Nitrogen Metabolism in Dairy Cattle. II. Soybran Flakes versus Oats as Principal Sources of Digestible Carbohydrates for Nitrogen Utilization

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Abstract
Nitrogen metabolism, food intake, and milk production were studied using 6 lactating dairy cows in a 3 X 3 Latin-square design. Soybran flakes and soybean oil meal were compared to either (1) soybran flakes, ground shelled corn, and soybean oil meal or (2) ground whole oats, ground shelled corn, and soybean oil meal as the principal sources of digestible nutrients. The 2 soybran flake mixtures were also compared to an alfalfa hay and grain concentrate ration, using 6 dairy heifers. The percentages of cellulose, starch, pentosans, polyuronides, and sugars were determined for each ration. Apparent protein digestibility reflected differences in the protein content of the complete rations. Cellulose digestibility was unusually high in the soybran flake rations, 77 to 79%. Higher average yields of milk nitrogen and larger positive nitrogen balances resulted in greater N efficiency in cows fed soybran flakes. However, differences in digestible dry matter intake were correlated with differences in N utilization, and adjusting for digestible dry matter intake accounted for the differences in N efficiency. Abrupt shifts to high levels of cereal grain in the ration depressed milk yield because of lowered feed intake. It was concluded that cellulose is equal to starch for supporting N metabolism in growing or lactating dairy cattle, provided the cellulose can be digested.