10 INFLUENCE OF ADMINISTRATION OF HUMAN CHORIONIC GONADOTROPIN ON DAY 4 AFTER FIXED-TIME AI ON CORPUS LUTEUM SIZE, FUNCTION, AND PREGNANCY RATE IN BEEF COWS

Abstract

Successful establishment of pregnancy in the cow depends on early rise of progesterone (P4) to improve embryo development. The present study was carried out to determine the corpus luteum (CL) function and pregnancy rate following the administration of 1.500 IU of hCG in suckled beef cows on Day 4 after fixed-time AI (FTAI). Crossbred Angus cows (n = 429), 40 to 70 days postpartum, were synchronized using 2 mg of oestradiol benzoate (EB) i.m. and an intravaginal progesterone-releasing device containing 1 g of P4 for 7 days. At device removal, prostaglandin F2a (150 mg of D-cloprostenol) was injected i.m.; 24 h later, 1 mg of EB was given and the FTAI were done 30 to 36 h after EB using semen of just 1 bull. Cows were assigned alternatively to receive 1.500 IU of hCG i.m. (n = 200) or 1 mL of saline i.m. (n = 229) on Day 4 after FTAI. Ovaries were examined by transrectal ultrasonography (n = 71) on Days 4 and 7 to detect the presence and to measure the area of CL. Blood samples were collected on Days 4, 7, 10, 14, and 30 post-oestrus to determine P4 concentration by RIA. Pregnancy diagnoses were done 30 days after FTAI. The effects of treatments on CL area and serum P4 levels were analysed by ANOVA (Proc GLM, SAS Institute Inc., Cary, NC, USA) and pregnancy rates compared by chi-square (Proc CATMOT, SAS). The induction of accessory CL determined on Day 7 was 44% in cows treated with hCG and not observed in the control group. The area of the CL on Day 7 was significantly higher (P < 0.01; 3.37 ± 2.56 v. 2.56 ± 0.22 cm2) in the hCG group. Despite the increase in CL size, the mean levels of P4 on Days 4, 7, 10, 14, and 30 post-AI indicated no difference (P > 0.05) in CL function between control and hCG-treated cows. On Day 30, pregnancy rates were not different (P = 0.08), at 46.7 and 55% for control and hCG-treated cows, respectively. In conclusion, administration of 1.500 IU of hCG on Day 4 post-FTAI did not improve pregnancy rate in suckled beef cows.