corrected milk. When only cows of the same breed were compared, the 

 production of herds with purchased cows exceeded that of herds with raised 

 cows by 165 pounds of milk. Thus small differences in average herd pro- 

 duction probably cannot be attributed to whether the cows were raised or 

 purchased. Apparently raised and purchased cows are equal in milk- 

 producing ability. 



Calves Born per Cow, Mortality, Sex Ratios and Sterility Rates 



In the economic analysis of dairy herd replacements that follows, it 

 is assumed that the calving interval ranges between 390 and 410 days, which 

 results in about 0.90 calvings per cow each year. This is based on Gilmore's 

 service-conception rates and an estimated 4-percent sterility in the cows of 

 breeding age. A calving interval of 400 days is realistic for New Hamp- 

 shire conditions if there are no special management decisions that lengthen 

 the calving period. 



Most dairymen attempt to breed a cow 21/4 to SYz months after calving, 

 which results in about 1 calving per year and, as shown by Peterson, the 

 highest milk production for the lactation.^ More specifically, a gestation 

 period of 280 days and conception at 21/^ months after calving results in 

 a calving interval of 355 days. But all cows do not conceive on the first 

 service. Boynton found from the records of the New Hampshire- Vermont 

 Breeding Association for the years 1950 through 1953 that of 152,109 

 cows that were bred, only 66.8 percent conceived on the first service."^ 

 Gilmore showed from the records of 16,954 fertile cows that about 65 

 percent conceived on the first service, 20 percent on the second service, 

 8 percent on the third, 4 percent on the fourth, and 3 percent on the fifth 

 or more service." li all farmers bred their cows about 2^ months after 

 calving, and they obtained the conception rate shown by Gilmore, the 

 calving interval would be about 365 days. However, farmers may not 

 detect the first heat period at around 2 months and thereby may delay 

 the opportunity to breed for another 18 to 21 days. They may also lengthen 

 the dry period for some reason — in most instances to change the freshening 

 dates to coincide better with the season when milk prices are highest. 



Results of other studies of calving interval appear to bear out the 

 existence of these management elements. A report on 87,058 calvings in 

 108,522 cow years showed 0.80 calvings per cow year, or a calving interval 

 of 456 days. ^ '* Another report showed that the calvings per cow year 

 were 0.82 and 0.86, which are equivalent to calving intervals of 446 and 

 424 days. 11 



Not all female calves born live to enter the milking herd. Table 7 shows 

 data on the mortality and sterility rates for dairy heifers which were sum- 



"^ W. E. Peterson, Dairy Science, J. B. Lippincott Company, Second Edition, 1950. 



^C. H. Boynton, Reproduction in Dairy Cattle, University of New Hampshre 

 Ext. Bui. 115, 1954. This level of 66.8 percent conception is probably 5 percent too 

 high because of the culling of some animals bred once and not settled and then sold. 



^L. 0. Gilmore, Dairy Cattle Breeding, J. B. Lippincott Company, 1952. 



i"L. H. Beard, "Relation of Bovine Age to Season of Calving," Unpublished Thesis, 

 Iowa State College, Ames, Iowa, 1933. 



lU. Ingals and C. Y. Cannon, "The Mortality of Calves in the Iowa State College 

 Dairy Herd," Journal Paper No. J 387, Iowa Agr. Expt. Sta., 1936. W. W. Yapp and 

 A. F. Kuhlman, "Breeding Results in a Herd of Cattle Infected with Contagious 

 Abortion," American Soc. Anim. Prod. Proc. 1932: 277-281, 1933. 



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