Immunization against somatotropin release inhibiting factor improves digestibility of food, growth and wool production of crossbred lambs

Abstract

Crossbred ewe lambs [Dorset Horn X (Border Leicester X Merino)] fed chopped 1ucerne:rolled barley in the proportions 2:1 at either 36 g kg-1 liveweight (moderate intake) or 40 g kg-1 (high intake) were immunized on four occasions at intervals of 25 days with a conjugate of somatotropin release inhibiting factor (SRIF) and keyhole limpet haemocyanin (KLH). Before the fourth immunization, some of the barley was replaced with fish meal so that the diet contained 10% by weight air dry of fish meal. Antibodies against SRIF were detected in plasma from 10 out of 12 immunized lambs after the second immunization; two lambs on the high feed intake failed to produce measurable antibodies even after four immunizations. In the lambs producing antibodies, plasma concentrations of growth hormone, insulin and non-esterified fatty acids were similar to concentrations for non-immunized (control) lambs, but plasma concentrations of glucose were increased in the immunized lambs on the moderate intake. Apparent digestibilities of dry matter and nitrogen as well as nitrogen retention rates were greater for immunized than control lambs, with differences being significant for lambs on the moderate intake. Liveweight gains and growth rates over the experiment were significantly increased by immunization in lambs on the moderate intake and tended to increase for lambs on the high intake, but there were no effects of immunization on body composition. Wool growth and diameter of wool fibres increased after the second or third immunization, and in most cases the increase was significant. When fish meal was included in the diet, plasma glucose and growth rates decreased and wool fibre diameter increased in all lambs. Differences between immunized and control lambs were maintained.