Metabolic responses to mid-pregnancy shearing that are associated with a selective increase in the birth weight of twin lambs.

Previous studies have shown that shearing pregnant ewes at mid- or late-pregnancy is associated with an increase in lamb birth weight. The present study was designed to investigate metabolic responses that may underlie this response. Single- and twin-bearing ewes were either unshorn or shorn at mid-pregnancy (Day 69 of pregnancy; P69), and insulin, glucose and epinephrine challenges were conducted on P109-111 and P132-134. Shearing increased the birth weight of twin lambs by over 1 kg (P < 0.001) without having any effect on singleton birth weight. This response was associated with a 10-20% reduction in the insulin response to a glucose challenge (P < 0.05) without a change in glucose clearance following either glucose or insulin challenges. The lipolytic response to epinephrine challenge increased as pregnancy progressed, but was not associated with the increased birth weight of twin lambs born to shorn ewes. By late pregnancy, a 25% reduction in maternal IGF-I concentration and a two- to threefold increase in maternal IGFBP-1 concentration (P < 0. 05) associated with shearing were observed. The increase in lamb birth weight associated with mid-pregnancy shearing may have been associated with an increase in the non-insulin dependent uptake of glucose by the placental-fetal unit.