Administration of human chorionic gonadotropin to improve luteal function in lactating beef cattle / Administração de gonadotrofina coriônica humana para estimulação da função luteal em vacas de corte lactantes

ABSTRACT

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 productionand short luteal phases. Treatments using human chorionic gonadotropin (hCG) after FTAI have been tested in aneffort to increase progesterone concentration, improve embryo development and consequently pregnancy rates. Theobjectives of this work were to evaluate the effects of administration of hCG on day 4 after FTAI on induction ofovulation of the first dominant follicle, corpus luteum (CL) development, progesterone production and pregnancyrate in lactating beef cows.Materials, Methods & Results: Five hundred and sixty nine multiparous Angus suckled beef cows, 45-70 dayspostpartum, were synchronized to FTAI. On day -9, intravaginal implants containing 0,750 g of progesterone wereinserted and 2mg of estradiol benzoate administered, im. Seven days later, cows had their implants removed andwere injected with 150 µg of d-cloprostenol, im, and 1 mg of estradiol cipionate, im. Cows were inseminated 52-56h after implant removal. Four days later, cows were assigned into two groups, hCG (1500 UI of hCG, im, n = 269) orControl (1,5 mL saline, im, n = 300). Blood samples were collected on days 4, 7, 10 and 14 after FTAI for analysisof serum progesterone concentration by radioimmunoassay. Ovarian ultrasonographic exams were done on day 0 todetermine 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 comparedby student-t test. The administration of hCG resulted in higher serum progesterone concentration, improved CL areaand the frequency of accessory CL...