Influence of ration physical form and nitrogen availability on ruminal morphology of growing bull calves.

Male Holstein calves were fed complete diets containing three physical forms (ground and chopped hay and all concentrate) and three rumen degradable protein percent. Calves were slaughtered at 20 wk of age. Weights of reticulorumen with and without contents wee recorded. Proportions of mucosa and muscle and gross morphology were determined on excised rumen wall. From nine additional calves rumen epithelial tissue was obtained for light microscopic examination of papillae to determined length, width, surface area, and morphological condition. Weights of reticulorumen with contents were heaviest for calves fed ground hay and tissue weights heaviest for calves fed concentrate. Ruminal content volume was greater for calves fed diets containing hay compared to all concentrate. Hay contents tended to be drier and less dense than concentrate contents. Ruminal epithelial lining was heavier in concentrate-fed and had more mucosa to muscle compared to rumen of calves fed ground or chopped hay. However, papillary length, number of papillae per field, and surface area/field was greater in calves fed hay. Morphological abnormalities of the rumen epithelium occurred more frequently in calves fed concentrate. Reticulorumen measures were unaffected by degradable protein percent. Absorptive capacity of calves fed all concentrate diets decreased compared to calves fed diets with 40% hay.