Fertilization Rates and Embryonic Death Rates in Cows Bred to Bulls of Different Levels of Fertility

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Abstract
Holstein and Guernsey bulls of different non- return rates in artificial insemination were compared to determine their fertilization rates and to estimate their embryonic death rates. The 64 bulls for which fertilization tests were made on 1st service heifers had an avg. nonreturn rate of 65.1%, a fertilization rate of 85.9%, and an embryonic death rate, estimated as the percent of fertilized ova that died, of 21.0%. When the bulls were divided according to nonreturn rates into a high half (67-79%) and a low half (40-66%), they had fertilization rates of 100% and 71.9% (P<0.01) and embryonic death estimates of 25.5% and 14.9% for the high and the low fertility bulls, respectively.[long dash]The differences were not significant between the fertilization rates of the Holsteins (89.2%) and the Guernseys (81.5%). The nonreturn rates of 65.8% and 64.0% were similar. [long dash]It appears from this study that approx. 40% of all potential young in dairy cattle are lost by 60-90 days after breeding. Their loss may be estimated as 3% due to genital abnormalities of the cow, 9% to defective ova, 12% to failure of fertilization resulting from undetermined causes, and 16% to death of the embryo, all calculated as percentages of the potential young producible by a female group.