198 NUTRITION OF FARM ANIMALS 



aging two or three successive daily weighings, as is often done, while 

 it reduces the error does not eliminate it. For example, a comparison 

 of the average of February 19-21 with that of March 3-5 shows a 

 gain of 8.7 Kgs., while if each average be taken a day later, viz., 

 February 20-22 and March 4-6, the comparison shows a loss of 4.8 

 Kgs. By increasing the number of single weighings averaged, the 

 uncertainty may, of course, be further reduced but not entirely elimi- 

 nated, even ten-day averages varying materially, as is illustrated 

 by the following figures. 



February 24-March 5, inclusive, 435.3 Kgs. 

 February 25-March 6, inclusive, 432.7 Kgs. 

 February 26-March 7, inclusive, 432.6 Kgs. 

 February 27-March 8, inclusive, 434.2 Kgs. 



A similar reduction of the error may be obtained by the use of a 

 number of animals combined into a group which is treated as an in- 

 dividual, the fluctuations in the single animals tending to balance 

 each other. 



These fluctuations are such as to preclude the use of the 

 gain in live weight as a measure of the nutritive effect in exact 

 scientific investigations, while it is evident that they must also 

 be considered in the planning and interpreting of commercial 

 experiments, as well as in judging the effects of rations in prac- 

 tice. Such experiments should extend over a considerable 

 length of time and include a considerable number of animals, 

 while the weights on which comparisons are based should be the 

 average of as many single weighings as possible. 



283. Composition of increase. In the second place, even 

 were it possible to ascertain the gain or loss in weight by the 

 body tissues proper, exclusive of the contents of the digestive 

 tract, i.e., the empty weight, the composition of the material 

 gained would still be unknown. An increase of a kilogram in 

 tissue weight, for example, might consist chiefly of adipose tissue 

 containing 10 or 12 per cent of water (95), or it might be largely 

 muscular tissue with 75 or 80 per cent of water (87). Moreover, 

 aside from the difference in water content, the dry matter 

 of adipose tissue carries more chemical energy than that of 

 muscular tissue, so that a gain of a kilogram in the former 

 case would be equivalent to the storage of seven or eight times 

 as much energy as in the latter. Finally, a knowledge of the 



