Factors Affecting the Nutritive Value of Range Forage

Abstract

The nutritive value of any forage is dependent upon its content of energy-producing nutrients as well as its content of nutrients essential to the body, namely, protein, minerals and vitamins. Forages supply energy mostly in the form of carbohydrates. The carbohydrate fraction makes up from 60 to 80 percent of the dry matter. The higher carbohydrates may be broken up into three classes : (1) starch, readily and nearly completely digested by the animal ; (2) cellulose, digested mostly ‘by bacteria in the rumen of ruminants; and (3) hemicelluloses, intermediate between starches and cellulose and broken down by weak acids and alkalies. In chemical analyses, these higher carbohydrates are arbitrarily grouped into two classes, crude fiber and nitrogen-free extract. Crude fiber consists almost entirely of cellulose and lignin together with some resistant hemicellulose. Lignin is essentially nondigestible by most domestic animals. However, Utah studies have shown that deer are able to digest up to 42 percent of this constituent in birchleaf mahogany (Cercolcarpus montanus) (Smith, et al., 1956). Nitrogen-free extract consists mainly of starch and most of the hemicelluloses. Protein commonly means nitrogen multiplied by the factor 6.25; however, 25 to 50 percent of the total nitrogen may be non-protein, essentially the amides and amino acids (Maynard, 1937). The value of protein is dependent principally upon its supply of certain amino a,ids which are required for the formation of body protein and which cannot be manufactured by the body. The amount of digestible protein furnished by a plant depends upon the plant species and the class of livestock using the plant. In addition, digestibility of protein varies appreciably between domestic animal species and somewhat between individuals o,f a given species. Minerals are essential for the proper functioning of the bodily processes. Aside from sodium, chlorine, calcium, and phosphorus, most of the essential elements are ample in forages except when grown on mineral-deficient areas, i.e., cobalt in Florida. The precursors of vitamin A and vitamin D are frequently limiting in range forage. Carotene is the precursor of vitamin A, which is manufactured in the animal body, and ergosterol is the precursor of vitamin D in plants. Cholesterol is the animal form of vitamin D precursor and like ergosterol, it needs sun exposure for transformation into vitamin D (Maynard, 1947). Both of these vitamins play an important role in body processes and growth.