Estimated Breeding Value or simply Value is based upon the difference of the individual to that of the group as a whole. It is expressed as a positive (above average) or negative (below average) number. The Genetic Value of a group of animals of a particular genotype relative to their base group is a function of the genes those animals carry and the combination in which they occur (Pattie & Restall, 1996). This is best illustrated when one of two genes that control a trait is incompletely dominant. The homozygous dominant condition (FF) is defined as having a genetic value of “0″. There will be a demonstrable benefit derived from the heterozygous condition (Ff) that is quantifiable. The homozygous recessive condition (ff) will exhibit an even greater benefit and it can be said that the genetic value of the Ff and ff condition is that value’s difference from the value measured at the FF condition. The effect can be said to be “additive” if the heterozygous conditions’ increase in the realization of genetic value is exactly half that of the homozygous recessive. Because one gene was not completely dominant over the other, there is no “dominance” effect. The genetic value then has two parts, the additive effect and the dominance effect. Only the additive part contributes to the breeding value. It is very important to remember that the magnitude of differences will be different for different populations. Breeding values cannot be compared between animals from different populations unless genetic links are established. These genetic links come in the form of highly controlled breeding schemes where a single nucleus buck farm services the needs of many satellite breeding doe herds.
The Breeding Value is defined as twice the average genetic value of an animal or group. Breeding Values are estimated after data regarding relative productivity from individuals within a population are gathered and after knowledge about the genetic control (heritability and correlations) of those traits is considered. Breeding Programs deal exclusively with variance of individuals from the average of their peers. Heritability is the amount of superiority of an individual that can be passed onto its offspring and its value can range from “0″, meaning no additive genetic differences, to “1″ meaning all differences are due to additive genetic effects (Pattie & Restall, 1996). Research has determined that cashmere goat heritability’s for down production can be said to be high, meaning that at 0.61, it has a value greater than 0.40. Heritability for down diameter is also high, having a value of 0.47, but clearly it is not as easily inherited as down production. The value for liveweight is 0.29, meaning its heritability is only moderate. Clearly it is very easy to manipulate down production, relatively easy to manipulate down diameter and not so easy to manipulate liveweight. Cashmere traits with the highest heritability’s are yield at 0.90 and down length at 0.70.
ESTIMATED BREEDING VALUE (EBV): The EBV of an individual is calculated by subtracting the value of the animal for a particular trait and the average value of the herd for that same trait. For example if the heritability of cashmere down weight is 0.60 and an individual superior buck yields 200 grams of cashmere when his herd average is but 120 grams, his EBV for cashmere down weight is 0.60(200-120)=48 grams. This means his progeny will be expected to average 48 grams of down more than if an average buck was used. Conversely, if the buck produced 200 grams of down when his herd average was 300 grams [0.60(200-300)=-60 grams] means that using this buck would on average decrease the production of his progeny by 60 grams. So data on the herd of origin for our elite bucks is a must.
ESTIMATED PROGENY DIFFERENCES (EPD): Sometimes breeders prefer to predict the merits of progeny using EPDs instead of EBVs because only half of the bucks’ genes are passed to its progeny. In other words, the predicted merit of the progeny will be the average of the EBVs of the parents: EPD= (EBVbuck + EBVdoe ) divided by two (Pattie & Restall, 1996). Going back to the example used above, if the elite buck were to be bred to average does, the expected increase in down production would be half that predicted, or 24 grams. Conversely, if the second inferior buck were to be bred to average does, his resulting progeny would suffer a decrease of only 30 grams of production. Clearly, it is to the breeders advantage to breed superior bucks to superior does, resulting in progeny that are even more superior. This is why it is important to keep intact only the bucks from the matings of these superior genetics.
The following is a series of examples used by Pattie and Restall in a breeding workshop presented at the 8th Annual Convention of the Cashmere Producers of America in Laramie, Wyoming in 1996. They illustrate the use of EBVs.
Example: A cashmere goat herd with an average down weight at the first shearing of 100 grams. The heritability of down weight is 0.60.
Question 1: A buck yields 220 grams of down at his first shearing. What is the average value of his progeny if he is mated to an average doe?
The EBV for the buck is (220-100) x 0.6 = +72 grams
The EBV for the doe is (100-100) x 0.60 = 0
Therefore, the EPD is (72+0) divided by 2 = 36 grams.
Answer: The predicted progeny average is 136 grams, the expected 100 grams plus the 36 grams contributed by using the superior buck)
Question 2 . What if the same buck is mated to a doe yielding 130 grams?
The EBV for the buck remains (220-100) x 0.6 = +72 grams
The EBV for the doe is (130-100) x 0.6 = 18 grams
Therefore, the EPD = (72+18 divided by 2 = 45 grams
Answer: The predicted progeny average is 145 grams.
NOTE: Not all the progeny of these parents will yield 145 grams of down because of the following factors:
- Genetic variation within families
- Individual environmental variation due to illness, injury, etc.
- Population wide environmental variation due to drought, etc.