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Selection among Nonlinear Models for Rate of Passage Studies in Ruminants¹

North Carolina State University⁴, Raleigh 27695

Abstract

Eight Toggenburg wether goats were fed either Coastal bermudagrass (Cynodon dactylon) or orchardgrass (Dactylis glomerata)-alfalfa. (Medicago sativa) hay (57:43%) in a reversal design. Four markers were used to study the movement of different digesta fractions: erbium for large particles >1,700 µm; ytterbium for medium particles <1,700 µm and >500 µm; chromium for small particles <500 µm and >32 µm; and cobalt to follow liquid passage. Fecal samples were collected every 4 h for 6 d. The excretion curves obtained for every marker in each animal were used for model selection. One-compartment gamma age-dependent models (gamma 2, 3 and 4 age-dependent), two-compartment (gamma 2 and gamma 3 age-dependent, age-independent) models and a biexponential model were fitted to the data. Residual sums of squares and the Davidson and MacKinnon test were used to select the most appropriate models. These procedures indicated that the models best describing the movement of different digesta fractions through the gastrointestinal tract of goats, in this experiment, were the biexponential model for liquids, gamma 2 two-compartment model for small and medium particles and gamma 3 two-compartment model for large particles. To accurately describe the behavior of different components of digesta in the digestive tract, different mathematical models are required. In general, models that describe the movement of large particles have higher orders of age-dependency than models that describe the passage of liquids or small particles.

¹ Paper No. 10544 of the Journal Series of the North Carolina ARS, Raleigh 27695-7601. Use of trade names in this publication does not imply endorsement by the North Carolina ARS or criticism of similar products not mentioned. This research was supported in part by the Small Ruminant Collaborative Research Support Program under a grant from the Agency for International Development. We wish to express our grateful appreciation to T. Johnson, Dept. of Stat. and Econ. and Bus., NCSU, whose comments have greatly improved the clarity of the manuscript.

² Present address: IDIAP, Apdo 958, David, Chiriqui, Panama.

³ Present address: Dept. of Biostatistics and Epidemiology, Univ. of Tennessee-Memphis, 877 Madison Ave., 38163.

⁴ Dept. of Anim. Sci.