Composition, Maturity, and the Nutritive Value for Forages

16 Composition, Maturity, and the Nutritive Value for Forages P E T E R J. V A N SOEST Downloaded by PENNSYLVANIA STATE UNIV on June 8, 2012 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0095.ch016

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U . S. Department of Agriculture, Agricultural Research Service, Animal Husbandry Research Division, Beltsville, M d . 20705

Nutritive value of forages depends in part upon the availability of nutrients in the plant for the animal. This availability is controlled by the chemical composition of the forage in respect to factors limiting the utilization of cellulose and hemicellulose. These include lignin, silica, and the total amount of plant cell wall substances. An important part of nutritive value is that of consumption or voluntary intake which is partly related to cell wall content and bulkiness of the forage. Consumption is much more variable due to the type and physiology of the animal and prediction from composition is much more difficult. The composition of the plant is controlled to a large extent by light, temperature, and maturity factors, but different plant species vary individually in this respect.

"Corages are important i n agriculture because they can be grown on lands where other crops are less easily grown and because they are the foundation of animal agriculture. A characteristic feature of forages is their fibrousness, which they have i n common with by-products of the cereals, including straw, hulls, and bran. Paper and wood pulp also share these attributes of forages, i n that they contain a potential supply of cellulosic carbohydrates for the ruminant animal. Cellulose is important to herbivores possessing the intestinal microflora that can digest it (27, 28); it is of no value to animals that do not harbor these cellulytic organisms i n their digestive tracts, for cellulases are not secreted by higher animals.

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Present address: New York College of Agriculture, Cornell University, Ithaca, New York, 14850. 1

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In Cellulases and Their Applications; Hajny, G., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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The same biological facts hold true for the hemicelluloses, which cannot be digested by animals unless the requisite bacteria reside i n their digestive tracts. M a n y grasses contain at least as much hemicellulose as cellulose. Thus, the large proportion of cellulose and hemicellulose i n forages determines the place of these plants i n the nutrition of animals, and a considerable amount of nutritional and physiological research has been aimed at understanding the utilization of cellulosic carbohydrates by niminants and other herbivores. Cellulose itself has sometimes been used as a negative measure of forage quality as an estimate of either fiber or of overall forage drymatter digestibility i n in vitro cellulose digestion. This usage has its weaknesses in that cellulose is seldom more than a third of the plant dry matter or roughly half of the total plant cell wall. The proportion of cellulose to lignin and hemicellulose varies widely, and thus cellulose may not be quantitatively representative of the fibrous constituents of the forage. In the discussion that follows, the term cellulosic carbohydrates w i l l be taken to include both cellulose and hemicellulose. Classification of Forage Components. In general, chemical constituents of plants may be divided into the structural components of the cell wall (lignin, cellulose, and hemicellulose) and the more soluble cellular contents (sugars, starch, fructosans, organic acids, lipids, and the nitrogenous fractions consisting of about two thirds protein and a third nonprotein nitrogen compounds). Cellular contents often exceed 50% of the dry matter of forages and represent material of very high nutritive availability. The fibrous cell wall is much more insoluble and contains very nearly all of the matter that is truly indigestible (see discussion below). Many tables of feed composition published during the past 100 years present proximate analyses involving ether extract, crude fiber, protein (total Kjeldahl nitrogen), ash, and nitrogen free extract ( N F E ) . The proximate system does not properly separate the plant carbohydrates, most of the lignin and hemicellulose and some cellulose being extracted into the N F E , which is supposed to represent soluble and hence available carbohydrates. Crude fiber is composed largely of insoluble cellulose and does not recover fibrous fractions of the plant. Its use is thus limited by the same problems listed for cellulose above (56, 61). Recent efforts to find a replacement for crude fiber have originated in the ruminant field, where the amount and nutritional quality of fiber is of particular importance. The remainder of this discussion w i l l explore aspects of plant composition and animal utilization that make the newer systems of forage analysis applicable.


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Forage Nutritive

Value

Analysis of the causal relationships between maturity, composition, and nutritive value is here approached through discussion of three types of problems—those involved in (a) defining and determining nutritive value; (b) relating chemical composition to various aspects of nutritive value; and (c) accounting for the effect of environment on plant composition and nutritive value. These problems are common to several fields, including nutrition, biochemistry, and plant physiology, and while it is impossible i n a chapter to do justice to them all, it is important to catalogue them and to provide a perspective. Problems in Measuring Nutritive Value. A basic problem i n the evaluation of feeds, and particularly forages, is the integration of different and not always compatible factors into a single numerical scale. Everyone w i l l agree that the best measure of nutritive value or feed quality is the animal productivity that can be obtained on a given feed. However, this may be broken down into digestibility, level of consumption, and efficiency—that is, an animal's production is limited by the digestible calories and nutrients that are consumed and the efficiency of use once absorbed. The most commonly determined measure of nutritive value is digestibility, which may be expressed in various ways, often as a percentage of dry matter or organic matter. The traditional calculation of total digestible nutrients ( T D N ) which attempts to place fat, carbohydrate, and protein on an equivalent energy basis by means of factors, is gradually being replaced by a system i n which energy digestibility is determined directly by bomb calorimetry of feed and feces. Estimates of the various energy losses on the part of the animal lead to the net energy value, which is expressed as megacalories available per 100 lb. feed. Division of this number by a measure of digestible energy or T D N yields an estimate of energy efficiency. Values for T D N , net energy and efficiency are given in Table I for a number of feeds. As can be seen, T D N and net energy do not evaluate feeds identically (25, 43), a unit of T D N i n forage having a lower net energy value than a T D N unit of a concentrate feed like corn or wheat. The reason for this difference is probably related to the energy lost i n fermentation and rumination of cellulosic carbohydrates and in the fermentation of methane. Methane loss and decrease i n efficiency are related to fiber content. The lower efficiency of cellulosic feeds relative to the concentrates is an economic factor limiting greater use of fibrous feedstuffs in the livestock industry in America at the present time. More research needs to be done on improving the efficiency of cellulose digestion by animals.


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Table I.

Relative Values of Feeds Expressed on a Total Digestible Nutrient (TDN) and Net Energy Basis (NE) * TDN Ib./lOOlb.

Feeds Corn Barley Oats Alfalfa Oat Straw Wheat Straw Downloaded by PENNSYLVANIA STATE UNIV on June 8, 2012 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0095.ch016

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Forages

80 78 70 51 45 41

NE therms/100 lb. 80 71 66 41 23 10

NE as percent of TDN

Cell Wall Constituents

100 91 94 80 52 25

10 19 30 45 70 82

•Reference 42. Despite the great value of efficiency and intake measurements, digestibility measurements predominate because of their ease and reproducibility under controlled feeding conditions. Net energy measurements (and therefore, efficiency estimates) are very expensive and time-consuming, and the much smaller body of net energy data has up to now been a primary factor limiting its wider use. The remaining aspect of nutritive value, the amount of food eaten, is one of the most problematic areas in nutrition, because no satisfactory measure of consumption as an attribute of a feed or forage has been developed. The range i n consumption among animals is dependent upon psychological and physiological factors that are not always related to the nature of the feed. Yet variation in consumption is more responsible for the range in animal productivity than is digestibility (3, 10, 11, 14). There are two aspects of food consumption: palatability and voluntary intake (47, 48). Palatability connotes the appealing characteristics of the feed and is measured as the acceptability or relative amount eaten when a number of forages are offered in cafeteria style. Voluntary intake means the amount that is consumed when a single feed is offered to the animal. Often forages that are rejected i n cafeteria feeding w i l l be consumed to a great extent in a voluntary intake trial. Ranking of forages by acceptability and voluntary intake w i l l not be the same, but there is obvious mutual interdependence involving animals, since it is the total consumption of digestible energy that determines the limits of the animal for growth, fattening, or milk production. Ranking of forages by acceptability and voluntary intake w i l l not be the same, but there is obvious mutual interdependence involving animals, since it is the total consumption of digestible energy that determines the limits of the animal for growth, fattening or milk production. The problem of intake measurement is that while reproducible values can be obtained for sheep (wethers) under specified conditions, the


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results w i l l be correlated only slightly or not at all with those from beef cattle or dairy cattle (7, 29, 48). Dairy cattle present the greatest dissimilarity i n that they consume much more feed in relation to body size than do other animals, including nonlactating dairy animals (11, 29). Intake measurements comparing dairy heifers and lactating cows rank forages differently (29). A further problem is that while all measurement trials consist of single-fed feeds or forages, practical feeding for production consists of carefully combined mixtures. Intake values of single feeds may not combine.to give a reliable prediction of the intake of mixtures.

DRY MATTER DIGESTED,

%

Figure 1. Showing the relationship of dry matter digestibility to adjusted feed intake (lb. per day) at two levels of milk production. Adjustments were proportional to weight below 66.7% dry matter digested and weight raised to the 0.51 power above 66.7% digested. Milk production expressed as pounds equivalent of 4% fat corrected milk (FCM). (Reference 13) M u c h current research on the intake problem has characterized many forages as having low intakes relative to more concentrated foods. Consumption suggests a positive relationship with digestibility—that is, the more digestible forages w i l l be consumed at a higher level. This tends to be an oversimplification. Conrad (12, 13) has shown that intake of



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Forages

highly digestible feeds is inversely related to digestibility, the dominating factor being the animal's requirement for metabolizable energy. Less of the more digestible feed is needed to meet this requirement. A t lower levels of digestibility—ca. 65-70—there is an inflection i n the curve (Figure 1) below which intake declines with digestibility. Intake and Composition. A number of researchers (14, 16, 17, 46) have related the sloping part of the curve to the volume and rate of digestion of the cellulosic fraction of the forage. These parameters are not always highly correlated with overall dry matter digestibility. The volume of coarse forages is related to cell-wall content, which has been related negatively to voluntary intake (57); cell-wall content above 6 0 % of dry matter is related to decline i n consumption. Levels of cell wall below 6 0 % are associated with diminished responses i n intake, as shown in Figure 2. Some forages with very high cell-wall volumes (grasses) have low lignin contents, thus promoting relatively higher digestibility with low intake; legumes present an opposite picture of high lignin, low cell-wall content, and higher intake (57).

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Composition, Maturity, and the Nutritive Value for Forages

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