service cows were used. Cows were observed for estrus and were inseminated under the "am-pm/pm-am" program. Five days later, they were randomly assigned to two treatments: P4 (n=282), cows received 500 mg of progesterone im; Control (n=279), they did not receive progesterone. Gestation diagnosis was performed by rectal palpation on d 45 after insemination. In order to know the blood profile of progesterone after its injection, five non-inseminated cows with corpus luteum were selected, which received two PGF2 alpha injections at 24-h interval; 24 h after the second PGF2 alpha injection, cows received 500 mg of progesterone im, blood samples were daily taken for the next 6 d and progesterone concentrations were assessed. There was no difference (P>0.10) between P4 and control groups in the CR (44 vs 41 % pregnant). Cows from group P4 which returned to estrus showed a CR higher (P<0.05) than cows from control group (P4=45 %; 31/69 vs control=31 %; 31/99). Progesterone concentrations were higher than 1 ng/ml between 48 and 72 h after treatment. It was concluded that progesterone administration 5 d after insemination did not affect CR; however, this treatment increased the conception rate in animals returning to estrus.