Synchronization of estrus in the cow following intramuscular injection of prostaglandin F_2α

Abstract

The minimum effective dose of prostaglandin F_2α (PGF_2α) for regressing the corpus luteum and the effectiveness of PGF_2α for estrous synchronization were investigated in the cow, by injecting it intramuscularly.
Seven cows with mean weight of 421 kg (ranging from 420 to 490 kg, except one weighing 280 kg) were injected intramuscularly with PGF_2α, at mid luteal stage (714 days after ovulation) of the estrous cycle. Either 6-30 mg of PGF_2α dissolved in distilled water, or 810 mg suspended in propylene glycol, was used. The corpora lutea regressed rapidly during the period of 13 days after the treatment. Ovulation occurred in 46 days, mainly in 4 days, after the treatment. Ho wever, in cows that were injected with 5 mg of PGF_2α dissolved in distilled water, and 4-6 mg of PGF_2α suspended in propylene glycol, the corpora lutea did not regress, or regressed slightly, during the period of 14 days after the treatment, and began to grow again thereafter. There were no marked changes in the treatment cycle length of these cows.
These results indicated that the minimum effective dose of PGF_2α for regressing the corpus lutetm of the cow was 6 mg, when it was dissolved in distilled water, and 8 mg when it was suspended in propylene glycol. There is no great difference in the minimum effective dose, between both vehicles.
In 17 heifers (10 Holstein and 7 Japanese Short Horn), weighing an average of 337 kg (ranging from 275 kg to 413 kg), those were injected intramuscularly with 10 mg of PGF_2α dissolved in distilled water, estrus occurred in 12 heifers (70.6 %) during the period of 2-5 days after the treatment. In 3 of the remaining 5 heifers, corpora lutea regressed during the period of 2-4 days after the treatment, and there followed follicular development. However, onset of estrus and ovulation in these heifers, were delayed until 7-9 days after the treatment. Corpora lutea of 2 other heifers did not regress following the treatment.
The results indicated that the single intramuscular injection of 10 mg of PGF_2α was applicable to synchronization of estrus and ovulation in the cow.