Plasma progesterone concentrations in cows and steers after Repositol Progesterone treatment

Repositol progesterone was injected intramuscularly into 4 steers and 17 postpartum cows at a dose of 2.2 mg/kg body weight. In steers the plasma progesterone concentration rose to 6.8 ng/ml 1 day postinjection and declined to a mean concentration of 2.1 ng/ml on days 3 and 4 postinjection. Postpartum, presumed anestrus, cows reached a lesser progesterone concentration of 2.9 ng/ml 1 day postinjection and then declined to a lower mean of .7 ng/ml on days 3 and 4 postinjection. These data suggest different rates of metabolism and/or excretion of progesterone by cows and steers.