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Relationship of Rate and Extent of Digestion to Forage Utilization-A Dynamic Model Evaluation^{1 ,2, 3,}

University of Georgia, Athens 30602

Abstract

Utilization of forages involves complex interactions among the plant, microbe in the gastrointestinal tract and the animal. A dynamic model of disappearance of forage from the digestive tract aids in delineating these interactions because processes are reduced to distinct and manageable components and relationships are precisely described in mathematical terms. Disappearance of feed from the digestive tract can be described by two major processes, digestion and passage. The digestive process can be divided into rates of digestion, digestion lag and potentially digestible fractions. Present information suggests that the size of the potentially digestible fraction is more important than rate of digestion or digestion lag in affecting digestibility and intake. It is also the component of die digestive process diat is most highly related to chemical composition, specifically lignin content. Factors affecting inherent rate of digestion and digestion lag of feeds have not been elucidated, although it has been observed that rate of digestion is related to morphological tissue types in plant leaves. The passage process can be divided into rates of particle size reduction, escape from die rumen and passage through the intestine. Although it has been observed diat retention time in the total digestive tract is related to intake level, physical form of the diet and rumination time, the specific component of the passage process affected and the magnitude of the effect have not been established. Use of a dynamic model to evaluate the relationship of rate and extent of digestion to forage utilization aids in clarifying conceptual frameworks for describing digestion and in stimulating well-defined and quantitative research efforts.

¹ Paper presented at the Symposium on "Production and Utilization of Forages and Fibrous Feeds," held on July 31, 1979 during the 71st Annu. Meet. of the ASAS, Univ. of Arizona, Tucson.

² The following donors are gratefully acknowledged for their financial support of the program: Agway, Box 1333, Syracuse, NY; American Colloid Co., Skokie, IL; A. O. Smith, Harvestore Products, Inc., 550 WEst Algonquin Road, Arlington Heights, IL; Bordin Inc.-Smith-Douglas Div., Nutritional Research Laboratory, RR 1, Elgin, IL; Iowa Limestone Co., Des Moines, IA and Occidental Chemical Co., Feed Products Div., Houston, TX

³ Publication of this paper was sponsored by the SEA, USDA, Administration Building, Washington, DC.