Experiments to determine the digestibility of the different kinds of food, a matter of the greatest importance to stock-owners, have not been carried on to any extent, if at all, in this country, and the author of The Chemistry of the Farm remarks that our knowledge concerning the digestion of food by farm animals is derived almost entirely from German investigations. He quotes from the work of Dr. E. Wolff, Die Ernahrung der landwirthschaftlichen Nutzthiere, and as the information is exceedingly valuable it is desirable to give a summary of it here.

The experiments were chiefly conducted, in the first instance, with oxen, cows, sheep and goats, but Dr. Wolff carried on special investigations on the digestive powers of the horse, in comparison with those of the sheep, the same food being supplied to each animal. The general results are shown in the two following tables, which indicate the proportion of each constituent digested out of 100 parts of each kind of food supplied: -

Experiments With Horses

Food.

Proportion of each constituent digested for 100 supplied.

Total organic matter.

Nitrogenous substance.

Fat.

Soluble carbohydrates.

Fibre.

* Pasture Grass ...

62

69

13

66

57

Meadow Hay (very good)

51

62

20

57

42

Meadow Hay (ordinary)

48

57

24

55

36

Red Clover Hay

51

56

29

64

37

Lucerne Hay (very good)

58

73

16

70

40

*Oats ... ' .........

68

86

71

74

21

* Beans ...

87

86

8

93

69

*Maize............

91

78

63

94

100

Experiments with Sheep

* Pasture Grass ...

75

73

65

76

80

Meadow Hay (very good)

64

65

54

65

63

Meadow Hay (ordinary)

59

57

51

62

56

Red Clover Hay

56

56

58

61

49

Lucerne Hay (very good)

59

71

41

66

45

*Oats ............

71

80

83

76

30

* Beans ...

90

87

84

91

79

* Maize ...

89

79

85

91

62

* Mean of Several Experiments.

On comparing these figures it is evident that a horse digests meadow grass and hay less perfectly than a sheep does, and the difference between them is apparently as great when the food is young grass as when the ordinary hay is employed. There is little difference in the proportions of albuminoids assimilated by the two animals, but the divergence becomes considerable when we come to the carbohydrates, fibre, and fat. Of the carbohydrates the horse digests 7 to 10 per cent, of the fibre 21 per cent, and of the fat and waxy matter 25 to 52 per cent less than the sheep. On the whole, the horse digests about 12 per cent less of the total organic matter of grass hay than the sheep. With red clover hay the results with the horse are better. With Lucerne hay of good quality the digestion of the horse is still better, and (save as regards the fat) practically ecpuils that of the sheep.

The smaller digestive power of the horse for vegetable fibre is plainly connected with the fact that it is not, like the sheep, a ruminant animal, and it is thus unprovided with the same means of attacking an insoluble food. In a trial with wheat-straw chaff, the horse digested 22.5 and the sheep 47.6 per cent of the total organic matter.

With the corn the digestion of the horse is apparently quite equal to that of the sheep. No stress must, of course, be laid on the digestion coefficients found for ingredients of the food present in small quantity, as the fat and fibre of beans and the fibre of maize. In French experiments on horses, in which maize or beans were consumed alone without the addition of hay, it was found that with maize 94.5 per cent of the total organic matter and 87'1 per cent of the nitrogenous substance, and with beans 90'4 per cent and 89'3 per cent respectively were digested. Of potatoes 93 per cent, and of carrots 87 per cent of the organic matter were digested by the horse.

A difficulty which attends all experiments of this kind, in which special kinds of foods are given exclusively, is that their digestibility will be necessarily affected more or less when they are mixed with other foods. This is proved by the following facts, recorded by the same authority.

If to a diet of hay and straw, consumed by a ruminant animal, a pure albuminoid, as wheat gluten, be added, the added food is entirely digested without the rate of digestion of the ordinary food being sensibly altered. The same result has been obtained in experiments with pigs fed on potatoes to which variable quantities of meal-flour were afterwards added. The albuminoids of the meal were entirely digested, while the proportion of the potatoes digested remained unchanged.

An addition of oil (olive, poppy, and rape oil) to a diet of hay and straw is also apparently without unfavourable influence on the rate of digestion; indeed, some experiments with small quantities of oil ( lb. of oil per day per 1000 lbs. live weight) show an improved digestion of the dry fodder; oil supplied in moderate quantities is itself entirely digested.

An addition of starch or sugar to a diet of hay or straw will, on the contrary, diminish its digestibility, if the amount added exceeds 10 per cent of the dry fodder. The albuminoids of the food suffer the greatest loss of digestibility under these circumstances. The fibre also suffers in digestibility if the amount of carbohydrate added is considerable. When starch has been added, it is itself completely digested if the ratio of the nitrogenous constituents of the diet is not less than 1 in 8.

These facts are of considerable practical importance. Nitrogenous foods, as oil-cake and bean-meal, may be given with hay and straw chaff without affecting their digestibility, but foods rich in carbohydrates, as potatoes and mangels, cannot be given in greater proportion than 15 per cent of the fodder (both reckoned as dry food) without more or less diminishing the digestibility of the latter. This decrease in digestibility may, however, be counteracted in great measure by supplying with the potatoes or mangels some nitrogenous food. When this is done, the proportions of roots or potatoes may be double that just mentioned without a serious loss of digestibility. Potatoes exercise a greater depressing effect on the digestibility of hay than roots, starch being more potent in this respect than sugar. The cereal grains are rich in starch, but contain also a fair propor-tion of albuminoids. They may be added to a dry fodder without seriously affecting its digestibility if the ratio of the nitrogenous to the non-nitrogenous constituents of the diet does not fall below 1 in 8.

Common salt is well known to be a useful addition to the food of animals. It is stated to quicken the conversion of starch into sugar by the saliva and pancreatic juice. When sodium salts are deficient in the food, salt supplies the blood with a necessary constituent. Sodium salts are tolerably abundant in mangels and small in quantity in hay; they are absent in potatoes, and generally absent in grain of all kinds.