Only the animal can integrate all of the known and unknown factors inherent in the forage that determine forage quality or the actual ability of the forage to meet the animal’s nutritional needs.
Thus, the one true test is to feed the forage to the animal and measure the animal’s response. Because both the quality of the growing forage and the needs of the animal are constantly changing, it is more practical to use indicators of nutritive value based on laboratory analyses, including percentages of crude protein, fiber, minerals and energy.
These measures help in predicting animal response. For stored forages, color, smell, absence of weeds, foreign material, mold and/or dust can provide further evidence of nutritive value. 
From Forage-Livestock Quotes and Concepts, D.M. Ball, et. al
