XIV ] VARIATIONS OF FAT IN MILK 277 



4. Period of Lactation. — Immediately after calving 

 the cow yields milk of abnormal composition, the 

 product being a thick yellowish liquid, which coagulates 

 on boiling and is known as colostrum or beastings. 

 The colostrum at first contains only about 70 per cent, 

 of water and something like 20 per cent, of proteins, of 

 which albumen is by far the most abundant, the casein 

 being present in only about the normal proportion. After 

 four or five days the colostrum, which is necessary for 

 the first nutrition of the calf, passes into ordinary milk. 

 During the first month or two after calving the yield of 

 milk is at its greatest, and then gradually falls off until 

 the cow becomes dry, after nine or ten months. During 

 the first flush of milk the proportion of butter fat is at 

 its lowest, it then reaches its average amount, and after- 

 wards rises again as the cow begins to dry off. The 

 influence of the period of lactation may be to some 

 extent disguised by the time of year at which the cow 

 calves down, and the effect of such changes as turning 

 the cow out to grass. 



5. Age. — While it is difficult to obtain strictly com- 

 parable results, there is evidence that the milk yield 

 improves up to about eight years of age, after which a 

 decided falling-off begins to set in about the twelfth 

 year. The percentage of butter fat drops a little for the 

 first few years, and then remains constant until the 

 eleventh year or so, after which it begins to fall 

 rapidly. 



6. Time of Milking. — As a rule, in Great Britain 

 cows are milked twice a day, at intervals separated by 

 very unequal lengths of time. It is not uncommon, for 

 example, to find cows milked at five or six o'clock in 

 the morning and again about two o'clock in the after- 

 noon, thus dividing the day into one period of sixteen 

 and another of eight hours. It is always found that 



