Animal Husbandry Investigations in 1919. 261 



hibitecl in milk production to be roughly about one-half of that 

 which we actually find when we measure this variation under 

 ordinary circumstances. 



In the case of fat content of milk, individuality has clearly 

 much more to do with determining variation. Here the effect 

 of the environment is extremelv small. 



The Accuracy with Which the Milk Production or But- 

 ter-Fat Percentage of one Lactation Describes Milk 

 Production or Butter-Fat Percentage of Sub- 

 sequent Lactations. 



The dairyman often asks this question as he looks over the 

 lactation record of one of his cows, will it pay to keep this cow 

 for another year ? Such a question involves three questions ; 

 (i) what amount of milk did the cow produce in the lactation 

 above referred to; (2) what was the cost of maintaining the 

 cow; (3) what reliance may be placed on the milk yields of 

 one lactation as a measure of the milk yield of a subsequent lac- 

 tation. . 



The first and second of these questions are those which the 

 dairyman can answer for each and every cow in his herd by 

 keeping a record of the cost and return on each cow. A good 

 deal of uncertainty appears to exist as to how far one lactation 

 is a measure of the milking capacity of future lactations. Dur- 

 ing the past year extensive records on a pure bred herd of Jer- 

 sey cattle have been analyzed to determine this point. 



The records included 88 cows which had completed five 

 lactation periods between the ages of 2 years and 8 years. Each 

 record was for the first eight months of the lactation period. 

 In each of these records the level of . each cow's record in the 

 first lactation was determined in relation- to the level in the 

 total milk production for the first five lactation periods. It was 

 found that the relative rate of milk production remained very 

 nearly the same for the total milk produced by a cow in her 

 first five lactation periods as it did in her first lactation. That 

 is, if the heifer was a high producer in her first lactation she 

 also was a high producer in the total production of her first 



