CALORIFIC VALUE OF FOODS 195 



the food as proportionate to the live weight is not so very 

 far out in practice. The surface law is more useful in com- 

 paring dissimilar animals at the same period of growth than 

 of similar animals at different periods of growth The surface 

 law enables one to equate the rations of a guinea pig and a 

 galloway, both three-quarters grown, but does not enable one 

 to equate the rations of a calf and a Cluristmas fat beast. 



An ox weighing 1200 lbs. needs 12,000 Calories per 

 diem for its maintenance, whilst a sheep weighing about 100 

 lbs. requires 2000 Calories. Directly any work or fattening 

 is needed, the amount of food must be increased. A horse 

 weighing 1125 lbs. required for maintenance 12,600 Calories, 

 but when doing fairly heavy work, required more than 

 double that quantity for its output of energy. 



Many different systems have arisen to use the piurely 

 theoretical considerations given above, and apply them to 

 the practical rule-of-thumb methods of feeding commonly 

 adopted. These systems have followed the needs of the 

 day. At the time when purchased cattle foods came into 

 common use there was much more corn grown than at present. 

 Much of this corn was grown on poor land, insufficiently' 

 manured, with a correspondingly big proportion of tail corn, 

 or with entire crops unsuited for the production of bread. 

 The beasts, therefore, received plenty of carbohydrates 

 in corn and straw whilst the albimiinoids were supplied by 

 good hay, but the oil was very deficient. Hence the " oil 

 theory " of the day. Later, as wheat was grovAix less and 

 less, and as the land fell back to grass of little fattening 

 value, the general feeding of the cows became low in albumi- 

 noids, but the increasing use of oil cakes removed the oil 

 shortage, and the ** oil theory " dropped out, and the 

 " albuminoid theorj^ " came in. Of recent years we have 

 had a dearth of carbohydrates, and the weak link in the 

 chain has occurred at that point. But carbohydrates are 

 too indefinite, being only a " difference figure," hence the 

 present use of the " starch equivalent " theory. 



Practical if rough ratios were studied in early research in 

 Agriculture. I^awes and Gilbert, at Rothamsted, deduced 



