Growth, Metabolic and Endocrine Responses of Broiler Cockerels Given a Daily Subcutaneous Injection of Natural or Biosynthetic Chicken Growth Hormone

Abstract

The effects of daily injection of natural chicken growth hormone (ncGH) or recombinant-derived chicken growth hormone (rcGH) on growth, heat production rate, plasma hormone levels and body composition were determined in rapidly growing broiler chickens. Beginning at 3 wk of age, eight broiler chickens were given a daily injection of either bicarbonate buffer (control), 100 or 200 micrograms ncGH/kg body wt, or 200 micrograms rcGH/kg body wt for 14 d. Blood samples were taken preinjection and 4 h postinjection on d 7 and 14 of chicken growth hormone (GH) treatment. Compared to preinjection levels, plasma GH levels at 4 h postinjection were significantly (P less than 0.05) elevated by daily injection (per kg body wt) of 100 micrograms ncGH (2.3-fold), 200 micrograms ncGH (5.5-fold) or 200 micrograms rcGH (6.4-fold). Although exogenous chicken GH treatment failed to increase body weight gain, ncGH injections did increase (P less than 0.05) body fat content to 117% that of the control group. Daily injection of chicken GH did not alter plasma levels of immunoreactive insulin-like growth factor-I (IGF-I), thyroid hormones, insulin, glucagon or glucose. Feed efficiency, heat production rate and respiratory quotient were also not affected by chicken GH treatment. Plasma levels of nonesterified fatty acids were elevated (P less than 0.05) by treatment with 200 micrograms ncGH/kg body wt. In contrast to domestic mammals, it is apparent that exogenous chicken GH can not be used to increase lean body mass or improve productive efficiency in chickens. Our results indicate that exogenous chicken GH exerts a strong lipogenic, rather than lipolytic, action in rapidly growing broiler cockerels.