THE RELATIONSHIP OF PLASMA SOMATOMEDIN (IGF-1) TO LAMB GROWTH-RATE

Somatomedin (IGF-1) was measured in plasma samples obtained from lambs undergoing SRIF immunization to improve their rate of growth. Weight gains and blood samples were obtained at predetermined intervals betwe en weaning and slaughter for a total of 78 crossbred ewe, ram and weth er lambs in three experiments. Plasma IGF-1 concentrations were not al tered by SRIF-immunization despite improved rates of gain in wether an d ram lambs. Ewe, wether and ram lambs had similar plasma IGF-I concen trations until 12 wk old. After 12 wk of age, ram lamb IGF-I plasma co ncentrations were significantly higher than those of ewe and wether la mbs. Fasting lambs for 24 h reduced plasma IGF-1 concentrations, havin g the greatest effect on heavier lambs. When lambs were allowed free a ccess to feed, plasma IGF-1 concentrations obtained between 7 and 18 w k of age were positively correlated, both with weight gains prior to s ampling, and with total weight gain from weaning to slaughter. Thus, a t each age examined, lambs with higher than average plasma IGF-I conce ntrations weighed considerably more than average at slaughter. This in dicates that plasma IGF-1 concentrations obtained at an early age may be useful indicators of growth potential and aid in the selection of f ast growing animals.