Selection--Causing or allowing certain individuals in a population to produce offspring in the next generation. Selection differential (reach)--The difference between the average for a trait in selected cattle and the average of the group from which they came. The expected response from selection for a trait is equal to selection differential times the heritability of the trait. Selection index-A formula that combines performance records from several traits or different measurements of the same trait into a single value for each animal. Selection indexes weigh the traits for their relative net economic importance and their heritabilities plus the genetic associations among the traits. Sibs--Brothers and sisters of an individual. Sire summary--Published results of national sire evaluation programs. Systems approach--An approach to evaluating alternative individuals, breeding programs, and selection schemes that involves assessment of these alternatives in terms of their net impact on all inputs and output in the production system. This approach specifically recognizes that intermediate optimum levels of performance in several traits may be more economically advantageous than maximum performance for any single trait. Terminal sires--Sires used in a crossbreeding system where all their progeny, both male and female, are marketed. For example F1 crossbred dams could be bred to sires of a third breed and all calves marketed. Although this system allows maximum heterosis and complementary of breeds, replacement females must come from other herds. Trait ratio--An expression of an animal's performance for a particular trait relative to the herd or contemporary group average. It is usually calculated for most traits as: Individual record ---------------------------- x 100 Average of animals in group The exception to this is birth weight ratio. It is calculated as: Adjusted birth wt. of individual 200 - ------------------------------------------------- Average adjusted birth wt. of contemporary group USDA Yield Grade--Measurements of carcass cutability categorized into numerical categories with 1 being the leanest and 5 being the fattest. Yield grade and outability are based on the same four carcass traits. Variance--Variance is a statistic that describes the variation we see in a trait. Without variation, no genetic progress is possible, since genetically superior animals would not be distinguishable from genetically inferior ones. Weight per day of age (WDA)--Weight of an individual divided by days of age.
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