Rumen degradation and fractional outflow rates of nitrogen supplements given to cattle eating sodium hydroxide-treated straw

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Abstract
1. Six rumen and abomasal cannulated heifers were used to study the effects of intake on the fractional outflow rates (FOR) of chromium-mordanted cotton-seed meal (Cr-CSM) and meat meal (Cr-MM), CrEDTA, ytterbium and lignin from the rumen. Values of FOR of Cr-CSM and Cr-MM were combined with values of nitrogen disappearance from the protein supplements, placed in porous synthetic (nylon) bags and incubated within the rumen (P), to calculate effective degradation (D)of CSM and MM when fed to heifers eating sodium hydroxide-treated straw. Also, N degradation in vivo (V) was measured as the difference between abomasal N flow and the sum of flows of microbial and endogenous N.2. FOR were positively related to intake and differences between supplements were significant (pD, 24 h after feeding, for CSM and MM respectively.4. With the exception of CSM at the high intake, estimates ofVwere underestimated byDand were 8.6–25.0% greater than theDvalues when time of incubation (t)= ∞. The two techniques, however, ranked the degradation of the two supplements in the same order at both levels of intake.5. Underestimation ofVbyDmay be attributable to underestimation ofP, overestimation of FOR (both resulting in underestimation ofD) or overestimation ofVdue to biases associated with the estimation of this part of the comparison. The relative importance of these factors remains to be determined.
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