Administration of human chorionic gonadotropin to improve luteal function in lactating beef cattle.

Background: Pregnancy rate after fixed-time artificial insemination (FTAI) in beef cattle is affected by ovulation rate, luteal function and embryo survival. Cows in early post partum period frequently have low progesterone production and short luteal phases. Treatments using human chorionic gonadotropin (hCG) after FTAI have been tested in an effort to increase progesterone concentration, improve embryo development and consequently pregnancy rates. The objectives of this work were to evaluate the effects of administration of hCG on day 4 after FTAI on induction of ovulation of the first dominant follicle, corpus luteum (CL) development, progesterone production and pregnancy rate in lactating beef cows. Materials, Methods & Results: Five hundred and sixty nine multiparous Angus suckled beef cows, 45-70 days postpartum, were synchronized to FTAI. On day -9, intravaginal implants containing 0,750 g of progesterone were inserted and 2mg of estradiol benzoate administered, im. Seven days later, cows had their implants removed and were injected with 150 μg of d-cloprostenol, im, and 1 mg of estradiol cipionate, im. Cows were inseminated 52-56 h after implant removal. Four days later, cows were assigned into two groups, hCG (1500 UI of hCG, im, n = 269) or Control (1,5 mL saline, im, n = 300). Blood samples were collected on days 4, 7, 10 and 14 after FTAI for analysis of serum progesterone concentration by radioimmunoassay. Ovarian ultrasonographic exams were done on day 0 to determine the diameter of ovulatory follicle (OF), on day 4 to measure the diameter of the first dominant follicle and, on day 7, to determine the CL area (cm2) and the number of accesory CL. Pregnancy diagnosis were done on day 30. Pregnancy rates were compared by chi-square test, progesterone concentration and CLs parameters were compared by student-t test. The administration of hCG resulted in higher serum progesterone concentration, improved CL area and the frequency of accessory CL, compared with Control group. Cows treated with hCG had higher (P = 0.07) pregnancy rate (53.9%; 145/269) than Control cows (46.3 %; 139/300) Discussion: Progesterone plays a key role in reproductive events associated with establishment and maintence of pregnacy in cattle. The use of hormone therapy to improve luteal function after FTAI in lactating beef cows is an alternative that can improve fertility, enhancing embryonic development and reducing early pregnancy loss. Low progesterone has been linked to early failure and poor embryo development while supplementing cows with progesterone has been shown longer conceptus and adequate production of interferon-tau. Several studies reported that the CL size is correlated with progesterone production and embryo development. In this study, cows treated with 1500 UI hCG on Day 4 after FTAI had increased concentration of progesterone on days 7, 10 and 14 and had greater size of CL on day 7 (P < 0.01). This improvent in luteal fuction was associated with luteotrophic effect of hCG in the original CL and the induction of accessory CL in 29.6% of treated cows. The pregnancy rate of hCG treated cows was 7.6% higher than Controls cows, indicating a tendency (P = 0.07) of increment in fertility. It was concluded that administration of hCG on day 4 after FTAI increased the volume of luteal tissue and progesterone concentration, improving pregnancy rate in lactating beef cows.