Impact of weaning age on rumen development in artificially reared lambs

This study examined the impact of weaning age (4 vs. 6 wk) on rumen development, differential morphology and ontogeny of the four ruminal sites in artificially-reared lambs. Thirty-two mixed-sex lambs (2-5 d old) were randomly allocated to one of two weaning groups: early weaning (EW; 4 wk) and control (Ctrl; 6 wk). Lambs were individually penned and fed milk replacer (MR; 24% CP and 25% fat, DM basis) at 20% of their corresponding initial BW. Weaning was achieved by gradual reduction of MR allowance over a period of 3 wk using a step-down procedure. Concentrate and meadow hay were offered ad libitum from 1 d of study until 6 wk, when lambs were transferred to a mixed sward pasture. At wk 4, individual intakes were recorded to estimate DMI and blood samples collected to measure β-hydroxybutyrate (BHBA). Eight animals per group were euthanized at wk 4 and 16 to evaluate short chain fatty acids (SCFA) and rumen wall histomorphometry of the dorsal (DS), ventral (VS), dorsal blind (DBS) and ventral blind (VBS) sacs. Hay intake tended to be lower in EW than Ctrl lambs at wk 4 (P=0.07), while no differences in concentrate and total solid feed intake were detected (P≥0.13). Fermentation profiles were similar between groups (P≥0.22) at wk 4 and 16. Plasma BHBA concentrations was 65% higher (P=0.01) in EW than in Ctrl lambs at wk 4. No effect of weaning age on rumen weight (full or empty), and papillae density, length, width, surface area ratio and muscle layer thickness (at any of the four rumen sites) was found (P≥0.14) at either wk 4 or wk 16, except for papillae epithelium thickness in the DBS which was greater (P=0.02) at wk 4 in EW than Ctrl lambs. Rumen site had a marked effect on rumen histomorphometry measurements at both wk 4 and wk 16 (P≤0.004), except for papillae density and surface area ratio at 4 wk (P≥0.09). Rumen ontogeny varied between 4 and 16 wk (P≤0.028) irrespective of treatments, but papillae epithelial thickness did not alter (P≥0.24). The results of this study indicate that morphological and physiological development of the rumen can be accelerated to support weaning of artificially-reared lambs at 4 wk, using a grain/fiber-assist step-down weaning system, without carryover effects to 16 wk. Morphological differences between rumen sacs indicate that future studies in lambs evaluating the impact of different diets should involve representative sampling across the rumen rather than a single site to more accurately study developing changes.