Roughages are bulky feeds containing relatively less digestible material i.e., crude fibre more than 18% and low (about 60%) in TDN on air dry basis. Most of roughages have a high content of cell wall material. The cell wall fraction may have highly variable hemicellulose, pectin, polyuronides, silica and other components. In contrast to cereal grains, roughages generally are low in readily available carbohydrates. The amount of lignin is a critical factor with respect to digestibility. Lignin is an amorphous material which is associated closely with the fibrous carbohydrates of the cell wall of plant tissue. It limits fibre digestibility, probably because of the physical barrier between digestive enzymes and the carbohydrate in question.
The protein, mineral and vitamin contents of roughages are highly variable. Legumes may have 20% or more crude protein content, although a most of may be in the form of non protein nitrogen (NPN). Other roughages, such as straw may have only 3-4% crude protein, most others fall between these two extremes. Mineral content may be exceedingly variable; some roughages are relatively good sources of calcium and magnesium, particularly legumes. Phosphorus content is apt to be moderate to low and potassium content high; the trace minerals vary greatly depending on plant species, soil and fertilization practices.
Roughages are sub-divided into two major groups; dry and green or succulent roughages based upon the moisture content. Green roughages usually contain moisture from 60-
90%, whereas, dry roughages contain only 10-15% moisture. For the sake of convenience, succulent feeds are again classified into various types such as pasture, cultivated fodder crops, tree leaves, roots and crops. Dry roughages have been further classified as hay and straw, based on the nutritive values and methods of preparation.