To understand what is an Estimated Breeding Value (EBV), we must first define it. The most basic definition is the sum of the average effects of the alleles that an animal possesses. In other words, it is the sum of all the additive effects of the genes, since they are what the animal transmits to its progeny. Therefore, an EBV is an indicator of the genetic merit of an animal for a particular trait and at the same time an estimate of the performance of its offspring for this trait. This has been used in the dairy cattle industry for decades, where it is called Predicted Transmitted Ability or PTA, and in the beef cattle where it is known as Expected Progeny Difference or EPD. Both the PTA and the EPD are half of the EBV since the offspring receive only half of the parent’s genes. In the pig industry, EBVs are more frequently used. The reader will have noticed that the definition of EBV uses the word “estimate” since the actual genetic value of an animal cannot be determined, but with current methods we can have an estimate of the EBV that is closer to the true Breeding Value than it was 10 years ago.
EBVs are expressed in the unit in which the characteristic is measured. For example, in kg for weight, or millimeters for backfat or loin muscle thickness. The EBVs are positive or negative differences between the genetic merit of the animal from the genetic base to which the animal is compared. A positive or negative EBV value by itself does not indicate anything since it depends on the particular trait being evaluated. For example, for weight at market age, we want a high positive EBV value. But for backfat thickness a negative EBV is good since it indicates that the animal was leaner than the population it is compared to.
It is a common error to think that the higher the heritability of a trait the higher the EBV. The value of the EBV does not depend on the value of the heritability. However, the magnitude of the EBV does depend on the unit in which a trait is measured. For example, backfat can be measured in millimeters or inches but in both cases, the EBVs will rank a particular animal exactly the same way even though the actual number of the EBV is different.
Why use EBVs?
Ranking or selection of animals only by doing a visual appraisal for economically important traits is very archaic and today we have much more advanced and better methods to rank animals than just visually. How can we differentiate two animals that visually look similar in every aspect with no other data? It is impossible and by doing that, we have the perfect recipe to introduce lower genetic merit animals into the herd by selecting animals based only on their phenotype (how they look) instead of the genotype (their genetic merit). Even if the animal is measured in some other way, for example by weighing it, the information is of limited value. The use of EBVs is a scientifically proven, effective method to select animals that will produce offspring that will perform better than their parents for economically important traits such as prolificacy (number of born alive, stillborn), growth, survival, carcass quality, meat quality or maternal ability among many others. However, phenotypic assessment remains crucial as the basis for the evaluation, either to be included in the estimation of the breeding values, or for functional purposes for traits like the number of teats on a gilt delivered to Choice’s clients.
How are EBVs estimated?
Let us start by defining what the phenotype is. The phenotype (P) is everything that we can measure or observe in the animal and is determined by the genotype (G) of the animal and the environment (E) where the animal is living. Environment in genetic programs includes climate, nutrition, health, herd management and among many other things. Genetic effects may be additive (A) or non-additive (NA) effects, and define the genotype (G). Of these components of the genotype, additive effects are the most important for the breeder since they are the only ones that can be transmitted from parent to offspring. Because the methods used to estimate EBVs focus on the additive component of genes and generally assume that each gene has a small effect on the trait of interest, the sum of all these effects is the EBV estimate.
Interpreting EBVs and practical use.
Geneticists are often asked to compare EBVs for animals of different breeds. The answer is that it is not possible. In addition, we cannot compare EBVs of animals of the same breed that belong to different populations since their EBVs are not estimated using the same base population. An example of this is that two pig breeding companies may each have a herd of Duroc animals. But because the populations are separate, an animal with an EBV of +10 kg for offtest weight is not equal to an animal with the same trait EBV value from the other herd.
When comparing two boars from the same population, Boar A with an EBV for Number Born Alive (NBA) of +1.25 piglets and Boar B with an EBV for NBA of +0.25 piglets, the difference in their EBVs is 1 more piglet for Boar A. As a result, the progeny gilts of Boar A would be expected to be 0.5 piglets more than those of Boar B for NBA. The EBV is divided by 2 because each offspring only inherits ½ of each parent’s genes. Similarly, if the EBV for offtest weight is -4 kg for Boar A and +10 kg for Boar B, the difference in their EBVs will is 14 kg and Boar B’s progeny are expected to be 7 kg heavier at offtest than the progeny of Boar A. These examples assume that the dams of the progeny had equal genetic merit.
It is important to remember that the expected progeny difference is half the difference in the EBVs of the parents since the progeny receives half the genes from the sire and half from the dam.
Genomic Estimate Breeding Value (GEBV)
A major advance in the calculation of EBVs is the use of genomic information. These GEBVs are genetic values estimated using information from the animal’s own DNA as well as DNA from relatives in a base population in addition to phenotypes. Again, the larger the base population, the better (more accurate) the estimates of the GEBV are.
GEBVs are most useful for improving traits that are difficult or expensive to measure, such as meat quality traits like color, pH, IMF, and tenderness. Another example are traits that are expressed in only by one sex, such as prolificacy in pigs where only females farrow litters. A third case is traits that are expressed later in life of the animal like stayability, productive lifetime performance, and longevity. And last, GEBVs are useful when traits have a low heritability, like survival and prolificacy.
Choice is committed to swine producers and that is why we use the most modern methods to estimate both the EBVs and GEBVs of our animals and thus deliver the best possible genetics to our customers.