Edible mushrooms are able to bioconvert a wide variety of lignocellulosic materials due to the secretion of extracellular enzymes [3, 11]. Literature reports that the bioconverted materials show an increase in their protein contents and a decrease in their fiber content[10,15] being possibly used as ruminant feed supplements [4, 5]. Traditionally, the evaluation of the quality of the converted materials is under the same criteria as those applied to the evaluation of conventional forages [14]. Nutrient availability is evaluated by chemical analyses such as dry matter, ash, protein, lignin, cellulose and hemicellulose determinations. Digestibility is evaluated through in vitro and in vivo assays and palatability suppose feed selection trials with whole animals [1, 8]. There is an increasing tendency to avoid animal experimentation. This implies the usage of even more indirect assays for the estimation of digestibility. For instance, Japan Livestock Technology Association recommends an enzymatic fractionation of dry matter into cell contents and cell wall. In turn, these are also fractionated in organic cell contents and organic cell wall. Cell wall is further divided into high digestibility and low digestibility fractions. Organic cell contents and the high digestibility fraction are assigned with a 100% and 95-100% digestibility respectively. Depending on the assayed material, the low digestibility fraction may have from 40% to 50% digestibility [9]