Control of estrus and progesterone levels in heifers given intravaginal progesterone coils and injections of progesterone and estrogen.

Fifty-three mature Hereford-cross heifers, which were between day 0 and 3 of the estrous cycle, each had silastic coils containing 2.1 g progesterone inserted into the vagina where they remained for 12 days. Animals were assigned to treatment groups and were injected with (1) 5 mg estradiol benzoate, (2) as (1) + 50 mg progesterone or (3) as (1) + 200 mg progesterone at the time of insertion of the coils. Blood samples were taken for radioimmunoassay of progesterone in plasma before insertion and daily while the coils were in the vagina. There were no differences among treatments in the percentage of heifers observed in estrus from days 1 to 6 after removal of the coils, but a greater percentage of animals in treatments 1 and 3 than in treat 2 were in estrus within 2 days of removal of the coils (P<.05). The mean daily plasma levels of progesterone during treatment for animals on the different treatments, which were in estrus within 2 days removal of the coils were not different. However, for animals in treatment groups 1 and 2 lower average progesterone levels over the first 2 days after insertion appeared to be associated with poor synchronization. The level of progesterone over the last 3 days of treatment was significantly related to