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Runt pigs grow more slowly and never reach the same body weight as age-matched littermates. We hypothesized that IGF-I would be reduced in the runts and that postnatal nutrition would alter IGF-I concentration and tissue expression. Runt and control littermates were removed from 20 crossbred sows 20 to 28 h after birth. Tissues were collected from a baseline group (n = 4). The remaining pigs were fed porcine milk replacer at either 70 or 120 g/kg BW for 14 d (n = 8). Feed intake and body weight were measured daily, with plasma samples collected by jugular venipuncture throughout the experiment. Expression of IGF-I mRNA was measured in the liver and gastrocnemius with an RNase protection assay. At d 0, runts were significantly smaller than controls in all measurements, except brain weight. During the 14 d, the relative rate of growth was significantly faster and more efficient in runts than in controls; however, runts never attained the same absolute body weight as controls. Circulating IGF-I was significantly reduced at d 0 but was similar to that in controls by d 2 of feeding. The IGF-I mRNA expression in liver or gastrocnemius muscle was not different between control and runts at d 0 or 14 and was not affected by dietary intake. This study has shown that runt pigs grow in a compensatory manner for at least the first 2 wk of life. However, this growth response does not seem to be mediated by IGF-I. Key Words: Runting, Compensatory Growth, IGF-I, Pigs

Runt pigs grow more slowly and never reach the same body weight as age-matched littermates. We hypothesized that IGF-I would be reduced in the runts and that postnatal nutrition would alter IGF-I concentration and tissue expression. Runt and control littermates were removed from 20 crossbred sows 20 to 28 h after birth. Tissues were collected from a baseline group (n = 4). The remaining pigs were fed porcine milk replacer at either 70 or 120 g/kg BW for 14 d (n = 8). Feed intake and body weight were measured daily, with plasma samples collected by jugular venipuncture throughout the experiment. Expression of IGF-I mRNA was measured in the liver and gastrocnemius with an RNase protection assay. At d 0, runts were significantly smaller than controls in all measurements, except brain weight. During the 14 d, the relative rate of growth was significantly faster and more efficient in runts than in controls; however, runts never attained the same absolute body weight as controls. Circulating IGF-I was significantly reduced at d 0 but was similar to that in controls by d 2 of feeding. The IGF-I mRNA expression in liver or gastrocnemius muscle was not different between control and runts at d 0 or 14 and was not affected by dietary intake. This study has shown that runt pigs grow in a compensatory manner for at least the first 2 wk of life. However, this growth response does not seem to be mediated by IGF-I.