Fig. 118.—Bag for
Collecting Feces.
Fig. 119.—Fecal Bag Attachment.
Healthy animals in the prime of life are used, and the feeding experiments are conducted with as large a number of animals as possible, in order to eliminate the effects of idiosyncrasy. The food used is previously prepared in abundant quantity and its composition determined by the analysis of an average sample.
The feeding period is divided into two parts. In the first part the animal is fed for a few days with the selected food until it is certain that all the excreta are derived from the nutrients used. In the second part the same food is continued and the excreta collected, weighed, the moisture determined, and the total weight of the water-free excreta ascertained. The first part should be of at least seven and the second of at least five days duration. The urine and dung are analyzed separately. Males are preferred for the digestion experiments because of the greater ease of collecting the urine and feces without mixing. For ordinary purposes the feces only are collected. The methods of analysis do not differ from those described for the determination of the usual ingredients of a food.
Example.—The following data taken from the results of digestive experiments, obtained at the Maine Station, will illustrate the method of comparing the composition of the food with that of the feces and of determining the degree of digestion which the proteids and other constituents of the food have undergone.
Composition of Maize Fodder and of Feces
Therefrom after Feeding to Sheep.
| Before Drying. | ||||||
|---|---|---|---|---|---|---|
| Food | Water, per cent. | Ash, per cent. | Proteid, per cent. | Fiber, per cent. | Fat, per cent. | Undetermined, per cent. |
| Sweet maize | 83.85 | 1.13 | 2.18 | 4.14 | 0.62 | 8.08 |
| Feces | 72.01 | ... | ... | ... | ... | ... |
| Dry. | ||||||
| Food | Ash, per cent. | Proteid, per cent. | Fiber, per cent. | Fat, per cent. | Undetermined, per cent. | |
| Sweet maize | 7.01 | 13.52 | 25.63 | 3.86 | 49.98 | |
| Feces | 14.42 | 17.52 | 19.34 | 2.68 | 46.04 | |
| Daily Weights. | ||||||
| Food | Green, grams. | Dry, grams. | ||||
| Sweet maize | 2521 | 407 | ||||
| Feces | 445 | 125 | ||||
| Per Cent Digested. | ||||||
| Food | Ash, | Proteid, | Fiber, | Undetermined, | Fat, | |
| Sweet maize | 37.0 | 60.2 | 76.9 | 71.8 | 78.3 | |
In the above instance it is seen that the coefficient of digestibility extended from 37.0 per cent in the case of the mineral components of the food, to 78.3 per cent in the case of the fats. These data are taken only from the results obtained from a single sheep and one article of food. The mean data secured from two animals and three kinds of maize fodder show the following per cents of digestibility: Ash 39.4, proteid 61.8, fiber 76.7, undetermined matters 72.1, fat 76.4. The undetermined matters are those usually known as nitrogen free extract and composed chiefly of pentosans and other carbohydrates.[569]
554. Natural Digestibility of Pentosans.—The digestibility of pentosan bodies in foods under the influence of natural ferments has been investigated by Lindsey and Holland.[570] The feeding and collection of the feces is carried on as described above and the relative proportions of pentosan bodies in the foods and feces determined by estimating the furfuraldehyd as prescribed in paragraph [150].[571]