Refinement of Feeding Experiments for Milk Production 245 



SUMMARY 



It is shown by the foregoing statistical studies that Holsteins in the 

 same stage of lactation, selected on the basis of approximately equal 

 production during a two- weeks period, are subject to a coefficient of 

 individual variability of 11.87 ±0.43 in a following period of eighteen 

 weeks. Under the conditions of selection used in which the eighteen- 

 weeks period was completed while the decrease in milk production due to 

 advancing lactation remained approximately uniform, the coefficient of 

 variability was not materially increased by using animals in various 

 stages of lactation. In a study with Jerseys in the same stage of lactation 

 the coefficient of variability was found to be 11.47zto.53. 



Studies as to the variation of groups resulted in coefficients for group 

 production which checked closely with the values calculated from the 

 figures for individual variation. This furnishes evidence as to the accuracy 

 of the values for individuals, and of the methods used. 



It is shown that groups of unselected individuals are more variable 

 than groups made up of individuals of equal production. Groups of 

 animals of the same age are shown to be less variable than groups including 

 all ages. A study of the first four lactations did not show any differences 

 in variability among them. 



Groups selected on the basis of yields during the preceding lactation 

 are shown to be more variable than groups selected on the basis of a 

 preliminary period during the lactation in question. 



A study of variation in fat production as regards Holsteins resulted in 

 a coefficient similar to that found for milk production. 



Yields varying within a 50-pound range during a two-weeks preliminary 

 period are considered as equal production as regards individuals. It is 

 shown that variation is not decreased appreciably by using a smaller 

 range. Similarly, it is shown that there is no, object in using a smaller 

 range than 100 pounds for the two-weeks period in selecting groups of 

 six of equal total production. It is shown further than when limits as 

 narrow as the ones stated above for individuals are employed, there is no 

 object in using percentage limits — that is, limits in proportion to the yield. 



Probably the maximum production of cows is reached later in life than 

 is commonly supposed. The cows used in this study did not reach their 

 maximum production until the eighth lactation, corresponding to an 

 age of from nine to ten years. 



APPLICATION OF RESULTS OF STATISTICAL STUDY 



In considering the application of the results of the studies described 

 in the foregoing pages, attention must be given to the conditions under 



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