Vet Column 8-9-10
August 9, 2010
Selecting animals for breeding is generally accomplished by first deciding on a breed or breed composition. Depending upon one’s breeding objectives, subsequent animal selections are focused on maintaining genetic diversity to optimize performance traits even if the same breed or breed composition is selected. Maintaining genetic diversity in a population is important because its loss limits mating choices and has negative effects on economically and biologically relevant traits.
Inbreeding is a measure of genetic diversity. Inbred animals exhibit inbreeding depression which can affect reproduction, conformation, and growth, among other traits. Other descriptive genetic diversity parameters of a population structure include the effective population size, effective number of founders and effective number of herds.
The Red Angus Association of America was established in 1954 with a focus on performance testing. Although Red Angus cattle are frequently selected for beef cattle breeding programs in the United States, little analysis of its genetic diversity has been published. Investigators from Colorado State University and Iowa State University recently published data summarizing the genetic diversity of animals registered with the Red Angus Association of America.
Data were obtained from the Red Angus Association of America for over 2 million (2,141,506) registered Red Angus animals born between 1927 and 2006. Identification numbers of the animal, sire, and dam were collected as was the birth year, sex, percentage Red Angus and herd of origin. Pedigree information in the decades before 1960 was limited since the breed did not have an official association until 1954. Inbreeding and average relationship coefficients, effective population size, effective number of founders, and effective number of herds supplying grandparents to the population were calculated from the recorded pedigree. In this study, 40,679 founders were identified.
Inbreeding for registered Red Angus cattle in the United States in 1960 was 10.7 percent and decreased until 1974 at a rate of 0.2 percent per year. In 1975, inbreeding was 3.2 percent and increased until 2005 at a rate of 0.02 percent per year. These inbreeding level estimates are less than most other breeds. Danish dairy cattle populations are reported as 0.9 to 1.1 percent and Japanese Black cattle as 5 percent. Both of these breeds are smaller in number than the American Red Angus. Contrastingly, inbreeding estimates of American Herefords indicate an inbreeding value of 9.8 percent even though there are almost 10 times more American Herefords registered over time than American Red Angus.
Conclusions from this study indicate that inbreeding of American Red Angus cattle is at a low level and the effective population size is large. In relation to the total number of animals and herds supplying grandparents, the effective number of founders and effective number of herds is small, indicating the disproportionate influence of a few founders and herds on the genetics of the breed.
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Based upon these data, satisfactory genetic diversity can be achieved by selecting Red Angus cattle registered with the American Red Angus Association.