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Ruminal Disappearance of Crude Protein and Dry Matter in Feeds and Combined Effects in Formulated Rations

Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061

ABSTRACT

Disappearance rates for feedstuffs were determined by suspension of feed-stuffs in polyester bags into a fistulated rumen of a lactating cow fed a corn-silage concentrate ration. Percent residual nitrogen and dry matter were measured at 2, 12, 24, 36, and 48 h, and fractional disappearances per hour were ascertained for all feedstuffs. Two-hour disappearance rates for feedstuffs ranged from .279 to .029 for nitrogen and from .259 to .023 for dry matter. Extended nitrogen disappearance ranged from .146 to .008 and dry matter from .088 to .007. Nitrogen disappearance rate before 2 h was uncorrelated with rate after 2 h while dry matter disappearance before and after 2 h was correlated .81. Nine rations were formulated for 30,45, and 60% degradable nitrogen with three physical forms (ground, chopped, and all concentrate) and subjected to ruminal degradation in polyester bags. Of individual feedstuffs, degraded nitrogen was more predictable in rations containing no forage and least in rations containing ground forage. Measured degradable nitrogen agreed most in rations containing 30, followed by 60, then 45% predicted degradable. Solubility or short term nitrogen degradation appears invalid for predicting ruminally degradable nitrogen and especially in mixed rations.