January 2, 1914] 



SCIENCE 



18 



lated to the needs of the soil for phosphoric 

 acid in pot experiments. The relation of 

 the pot experiments, and the analysis, to 

 field needs, must be worked out. Soils also 

 deviate from the average, as regards their 

 plant food content and behavior to pot ex- 

 periments; such deviations must be stud- 

 ied and their causes ascertained. There is 

 much to be done, but progress is being 

 made. 



In the field of animal chemistry, decided 

 progress has been made in recent years. 

 "We must now recognize the possibility, that, 

 in digestion, proteids of different kinds 

 may be split into different products, some 

 of which may be unfit for use as struc- 

 tural material in building up animal pro- 

 teids, and so must be discarded. We know 

 that this is possible, but we have not yet 

 secured positive evidence that such occurs 

 with any of the various proteids fed do- 

 mestic animals. Such studies may be ex- 

 pected in the future. 



It has been shown, without doubt, that 

 the digested materials of different feeds 

 have different values to the animals. One 

 pound of digestible nitrogen-free extract 

 in corn has a much greater value than one 

 pound of digestible nitrogen-free extract 

 in straw. The fact that there is a differ- 

 ence in the values of digested nutrients of 

 the same class but from different feeds 

 has been clearly shown by the work of Kell- 

 ner and of Armsby. There is no doubt 

 about it. It is a step forward to recognize 

 the differences in the values of the digested 

 nutrients and to adjust our tables, our ra- 

 tions and our calculations accordingly. 

 There is abundant room for work along 

 this line, but enough work has already 

 been done to justify this advance. Nearly 

 every American book which deals with the 

 feeding of animals still assumes that the 

 digestible nutrients of one feed are equal 

 in nutritive value, pound for pound, to 



the digestible nutrients of the same class 

 in any other feed. These books must be 

 re-written and adjusted to our latest ad- 

 vances in knowledge. This advance will, 

 to a certain extent, reconcile the differen- 

 ces between the effects of feeds or of ra- 

 tions in feeding experiments which, under 

 the old standards, should have apparently 

 the same nutritive values. 



We are now able to state the nutritive 

 value of a feed in terms of three factors: 

 its bulk, which satisfies the hunger of the 

 animal; its proteids, which repair flesh or 

 tissue, or which, in excess, may be used for 

 fat or energy; its fat-producing value, 

 which is its ability to furnish the animal 

 with heat or energy or to form fat. The 

 fat-producing value of a feed or nutrient 

 is determined experimentally. First, the 

 fattening animal is fed a ration which pro- 

 duces a slight gain of fat, and the gain of 

 fat is measured by determining the in- 

 come and outgo of carbon and nitrogen. 

 Next, the nutrient or feed is added to this 

 ration, and the gain in fat again deter- 

 mined. The difference in the quantity of 

 fat produced is due to the added feed or 

 nutrient. 



The results of such work can be readily 

 compared with calculations based on the 

 assumed equality of the same group of nu- 

 trients in different feeds. While the cal- 

 culated value of peanut meal or linseed 

 meal is practically equal to that found, 

 the value for a wheat straw is only 20 per 

 cent, of that calculated, of meadow hay 54 

 per cent., of rye bran 79 per cent, of that 

 calculated. 



It should be clear that the recent ad- 

 vances in the chemistry of animal nutri- 

 tion compel us to modify materially tables 

 of feeding values, rations, and methods of 

 calculation. There is opportunity for use- 

 ful and valuable work along the lines of 

 determining exactly the productive values 



