DURATION OF ESTROUS CYCLES SUBSEQUENT TO 2 INJECTIONS OF PROSTAGLANDIN-F2-ALPHA GIVEN AT A 14-DAY INTERVAL IN NONLACTATING HOLSTEIN COWS

Data from previous experiments in our laboratory indicated that estrous cycles following synchronization with two injections of prostaglandin F2-alpha often averaged more than 21 d. To investigate whether this could be an effect of prostaglandin F2-alpha, 54 mature heifers and nonlactating Holstein cows were used in a crossover experiment in which estrous cycles either occurred spontaneously or followed two injections of prostaglandin F2-alpha at a 14-d interval (induced estrus). One estrous cycle of each type was recorded for each animal. Blood samples were collected twice weekly for analysis of progesterone. Cattle were observed twice daily for estrus with the aid of tail chalking and an androgenized cow. Estrous cycles following induced estrus were longer than those following spontaneous estrus (24.2 vs. 22.7 d). Ninety-two percent of the cycles following spontaneous estrus were 17 to 25 d in duration compared with 71% of the cycles following induced estrus. If the first prostaglandin F2-alpha was given when progesterone was greater than 1 ng/ml, the estrous cycle following treatment was longer than if the first prostaglandin F2-alpha was given when progesterone was less than 1 ng/ml (26.0 vs. 22.6 d). This experiment demonstrates that two injections of prostaglandin F2-alpha increase the incidence of estrous cycles greater than 25 d. Stage of the estrous cycle at time of the first injection of prostaglandin F2-alpha may contribute to this effect.