Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2005
Publication Date: 5/1/2005
Citation: Cleveland, M.A., Blackburn, H.D., Enns, R.M., Garrick, D.J. 2005. Changes in inbreeding of u.s. herefords during the twentieth century. Journal of Animal Science. Vol. 83 #5, MS#4708, pp. 992-1001.
Interpretive Summary: Over 20 million parentage records of Hereford cattle were used to evaluate inbreeding levels and rates of change from 1900 to 2001. Inbreeding levels increased on an annual basis until 1966, where a maximum level of 11.5% was calculated. During the 1970s and 1980s inbreeding levels decreased and by the 1990's inbreeding was increasing and reached a level of 9.8% by 2001. The reported decrease in inbreeding is unusual since the Hereford population is relatively closed. Additional analysis suggested that there were two potential causes for the decreased inbreeding levels. These were a shift to types or lines of Herefords that were not as popular before 1966 and/or the importation of genetic resources from Canada. While one or both of these causes led to a reduction in inbreeding that reduction was transient, as inbreeding increased again in the 1990s. From 1990 to 2001 the effective population size of the breed was calculated and found to be 85 animals. The study provides a benchmark of current genetic diversity in Herefords and can be used in monitoring future changes and if need be developing conservation strategies.
Technical Abstract: Genetic diversity in the U.S. Hereford population was characterized by examining the level and rate of inbreeding and effective population size. Pedigree records for 20,624,418 animals were obtained from the American Hereford Association, of which 96.1% had both parents identified. Inbreeding coefficients were computed for the entire pedigree and mean inbreeding was calculated by year from 1900 to 2001. From 1946, inbreeding increased linearly to a maximum of 11.5% in 1966. Through the 1970's and 1980's mean inbreeding declined to mid-century levels. To explain this decline we examined the relationship of popular bulls, constructed complete 12-generation pedigrees, and simulated the impact of imported animals. The mean pedigree relationship between prominent sires declined concurrently with inbreeding, an indication that a shift in line selection may have impacted the inbreeding trend. Mean inbreeding for a 12-generation pedigree containing only animals tracing to a pre-1950 base increased rapidly from the 1970's, without the decline observed when all records were used. This result in conjunction with an increase in unknown sire parentage over the same time period suggests an effect on inbreeding due, in part, to importation of animals into the pedigree and additionally provides an estimate of the upper limit of inbreeding in this population. Further analysis considering different strategies for the replacement of unknown parents with simulated parents showed the impact on inbreeding of the coancestry between progeny of imported sires and dams. The decrease was transient, however, and few generations were needed for inbreeding levels to increase. In 2001, was 9.8%, and approximately 95% of individuals were inbred. The maximum inbreeding coefficient was 76%. The annual change in mean inbreeding was estimated for Herefords born during five time periods from 1946 to 2001, where inbreeding was changing at different linear rates. The annual change in mean inbreeding for the most recent generation (1990 to 2001) was 0.12% per year. Assuming a generation interval of 4.88 yr the estimated effective population size was 85. This study provides a benchmark of current genetic diversity in the Hereford population. Results indicate that inbreeding is accumulating linearly and below critical levels. Increases in the adoption of technology could reduce genetic diversity and strategies to minimize inbreeding in the future may need to be considered. Key Words: Hereford Cattle, Inbreeding, Effective Population Size, Genetic Diversity