The influence of naturally occurring and synthetic anabolic hormones on growth and reproduction in female cattle and guinea-pigs
A series of six experiments was conducted on female cattle and guinea-pigs to investigate the effects of some anabolic compounds on both growth and reproductive functions in the two species. The results indicate that trenbolone acetate increased significantly, the live-weight gains of heifers and improved the efficiency of food conversion. Zeranol and oestradiol-17 treatments had no effect on growth performance. Trenbolone acetate inhibited or delayed ovulation and resulted in elevation of plasma oestradiol-17 concentrations. Zeranol and oestradiol-17 had no effect on estrous cycle occurrence nor ovulation as determined by plasma progesterone concentrations. It is concluded from studies investigating the response to gonadotrophin releasing hormone (Gn-RH), oestradiol benzoate (OE2-B) and pregnant mares' serum gonadotrophins (PMSG) that trenbolone acetate acts (1) on the pituitary gland to decrease the sensitivity to Gn-RH, (2) on the ovary to decrease the sensitivity of the ovarian follicles to gonadotrophins and (3) acts on the pituitary gland and/or the hypothalamus to block the positive feedback effect of oestrogen to release the LH-surge. In the guinea-pig, trenbolone acetate at dose levels of 2 and 10 mg/kg body-weight inhibited ovulation. At a dose level of 0.4 mg/kg body weight, trenbolone acetate prolonged the length of the oestrous cycle. When trenbolone and testosterone, at dose levels of 3.1 and 15.7 mmol/l each, were compared, trenbolone was shown to have more general promoting activity than testosterone. The high dose of both hormones inhibited ovulation and increased the rate of occurrence of atretic follicles. However, only testosterone at the higher dose decreased the weight of the ovaries and lowered the number of follicles. From studies measuring the response to follicles stimulating hormone (FSH) and human chorionic gonadotrophin (HCG), it is concluded that, in the female guinea-pig, trenbolone acetate had no effect on LH-surge mechanism, but it may act on the pituitary gland to block the release of FSH.