The results show that the dry matter of the corn silage was found to be 62.9 per cent digestible, the protein 55.3 per cent, the fat 82.1 per cent; i.e., the digestion coefficients for the different components in the feed were as follows (leaving off fractions) : Dry matter, 63; protein, 55; fat, 82; fiber, 45; nitrogen-free extract, 72, and ash, 18.

If the digestion coefficients for the organic matter in silage is wanted, it is readily obtained by calculating the amount of this component in the feed and feces, as follows: In silage, 20.55 (dry matter) minus 0.88 pound (ash) equals 19.67 pounds (organic matter); in feces, 7.62 less 0.72 equals 6.90; 19.67 less 6.90 equals 12.77; percentage digestible, 12.77 divided by .1967 equals 64.4. It was found, therefore, that 64 per cent of the organic matter of the silage was digestible.

In the case of feeding stuffs that cannot be fed alone (i.e., a grain feed for ruminants) it is necessary to feed it along with some feed of known digestibility that will supplement it so that when fed together they will make at least a fairly normal ration. The calculated amounts of digestible components in the second feed are then deducted from the total digestible amounts of the various components in the ration fed, and the difference is calculated on a percentage basis of the total amounts present in the feed whose digestibility was to be determined (Fig. 8).

Interpretation of Results.—The figures obtained in digestion trials show the proportions of the components of the feed that have been dissolved by the digestive fluids of the body and retained for the uses of the animal. This is true only in a general way, for various factors render the matter much more complicated. There reappears in the dung not only the undigested matter of the feed, but small amounts of residues of the digestive juices, waste products in the activity of the digestive organs, and intestinal mucus. The amount of these waste products in the dung can, however, be determined in the laboratory and a correction made accordingly. This is done by means of artificial digestion of the dung with a pepsinhydrochloric-acid solution (Kühn's method), and making proper deductions for these in the calculations. Another and more serious source of error is introduced by the fact that the feed is subjected to the action of bacteria and ferments in the paunch and intestines through which gaseous products are formed, as previously stated (p. 30). These attack especially the fiber of plant materials, and the figures obtained for the digestibility of these components, therefore, include a portion which has not been dissolved by the digestive fluids of the animals and taken into circulation. This portion does not contribute to the maintenance or the growth of the body, and is of value to the animal only in so far as the heat generated by the fermentation processes helps to maintain an even body temperature. In spite of these errors to which digestion trials are subject, the result obtained by them are of greatest value to both the feeder and the student of nutrition problems. Until toward the end of the last century, our theories of these problems and the science of animal nutrition rested almost wholly on the knowledge of the feeding stuffs gained through chemical analyses and digestion trials.

Coefficients of Digestibility.—The average digestion coefficients for the more important American feeding stuffs as determined in digestion trials with ruminants, are given in Appendix Table II. Complete compilations of digestion coefficients determined for American feeding stuffs with different classes of farm animals will be found in several U.S. Department of Agriculture and State publications, as well as in standard reference books on the subject; these compilations also give the number of separate trials conducted and the number of animals experimented with in each case, as well as the variations in the results of the separate trials. The following table shows the digestion coefficients for a few representative feeds:

Digestion Coefficients for Ruminants

B. THE ENERGY OF FEEDING STUFFS The feeds which animals eat are directly or indirectly derived from plants. During their growth plants store up chemical energy in the stems, leaves and fruit, which becomes available as food for man and beast. This latent or potential energy in plant materials is again set free on combustion (oxidation) and is utilized by the animals in one or more of three ways, either directly as kinetic energy for maintenance of body heat, or transformed into motion as mechanical energy for doing mechanical work, or changed into chemical energy. If the energy supplied in the feed is more than sufficient to cover the demands for the first two purposes given, the excess may be stored up in the body as chemical energy in the form

10 20 30 40 50 60 70 80 90 NUTRITIVE RATIO COTTON-SEED MEAL

1:1.2 LINSEED MEAL

1.5 SOYBEANS

1.8 DRIED BREWERS' GRAINSI

2.3 GLUTEN FEED

2.4 COW PEAS

3.1 WHEAT BRAN

4.0 ALFALFA HAY

4.3 WHEAT MIODLINGS

4.6 RED CLOVER HAY

5.9 OATS

6.2 RYE

7.8 BARLEY

8.0 INDIAN CORN

9.9 DRIED BEET PULP

11.9 TIMOTHY HAY

16.2 CORN STALKS

23.6 OAT STRAW

383 SKIM MILK

2.0 COWS' MILK

38 PASTURE GRASS

4.5 MANGELS

4.9 RAPE

4.3 TURNIPS

7.7 CORN SILAGE

120 I PROTEIN W CARBOHYDRATES AND FAT Fig. 8.-Digestible components and nutritive ratios of common feeds, in per cent.

(See also Frontispiece)