The evaluation of PDI concentrations in some ruminant feedstuffs: a comparison of in situ and in vitro protein degradability

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Abstract
The effective rumen degradability (Dg) of dry matter (DM), nitrogen (N) and non-protein organic matter (NPOM) of 16 commonly used ruminant feedstuffs was measured in situ. The PDIA, PDIE and PDIN contents of the feedstuffs were calculated either using the DgN and DgNPOM in situ values or using the DgN and fermentable organic matter (MOF) data derived from INRA tables. The regressions between values from the 2 systems had high r2 values,s but only agreed closely for PDIN values. The difference between PDIA values calculated using the 2 systems was small, but PDIE values differed considerably. The in vitro enzymatic degradability of the same 16 feeds was also measured, using protease extracted from Streptomyces griseus. The 1-h enzymatic degradability of the feeds was higher than the value in situ, except for the maizemeal and forages. However, protease degradability after 24 h was lower than the effective degradability in situ. Only the animal proteins deviated from this rule. Significant correlations were found between the coefficient a in situ and as and bs in vitro, and between bs in vitro and b in situ. The effective degradability in situ was highly correlated with the enzymatic degradability.