Synchronized estrus and subsequent conception in dairy heifers treated with prostaglandin F2 alpha. I. Influence of stage of cycle at treatment.

When prostaglandin F2 alpha (PGF2 alpha) was injected (im) at three different stages of the cycle, estrus was synchronized within an 80-h period in a progressively, but nonsignificantly, higher percentage of heifers with each advancing stage: 86.0% at d 7, 90.0% at d 11 and 98.0% at d 15. Estrus was exhibited within a 48-h period for heifers treated on d 7, but extended over a 72-h period for heifers treated on either d 11 or 15. Stage of cycle when PGF2 alpha was injected not only influenced the degree of estrous synchrony, but also influenced (P less than .001) the time of estrous onsets. Within the 24-h interval from 32 to 56 h postinjection, the percentage of heifers exhibiting estrus from treatment on d 7, 11 or 15 varied widely: 88.4, 13.3 and 73.5, respectively. Induction of estrus was complete, or nearly so, by 72 h postinjection for heifers treated during the early (d 7 = 100.0%) or late (d 15 = 95.5%) stage of the cycle, but reached only midpoint for midcycle injections (d 11 = 48.9%), with the remaining one-half (51.1%) occurring 72 to 104 h later. Pregnancies arising from the first through the fifth services among the 150 PGF2 alpha-treated heifers were as follows: 76.0, 10.0, 6.7, 4.0 and .7%, plus failure of pregnancy in 2.7%. These percentages were not different (P greater than .50) from those among the 50 control heifers.