Managing Genetic Defects in Beef Cattle
Over the last five years the beef seedstock sector has had to deal with a number of recessive genetic de-fects. The utilization of embryo transfer and AI has allowed breeders to concentrate selection to a small number of animals. While many breeders avoid mating half-sibs or sires to daughters to reduce inbreeding, it is not unusual for prominent sires to appear several generations back in pedigrees of both the sire and dam. It is in this case when there is an increased chance for the progeny to be affected by a recessive genetic defect.
Autosomal recessive genetic defects are inherited abnormalities. These genetic mutations occur on one of the 29 pairs of autosomal chromosomes. Animals that inherit two copies of the defective gene are homozygous for the recessive forms of the gene and are phenotypically affected by the abnormality. The carriers (one mutated and one normal copy of gene) and normal animals do not exhibit the condition and are visually indistinguishable. The fact that the normal animals and carriers are indistinguishable makes phenotypic selection to eliminate defect carriers ineffective.
Management of genetic defects can be quite challenging. In the case of seedstock herds, suspect animals or those known to be the progeny of carriers should be tested when DNA diagnostic tools are available and practical. Carrier animals may be retained in the breeding herd, but breeders should test all progeny to determine carrier status prior to marketing as breed-ing stock. Carrier calves should only be sold to feeders and designated for slaughter only. If you have a calf born with a suspected defect, photograph the calf and contact your breed association to arrange for tissue collection and reporting.
In a commercial herd that has had affected calves or is striving to prevent introduction of a known defect, bulls should be DNA tested as the primary means of control. Mating of carrier cows to non-carrier bulls will result in the production of no affected calves. If a DNA test is available, all new sire purchases or semen used for AI should be from sires that are not carriers. Non-carriers may also be determined by pedigree. The use of a crossbreeding system may eliminate the appearance of affected calves.
Care should be taken to select a breed that has not had any calves produced in recent generations that are af-fected by the same defect one is trying to eliminate. At the commercial level, autosomal recessive defects can be effectively managed through careful sire and/or breed selection without extensive culling of the cow herd.
Dr. Robert Weaber, Assistant Professor Beef Cattle Genetics
University of Missouri, Columbia, MO