Evaluation of the lytic response of prostaglandin F2 alpha in Zebu cattle based on serum progesterone

To evaluate by means of blood progesterone levels and estrous expression the effect of the injection of 25 mg of PGF2alpha in Zebu cows, 17 Indobrazil cows with a palpable corpus luteum (CL) were injected with PGF2alpha and blood samples taken every 6 h in Experiment I. In Experiment II, 15 cows from the previous experiment at the same stage of diestrus were injected with PGF2alpha and bled every 4 h for 5 d. Progesterone levels had declined by 50% 6 h after injection in all cows and dropped to below 1 ng/ml by 30 h. Estrus was observed in 47% of the animals in Experiment I and 60% in Experiment II. Ovulation was detected accurately in 29% of the animals at 33.6 +/- 11.6 h after the onset of estrus. The correlation between finding a CL by rectal palpation and levels of progesterone higher than 1 ng/ml of blood was 86 and 93% in Experiments I and II, respectively. These studies indicate that the injection of 25 mg of PGF2alpha is sufficient to produce luteolysis in Zebu cattle, although estrous expression is poor.