Relationship between the Concentrations of Crude Protein and Apparently Digestible Protein in Forages

A study was made of the relationship between the concentration and digestibility of crude protein in forages representing a wide range of plant species, growth stages, preservation treatments, and sources of data. The digestibility of protein increases exponentially as the concentration of crude protein in forages increases. On the other hand, the percentage of digestible protein (Y) increases rectilinearly as that of crude protein (X) increases, and the relationship is expressed by the equation, Y=0.929×−3.48, for which the standard error of estimate was found to be 0.46%. It would appear that interrelationships exist among the constituents of forages which result in the excretion of metabolic nitrogen in a manner correlated with the crude protein concentration of forages. Thus, the true digestibility of protein is relatively constant regardless of the concentration of crude protein in forages. The relationship between the concentrations of crude and digestible protein in the leaves and stems of alfalfa and in grasses fertilized with high levels of nitrogen conformed closely to that derived from the study of a variety of other forages.