10 MASS. EXPERIMENT STATION BULLETIN 406 



Table 9. — Shrinkages From One Period to the Next in the 

 "Double Reversal" Groups * 



Average Percentage 

 Shrinkage 

 Change 



In Milk In Fat 



From regular ration to urea ration 



1940 18.7 19.1 



1941 11.0 12.7 



From urea ration to regular ration 



1940 9.7 9.9 



1941 5.7 5.6 



*The periods were eight weeks in 1940 and seven weeks in 1941. 

 The basis of comparison is the production for one entire period contrasted with the period im- 

 mediately following it. Each average figure is based on eight individual values. 



Length of Lactation and Dry Period 



Obviously only the data from the "continuous" groups can be used in studying 

 this phase of the results. These have been summarized in Table 10: in general 

 lactation was shortened and dry period lengthened during the experimental 

 program, but differences between the groups were not sufficient to be of any 

 significance. 



Table 10. — Average Length of Lactation and Dry Periods 



Lactation (Days) Dry Period (Days) 



8 Cows 6 Cows 8 Cows 6 Cows 



Regular ration 



1939 -* 301 -* 61 



1940 258 249 120 131 



1941 291 286 86 82 



Urea ration 



1939 -* 313 -* 71 



1940 259 252 84 92 



1941 293 291 105 120 



*Two of the eight cows were first-calf heifers in 1940; hence had no record in 1939. In order to 

 make the comparisons more accurate, the averages for the six cows carried along from 1939 are 

 given for each year as well as the average for all eight. 



Reproductive Performance 



Here also only the records of the "continuous" groups can be used. The 

 average results shown in Table 11 indicate such nearly identical performance as 

 to be dismissed without further comment. 



Urea in Blood and Milk 



Urea determinations were made during the summer of 1941 in the blood and 

 milk of all cows in the experiment: twice in the blood (in May and July), and 

 once in the milk (in August). Average results appear in Table 12. Differences 

 were slight, but on the whole the values were higher for the cows on the urea 

 ration'than for those not receiving urea, which is what would naturally be ex- 

 pected. 



