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This study examined the impact of weaning age (4 vs. 6 wk) on rumen morphological and metabolic development in artificially reared lambs. Thirty-two mixed-sex lambs (2 to 5 d old) were randomly allocated to 1 of 2 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 the study until 6 wk, when lambs were transferred to a mixed sward pasture. At week 4, individual DMI were recorded and blood samples collected to measure β-hydroxybutyrate (BHBA) and NEFA. Eight animals per group were euthanized at week 4 and 16 to evaluate short chain fatty acids (SCFA) and histomorphometry of the rumen walls: dorsal (DS), ventral (VS), dorsal blind (DBS), and ventral blind (VBS) sacs. Linear mixed models were used to analyze the effect of weaning treatments at week 4 and 16, and to compare between timepoints and between rumen sites. Dry matter intake and rumen SCFA profiles were similar between groups (P &gt; 0.10) at week 4 and 16. Plasma concentrations of BHBA were greater (P = 0.03) in EW than in Ctrl lambs, while NEFA did not differ (P &gt; 0.10) between groups at week 4. No effect of weaning age on rumen empty weight, and papillae density, length, width, surface area ratio, and muscle layer thickness at any of the 4 rumen sites was found (P &gt; 0.10) at either week 4 or week 16, except for greater (P = 0.02) papillae epithelium thickness in the DBS at week 4 in EW than Ctrl lambs. Papillae morphology and muscular thickness differed across rumen sites at week 4 and week 16 (P &lt; 0.05), except for papillae density and surface area ratio at 4 week (P &gt; 0.10). Rumen papillae length, width, and muscle layer thickness increased while papillae density and surface area ratio decreased (P &lt; 0.05) between 4 and 16 wk. 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 step-down weaning system. Morphological differences between rumen sacs denote 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 rumen development and ontogenic changes.

Impact of weaning age on rumen development in artificially reared lambs1