476 NUTRITION OF FARM ANIMALS 



Stage of lactation 



567. Milk production a periodic function. Milk produc- 

 tion has as its object the nourishment of the offspring. In a 

 state of nature it is a periodic function, beginning at the birth 

 of the young or shortly before, while as the young animal gradu- 

 ally becomes less dependent on the mother it diminishes in 

 intensity and finally ceases. Although man has greatly pro- 

 longed the period of milk production of the cow, so that the 

 time during which she goes dry is relatively short and in some 

 instances is eliminated altogether, nevertheless, milk production 

 still retains its periodic character and undergoes marked changes 

 during the progress of a lactation. 



568. Influence on yield of milk. The most evident effect 

 of advancing lactation is the gradual decrease of the amount 

 of milk produced, but the rate of decrease may differ widely 

 at different stages in the same animal and in different animals 

 at the corresponding period in lactation. As a rule, the amount 

 of milk does not reach its maximum immediately after the birth 

 of the young, but shows an increase for one or two weeks in 

 the case of the cow. Following this maximum, there is typi- 

 cally a slow falling off for several months followed by a more 

 rapid decrease as the time of the next calving approaches, all but 

 exceptional Cows going dry for a longer or shorter time. In the 

 case of farrow cows, the milk production may continue to show 

 a comparatively slow decrease for a much longer time. 



The curves of lactation as they may be called, however, vary 

 greatly from cow to cow and from year to year with the same 

 animal and show marked irregularities often not readily ex- 

 plained by any observed conditions. 



569. Influence on composition of milk. In general the 

 percentages of total solids and of fat tend to increase, especially 

 toward the end of lactation when the quantity of milk falls off 

 rapidly. Like the changes in quantity, these variations in 

 composition are often irregular and sometimes are scarcely 

 manifest at all until the rapid falling off in quantity sets in 

 toward the end of the lactation. 



570. Bearing on experimental methods. The unavoidable 

 changes in the yield and composition of milk with the advance 

 of lactation must be taken account of jn all experiments on 



