The Effect of Starch on Forage Fiber Digestion Kinetics In Vitro

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Abstract
Purified corn and wheat starch were added to alfalfa, Coastal bermudagrass, fescue and orchardgrass hays [ruminant forages] at 0, 40, 60 and 80% of the total as-fed substrate and fiber digestion kinetics were determined in vitro. Kinetics were estimated by the model R = Doe-k(t-L) + U where R is residue remaining at time t, Do is digestible fraction, k is digestion rate constant, L is discrete lag time and U is indigestible fraction. Parameters of the model were estimated by logarithmic transformation and a direct nonlinear least squares procedure. Corn and wheat starch did not differ in their effect upon lag time of fiber digestion, digestion rate or potential extent of digestion. Alfalfa had a shorter lag time of fiber digestion (0.86 h) than Coastal bermudagrass (3.05 h) but not orchardgrass or fescue (1.66 and 2.42 h). Orchardgrass differed in fiber digestion rate (0.0542 h-1) from Coastal bermudagrass (0.0698 h-1)but not alfalfa or fescue (0.0670 and 0.0658 h-1). Potential extent of fiber digestion was similar for fescue (75.8%) and orchardgrass (76.0%). Potential extent of fiber digestion for alfalfa (50.9%) differed from Coastal bermudagrass (64.3%) and both differed from fescue or orchardgrass. Addition of starch resulted in a linear increase in lag time of fiber digestion but digestion rate was not affected. Potential extent of digestion was decreased when starch was added.