Effect of stage of the estrous cycle on interval to estrus after PGF 2α in beef cattle

Abstract

Effect of stage of the estrous cycle at the time of prostaglandin F 2α (PGF 2α) (b) (b) Lutalyse (prostaglandin F α; dinoprost tromethamine) was graciously provided by the UpJohn Company, Kalamazoo, Michigan, 49001. injection on subsequent reproductive events in beef females was studied in four trials involving 194 animals. Cycling animals were given two injections of 25 mg PGF 2α 11 days apart or, in some cases, the interval was altered to allow the second injection to fall on a specific day of the cycle. Day of estrous cycle at time of the second injection was determined by estrous detection. Interval from the second PGF 2α injection to the onset of estrus (interval to estrus) was shorter (P<.01) in heifers than in cows. Both cows and heifers injected on days 5 to 9 (early cycle) had a shorter (P<.01) interval to estrus (estrus = day 0) than did those injected on days 10 to 15 (late cycle). Conception rate was lower (P<.05) for early-cycle heifers than for late-cycle heifers inseminated by appointment at 80 hours. There was no significant difference in conception rate of early-or late-cycle heifers or cows inseminated according to estrous detection or early- or late-cycle cows inseminated at 80 hours. Progesterone concentrations in blood samples collected in heifers at 4-hour intervals after the second PGF 2α injection on either day 7 or day 14 declined linearly (P<.05) through 36 hours. Day of the estrous cycle at PGF 2α injection had no effect on rate of progesterone decline, even though heifers injected on day 7 had a shorter (P<.05) interval to estrus. All animals whose cycle length was not affected by the second PGF 2α injection were treated on days 5 through 8 of the cycle, indicating that PGF 2α was less effective in regressing the corpus luteum between days 4 and 9 of the cycle than later in the cycle.