Impact of early weaning on small intestine, metabolic, immune and endocrine system development, growth and body composition in artificially reared lambs

This study evaluated the effect of early weaning (EW) of artificially-reared lambs using a restricted milk replacer (MR) feeding and step-down weaning system on the short- and long-term effects on growth, feed intake, selected blood metabolites and hormones, body composition and small intestine development. Mixed-sex twin-born 2-5 d old Romney crossbred lambs were randomly allocated to groups (n=16/group) with MR weaning at either 4 (EW) or 6 wks (Control; Ctrl) of artificial rearing. Individually penned lambs were fed MR at 20% of initial individual BW, and were step-down weaned over a 3 wk period. Concentrate starter and fiber diets were offered ad libitum to week 9, then gradually removed over a 10 d period. All lambs were managed as a single group on ryegrass/white clover pasture from week 6 to 16 of the trial. Feed intake was recorded daily in the first 6 wks, and BWs recorded weekly during the trial. At weeks 2, 4, 6 and 8, and pre- and post-clostridial vaccination at week 8, blood samples were collected for analysis of selected blood metabolites, IGF-1 and immune function. Body composition was evaluated in 8 animals per group at week 4 and 16 following euthanasia, and duodenal samples collected for histomorphometric evaluation. Early weaned lambs had lower DM, ME, CP and NDF intake than Ctrl lambs from 21, 15, 21 and 36 d of rearing respectively (P<0.001) driven by the lower intakes of MR from day 15 (P<0.001) as per the experimental design, and lower total DMI of fiber (P=0.001) from 21 to 42 d of rearing. BW tended (P=0.097) to be lower in EW than Ctrl lambs from five to 10 wks of rearing only, with lower ADG in EW lambs from week 3 to 6 (P=0.041). Early weaning had negligible effects on duodenal morphology, organ and carcass weights at week 4 and 16. Plasma metabolites (urea nitrogen, triglycerides, NEFA, glucose and total protein) were similar between groups, while beta-hydroxybutyrate was greater in EW than Ctrl lambs at week 4 and 6 (P=0.018) but not week 8 indicative or early rumen development. Serum IGF-1 tended to be lower in EW than Ctrl lambs from week 2 to 6 only (P=0.065). All lambs developed antibody responses post-vaccination but there was no effect of treatment (P=0.528) indicating similar immune function. The results of this study illustrate that artificially-reared lambs can be weaned off MR by 4 or 6 weeks of rearing without compromising growth, small intestine morphology, major organ development and body composition, nor immune function at either 4 (pre-weaning) or 16 (post-weaning) weeks of age.