56 A Study of Nitrogen Metabolism i)i tlie Dalrij Cov 



cow (Cow C) was maintained "dry" and not in calf, as control cow, 

 whilst the other cow (Cow D) after 302 days became pregnant and its 

 record was followed throughout the stages of pregnancy and parturition 

 and for the first 13C days of active lactation. 



The outstanding features of the results are as follows: 



(1) With the progressive increase of nitrogen-consumption beyond 

 the fundamental requirements of the dry cow the rate of nitrogen-reten- 

 tion steadily increases to a maximum and then falls. The maximum 

 appears to be attained under the con<litions of our experiment with a 

 protein-supply in the neighbourhood of 2-4 kg. crude protein per 1000 kg. 

 live-weight. There are indications that this figure may be independent 

 of the nature of the foods fed along witli hay. 



(2) When the cow is maintained ujion a ration which causes an initial 

 nitrogen-retention the rate of retention falls steadily, but a very pro- 

 longed period — up to 90-100 days — may be necessary before nitrogen- 

 equihbrium is attained. 



(3) Even after nitrogen-equilibrium is estaliUshed and a relatively 

 constant nitrogen-consumption is maintained, there may arise from time 

 to time considerable deviations from equilibrium either in the positive 

 or negative direction. It would appear therefore that for reliable work 

 of this character long experimental periods are essential. 



(4) The very earliest stages of pregnancy are marked by a profound 

 disturbance of nitrogen metabolism, the requirement for maintenance 

 of nitrogen -equilibrium being very sensibly increased. This additional 

 requirement persists at a steadily reduced rate for some 1.") to 20 weeks, 

 after which it is very small. Over the whole jieriod of pregnancy the 

 average rate of nitrogen-retention was only about 2-4 gm. per day. 



(5) During parturition and for a few days subsequently the output 

 of nitrogen is very great and more than can be restored rapidly by food- 

 consumption. With the experimental cow, giving barely three gallons 

 of milk per day at most, some two to three weeks elapsed after calving 

 before nitrogen-equilibrium was restored. 



(6) It would appear that to maintain nitrogen-equilibrium during 

 lactation, the food must supply from twice to three times the amount 

 of nitrogen secreted in the milk, in addition to that required for the 

 maintenance of equilibrium in the "dry" state. This represents a food- 

 consumption which would be difficult to attain in the case of cows giving 

 large yields of milk, and accounts for the familiar difficulty of main- 

 taining the "condition" of such cows in the earlier stages of lactation. 



{Received l&th November, 1921.) 



