How does solid feed composition affect reticular development in calves receiving high milk replacer? A comparison of starter concentrate and alfalfa hay.

Reticulum, the second compartment of the ruminant stomach, is crucial in the stratification and sorting of ingested particles. Early-life nutrition influences forestomach development; however, its specific effects on reticular morphology are not well understood. This study evaluated the impact of two solid feed regimens-starter concentrate (Group A) versus alfalfa hay (Group B)-on reticular development in 60-day-old Holstein calves receiving 8 L/day of milk replacer. Twenty male calves were randomly assigned to the dietary treatments, and feed intake, body weight, and reticular morphometry were assessed. Reticulum samples were examined macroscopically, histologically, and using scanning electron microscopy. No significant differences were found in gross reticulum weight or linear dimensions between groups. Histological analyses, however, revealed greater structural complexity in Group B. Forage-fed calves exhibited more developed primary, secondary, and tertiary reticular crests (cristae reticulares), larger papillae along free margins, and more prominent unguiculiform papillae. These structural features likely improve mechanical sorting and retention of fibrous particles, promoting efficient fermentation. Starter-fed calves showed simpler crest architecture and smaller papillae, consistent with lower fiber intake; however, papillary development was still evident, indicating that solid feed contributes to epithelial growth. These findings emphasize the reticulum as a distinct anatomical and functional unit within the forestomach. Early inclusion of adequate dietary fiber through alfalfa hay enhances reticular complexity, which may support more effective particle sorting, improved fermentation, and long-term digestive efficiency. Overall, pre-weaning feeding strategies that incorporate fiber can positively influence reticular development, with potential benefits for nutrient absorption, digestive function, and calf growth.