Decreasing generation interval to increase genetic progress
Abstract
"The accuracy of selection (r) is the correlation between the true breeding value and the estimate used for selection decisions. When observed measurements, such as actual weaning weight, are used to make selection decisions, r is the square root of the heritability (i.e., the portion of variation in the trait due to genetics). When EPDs are used for selection, the accuracy of the EPD determines r. The accuracy of selection using EPDs is always larger than the accuracy from using observed measurements (i.e., phenotypes) (see https://www.nbcec.org/producers/sire_selection/chapter10.pdf). The genetic variability of the trait within the population is measured by the standard deviation ([sigma]g). The genetic standard deviation measures how much observations range around the average. If most animals are close to average, the standard deviation is small. If there is more variation and animals are not as close to average, the standard deviation is larger. In terms of the response to selection ([delta]G) the amount of genetic variability is the one component that breeders do not typically control. The selection intensity measures how different the selected parents are from the overall population average. If the selected parents are close to the population average, the selection intensity is small. If the parents are quite different from the population average, the selection intensity is large. In other words, the selection intensity reflects whether the parents are from the top 25%, 5%, 1%, etc. of the population (i.e., percentile rank). The focus of this fact sheet is changing the generation interval. The generation interval is the average age of parents when the next generation is born. If older bulls and cows are used as parents, the generation interval is longer and genetic change is slower. But if we use younger bulls and cows and replace older generations with younger generations, the generation interval is shorter and genetic progress is more rapid. From the equation presented earlier, L is in the denominator and thus can have a large impact on the amount of genetic progress that can be made."--First page.
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