Site and Extent of Protein Digestion in Lactating Cows Fed Alfalfa Silage or Baled Alfalfa Hay

Four lactating Holstein cows fitted with T-type cannulae in the proximal duodenum and terminal ileum were in a three-period switchback experiment to monitor nitrogen digestion and sites and extent of nutrient digestion when alfalfa, preserved as hay or as silage (47.9% dry matter), was served as the forage and major source of nitrogen in the ration. Cows were fed four times daily, and diets contained 65% forage and 35% grain. Lanthanum and samarium served as indigestible markers for calculating digesta flow rates and nutrient digestibilities. Apparent digestibility of organic matter tended to be greater for the silage diet, primarily due to increased disappearance of organic matter in the small and large intestine. Nonammonia nitrogen flows to the duodenum were 79 and 74% of nitrogen intake for hay and silage diets. The apparent degradability of dietary crude protein in the rumen was 80% for both forage sources. This indicates that alfalfa hay and alfalfa low-moisture silage were equal in ability to supply protein to the small intestine, even though nitrogen solubilities for hay and silage were 40 and 63%. Digestibilities of amino acids in the small intestine were not affected by source of forage and were 62.3 and 58.1% for hay and silage diets. Apparent digestibilities of individual amino acids in the small intestine differed.