Effects of gonadotrophin-releasing hormone and bovine somatotrophin on hormone profiles and ovarian function in postpartum beef cows

Reproductive efficiency in beef cows is limited by the length of the postpartum anoestrous period which in turn is controlled by pituitary gonadotrophins and gonadal steroids and/or proteins. The first aim of this study was to elucidate the role of LH pulses in the development of ovarian activity in postpartum beef cows, specifically to determine whether or not the normal, relatively slow process of follicular development could be accelarated in cows in low body condition (BC) with pulses of gonadotrophin-releasing hormone (GnRH). The second aim was to determine the role of the metabolic hormones and in particular growth hormone (GH) in ovarian follicle development and function.Treatment of cows in low BC with pulsed intravenous infusions of 2 pg every 2 hours from 5 to 7 weeks postpartum induced ovulation in 10 out of 12 cows. Saline infusions resulted in ovulation in only 1 of 12 cows in high BC and 1 of 11 cows in low BC groups. Plasma concentrations of GH, IGF-I and insulin were not affected by GnRH treatment, but cows in high BC had higher IGF-I and lower GH levels than cows in low BC. Gonadotrophin profiles, luteinizing hormone (LH) pulse frequency and LH pulse amplitude were not affected by either GnRH treatment or BC. Numbers of small (3-7.9 mm diameter) and large (> 8 mm diameter) follicles present at week 7 postpartum and numbers of LH receptors in theca and granulosa tissue were not affected by GnRH treatment or BC. The number of granulosa cells present in large follicles at week 7 postpartum was also unaffected. Intrafollicular concentrations of oestradiol, testosterone and IGF-I in large follicles at 7 weeks postpartum were not significantly affected by GnRH treatment or BC but there was a marked trend towards higher concentrations of oestradiol in cows in high BC compared with cows in low BC.In Experiment 2, depot injections of 320 mg bovine somatotrophin (bST) (each designed to release 23 mg/day for 14 days) administered at weeks 2, 4, 6 and 8 postpartum induced ovulation in 4 out of 17 cows while 0 out of 15 ovulated in the control group (p= 0.10). While peripheral concentrations of GH and IGF-I were significantly increased throughout the postpartum period, there was no effect of bST on circulating concentrations of insulin, glucose, follicle-stimulating hormone (FSH), LH, LH pulse frequency or LH pulse amplitude. Numbers of small (3-7.9 mm diameter) and large (≥8 mm diameter) follicles and numbers of LH and FSH receptors in both theca and granulosa tissue at week 9 and numbers of granulosa cells present in large follicles was also unaffected. bST treatment significantly enhanced intrafollicular concentrations of oestradiol and IGF-I in large follicles (≥8 mm diameter) at 9 weeks postpartum but no difference in testosterone concentrations was observed.It is concluded that infusion of exogenous GnRH pulses enhanced the process of follicular development in cows in low BC. While it was not possible to identify causal relationships between BC, plasma GH and IGF-I concentrations and intrafollicular oestradiol concentrations, bST treatment was found to increase oestradiol production in large follicles. This effect was not mediated through changes in gonadotrophin profiles or receptors, indicating that GH has a more direct role in the control of ovarian function. It is suggested that this effect may be mediated through changes in intrafollicular IGF-I concentrations