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Digestion Kinetics of Sequentially Extracted Cell Wall Components of Forages¹

Rate and extent of cell wall digestion are important factors limiting utilization of forages in ruminant diets. The objectives of this experiment were to compare two methods of calculating these parameters and to compare the in vitro digestion kinetics of cell wall, cellulose and hemicellulose sequentially extracted from alfalfa (Medicago sativa L.), tall fescue (Festuca arundinacea Schreb.), sorghum x sudangrass [Sorghum bicolor (L.) Moench], and wheat (Triticum aestivum L.) straw. Plant tissue was incubated in a rumen fluid-buffer medium for 3,6, 9,12, 24, 36, 48, 72, or 96 h. Cell wall components were extracted sequentially using glass microfiber filters with sintered glass crucibles. The kinetic model used was: C = C_De ^–k(t-L) + C₁; where C = concentration remaining at time t, C_D — digestible concentration, k = rate constant, L = discrete lag time, and C₁ = indigestible concentration. Parameter estimates were obtained using a In-linear least squares procedure and the multivariate secant nonlinear least squares procedure to determine if one method was more appropriate than the other. Both methods resulted in equations that described the data very well. Predicted and observed values were highly correlated (r > 0.99, P 0.01), and residual errors were similar for both procedures. There was, however, a bias associated with the nonlinear procedure, which tended to result in lower estimates of k and L when long lag times occurred. Within the grass species, there was no difference (P 0.05) among rate constants for digestion of cell wall components. Average rate constants for digestion of tall fescue, sorghum x sudangrass, and wheat straw cell wall components were 0.046,0.059, and 0.045 h^–1. Rate of digestion of alfalfa hemicellulose was 1.6 times faster than that of cellulose. Rate constants were 0.200 h^–1 for alfalfa hemicellulose and 0.125 h^–1 for cellulose. The results of this experiment indicate that use of the In-linear calculation procedure is preferable when the digestion kinetics of species with a wide range of lag times are compared. Application of both calculation methods allowed for a comparison of cell wall digestion kinetics in vitro.

Key Words: Rate constant • Extent of digestion • Lag time • Alfalfa • Medicago sativa L. • Tall fescue • Festuca arundinacea Schreb. • Sorghum x sudangrass • Sorghum bicolor (L.) Moench • Wheat straw • Triticum aestivum L.

² Assistant professor, Dep. of Agronomy, Univ. of Illinois and assistant professor, Dep. of Agronomy, Purdue Univ., respectively.

Received for publication October 11, 1985.