This would leave only 13 potentially fertile females. Thus, of the singleton 

 and twin births, 455 would be sold as males or infertile females. Of the 

 417 potentially fertile female calves alive after birth, 10 percent would die 

 before they were 6 months old. This leaves 375 females, of which 2 percent 

 would die between 6 and 18 months of age. Of the 367 females that reach 

 18 months, 8 percent are sterile and are culled. This leaves 338 bred heifers, 

 of which 1 percent would die before freshening. This would leave a total 

 of 335 to enter the herd. Thus with 1,000 cows and with a normal sex 

 ratio of calves. 335 replacements can be raised under good management. 



The replacement rate shown in Figure 5 is an estimate of the number 

 of replacements that could be raised per 1,000 cows. Under farm conditions, 

 however, both heifer calves and heifers are culled for economic reasons. 

 Table 8 shows the estimated number of heifer calves born and the number 

 raised in New Hampshire from 1945 through 1954. It also shows the num- 

 bers of heifers 1 to 2 years old and heifers 2 years and over per 1,000 cows. 

 Taking mortality and sterility into account, an estimate was made of the 

 number of heifers calving per 1,000 cows and heifers 2 years old and over. 

 This is an estimate of the actual replacement rate, which covers culling 

 for economic reasons, such as adjustments to other resources on the farm 

 and current and prospective changes in prices. 



A comparison of the actual replacement rate (Table 8) with the 

 potential replacement rate (Figure 5), gives an approximation of the addi- 

 tional number of heifers that could have been raised for replacements. The 

 number varied annually from 47 to 122 per 1,000 cows and heifers 2 years 

 old and over for 1945 through 1954. Apparently farmers have culled around 

 5 to 12 percent of the annual heifer crop for herd improvement or other 

 economic reasons. This indicates roughly the possibilities for expansion or 

 improvement of herds through breeding and closer culling of cows. That 

 is, only 5 to 12 percent of the annual heifer crop that was culled could be 

 used to increase the size of the herd or for selection of female calves to 

 increase milk production per cow, as most of the female calves born must 

 be used to maintain the cow population. 



The net reproduction rate or ratio of animals in two consecutive 

 generations is a more accurate measure of the amount of selection avail- 

 able in heifer calves. With the assumed average levels of management, 

 fertility (Figure 5). mortality and culling (Tables 9-10), 1,000 2-year-old 

 females would produce 1,559 2-year-old females by the end of their pro- 

 ductive lifetime. ^ '^ 



The increase from 1.000 to 1,559 2-year-old females in one generation 

 is at the rate of 559 per 1,000. This may be interpreted to mean that dur- 

 ing the average generation life of 4.69 years for New Hampshire cows, 

 each cow could provide a replacement and 0.559 of a cow for an increase 

 in the cow population or for selection and upbreeding through culling. 



The true rate of natural increase may be derived from the net repro- 

 duction rate. This annual rate of increase is the ultimate rate of increase 

 for New Hampshire cows in this sample with the assumed conditions of 

 management, fertility, mortality, and culling. It measures the actual ca- 

 pacity for growth of the State's cow population without purchase of out- 

 side animals. For the sample of New Hampshire Dairy Herd Improvement 



1^ For a discussion of reproductive rates and methods for determination, see L. 

 I. Dublin, A. J. Lotka, M. Spiegelman, Length of Life, The Roland Press Co., New 

 York, 1949. 



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