Selective Breeding for Herd Improvement
Maxine Kinne

Unfortunately, there is no magic wand to wave around to produce the perfect goat. It's hard work. Success depends on establishing your goal, developing a well-planned breeding program, then following through with it. To make consistent improvements in your animals, you need to be familiar with anatomy (phenotype) - the terminology, how the parts are supposed to work, and understand how you want to modify things.

A cardinal rule of selective breeding is that two goats with the same fault are never mated. For instance, if both parents have narrow chests, chances are very remote that their kids will have wider chests than the parents. Instead, the trait of narrow chests will be fixed in your kids and be harder to improve. If a doe has a narrow chest, her mate should be a wide-chested buck. This example applies to all parts of the goat.

Always breed does to bucks that are correct in the traits you want to improve in the offspring. If you have a doe with a very steep rump, don't breed her to a buck with a very level rump; breed her to a buck with a correct rump.

Due to the small number of foundation animals, there is a certain level of homozygosity (identical genes at the same location) in purebreds, including Pygmy goats. Some of those genes define breed type, enabling you to look at a Pygmy, a Doberman or a Holstein and recognize the breed. In the early days of all breeds, few animals were available and mating related animals was probably unavoidable. Sometimes, it was purposely done, and sometimes it still is.

The following are descriptions of various mating systems, including the advantages and limitations of each type.
 

Outcrossing

Mating two animals that are unrelated for at least 4 to 6 generations back is called an outcross. This method works best when the heritability factor for a trait is high. Unfortunately, few heritability rates have been established in goats, although it has been shown that inheritance rates for specific traits of other livestock are similar to each other. When dominant genes are the desirable ones, outcrossing works perfectly well. Linebreeding is more helpful when recessive traits are involved. It is thought that up to 0.5% of genes become homozygous in mating unrelated animals of the same breed.

Advantages: There are higher degrees of performance and fitness traits in outcrosses. Outcrossing two highly linebred lines introduces hybrid vigor and combines the desirable traits of both lines as it provides new combinations of genes that can be used in linebreeding. One of the best advantages of outcrossing is that it hides detrimental traits by keeping them recessive. Inbred and highly linebred herds may need to outcross if and when they develop defects or when further progress cannot be made. Outcrossing improves fitness traits such as reproductive ability, milk production, kid survivability and longevity.

Limitations: There is usually a high degree of variation because gene frequencies for dominant and recessive traits are heterozygous (not matching). Another way of looking at it is that there is a low degree of uniformity in genotype. Your rate of progress depends on your ability to choose superior parents. Phenotype does not accurately indicate genotype when the heritability rate is low.

 
Linebreeding

Linebreeding involves mating related animals like half-brother/half-sister, cousins, aunt/nephew, and other more distant relationships. This is usually done to capitalize on a common outstanding ancestor who appears in recent generations of the pedigree. If the desirable ancestor appears only once, several generations back, it will not contribute many genes to the current breeding for them to become homozygous and noticeable in the phenotype or performance of the offspring. The genetic relationship between offspring and ancestor decreases by half for each generation that separates them. Each parent should be chosen for its individual merit, not just because it has an oustanding ancestor. Linebreeding average animals is counterproductive as you are certain fix mediocrity firmly in your herd.

Advantages: The good traits of an outstanding ancestor can be perpetuated. There is a higher degree of uniformity with linebreeding than in outcrossing, and a reduced possibility of harmful genetic defects than inbreeding. You can make reasonable progress without the bigger risks of inbreeding.

Limitations: Compared to outcrossing, this system is not progress oriented when carried to extremes. It is like like saying that the ancestor is better than the animals currently being produced. Harmful recessive genes can be expressed through homozygosity, though with less frequency than inbreeding. Linebred animals can suffer from inbreeding depression (below average performance in fitness traits).

 
Inbreeding

This breeding method has been called a "trial by fire" because there are more chances for disaster than for good. It is mating directly related animals, like mother/son, father/daughter, and full brother/full sister (full siblings). Each of these relationships makes an additional 25% of the genes homozygous in the offspring, including recessive genes. Recessive genes are more of a factor than dominant genes in genetic faults, so you risk getting kids with problems. An inbreeding program must start with a number of above average animals due to the amount of culling needed, especially in the early generations, although the rewards for ruthless culling can be great. Don't try inbreeding if you lack the fortitude to deal with a little disaster once in a while. Successful inbreeding is based on the merit of the foundation stock, the rate of inbreeding, rigid culling, and selection for fitness traits and structural soundness.

Advantages: Homozygosity rapidly increases, creating uniformity and prepotency. Prepotency means that more desirable genes are present and that they transmit to offspring with greater uniformity and frequency. Inbred and/or highly linebred lines can be crossed for superior results. Bucks from these lines can be used to great advantage in outcross herds. Although it may look like a disadvantage on the surface, inbreeding forces out latent weaknesses so carrier animals and defective ones can be eliminated from the gene pool.

Limitations: Lethal genes and abnormalities may be produced in much greater numbers than other breeding schemes, so culling the defects is imperative. Inbreeding reduces the pool of available genes and can cause some lines to become extinct. When very popular lines are used extensively, less popular lines can decline greatly in number. Care must be taken that these lines do not develop problems. Fitness traits are especially at risk with this breeding scheme. There are many single-gene diseases that only show up in later life, so be prepared to deal with them.

 
Summary

The first step in selective breeding is choosing the right buck to breed to each doe, regardless of the mating system you choose. Avoid mating two animals with the same fault. Always try to improve the kids over the parents. The trick to reproducing desirable qualities with great regularity is to pair dominant genes by the method of breeding you choose - outcrossing or linebreeding.

The most important thing to remember about a breeding scheme is that if you run into problems, you can always change direction. The goats aren't married. You can try a different buck next year if things don't work out. And, no divorce lawyers to deal with. Cool.

 

Related Reading

 Heritability Percentages of Many Traits

 
 


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