Making Genetic Progress

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As breeders of purebred livestock, attention to genetic progress should be at the forefront of our selection programs. The equation below summarizes the components that contribute to genetic progress. indicates change in genetics ( stands for change, G stands for genetics). Genetic progress can be improved by increasing those components in the numerator (Accuracy, Selection Intensity, and Genetic Variation) and decreasing those traits in the denominator (Generation Interval). Each component is described in greater detail below.

Accuracy: Accuracy values represent the relationship between the “estimated” breeding value and “true” breeding value. Increased accuracy results from greater records in the evaluation (individual and progeny records). Accuracy can also be improved through genomic testing. Parentage verification can ensure accurate sire and dam identification. Genomic-enhanced EBVs (GEBVs) provide improvements in trait accuracy as well. Remember, genomic data is only relevant as long as phenotypic records support it. A genomic test does not replace the need for data collection. Additionally, accuracy or genomics alone do not make an individual more genetically superior. They simply allow us to more accurately identify those individuals with superior genetic merit based on EBVs.

Selection Intensity: Selection intensity is reflected in the selection differential. The selection differential is the difference between the selected population for breeding and the average of the population. By selecting individuals further from the average, greater intensity is applied to selection and greater progress can be made. Breed percentile reports can be used to identify superior individuals within the breed for particular traits (Top 5% or 95th percentile for example) that will have a greater selection differential and allow for greater selection intensity.

Genetic Variation: Genetic standard deviation describes the variation in genotypes for a given trait. Traits with more variation give us more opportunity to identify and select superior individuals. However, this component is relatively constant for a population and difficult to change.

Generation Interval: The generation interval is the average age of the parents when the offspring are born. To increase genetic improvement, generation interval needs to be decreased. Therefore, greater utilization of ram lambs and breeding ewe lambs can be very beneficial. Genomics can assist in more accurately identifying those ram lambs to use. Ram lambs and ewe lambs should be managed in a way that improves their early reproductive success.

When making breeding decisions this fall, consider these components in your selection program. Understand tradeoffs may be necessary in some components to improve other components. For example, accuracy may need to be sacrificed in order to improve selection intensity and decrease generation interval. Providing the commercial industry with breeding stock that has superior genetics for economically relevant traits is an important role for purebred breeders and necessary to move the sheep industry forward.     

Dr. Andrew Weaver, NC State Extension Small Ruminant Specialist


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