In Situ Degradation Kinetics: Evaluation of Rate Determination Procedure

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Abstract
In situ digestion profiles from dry matter, acid detergent fiber, neutral detergent fiber and alfalfa hay, and dry matter from orchardgrass hay were selected to compare methods to describe kinetically degradation rates. The four methods were (1) least squares regression of logarithmic-transformed residuals without correction for ruminally undegradable residue, (2) method 1, with correction for undegradable residue, (3) curve peeling, and (4) nonlinear iterative least squares regression. Not correcting data sets for undegraded residue resulted in rate constants for all chemical components of forage approximately one-third as fast compared with results correcting each fermentation point. Neutral detergent fiber of alfalfa hay and dry matter from orchardgrass hay exhibited single component linear degradation profiles; however, alfalfa hay dry matter and acid detergent fiber each exhibited 2 distinct linear component profiles. Methods 2, 3, and 4 estimated the following rate constants for alfalfa hay dry matter and acid detergent fiber: 7.5, 22.3, and 17.4%/h and 4.4, 25.2, and 8.8%/h, respectively. The nonlinear method tended to underestimate lag and potential extent of degradation to achieve a best fit curve for alfalfa acid detergent fiber and tended to overestimate potential extent of degradation when on linear component existed. One specific method may not be adequate to describe all degradation profiles.