538 



Bulletin 169. 



The average of all (I) includes 135 yearly records and such a 

 large number must give a fair representation of the average 

 decrease in yield of milk and increase in per cent of fat as the 

 period of lactation advances. This average shows a gradual 

 and fairly regular decrease in milk flow from the time of calving 

 until the cow is dry. The decrease from one period of four weeks 

 to the next varies from 3.1 to 11.3 per cent, while the average 

 decrease is 5.3 per cent. This calculation also shows a decrease 

 in the per cent of fat from the first month to the second and then 



Diagram showing average variation in yield of milk and fat as 



lactation progresses. 



a gradual rise to the end of the lactation period. The variation 

 from one period of four weeks to the next is from minus 4 per 

 cent to plus 4. i per cent, while the average increase is slightly 

 more than one-half of one per cent. The average per cent of 

 fat during the eleventh month is only six per cent greater than 

 during the first month. At no time during the milking period 

 does the increase in per cent of fat compensate for the loss in 

 yield of milk, in the production of total butter fat. This may be 

 the case for a time with some individual cows but as a rule it. does 

 not occur so far as our records show. In other words a cow may 

 generally be depended upon to give a larger yield of butter fat 



