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Evaluation of Forage Digestibility Prediction Equations Using Chromogen and Nitrogen as Fecal Index Indicators¹

Regression analyses in a series of digestion studies involving three forage species of varying lnaturitles fed to sheep revealed significant relationships between the following: 1) feed-to-feces ratio and percent nitrogen in the feces (linear and curvilinear); 2) percent DOM in the forage and percent nitrogen in the feces (linear); 3) units of chromogen per gram of forage organic matter and units of chromogen per gram of feces organic matter (curvilinear); and 4) percent DOM of forage and units of chromogen per gram feces organic matter (linear). The standard errors of estimate (SEE) and standard errors of a predicted value (SEY) were quite high for all equations, although often similar to those reported by other workers.

The nitrogen fecal-index method for prediction of feed-to-feces ratio on a linear basis afforded a more reasonable estimate of DOM in forage than methods employing fecal chromogen. However, even the validity of this equation (Y = 1.07 + 0.72N, where Y = feed-to-feces ratio and N = percent nitrogen of fecal organic matter; SEE of 3.06%) would appear dubious for prediction of relative DOM of forage species, due to its bias against reed canarygrass. The need for local regressions developed for use with specific forage species is indicated.

Key Words: plant pigments • fecal nitrogen

² Present address: N. E. Missouri State Teachers College, Kirksville, Missouri 63501.

³ Former Research Assistant, Professor of Animal Science, and Research Agronomist. Grateful acknowledgments are given to C. E. Gates and Kathleen M. Keenan who assisted with the analyses of the data.

Received for publication January 8, 1968.