Gastrointestinal structure and function of preweaning dairy calves fed a whole milk powder or a milk replacer high in fat

The composition of milk replacer (MR) for calves greatly differs from that of bovine whole milk (WM), which may affect gastrointestinal development of young calves. In this light, the objectives of the current study were to compare gastrointestinal tract structure and function in response to feeding liquid diets having a same macronutrient profile (e.g. fat, lactose, protein) in calves in the first month of life. Eighteen male Holstein calves (46.6 ± 5.12 kg; 1.8 ± 0.55 d of age at arrival; mean ± SD) were housed individually. Upon arrival, calves were blocked based on age and arrival day and within a block were randomly assigned to either a whole milk powder (WP; 26% fat, DM basis, n = 9) or a MR high in fat (MR, 25% fat, n = 9) fed three times 3.0 L daily at 135 g/L through teat buckets. On d 21, gut permeability was assessed with indigestible permeability markers (Cr-EDTA, lactulose, and D-mannitol). On d 29 after arrival, calves were euthanized: the weight of the total forestomach without contents was greater in WP-fed calves. Furthermore, duodenum and ileum weights were similar between treatment groups, but distal jejunum and total small intestine weights were greater in WP-fed calves. The surface area of the duodenum and ileum did not differ between treatment groups; however, the surface area of the proximal jejunum was greater in calves fed WP. Urinary lactulose and Cr-EDTA recoveries were greater in calves fed WP in the first 6 h post marker administration. Tight junction protein gene expression in the proximal jejunum or ileum did not differ between treatments. The free fatty acid and phospholipid fatty acid profiles in the proximal jejunum and ileum differed between treatments and generally reflected the fatty acid profile of each liquid diet. Feeding WP or MR altered gut permeability and fatty acid composition of the GIT and further investigation are needed to understand the biological relevance of the observed differences.