There are several different Breeding Program designs, each of which has its advantages and disadvantages. They include:
1) Tiered Breeding Structures
2) Index Selection System
3) Independent Culling Levels
4) Tandem Selection.
It is very important to note that any of the above systems requires and assumes that the animals selected have already been through a cull based on conformation. Poorly conformed animals have no place in a breeding scheme.
A Tiered Breeding Structure involves a herd or herds that only receives bucks from an established nucleus herd. Bucks that are used over the does are average within their nucleus herd and the does are bred to them in a random manner, meaning there is no elite doe herd and all the bucks have the opportunity to breed any doe. Within a generational interval of 4 years (meaning that culling and natural attrition have completely replaced all animals within the herd in 4 years time), the annual rate of progress would be about 1.25% and the difference between the nucleus herd and the doe herd would be 10%. According to Pattie and Restall, the long term rate of genetic progress is the same between the two groups and the lag between them may be reduced if heavy selection is applied to the doe herd and/or the bucks selected are above average.
An Index Selection System requires that all characteristics of interest be measured on each animal every year. Then, what is called the phenotypic value, expressed as the deviation of the individual from the population mean are multiplied by weighting factors and added together to give an overall index value for each animal. Then, animals with the highest index are selected and bred exclusively to produce bucklings that can be used for breeding next year.
Independent Culling Levels requires the adoption of a set of standards for each trait and animals meeting those standards are selected. The rest are culled. For example, if we have 100 replacement does and we want to select 30 from that group, we might select the top 50, based upon which are superior for micron diameter. Of this group, we would then select the top 30 having the greatest down production.
Tandem Selection is a selection process that selects for one trait at a time for varying periods of time. For example, we might select for down weight for four years and then for down diameter for two years. Tandem selection is very easy but it can result in slow genetic progress, as it is inefficient.
Index Selection Systems are generally more efficient than either Independent Culling Level Systems which are in turn more efficient than Tandem Selection Systems. However, Index Selection requires complete record on each animal, and the maintenance of that complete population throughout the life of the program. This means that animals cannot be culled in order to preserve the entire dataset to which an individual can be compared. The Independent Culling Level System also requires good measurements and record keeping, but as animals are selected, the others can and should be culled. There is much more flexibility within this system.
Most of the above systems usually depend upon random mating. This is essential if the object is to determine progeny differences and identify superior breeding bucks. However, assortive mating systems can be also used within any of the above systems. This means simply that the best bucks are used exclusively over the best does. This is the basis for nucleus breeding programs. While this precludes the calculations of sire evaluations, it does result in a small genetic effect. But because the does will consistently breed back to the average of the herd, genetic progress will be slow but steady.
Another type of mating system is disassortive, or corrective. This involves mating dissimilar animals in an attempt to correct a deficiency within a population by balancing its weakness with a buck or bucks that are strong in that trait. This system reduces variation, results in a more uniform population and has small genetic effects.
The remaining two breeding program types, inbreeding and cross breeding will not be addressed here. Inbreeding is dangerous because of the ease with which undesirable traits, especially conformational traits can be imbedded within the gene pool. Cross breeding, in this case meaning crossing with another goat breed, the Angora, has already been done and from all sources can be pinpointed as the cause of a decline in fiber quality.
SUMMARY: According to Pattie and Restall, there are five important steps in determining a breeding program.
Breeding Objectives must be carefully selected so as to contribute to profit. We are not trying to breed a perfect goat; we are trying to breed a profitable one. To do this, we must select traits that are amenable to genetic manipulation, meaning their heritability is moderate to high.
Selection criteria for goats within herds, if used, will be based upon performance recording, as we are not especially interested in providing sire evaluations to the buck breeding farms. Perhaps the best way to proceed will be to establish a co-operative breeding nucleus herd with multiplier herds around the country. Either way, it is important that we measure as many traits as we can.
Genetic improvement is an efficient means of improving overall performance as any small change that is effected will build exponentially upon the last in a structured breeding scheme. When using a group breeding scheme, the outlying flocks will lag behind the nucleus herd by about two generations, but the rate of progress will be the same.