Immunization of pregnant ewes against somatotropin release inhibiting factor increases growth of twin lambs

Multiparous ewes, pregnant with twin foetuses, were immunized with a complex of somatotropin release inhibiting factor/bovine serum albumin emulsified in Freund's Complete Adjuvant, or placebo on about days 90, 111 and 132 of pregnancy. At lambing, ewes with viable twin lambs and no obvious abnormalities were penned indoors with their lambs and fed restricted amounts of a good quality ration. Birthweights of lambs were greater for immunized than control ewes (4.6 v. 3.6 kg, P < 0.05) and 6 weeks later differences were maintained (13.6 v. 10.8 kg, P< 0.001). Liveweights of ewes from both groups were similar at parturition and 6 weeks later; even so, immunized ewes tended to lose more weight post partum than control ewes (10.9 v. 8.8 kg, P > 0.10). Milk yields of immunized ewes were 20-30% higher than those of control ewes during the 6 weeks post partum and milk of the immunized ewes contained lower proportions of lactose than in control ewes at day 14 of lactation. Feed intakes of both groups were similar, resulting in improved efficiency of use of food for milk synthesis in immunized ewes. Plasma concentrations of insulin were similar for both groups of ewes and lambs at all times, but plasma concentrations of glucose at parturition and 6 weeks later and urea during late pregnancy were reduced and concentrations of non-esterified fatty acids tended to increase during late pregnancy in immunized ewes compared with control ewes. The data are consistent with more efficient feed use by immunized than control ewes enabling greater foetal growth, increased milk yields and greater lamb growth.