In the last four lectures we looked at the basic principles of genetics and learned some of the terms that are used in that science. In this lecture we are going to see how knowledge of genetics can be used in animal breeding programmes.
The objective of any breeding programme is to improve the commercial production of farm stock, and this applies to the individual farmer with a single herd or flock, as well as to the large farming company with many herds. In genetic terms, this is done by fixing a high proportion of the genes responsible for desirable characteristics in a herd, and getting rid of as many undesirable genes as possible. Once this has been done, the farmer will have a herd of good performers (good phenotype) and good breeders (good genotype).
The starting point in any breeding programme is the selection of the animals from which to breed – the basic foundation stock. The criteria used to select these animals will vary according to the type of animal the farmer is aiming to breed from his programme. Some of the points for selection for different classes of farm animals are given below.
POINTS FOR SELECTION
|Milk Yield Total Solids Longevity Udder Shape Speed of Milking Good Feet and Legs
|Birth Weight Live Mass Gain Food Conversion Carcass Grades
|Birth Weight Litter Size Food Conversion Live Mass Gain Length Back Fat Thickness Carcass Grades
|Birth weight Single/Twins Milkiness Hardiness Carcass Quality Wool Quality Wool Quantity
After looking at this list, there are two important points to remember:
- It is essential to have very clear objectives in any breeding programme. For example, the ideal dairy cow is one with a high milk yield, good solids, a well shaped udder, a fast milker, good feet and legs so that she can travel to find her food, and a resistance to disease so that she will have a long, productive life in the herd. The basic foundation stock that the farmer is using to start his breeding programme may have few, if any good characteristics. The farmer or breeder has to decide which of the characteristics are the most important and select his stock and carry out his breeding programme so that those characteristics are given a priority. If he tries to improve everything at the same time, he will not succeed and may end up with animals that are worse than his foundation stock. Once he has decided on his priorities, he may have to sacrifice some less desirable characteristic e.g. he may improve the milk yield of his cows but not the udder shape;
- The selection points for dairy cattle have nothing in common with those of beef cattle. This makes the selecting and breeding of dual purpose animals, almost impossible because there is a conflict of aims. A programme to .improve the beef qualities of a dual purpose breed will result in losing the milking capacity of the cows. With dual purpose cattle, the strain within the breed is often more important than the breed itself, some strains being good milkers and other strains being good beef producers. The old triple purpose cattle that were kept for milk, beef and work did not do any of these things particularly well; and
- In some cases, certain selection points are related to each other, others are unrelated and still others are antagonistic. This means that if you select for one point, another point that is antagonistic will be adversely affected. Some examples taken from dairy cattle are:
- High milk yield and size of animal: In general, the larger the cow, the greater its ability to produce milk.
|AVERAGE MILK YIELD
- Butterfat and Solids-not fat: Improving the B. F. also improves the amount of S.N.F. in milk.
- High Yield and Quality and Length of Udder: Provided the udder is made up of milk tissue and not fat.
- High Yield and Length of Head: A long, narrow head is an indication of a good milking cow.
- High Yield and Type: Straight back, legs, tail set etc. have no connection with milk yield. In fact there can only be a negative effect because many show judges go for constitution which is associated with fleshing or fattening ability and is the wrong type of food conversion for milk production.
- High Yield and Longevity: The high yielding cows usually have a short productive life, because they produce a lot of milk and often have to be culled for infertility or chronic mastitis.
- High Yield and Fertility: The high yielding cow is often the most difficult to get in calf.
- High Yield and Percentage Total Solids: If one selects for yield, the yield and the total solids will rise, but the percentage solids will fall. If one selects for percentage solids, the yield and the total solids will fall but the percentage solids will rise.
- High Yield and Fleshing Ability: Good milking cows are thin because they convert their food into milk. Very few cows put on body fat and give high milk yields.
- High Peak Production and Persistency: Cows that peak with a high daily milk yield show a fairly rapid drop in daily production. Cows with a lower peak yield persist longer and maintain a reasonable daily yield until the end of their lactation.
Cattle have been used by man for a very long time, and until the last 100 years, all mating was done by natural selection. The Afrikaner breed is indigenous to South Africa, and when Jan Van Riebeeck landed in the Cape in 1652he found these cattle in the possession of the local inhabitants. The breed originated in Central Asia and entered North Africa about 2 000 years ago.
The European breeds, the so called exotics, are descended from two sub-species:
- The Wild Ox, was large with thick horns, a very bad temper and is now extinct; and
- A small, fine boned species with a short, thick head and neck, curved horns and a docile temperament.
These two species crossed and in-bred and they formed many local types of cattle from which today’s breeds are descended. Cattle circulated around Europe and North Africa because all the ancient people carried their cattle with them; the Romans, Greeks, Vikings, Goths, Vandals etc.
The earliest milking cows were kept for cheese making. Butter, like sugar, is a recent invention. Records from Classical Greece show that they had black and white cows giving high yield of milk, the milk .being made into cheese. The making of cheese spread into Italy and later the Swiss and Dutch people became great cheese eaters.
In Britain, many different types of cattle were introduced by people who invaded the country, and this accounts for the many different breeds to be found there. The type best suited to a district survived and the less adaptable died out. An example is the North Devon breed which is found in the harsher climate north of Dartmoor, and the South Devonfound in the warmer, drier more fertile area South of the moor.
Cattle were kept for meat and hides, in general, and they were the poor relations of the sheep and the horse. The wealth of Britain was founded on wool, the Golden Fleece, and even today, the Lord Chancellor in the House of Lords, sits on the Woolsack as a symbol of this fact.
Breeding of all these domesticanimals was by random mating, which is the best way to produce animals purely for survival. Animals produced by this type of mating tend towards the average for the species or breed in both their appearance and performance. Each species of wild buck look alike, run at the same speed, have the same stamina, eat the same food and generally perform close to the average for that species, and the same applies to cattle, sheep, pigs etc. In herds of buck, and in closed herds of cattle, the strongest bulls serve all the females until displaced by younger bulls that fight and defeat the older animals. In this way the strongest animals in the herd are producing the offspring, and this is what is termed natural selection. Any male that is weak or infertile and any female that cannot conceive or give birth will die out so that the weakness will not be continued withinthe breed.
In Europe and particularly in Britain, no cattle breeding policy could be adopted because of the type of peasant farming carried out. In most cases, each householder in a village had a plot of arable land and all the village cattle were grazed together on a common pasture. A fewbulls served all the cows on a random mating system. Furthermore, a lack of fodder meant that many cattle were slaughtered in the autumn and in a hard winter, many died. Most of the hay made was kept for the horses and sheep to eat during the winter. As there were no towns of any size there was no demand for liquid milk and villagers produced only enough for their own use.
During the years 1700 to 1800, and especially during the Napoleonic wars, much of the village land was enclosed into larger units. The Enclosure Movement which continued for 150 years, caused great hardship to the village peasants who were dispossessed of land, but it resulted in great improvements in the agriculture of the country. The introduction of root crops for winter fodder, sound rotations, land drainage etc. resulted in good profits in farming. The fending of land meant that the mating of cattle could be controlled and this lead to the beginnings of the Artificial Selection System for livestock breeding.
The pioneerof animal breeding is Robert Bakewell of Leicestershire in England who worked with cattle, sheep, fighting cocks and pigs, on his farm. He saw that there was a demand for meat, wool and milk from the people in the towns that were growing in numbers due to the Industrial Revolution. Working with the old Leicester sheep, he selected the type which he thought were desirable, and then he fixed this type by inbreeding. Until this time, breeders had always avoided mating closely related relatives (inbreeding) because of the teaching of the church, which said that this practice was sinful in both man and animals. Bakewell produced the Dishley Leicester sheep which was finer boned and matured earlier than the old type of Leicester. He hired rams out to other farmers and breeders and was able to getback information about their progeny – the first form of progeny testing ever used. The Border Leicester sheep of today are descended from the Dishley strains. Bakewell also improved the Longhorn cattle, a breed which is almost extinct today. Bakewell was showing positive results from his breeding programmes by 1760 which is five years before Mendel had completed his work on the genetics of sweet pea plants. By 1784, the Collings Brothers of Darlington, England, were breeding Shorthorn cattle selected from the local imported Dutch cattle, and bred along the lines that Bakewell had shown to be successful. Their first inbred bull was called Favorite, and he was bred back to his mother, or dam.
|HERITABILITY: The proportion of observed variation in a particular trait (e.g. height) that can be attributed to inherited genetic factors in contrast to environmental ones.
The result of the mating between Favorite and his dam was a heifer called Young Phoenix, and this heifer was mated back to her sire, Favorite. This mating produced a bull calf called Comet and this bull became one of the best known bulls of the Shorthorn breed and was sold for £1 000 which was an incredible price for an animal at that time. The Shorthorn breed was further improved by Thomas Bates and Thomas Booth of Yorkshire. Amos Cruikshank of Aberdeen, Scotland founded the Scottish Beef Shorthorn breed using stock from Booth and inbreeding. From these beginnings there developed distinct breeds of cattle, sheep and pigs, and later came the formation of Breed Societies to look after the interests of each breed.
These pioneers based their selection on what they could see, selecting the animals which showed the characteristics which the breeder wanted. In other words, they looked at the Phenotype of the animal. This worked well because they were working with beef animals and sheep, and the characteristics they wanted had a high Heritability.
If the phenotype for a characteristic is strongly influenced by the genotype of that characteristic, and the influence of the environment is small, selection by phenotype will work well. The characteristic influenced by both phenotype and genotype has a high heritability, although the early breeders did not realise that fact at the time. Characteristics with a high heritability (40 – 60%) can be selected by inspection of the animal, because there is a very good chance that this characteristic will be passed on by a parent to its offspring.
Where the characteristic is influenced greatly by theenvironment, and therefore has a low heritability, selection by phenotype or by eye will not be very successful. The best example of this is milk yield which has a heritability ofabout 20% and depends mainly on the feeding and management of the cow i.e. the environment. Trying to select dairy cows by phenotype never works because one cannot tell how a heifer is going to milk just by looking at her. Furthermore, there is a pull towards breed average that the breeder has to overcome. Even today most dairy herds have a few very high yielding cows, a few low yielding cows, and the majority of the cows giving about the average milk yield for the breed.
When selecting by phenotype for a breeding programme, the breeder must understand the following:
- The Sire is themost important single animal in the herd. The male will sire a large number of animals in the herd, whereas any individual dam will produce only a few animals during her breeding life;
- The greater the number of characteristics which the breeder tries to improve at once, the less his chances of success because his Selection Intensity will be low. The farmer who tries to select for a number of characteristics in a small herd will have a very low selection intensity. The ideal situation is the breeder selecting for one or two characteristics, and working with a large number of animals.
- The Generation Interval This is the interval between one generation and the next, and the closer the generation interval, the faster improvement can take place. Improvement is much faster in breeding programmes for poultry or pigs than for cattle because a sow will produce 2 litters of pigs a year while a cow will produce 1 calf a year; and
- The Heritability of the selected characteristics. Those with a high heritability will show good results in any breeding programme based on selection by phenotype, while those with a low heritability will show poor results. In general, the factors which affect carcass quality have high heritabilities, so that beef animals can be improved by selecting for phenotype; good conformation equates to good phenotype. Characteristics which have very high heritabilities are coat colour and horns, but they have no economic value.