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E p i c e n t r e
Issue 3

Breeding Strategies

Breeding strategies used in animal production depend upon the final targets of the breeder, and are related to whether the product is required for meat, milk production, hide, wool, doing work, survival in certain environments, or merely to be attractive.

Systems of Mating

Unlikes Mated
Random Mating
Like with Like
As Individuals
Pure Chance
As Individuals
By Pedigree
- - -
By Pedigree

The mating of unlikes together as individuals, is used by some misinformed breeders attempting to compensate for defects. Sheep breeders with a problem of undershot lower jaws in their flock seek tups with overshot jaws. If the problem is tight "cotty" fleeces then they seek tups with open fleeces. This is not how inheritance operates, all that these breeders are doing is introducing more defects into their flocks. This is a method to be condemned.

Mating unlikes together by pedigree is outbreeding or crossbreeding achieved by crossing strains within a breed or crossing breeds or even species. This method has a place in commercial animal production but it destroys at a single stroke the generations of selection which produced the characteristics of that breed or strain. It is not a system to be recommended with pedigree animals. A computer programme written for the Exmoor Pony Society calculated the most distantly related stallion to the mare to be served. Use of this would have ensured that in two to three generations all ponies in the breed would be equally related, the only ones to benefit being the haulage industry.

Random mating is assumed to occur in Nature, it happens purely by chance as if by drawing lots within the group to be mated. However modern work in observing wild populations suggests that in Nature there is a high chance of assortive mating, that is preference is shown towards partners already known. e.g. mating within a covey of partridges. Recent genetic studies have shown that wild species of garden birds can have coefficients of inbreeding in excess of 10%. Assortive mating certainly occurs in Exmoor ponies. Frequently a strange mare introduced to a stallion is rejected and not served. Similarly a new stallion joining a herd will have an apparent reduction in fertility until he has lived with that herd for a season or two.

The mating of like with like as individuals, e.g. Big with big, little with little, has some merit. It has been the basis in breeding racehorses for many generations. i.e. Fast with fast, jumper with jumper, or stamina with stamina. The main limitation of this scheme is that it assumes all genetic characters blend together. That is they are additive. This is certainly not true, and makes no allowance for epistasis, in which genes interact with each other in many different ways often to the extent that one gene if present may "switch off" the action of another gene.

Mating like animals together by pedigree is inbreeding. Mating is done within the breed, within the strain, or within the family. Most modern breeds have been established in this way, many using line-breeding to establish the breed founders. Line-breeding involves using the same sire on his daughters, grand-daughters, great grand-daughters, etc. This establishes in a relatively short time, a more homozygous population. This is the method most recommended.

Stock which is homozygous for a genetic character breeds true for that character, and as homozygosity increases then animals become more prepotent, that is it is more likely to pass on characteristics to their offspring than average parents.

With continued inbreeding, every new generation becomes more homozygous, and the population breaks up into a number of lines, the individuals within each line becoming progressively more similar genetically. This is called True-breeding. It preserves the breed and the strains within that breed, and involves a high degree of inbreeding. Not only is this necessary to maintain the purity of the strain but the very basis for cross-breeding production is destroyed as soon as the original breeds or strains cease to exist

Degeneration may be observed after continued inbreeding. This is caused by the increasing homozygosity of deleterious genes, generally recessives. The degree of this problem depends upon the skill of the stockman in his selections, and the abundance of undesired genes in the stock with which he begins. If such a problem arises then the introduction of another strain generally solves the problem. This out-breeding involves the crossing of animals within the same breed but distantly related. It is the last resort of those whose selection of parental stock has been faulty.

Robert Bakewell 1725-95, the great improver stated "Inbreeding produces prepotency and refinement". Inbreeding rates are approximately 12%, 6%, 4%, and 3% for 1, 2, 3, and 4 sire herds. On a herd basis with moderate care inbreeding rates as high as 6% may be pursued for many generations without noticeable harm. Inbreeding should be regarded as the process that converts genetic variation within a random population into differences between groups of that population by making each strain breed true for chosen characteristics. We should endeavour to keep the different Exmoor Pony types by managing them within inbred lines.

Selection

In Nature with a random mating situation, the initial supposition that all individuals contribute equally to the next generation is incorrect. Some individuals differ in viability and fertility. The weak and the sickly die before breeding age is reached. This occurs even with domestic animals receiving the best of care. If such differences in fitness are associated with the presence or the absence of a gene, then selection is said to operate on that gene.

Among the animals bred by man, natural selection is merely supplemented by man's selection. Man, in deciding as to which animals should leave many offspring or which should leave no offspring, is only emphasising characteristics which may be of little worth in nature. Favouring for breeding purposes those animals, which in the owner's opinion are the most desirable, must have began with domestication. Man's selection generally differs from natural selection both in intensity and direction, his influence merely intensifies forces and processes already existing in nature. The intelligent use of these choices can result in a much more rapid progress towards the goals of the breeder than could ever have been possible in terms of the geologic time that nature requires.

Quantitative Genetics permits us to detect, measure, and evaluate genetic structure and to assist the planning of breeding strategies, that provide sound principals for the promotion of certain individual animals as parents.

Selection of suitable breeding stock can be made by Production, Conformation, Pedigree, and by Progeny testing.

  1. Production
  2. There are no records kept of production of Exmoor ponies. Other species may have records of milk production, carcass size, carcass quality, wool grades, work done or loads carried. But the EPS has no such information, even records of fertility can only be inferred. Where such records exist then the poor producers are culled and not allowed to breed.

  3. Conformation
  4. The show-ring assesses animals by conformation. At best, if all judges were equal and had consistent standards, we would be evaluating an animal only by its Phenotype, i.e. by what it appears to be. Conclusions reached by this kind of estimation are of little or no use to the animal breeder. The breeder requires an evaluation giving an indication of the Genotype. i.e. the genetic characters the animal is likely to pass on to its offspring.

    At best the show ring offers a channel for publicising the breed but it only reflects the ideals held by a minority of breeders. The show ring does not lead, it merely follows current views on what an animal should look like.

    In the spring 2000 Newsletter of the EPS an article appeared in which the author awarded points and classified stallions according to prizes won by their progeny at agricultural shows. Someone, with money and time on their hands, could trail the same pony around every show in the country. This would gain its sire the maximum number of points, even if it were the only progeny of that sire that had a conformation acceptable to the judges. Every other sibling of that prize-winner could not only have failed inspection but have four white feet and a white star. Yet strangely, the autumn 2000 Newsletter states, "very few members are interested in showing".

    Shows, publicity, advertising and other sales efforts make some animals more popular than others. This has the effect of increasing selection towards them, usually to a greater degree than any deliberate breeding policy.

    The Exmoor Pony Society has practised a system of visual inspection of foals at branding time since the foundation of the Society in 1922. If foals are not of prescribed standards then they are NOT registered.

    Careful and critical studies of this Selective Registration system show the following:

    1. Ideals set up to guide the inspectors must be of real merit.
    2. The inspectors must operate such as to unify standards. Assessments must be objective regarding qualitative features and direct measurements made if possible of quantitative characteristics without relaying on 'eye' alone.
    3. There is increased selection for those characters desired by the inspectors.
    4. From studbook entries information on collateral relatives can provide a degree of assessment.
    5. There is a slight improvement of the breed.
    6. The average breeding worth of registered animals is higher than those not registered but the difference may not be great.
    7. In the first four to five generations selection is easier and genetically rewarding but in subsequent generations it becomes progressively difficult with less to be gained. As a population approaches its selection limit it becomes so uniform that the difference between those selected and those rejected is small. The environment or management must then be changed so that genetic differences can express themselves.

    The last three points are most important, they throw doubt on whether selective registration is worth the money and effort it costs. A herd book of names and parentage of only the acceptable progeny provides but little information to assist a breeder planning strategies for genetic improvement of his stock.

  5. Pedigree
  6. The EPS has its Studbook information stored in a Computer Database with the capacity to provide studbook information and pedigrees. Pedigrees can be followed back for six or more generations, certainly to the Second World War when the breed experienced a genetic bottle-neck.

    The decision whether to reject or keep an animal for breeding may be modified by its ancestry, when this is done then the intensity of individual selection is lowered, an animal may be retained and bred from because of unusually excellent ancestors.

    Especially if the animal is immature and we estimate its breeding value from its own appearance or performance, then mistakes will be made on account of environment and the complex interactions of genes. Some of these mistakes may be avoided if we estimate its worth by the conformation and performance of its relatives.

    It should be emphasised here that relatives could be ancestors, or living relatives like brothers and sisters. If we consider sibs or half-sibs then we are using collateral relatives.

    Pedigree information is most useful when studbooks publish not only parents, grandparents, etc. but also as much information about these ancestors as possible. It is of little use having a studbook giving a meaningless genealogical jumble of names and numbers about an animal but with no indication of how meritorious its ancestors were.

    In 1832, it was commented in 'The Thoroughbred Horse in Prussia': "If they do not contain production tests, such herdbooks will be useless and without interest, since they would only contain names of which no one knows anything and which mean nothing."

    In choosing breeding stock, rarely should the pedigree receive as much weight as an animal's own conformation or performance. But pedigrees assist in the immature and help to reduce errors introduced by environment and epistasis. Using pedigrees for selection should be regarded as a minor accessory to individual selection, but beware those who would select on apparent rarity within a studbook.

  7. Progeny
  8. In the first century B.C. , Varro wrote in his treatise on "The Husbandry of Livestock":- "The quality of a ram can usually be determined from his conformation and from his get. You may judge them by their get if their lambs are of good quality."

    This statement indicates that over 2,000 years ago intelligent breeders of livestock understood the principals of selection of livestock not only from their conformation but also from what would now be called a Progeny Test.

    By the late 1930's the livestock industry was using progeny testing in both Britain and North America. Modern Genetics now allows us to understand, analyse and utilise the results of such a test to a far greater extent than in Varro's day or even in the 1930's.

    A progeny test provides a means of estimating the heritable characteristics of an individual by studying its offspring. The test is not without limitations, these are:-

    1. The sampling nature of inheritance - a parent can transmit genes which are not typically expressed in that parent.
    2. The offspring receives half of its genes from its other parent - that parent may be atypical of the breed.
    3. Environmental effects, dominance deviations, and epistatic interactions - these could confuse the situation.

    These errors are minimised if certain rules are adhered to. By relying upon the sciences of Statistics and Quantitative Genetics we can reduce these errors and gain help in the interpretation of results. These limitations are the result of the random nature of inheritance. This problem and that of sampling errors is overcome by using a sufficiently large number of offspring in the test.

    Statistically a progeny test becomes more accurate than:-

    1. An estimate of heritability from a pedigree, when there are FOUR or more offspring,
    2. The individual merit of a parent, when there are FIVE or more offspring. Also progeny tests are most useful for characteristics which are only slightly heritable and for which individual selection is therefore not accurate.

    At registration of foals the EPS inspectors examine each animal and it must comply with certain minimum standards before acceptance in the studbook.

    In recent years this system of selective registration has required that :-

    1. All foals are put forwards for inspection.
    2. All failures are recorded with details of the reasons for failure, and the parents.

    Such records are an ideal for progeny testing, and although the requirement for having five offspring for the test could be difficult for many mares in the studbook, it is certainly not so for stallions.

    In order to maximise the value of such a progeny test it would be necessary to ensure that:-

    1. The standard of inspection and the inspectors are consistent.
    2. The rules for inspection and re-inspection are fully enforced.
    3. Results of inspections are properly recorded.
    4. Results are made freely available to breeders, owners, and potential buyers.
    5. The present inspection standards are very minimal, there has been little change since 1922.

    A revision of standards should include provision to record 'mares put to stallion' so that fertility of herds can be monitored.

    Breeders have already a form in which they register mares put to stallion, it only requires that proper records are prepared from these returns. In addition other facilities exist which can record and evaluate the failure information gathered at foal inspections and calculate factors such as coefficient of inbreeding. Such records would be of immense value to the breeder and researcher.

Strategy

Some recent breeding of Exmoor Ponies has tended to produce animals of a more uniform conformation, and which are less prepotent. This is the ability of a parent to pass characteristics to its offspring to such an extent that they resemble that parent, or each other, more closely than is usual. This situation is probably the result of searching for a "typical" or "average" pony or catering for the judge's eye, and caused by breeding together lines which were kept separate in the past by deliberate policy, geography, or animosity between owners.

Characters like size, colour, and conformation although obvious, are of no more importance to an animal than the less obvious such as fecundity, digestive capacity, or longevity, this is well understood by geneticists, and must be stressed. The main endeavour in breeding Exmoor ponies should be to protect the original stocks and produce hardy animals of the Moorland type.

The EPS must formulate clear policies for the future, revise the standards for the Exmoor breed and take steps to ensure the full and proper enforcement of the rules.

The future management of the Exmoor pony breed requires that clear standards and rules are laid down so that inspectors efforts can be maximised. It has been noted that even a six month old foal, with bad sweet itch, has been passed as fit to breed, apparently on re-inspection. It is known that a genetic predisposition exists for the condition.

The Exmoor ponies already exist in herds. These herds vary in size from one to nearly fifty brood mares. An ideal system for the most rapid improvement of the breed as a whole would be:-

  1. Retain the herds in individual breeding groups, organise local herds into groups which require the services of about three stallions.
  2. Each group rarely introducing any breeding animal from any other group and then with caution.
  3. The smaller the group the higher would be the rate of gene fixation on account of inbreeding.
  4. The latter would lead each group towards uniformity within the group and towards distinctness from group to group.
  5. This group differentiation is necessary for effective inter- group selection.
  6. The consequence of the above is that each group will more quickly become more uniform than herds are today, and each group becomes different from other groups.
  7. Selection between the groups would be effective to an extent impossible today.
  8. Selection of individual ponies for low to moderate heritable traits would become much more obtainable.

Some groups would begin to show undesired characteristics varying in severity. Other groups would show highly desirable traits more uniformly than present herds do. Mild out crossing of these desired groups to the poorer groups would cause an immediate improvement.

Linebreeding with rigid selection for the desired characteristics would then genetically fix these traits. Groups showing few desirable characteristics and many undesired could either be discarded or upgraded by the continued use of imported sires.

In general the more successful the group the less necessary would be any outcrossing.

Written by Alex N. Copland

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