Effects of recombinant bovine somatotropin on luteinizing hormone and ovarian function in lactating dairy cows

Thirty-two lactating Holstein cows were assigned to 1 of 4 groups in a randomized block design using a 2 x 2 factorial arrangement of treatments. Recombinant bovine growth hormone (rbSt; 25 mg/day) or placebo was administered beginning at Day 35 or 70 postpartum. All cows began treatment approximately 3 days post-estrus. Blood samples were collected at least once daily for a 70-day period to determine the concentration of progesterone and the duration of the luteal and follicular phases. During estrous cycles 1 and 3, frequent blood samples were taken (every 10 min for 8 h) 24 and 60 h after the onset of luteal regression. These samples were assayed for luteinizing hormone (LH), and samples coincident with the second LH pulse detected were asayed for estradiol. Ultrasonography was used to determine the size of the largest ovarian follicle from Day 17 until ovulation in estrous cycles 1 and 3. Luteal life span, length of the follicular phase, and diameter of the largest follicle were not affected by treatment with rbSt. Administration of rbSt increased the concentration of progesterone in plasma during the first two luteal phaes (p < 0.01). Progesterone was elevated during the mid-luteal phase of cycle 3 in rbSt-treated cows that began treatment about Day 35 postpartum but not in cows that began treatment on Day 70 postpartum (Treatment x Stage x Day, p < 0.01). during the first follicular phase studied, LH pulse frequency was higher (p = 0.06) in rbSt-treated cows than in cows receiving the placebo. In contrast, baseline and average concentrtions of LH were lower (p < 0.01) in cows given rbSt. No differences in the secretion of LH were found among the treatment groups at the end of cycle 3. Average concentrations of estradiol were not altered by rbSt. But, there was a Treatment x Stage x Min interaction (p < 0.05) 24 h after the onset of luteal regression in cycle 1. The reason for this interaction is not clear. In summary, rbSt increased the concentration of progesterone in plasma and this response was diminished after approximately 120 days postpartum. During the first follicular phase following initiation of treatment, rbSt altered the pattern of LH secretion, but this effect did not result in changes in peripheral plasma concentrations of estradiol, follicle size, or the length of the follicular phase.