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Analytical Estimates of Nitrogen Digestibility in Heat Damaged Forages¹

Animal Science Research Division, USDA, Beltsville, Maryland 20705
and University of Maryland, College Park, Maryland

ABSTRACT

Alfalfa, orchardgrass, native grass, and mixtures of 31 hay and 16 silage heat damaged samples on which in vivo digestibility had been determined, were assembled to relate laboratory estimates to nitrogen digestibility. They represented a wide range of nitrogen concentration in dry matter (1 to 4.2) and nitrogen digestibility coefficients (6 to 85). The regression of digestible nitrogen concentration (g/100 g forage dry matter) on total nitrogen concentration explained only 71% of the variation in observed digestible nitrogen. Several alternative approaches were more effective for expressing and predicting digestible nitrogen concentration of these forages. Determining either acid-detergent soluble or pepsin soluble nitrogen as a concentration in dry matter and regressing digestible nitrogen on these fractions explained 89 and 93% of the variation in digestible nitrogen concentration. Multiple regressions predicting digestible nitrogen from both soluble and insoluble nitrogen concentrations in dry matter of either acid-detergent or pepsin explained 93 and 94% of the variation. Acid-detergent insoluble nitrogen and pepsin insoluble nitrogen as a percent of total nitrogen explained 86 and 83% of the variation in nitrogen digestion coefficients. Acid-detergent insoluble nitrogen concentration explained 81% of the variation in energy digestion. Energy digestion coefficients regressed on pepsin insoluble nitrogen concentration explained 63% of the variation.

¹ Data from a thesis presented by H. K. Goering in partial fulfillment of requirements for the Ph.D. degree, University of Maryland.

² Present Address: Department of Animal Science, University of Kentucky, Lexington 40506.

³ Present Address: Department of Animal Science, Cornell University Ithaca, New York 14850.

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