Selection and Mating
Selection and mating systems are the foundation for genetic improvement. Selection allows breeders to positively alter genotypes, with an additive mode of inheritance, that influence a trait and mating systems lets breeders take advantage of heterosis and breed complementarity. Before deciding on a selection and mating scheme it is first important to set goals for your operation, often called Breeding Objectives. Determining breeding objectives should start with assessing your management, resources and market opportunities. Well developed breeding objectives should be the focal point of determining your mating scheme, traits that should be selected for, and magnitude and direction of selection pressure to be placed on those traits.
In commercial cattle production, when calves will be marketed and the replacement strategy of the herd are important factors in determining breeding objectives. Marketing endpoint will influence the growth trait that should be selected for. For example, if calves will be sold at weaning then the Weaning Direct EPD will be the most appropriate selection tool for growth, but if retaining ownership and selling on the rail, then the carcass weight EPD would be the proper growth selection tool. Replacement strategy will determine if maternal traits should be included in your breeding objectives or not. If daughters of a bull will be kept as replacements then maternal EPD will need to be considered. For example, Calving Ease Maternal EPD can be used to decrease calving difficulty in future retained heifers. The level of milk necessary in replacement females would also need to be considered in relation to management practices and the availability and quality of nutrients
When considering selection and mating decisions it is essential to understand the genetic mechanisms of the traits under consideration. For example, many important traits are monogenic traits - traits influenced by only a single gene locus. Monogenic traits include traits such as coat color and many genetic defects. Other traits are influenced by many genes, such as those for which EPDs are computed, and they are called polygenic traits.
Mating systems are dependent on the objectives of the operation (seedstock or commercial). Mating systems may include pure/straight breeding, crossbreeding and composite breeding, each having advantages and disadvantages depending on the intensity of management and level of heterosis desired. Breed selection is another factor to consider when setting up a mating scheme.
Finally, selection at its foundation depends on the accurate identification of bulls and/or semen, replacement females, and cows to cull to meet your breeding objectives. Understanding monogenic traits and utilizing tools such as EPD and decision support tools can assist in this process.