When one trait or factor that is present in a group of goats is plotted on a graph, a characteristic bell-shaped curve results.
The highest point on the graph is the `mean’, and the shape of the curve shows the range of difference from the mean for individual animals in the group.
Standard deviation is the term used to describe the distribution, or the shape of the curve, for a particular group of goats compared with a ‘normal’ curve. Two-thirds of the group will lie within two standard deviations of the mean.
Coefficient of Variation (COV) states the size of the ‘standard deviation’ in relation to the mean value, so it can be used to assess the relative variability of factors.
For example, COV for mohair weight is twice that for staple length, and so gives greater scope for manipulation.
Variation may be good news or bad news for the farmer. If a buck has a large COV for fleece fibre diameter, it means that there could be a large variation in mean fibre diameter, which would make clip-grading difficult, and even depress the price if the fibre was too coarse.
However, if a farmer was trying to produce progeny with very fine, or very coarse fibre, a buck with a large COV for this character will be more likely to produce some animals with that characteristic than one with a low COV.
It is here that variation in genotype (the genetic make-up of the animal) or phenotype (what an animal looks like) is important. A single kid will grow faster than a twin, purely because it gets more milk to drink. Well-fed Angora goats will produce coarser fleece than poorly-fed Angoras.
Remember that the mean, and the variation from it, are measurements within one group of goats. Those measures cannot be directly compared with those of other groups, because of the difference in environment.
This variation in the population is the key to selection. But the variation must be measured and such measurements must be recorded.
Reference has been made to performance and pedigree records. In respect of the latter, bear in mind that half the genes in a goat come from each parent. Each grandparent contributes only 1/4th of those genes, so it is seldom worth considering early ancestors “unless one is searching for a particular factor. With highly inheritable factors, it is better to look at the animal itself, and others of the same breeding of the same generation (e.g., sister or half sister).
Progeny records are helpful for poorly-inherited characters.
Progeny testing is expensive and usually only bucks are tested, as their influence is far greater. Measuring offspring to assess their parents is most helpful when the characters are sex-linked to the doe, such as milk production.
Performance test records of bucks run in the same environment, are often influenced by the conditions which pertained before the test affecting different bucks to a different degree.
Some characters, such as milk production, are more difficult to measure than early growth rate, because the doe has to produce for a whole lactation, and this has to be done without adverse influence of mastitis or milk fever, drought or accident or any other environmental factor. If the character to be measured has low repeatability, such as kidding interval, several years of records are needed.
Inevitably farmers try to find other (usually visible) characters to help selection, and over the years relationships have been established. A thin skin is supposed to be related to good milk production; weak horns are supposed to indicate a weak constitution.
The ranking of goats by merit for different characters is often more useful than the absolute figures themselves, as this demonstrates the variation.
To complicate the whole matter of breeding further, there is the question of whether to concentrate the genes or spread them. Concentration is achieved by (a) inbreeding, or (b) line breeding.
Spreading the genes can be achieved by (a) cross-breeding, (b) out-crossing, (c) back-crossing, (d) top-crossing, or (e) up¬grading.
Inbreeding mates goats that have some ancestors in common. It rapidly creates uniform families in a flock. Production can be depressed over a period of time, especially if too many factors are being selected for.
Line-breeding is a form of inbreeding that concentrates on one common ancestor, and has the same effect.
Success of inbreeding and line-breeding depends on having adequate records, relationship between the genes, the genetic makeup of the interbred stock (often unknown), and especially the size of the breeding group.
Cross-breeding — Goats can be bred
across breeds, or strains, or lines within a breed. After the first cross (F1 generation), farmers have to decide about the next generation; whether to cross back to one of the original breeds, or both of them in rotation, or whether to introduce other breeds or strains.
Interbreeding of the Fl generation is a helpful technique to create variation for intensive selection. Often farmers interbreed on the doe side only, getting bucks from outside their flock.
Developing Angora flocks in New Zealand have particular scope for inter-breeding. As an example, using an Angora buck clipping 5kg of mohair, and a crossbred doe with 2kg mohair fleece weight, you can assume a mean fleece weight of 3.5kg in the progeny.
However, inter-breeding of these progeny will concentrate genes for high fleece weight in some animals, and for low fleece weight in others. It is necessary to consciously select the best to make the most of this variation.
Out-crossing also brings in new genes, but is only of value if the farmer can measure performance of progeny and has a large enough flock for the variation between the progeny to show.
Up-grading — both Angora and dairy goat farmers are familiar with up-grading — mating purebred bucks to feral and other low-grade stock, and then mating the progeny to purebred bucks, and continuing the process: the Fl generation is 50% pure, the F2 generation is 75% pure, the F3 generation is 87.5% pure, and the F4 generation is 93.77% pure.
However, genetic makeup is so complex, and variation in a group is so great, that some animals will appear more `pure’ than others in each generation.
Top-crossing and back-crossing are variations of up-grading, where the policy is to mate to a particular breed or strain.
Hybrid Vigour is often increased by crossbreeding, and shows in the Fl generation, but crossbreeding can have a negative effect on some progeny. Hybrid vigour is said to be shown by an animal if it is better than the mean of its parents; offspring are rarely better than both parents.