PLASMA-LEVELS OF GROWTH-HORMONE AND INSULIN-LIKE GROWTH FACTOR-I AND FACTOR-II FROM 2 TO 6 WEEKS OF AGE IN MEAT-TYPE CHICKENS SELECTED FOR 6-WEEK BODY-WEIGHT OR FOR FEED CONVERSION AND REARED UNDER HIGH OR NORMAL ENVIRONMENTAL-TEMPERATURE CONDITIONS

The aim of this study was to compare the effect of high (33-degrees-C) and normal (33-20-degrees-C) rearing temperature on growth and plasma levels of the somatotrophic hormones of 2 genetic lines of broiler chickens selected for 6-wk body weight (GL-line) or for feed conversion between 3 and 6 wk of age (FC-line) or for feed conversion between 3 and 6 wk of age (FC-line). Blood samples were collected weekly and analysed for growth hormone (GH) and insulin-like growth factors ((IGF)-I and II levels by RIA. The growth-depressing effect of the HT-treatment was more pronounced in the heavier GL-line and in males. A similar age-related pattern for all hormones studied was observed with the highest levels between 2 and 4 wk of age. FC-line chickens and males had consistently higher plasma GH levels than GL-line chickens and females respectively. No consistent effect of rearing temperature on plasma GH levels were observed. At 2 wk of age, HT-treatment resulted in higher plasma IGF-I levels while this was reversed from 3 wk of age onwards. GL-line chickens had significantly higher plasma IGF-I levels at 2, 3 and 4 wk of age. No consistent effect of sex on plasma IGF-I levels could be observed. For the whole period studied, GL-line chickens had significantly higher plasma IGF-II levels than FC line chickens. No consistent effect of sex or temperature treatment on plasma IGF-II levels was observed.