raarized chiefly from records for well-managed college herds. The incidence 

 of mortality and sterility probably was less than in commercial herds. 

 Results of the various observations differ, but even so the percentage of 

 calves that were born dead or died at birth represented a considerable part 

 of the total number of animals born. Three herd records, each of which 

 covered a group of heifers from birth to calving, showed that 66 to 83 

 percent of the females born lived to calve. 



Many of the animals that were born dead or died at birth were twins 

 or a twin. About 3 percent of all births are twin births. i- Twins are 

 smaller and less vigorous than singleton calves and according to Peterson 

 they have a death rate three times that of single-birth calves. ^^ In a 

 sample of 34 New Hampshire twin births, 17 of the 68 animals, or 25 

 perecent, were born dead or died at birth. In the New Hampshire sample 

 reported in Table 7, twins accounted for 3 percentage points and singleton 

 calves for 3.5 percentage points of the 6.5 percent of the calves that were 

 born dead or died at birth. The death rate for twin calves was about seven 

 times that for singleton calves in this New Hampshire Herd. 



Another problem associated with twin calves is that the female in 

 a bisexed birth is sterile 9 in 10 times. ^"^ This means that of the twins 

 that live, many of the females must be culled for sterility. Theoretically, 

 fraternal twins occur in the ratio of 1 male pair, 1 female pair, and 2 male- 

 female pairs. In dairy cattle, the ratio of fraternal twins to identical twins 

 is 96 to 4.^^ Thus the expected frequency of different sex combinations 

 would be 27 percent male pairs, 43 percent male-female pairs, and 25 

 percent female pairs."' This includes correction for both the incidence of 

 fraternal and identical twins as reported by Bonnier and the sex ratio for 

 multiple births as reported by Gilmore.^'' From the standpoint of producing 

 replacements, twinning is undesirable. One hundred twin calvings normally 

 produce 43 potentially fertile heifers while 100 singleton calvings produce 

 46 potentially fertile heifers. 



The sex ratio of single-birth calves also affects the normal increase in 

 numbers of dairy cows. Records on 213,698 calves showed 51.2 percent 

 males and 48.8 percent females.^" For twin births, the sex ratio decreases 

 to 49.1 percent males and 50.9 percent females. Even so, the number of 

 male calves born exceeds the number of female calves born. 



The compound effect on the natural increase in the cow population 

 of calves born per cow, calf mortality, twinning, sex ratios, and sterility 

 is shown in Figure 5. For each 1,000 cows, there would be 900 calvings 

 each year, 873 singleton calves and 54 twin calves, or a total of 927. Of 

 the 873 singleton calves, 5 percent would be born dead or die at birth — - 

 a loss of 44 calves. The ratio for singleton calves would be such that of 

 the 829 calves alive. 404 would be potentially fertile females. The rest of 

 the singleton calves would be sold for veal. Of the 54 twin calves, 20 per 

 cent would be lost and 70 percent of the 43 calves alive would be sterile. 



l^See Appendix Table 8 for a summary of the proportion of twin births among 

 all births of dairy cattle. 



l^W. E. Peterson, Dairy Science, op. cit. 



i^Gert Bonnier, "Studies on Monozygous Cattle Twins. II. Frequency of Monozygous 

 Cattle Twins." Acta. Agr. Suec. I, 147-151, 1946. 



15 (P + Q)2 where P equals percent males and Q equals percent females. 



16 Gert Bonnier, op. cit., and, L. 0. Gilmore, Dairy Cattle Breeding, op. cit. 

 1^ See Appendix Table 7 for a summary of the sex ratio of dairy cattle. 



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