CIRCULATING IGF-I IN PLASMA OF GROWING MALE AND FEMALE TURKEYS OF MEDIUM AND HEAVY WEIGHT LINES

Plasma concentrations of insulin-like growth factor-I (IGF-I) were det ermined in male and female turkeys from a medium weight (RBC2) and a r elated heavy weight line (F) from 1 to 28 wks of age. At hatch, the co ncentrations of IGF-I were relatively low and not different between li nes or sexes. During the neonatal period (1 to 7 wks), the concentrati ons of IGF-I increased and were higher in the faster growing F line an d in males. During the juvenile period (8 to 15 wks) the concentration s of IGF-I were higher in males but not different between lines. Durin g the preadolescent period (16 to 21 wks), the concentrations of IGF-I were higher in males but was not different between lines in males whi le the females of the RBC2 line had higher concentrations than females of the F line. During the adolescent period (22 to 28 wks) the concen trations of IGF-I were higher in males but was not different between l ines in males while the femaLes of the RBC2 line had higher concentrat ions than femaLes of the F line. A phenotypic correlation (+.25) betwe en plasma IGF-I and growth rate was present after statistical absorpti on of model effects during the neonatal period but not at the later ag es. We conclude that IGF-I concentration was positively correlated wit h growth rate during the neonatal period, but that this relationship c hanged during the preadolescent and adolesCent periods so that IGF-I c oncentrations were not related to growth rate in males but were negati vely related to growth rate in females.