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

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Abstract
Disappearance rates for feedstuffs were determined by suspension of feedstuffs in polyester bags into a fistulated rumen of a lactating cow fed a corn-silage concentrate ration. Percent residual N 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 0.279-0.029 for N and from 0.259-0.023 for dry matter. Extended N disappearance ranged from 0.146-0.008 and dry matter from 0.088-0.007. N disappearance rate before 2 h was uncorrelated with the rate after 2 h, while dry matter disappearance before and after 2 h was correlated 0.81. Nine rations were formulated for 30, 45 and 60% degradable N with 3 physical forms (ground, chopped and all concentrate) and subjected to ruminal degradation in polyester bags. Of individual feedstuffs, degraded N was more predictable in rations containing no forage and least predictable in rations containing ground forage. Measured degradable N agreed most in rations containing 30, followed by 60 and then 45% predicted degradable. Solubility or short term N degradation appears invalid for predicting ruminally degradable N and especially in mixed rations.