Birth weight effect in twin‐born lambs from mid‐pregnancy shearing is associated with changes in maternal IGF‐I concentration

Abstract

A number of studies have shown an increase in the birth weight of lambs in response to mid pregnancy shearing. The present study sought to determine whether the birth weight effect of mid‐pregnancy shearing is mediated through changes in maternal insulin‐like growth factor‐I (IGF‐I) and insulin‐like growth factor binding protein‐I (IGFBP‐I) concentrations. Single‐ and twin‐bearing ewes from three genetically divergent IGF‐I lines were either left unshorn or shorn at a mean day 90 of pregnancy (P90). Maternal plasma concentrations of IGF‐I and IGFBP‐I were measured at P89, P108 and P136. Lamb liveweight at birth and weaning weremeasured, andplasmaconcentrations of IGF‐I determined at weaning. Shearing increased the birth weight of twin lambs by over 350 g (P < 0.05) without having any effect on singleton birth weight. The increased birth weight of twin lambs born to shorn ewes was associated with a decline in maternal plasma IGF‐I concentration (P < 0.05) from P108 toP136. Low IGF‐I line ewes had lower (P < 0.001) plasma IGF‐I than high IGF‐I line ewes at each stage measured but higher plasma IGFBP‐I at P89 and P108. Birth weights were higher in lambs born to low IGF‐I line ewes than in those born to high IGF line ewes. These results suggest that the difference in birth weight between twin lambs from shorn and unshorn dams was mediated through a decline in maternal plasma IGF‐I concentrations in late pregnancy. In support of previous studies, there was an inverse relationship between IGF‐I selection line and lamb birth weight.