The in situ technique is a direct methodof measuring the rumen degradation kinetics of a feed. It was suggested to describe the data obtainedby this technique by an exponential curve (0rskov and McDonald, 1979).This first order exponentialmodel is a most commonprocedure for detennining crude protein and dry matter degradationcoefficients. However, very low attention has been paid to the choice of mathematical model to fit the curves and goodness-of-fitof the model. Different mathematical models were evaluated to describe ruminal dry matter (DM) and crude protein (CP) degradation kinetics ofvarim,ls feeds (Lopez et al., 1999; Fathi et al., 2006). In the present study, two different mathematical models of a straight line or a negative exponential (Fathi et al., 2006) were selected to evaluate crude protein(CP)degradation kinetics of variousruminant feeds including alfalfa hay, corn and whole barley silages, barley and coni grains, cottonseed ~mdsoybean meals using data obtained from the in situ technique.

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