Developmental patterns of plasma insulin-like growth factor-1 concentrations in sheep

A study was undertaken to examine the ontogeny of circulating insulin-like growth factor-1 (IGF-1) concentrations in sheep. The trial was a balanced 2 x 2 factorial design incorporating the effects of sex and rearing rank with a total of 48 animals. Blood sampling was initiated four weeks post-weaning (about 3 months of age) and continued every 2 weeks for 6 months and then every 4 weeks for 7 months. Fecal egg counts and plasma concentrations of non-esterified fatty acids (NEFA) were also determined. IGF-1 concentration was positively related to live weight throughout the trial, even when adjusted to common sex and rearing rank. Sex of lamb had a significant effect on plasma IGF-1 concentrations with concentrations in males being greater than those in females. Puberty in females, as determined by date of first estrus, was associated with an increase in plasma IGF-1 concentrations. Although puberty in males was not measured, a surge in plasma IGF-1 concentrations around the approximate time of puberty in males resulted in a marked divergence between the sexes which remained throughout the study. Rearing rank did not influence plasma IGF-1 concentrations beyond 7 months of age. Plasma IGF-1 concentrations tended to be negatively associated with plasma NEFA concentrations and fecal egg counts but relationships were only occasionally significant. IGF-1 levels were highly repeatable, concentrations at the first sample being significantly correlated with those at all subsequent samples. Implications of these results in relation to potential use of plasma IGF-1 concentration as a genetic marker for productivity are discussed.