Estimated Breeding Values (EBVs) and their benefits to the Australian Cashmere Industry.
Selection for the best Cashmere producing animals.
The Australian cashmere industry is a relatively young industry. Twenty years ago our industry was where the sheep industry was two hundred years ago. Selection by dedicated breeders has seen large increases in the production and quality of fibre in the Australian Cashmere goat. When these breeders perform selection on their animals they are affecting the rate of genetic change in their herds.
There are four main factors that affect the rate of genetic change in a herd; genetic variation, selection intensity, generation interval and accuracy of selection.
Genetic variation is the variability of the genetics in the herd. An inbred herd will have a low genetic variation and therefore will have a low rate of genetic change.
Selection intensity refers to how many of the animals in the herd are culled. The more intense the culling program, the greater the genetic gain. This occurs because there will be a greater percentage of exceptional animals selected to be used in the breeding program.
Generation interval refers to how long it takes to replace one generation with the next.
Selection accuracy refers to how accurate you are when selecting the animals to keep as replacements for the current herd.
Selection intensity, genetic variation and generation interval are all management decisions and will be dealt with differently by different herd owners. There are tools all cashmere breeders can use to increase the accuracy of selection and therefore improve the rate of genetic change in their animals. Increasing genetic gain is something which all serious cashmere producers are keen to achieve as increased genetic gain will in turn improve the productivity & profitability of their herds.
Currently most cashmere breeders would be selecting their breeding animals on the performance of those animals. Although this is a perfectly legitimate way of selecting animals it can be inaccurate. An animal's phenotype (way an animal looks) is affected by the animal's genotype (genes) and environmental factors. Therefore, if you are selecting on phenotype you may be selecting animals who produce more because they have been getting more feed. You may also end up culling an entire year's kids because none of them ever seem to do well (just because they were born in a poor year.) So people who use this form of selection will always have the best animals in their herd, phenotypically, at any point in time.
However, if we want fast productivity gains in the cashmere industry, we need the animals which are genetically the best possible animals available for selection. This may mean that people will need to start keeping those animals which have been raised under poor environmental conditions and don't do well themselves but have superior kids.
Nobody can afford to keep all of their animals and breed from them to find which are the genetically superior animals. However by using information from all the known relatives of an animal, you can obtain an estimated breeding value of that individual animal. This method increases the accuracy of selection for any individual animal.
What are EBVs (estimated breeding values)?
As mentioned before, an animal's phenotype is made up of the genotype and the environmental effect. The environmental effect is what makes selection on phenotypes alone more inaccurate when trying to select the animals which will have the best effect on the next generation.
An estimated breeding value is an estimation of how much better than the average an animal's genetics should be, based on the animal's performance, as well as the performance of all its relatives. The more closely related the relative is to the individual, the more it can contribute to the EBV. Therefore siblings, progeny and parents are usually used for EBVs, as they share the most genes with the animal. The accuracy of the EBV increases as you have more relatives contributing to it. Therefore experiments such as progeny trials are a good way of increasing the accuracy of EBVs in animals which are considered phenotypically exceptional. They also create a quick and easy way to do cross heard comparisons.
How to use EBVs to increase productivity & profitabilty of your herd
Let us assume that a cashmere producer wanted to increase the Cashmere down weights of his / her cashmeres.
This producer was considering buying a buck with an EBV for cashmere down weight of 200g and the industry average was 200g of cashmere down per animal. This means that genetically, that buck should produce 400g of down (This is an estimate of what it should produce according to its genotype, not exactly what it does produce; its performance being influenced by environmental factors).
If the grower wanted to use this buck with a doe which had an EBV of -100g the kids from this mating wouldn't all have 400g of down. The kids expected breeding values would be half of the does and half of the bucks (just like their genes). Therefore the grower could expect the kids to have (1/2)X(-100) + (1/2)X(+200) grams of down or 50g of down more than the industry average (250g).
So by using EBVs , Cashmere Growers can increase the rate at which they improve their herds, by choosing the bucks that would have the greatest genetic potential for the traits for which they want to select.