Effect of human chorionic gonadotropin administered early in the estrous cycle on ovulation and subsequent luteal function in cows.

Cows of mixed beef breeds were randomly assigned to receive one of three treatments: 1) i.v. injection of hCG on d 6 of the estrous cycle (estrus = d 0) and slaughter on d 12 to 14, 2) i.v. injection of saline on d 6 and slaughter on d 12 to 14, or 3) slaughter on d 5 to 7. Corpora lutea (CL) were collected at slaughter and weighed. A portion of each CL was cultured for 6 h with no hormone or various doses of LH (25, 50, or 100 ng/mL of medium). All cows that received an hCG injection on d 6 (Treatment 1) exhibited a luteal structure in addition to the CL present at the time of hCG administration. Thus, the following CL were evaluated: 6-d-old (induced) CL from Treatment 1 (d-6 hCG) and 13-d-old (natural) CL from Treatment 1 (d-13 hCG), 13-d-old CL from Treatment 2 (d 13), and 6-d-old CL from Treatment 3 (d 6). From d 9 to 13, daily plasma progesterone concentrations were greater (P < .01) for cows on Treatment 1 than for cows on Treatment 2. Whereas LH stimulated (P < .01) progesterone secretion regardless of treatment, progesterone secretion in vitro was greater (P < .01) for CL from saline-treated cows (Treatments 2 and 3) than for CL from hCG-treated cows (Treatment 1). No effect of hCG was observed for luteal DNA, RNA, or protein concentrations, or for luteal RNA:DNA or protein:DNA ratios.(ABSTRACT TRUNCATED AT 250 WORDS)