Does the nature of solid feed influence rumen development? A comparative study of calves fed a high milk replacer with starter concentrate or alfalfa hay

This study evaluated the effects of two milk replacer-based diets, one supplemented with starter feed and the other with alfalfa hay, on rumen development in preweaned Holstein calves. Twenty newborn male calves were fed 8 L/day of milk replacer plus either starter feed (group A) or alfalfa hay (group B) ad libitum for 10 weeks. Rumen morphometry and histological features were assessed via gross anatomical evaluation, histology, and scanning electron microscopy. The calves in group A consumed significantly more solid feed and exhibited greater papillae development, with higher papillae height, width, and density per cm2, than those in group B. Moreover, the surface enlargement factor was consistently greater in group A across ruminal regions, except in the caudodorsal blind sac (saccus cecus caudodorsalis; p < 0.05). Empty rumen weight was also higher in starter-fed calves, suggesting enhanced mucosal development and absorptive capacity. Notably, keratin layer thickness did not explain the observed differences between the groups. Despite similar final body weights and growth, calves supplemented with starter feed showed greater rumen papillae development and absorptive surface area than those supplemented with alfalfa hay. These findings suggest that starter feeding during high milk allowance promotes epithelial maturation and enhances post-weaning absorption of fermentation products.