Genetic Markers for Reproduction in Beef Cattle
R. Mark Thallman
U.S. Meat Animal Research Center, Clay Center, NE

Reproductive traits of females are economically important in beef cattle, but have not received much emphasis because they are low in heritability. The accuracy of estimated breeding values improve as repeated records are obtained over the lifetimes of the cows, but only after the selection decisions have been made. Genetic markers could enhance the accuracy of selection for female reproductive traits by indicating which alleles at important loci were inherited prior to selection decisions.
A large number of animals is required to detect quantitative trait loci (QTL) for traits that are low in heritability and it would take about ten years to design a resource population based on repeated breeding records. Therefore, the use of existing populations is an attractive alternative. The U.S. Meat Animal Research Center has a population of cattle with extensive reproductive data that has been selected for increased twinning rate for 20 years. The dataset includes 2661 cows with parturition records, 3682 females with approximately six ovulation rate records each, and 2817 individuals with DNA.
The primary challenge in using existing populations for QTL mapping is that they typically do not have the simple family structures with large numbers of sibs that most resource populations are designed to have. Most existing populations, including the twinning population, have complex, multigeneration pedigrees with relatively small sibships. Therefore, the usual within-family statistical methods do not work well and more general, across-family statistical methods are being developed for use in complex pedigrees.
Three QTL affecting ovulation rate and twinning rate have been identified in this population and are being used in a marker assisted selection program. Loci that affect ovulation rate are likely to be involved in follicular recruitment or ovulation. Those that affect twinning rate but not ovulation rate may be involved in fertilization, implantation or embryo survival.