Influence of Nonprotein Nitrogen and Protein of Low Rumen Degradability on Nitrogen Flow and Utilization in Lactating Dairy Cows

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Abstract
Four lactating cows, each fitted with a rumen cannula and duodenal and ileal t-cannulae, were used to measure flow and digestion of nitrogenous compounds in the digestive tract. Dietary dry matter contained 17% crude protein and 50:50 forage:concentrate. Treatments were: corn silage-soybean meal, corn silage-heated soybean meal, NH3-treated corn silage-soybean meal, and NH3-treated corn silage-heated soybean meal. Flow of organic matter to the duodenum was overestimated when La or Cr EDTA was used as an indigestible marker. This resulted in low estimates of ruminal digestion of organic matter and high estimates of N flow to the duodenum. Calculations using La or lignin as markers yielded similar organic matter flow to ileum and feces. With acid-detergent lignin as a marker, estimates of dietary N degraded in the rumen were: corn silage-heated soybean meal, 55.0%; NH3-treated corn silage-heated soybean meal, 58.8%; NH3-treated corn silage-soybean meal, 63.3%; and corn silage-soybean meal, 66.0%. Digestion in the small intestine of non-NH3 N was equal for all treatments suggesting that availability of heated soybean meal in the intestine was not different, although ruminal degradability tended to be lower. Feeding diets containing nonprotein N did not decrease available N at the duodenum.