Estrus synchronization systems involving prostaglandin F(2alpha) and progesterone pretreatment in beef heifers.

Two trials were conducted to evaluate treatments combining progesterone pretreatment and prostaglandin F(2alpha) (PGF(2alpha)) on estrus response, pregnancy and calving rate in heifers. Treatments in Trial 1 were 1) control (T(1); n=59), 2) 25 mg PGF(2alpha) on Day 0 (T(2); n=58), 3) 150 mg progesterone (P(4), i.m.) in corn oil on Day -24 plus PGF(2alpha) (T(3); n=61), and 4) 150 mg P(4) on Day -5 plus PGF(2alpha) (T(4); n=59). Trial 2 had T(2) and T(4) only. Heifers were artificially inseminated 8 to 16 h after detection of estrus for 10 and 5 d in Trials 1 and 2, respectively. In Trial 1 more heifers in T(3) and T(4) showed estrus by 72 h compared to T(1) and T(2). In T(3), percentages were greater at 84 and 96 h than in T(1) and T(2). There were no differences between T(3) and T(4) or T(1) and T(2) over time. Cumulative distributions of responses showed that more heifers in T(3) and T(4) were in estrus by 84 h after PGF(2alpha) than after other treatments, while T(3) showed the greatest total number of heifers in estrus by 84 h; this difference persisted for 180 h. In Trial 2, percentages of heifers observed in estrus for T(1) and T(4) were not different. Average interval from PGF(2alpha) to estrus was shorter in Trial 1 for T(3) heifers compared to other treatments. No difference was observed in interval to estrus for T(2) and T(4) in Trial 2; this interval averaged 58 h. Artificial insemination pregnancy rates were not different among treatments in either trial and averaged 67.4%. In Trial 1, a greater proportion of heifers in T(2), T(3) and T(4) calved by 35 days into the calving season compared to T(1), but in Trial 2 calving rates for T(2) and T(4) were not different. Progesterone pretreatment combined with PGF(2alpha) appeared to enhance estrus synchronization without influencing either pregnancy or calving rates.

Effect of pretreating heifers with human chorionic gonadotropin and estradiol on subsequent in vitro luteal tissue progesterone biosynthesis.

PIP: The use of in vivo treatments of human chorionic gonadotropin (HCG), estradiol, or HCG plus estradiol was investigated as a method to provide large amounts of LH-sensitive luteal tissue for more extensive in vitro studies. HCG (total dose 11,000) treatments to heifers on Days 1 to 7 of the estrous cycle produced tissue that was senitive to LH added in vitro, but Day 11 corpora lutea of the heifers were only slightly larger than normal. Total doses of 13,000 and 15,000 IU HCG increased the weight of the corpora by factors of 2 and 3, but reduced or abolished the LH response in vitro. Treatment with 5000 IU HCG on either Day 8 or 9 did not substantially increase the weight of the corpora. Basal Day 11 in vitro progesterone synthesis was high, probably due to the recent in vivo luteotrophic treatment. However, the tissue was not capable of further stimulation with LH in vitro. Pretreatment with HCG plus estradiol in 2 experiments resulted in tissues with poor in vitro LH sensitivity. Pretreatment with 3 mg estradiol at 4, 6.5, or 12.5 hours before removal of the corpus on Day 11 also reduced the sensitivity of the tissue to LH IN vitro. These data support the theory that in vivo luteotrophic treatment may deplete a preformed precursor of progesterone, which reduces or abolishes LG sensitivity in vitro. However, results obtained when estradiol was given at levels and times used in these experiments did not support the idea that estradiol reduces endogenous gonadotropins.^ieng