In baby pigs that were fed milk, giving them extra IGF‑1 or a stronger version called LR3‑IGF‑1 didn’t change growth when food was limited, but both increased how much milk they drank. In the second half of the study, the LR3 version helped the pigs gain weight faster than the regular IGF‑1. Blood levels of a binding protein (IGFBP‑3) were linked to how quickly they grew.

Exogenous insulin-like growth factor (IGF)-I has been shown to increase growth rate in neonatal pigs while an analogue of IGF-I, long arginine (LR3) IGF-I, has been shown to be more potent than IGF-I in the rat. Therefore, two studies were conducted to determine whether IGF-I and LR3IGF-I increase growth in the artificially-reared neonatal pig. Expt 1 involved forty-two (2 kg initial weight) pigs infused with either control, IGF-I (2, 4 or 8 microg/h) or LR3IGF-I (2, 4 or 8 microg/h) infusions for 8 d. Pigs were weighed and then offered 1.7 MJ (gross energy) milk replacer/kg0.75 per d. Expt 2 involved eighteen pigs (2 kg initial weight) treated with control saline, IGF-I (8 microg/h) or LR3IGF-I (8 microg/h) infusions. After 9 d an additional pump was inserted to increase the infusion rates of each of the growth factors (16 microg/h) for a further 9 d. Cows' milk was provided ad libitum. In Expt 1 there was no overall effect of growth factors on daily weight gain or slaughter weight. However, milk intake was greater in pigs infused with growth factors (909 v. 867 g/d, P=0.027), with an apparently greater milk intake by the pigs infused with IGF-I compared with LR3IGF-I (920 v. 898 g/d, P=0.12). Infusion of LR3IGF-I decreased plasma IGF-I concentrations, but had no effect on plasma IGF-II concentrations. In Expt 2, neither IGF-I nor LR3IGF-I infusion had any effect upon daily weight gain over the first 9 d of the study. However, over the second 9 d of the study, daily weight gain was increased in LR3IGF-I-infused pigs (457 v. 386 g/d, P<0.01), but not in pigs infused with IGF-I (413 v. 386 g/d, P=0.15). Milk intake was not different during the first 9 d of the study but was significantly greater in pigs infused with growth factors over the second half of the study (3407 v. 2905 g/d, P<0.01). Plasma IGF-binding protein-3 concentrations were highly correlated (R=0.85) with average daily gain over the 3 d preceding blood sampling. In conclusion, exogenous IGF-I and particularly LR3IGF-I can increase growth rate and milk intake in artificially-reared pigs fed ad libitum but not in limit-fed piglets.