Effect of Forage: Concentrate on Kinetics of Forage Fiber Digestion In Vivo

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Abstract
With five rumen-fistulated Holstein steers in a Latin square design, the effect of dietary concentrate (0, 20, 40, 60 or 80% cracked corn) on kinetic characteristics influencing forage fiber digestion in vivo was determined. Rate and potential extent of neutral deterent fiber degradation were determined for fescue hay in situ by nylon bag technique. Rate of fiber passage from the rumen was measured by fecal excretion of Cr-mordanted fescue cell walls. Apparent extent of forage fiber digestion was predicted by a model in which fiber disappearance from the rumen is conceptualized as the sum of 2 competing 1st-order processes, digestion and passage, modified further by a discrete lag time during which fiber passes from the rumen before digestion commences. Stepwise multiple regression analysis revealed that potential extent of fiber degration in situ was the primary determinant of depression of fiber digestibility as dietary concentrate increased. Rate of digestion, rate of passage and lag effects collectively accounted for a small portion of the depression of fiber digestibility.
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