INTRODUCTION
Before discussing pig breeding in more detail, it is necessary for you to know the meaning of three important terms. These are Heritability, Performance Testing and Progeny Testing
HERITABILITY
Due to genetic factors which are explained more fully in the Animal Breeding Course, some characteristics or traits are highly heritable. This means that there is a good chance of parents passing on the trait to their offspring. Some traits that are highly heritable have little or no economic value, examples being skin colour, shape of the ears etc. However, other traits do have a considerable economic value, an example being the body weight of the mature animal. The number of muscle cells is fixed by the end of the third month of growth of the foetus – the young animal growing inside the womb. The number of muscle cells determines the maximum weight of the mature animal, and this trait is highly heritable.
Table 1: Traits that have a high heritability in pigs are the following:
| TRAIT | PERCENTAGE HERITABILITY |
| Daily Live-mass gain after weaning | 30% |
| Food conversion | 35% |
| Backfat thickness | 50% |
| Lean meat percentage in the carcass | 45% |
| Carcass length | 50% |
These traits have a high economic value and because they are highly heritable they can be improved rapidly in a pig herd by direct selection and performance testing. For example, a good long boar used in the herd will produce longer pigs with a better carcass length, and provided that long boars are selected and used in the herd, the length of all the pigs in the herd can be increased in two or three generations.
Examples of traits that have a low heritability are litter numbers and weaning weights, because these depend on management and feeding, rather than genetic factors. Litter numbers are affected by the feeding of the sow during her pregnancy, and weaning weights of little pigs, depend on the milking capacity of the sow while she is suckling the little pigs. Traits that have a low heritability cannot be improved by selection, but they can be improved by cross breeding.
PERFORMANCE TESTING
This is done by measuring the performance of an animal while it is growing. Pigs are performance tested during the period from weaning to bacon weight, which is from a live-mass of about 22kg to 85 – 90kg. During the period of the performance test the pigs are fed on a standard ration and records are kept of the live-mass gain, food intake and food conversion, days to bacon weight, back fat thickness etc. In this way the performance of one pig can be compared to that of another.
PROGENY TESTING
The performance of the young pigs belonging to a boar is measured to see if the boar is passing his good traits onto his offspring. The young, growing animals are performance tested, and the boar is progeny tested at the same time.
| BOAR | He is being progeny tested |
| as | |
| Litters of young pigs | are being performance tested |
Performance and progeny testing can be accurate only with traits that have a high heritability, and these are mainly the fattening qualities of beef and pigs. The conditions of the test must be similar to that of the normal feeding and management of the animals under commercial conditions. A boar that does well in a progeny test under good conditions cannot produce offspring that will do well under conditions of poor feeding and poor management.
2. PURE BREEDING
The mating of two animals of the same breed is called pure breeding, and mating two animals registered by the same Herd Society is called Pedigree Breeding. Two Large White animals, a boar and a sow, mated together would be pure breeding. If the two animals were registered with the Large White Herd Society, their mating would be pedigree breeding, and their progeny would be eligible for registration with the Society.
Pure breeding depends on Selection and Heritability to be effective. The breeder must know the characteristics or traits that he wants in his pigs, and he must select for those traits. Animals that have the traits that are desired are kept in the herd, and those animals that do not have the correct traits must be culled from the herd. Points for selection in pig breeding are:
- The Birth weight of the litters that are born in the herd;
- The Litter Size – the number of little pigs born in each litter;
- The Live-mass Gain of the pigs from birth to weaning, and from weaning to slaughter at either pork or bacon weights. The Live-mass Gain from birth to weaning is the measure of the milking capacity of the sow, while the Live-mass Gain from weaning to slaughter is the measure of the pig’s ability to grow quickly provided that feeding and management of the herd is good. Pure breeding under poor management will never succeed, because the genetic potential of the pigs can never be realised. Their performance will be poor because their management is poor;
- Food conversion, which is the amount of food, required to put on 1kg of live-mass gain. A Food conversion of 3,5 to 1 means that 3,5kg of meal is required to put on 1kg of live-mass gain. As 80% of the cost of producing a bacon pig, is the cost of the food eaten by the pig, food conversion is an important factor in pig production;
- Length of carcass. A long carcass is required in bacon production because it produces more bacon; and
- Back fat Thickness. This is used in the grading of pigs, and too much back fat will cause the pigs to be down-graded.
If you look at the heritability’s table on page 1 of this lecture, you will see that, apart from the first two selection points, the remainder has high heritability’s. Birth weight and Litter Size have very low heritability’s and cannot be improved by selection. The other traits are highly heritable and can be improved quite quickly by careful selection. Using boars that have long bodies, a low backfat thickness and good food conversion in the herd will improve these traits in the progeny of those boars.
Pure breeding is a highly specialised form of breeding requiring great skill and very good management on the part of the breeder. It is dealt with in more detail in the Animal Breeding Course.
3. CROSS BREEDING
The mating of two animals of different breeds is called cross breeding and is practiced widely with cattle, sheep, pigs and poultry. Cross breeding produces improvements in the crossbred animal due to a large number of fresh genetic combinations coming into existence. This condition is called Heterosis, or Hybrid Vigor, and it affects performance of the animal which in turn improves the commercial value of the crossbreed. The commercial farmer is looking for animals that perform well, whereas the pure or pedigree breeder is looking for animals that both perform well and transmit their good performance to their offspring. It is far easier to breed animals that are good producers, than animals that are both good producers and good transmitters.
The advantages of cross breeding are:
- The crossbred can combine the best qualities of each of its parents. If the parent’s pure breeds have been selected for good economic traits, these can be brought together in the crossbreed.
- The effect of hybrid vigor can improve the commercial performance of the crossbred animal. Hybrid vigor has most effect on those traits that have a low heritability, such as litter numbers and birth-weights. A crossbred sow usually produces larger litters and more milk than a purebred sow.
The disadvantage of crossbreeding is that the crossbred animal will not pass on its improved performance to any offspring. Hybrid vigor is strongest in the first cross (the F1 generation) and it has much less effect in the second and subsequent generations of crossbreeds. To give you an example, if a Large White boar is mated to a Landrace sow, the offspring will show the effects of hybrid vigor. If two of these crossbreeds are then mated, the crossbreeds produced by this mating (the F2 generation) will not have the performance of either of their parents. However, if a crossbred sow from the F1 generation is mated to a boar from a different pure breed (e.g. a Welsh Boar) the offspring of that cross will show hybrid vigor and perform well. It is for this reason that the breeders of hybrid pigs have to look for a number of pure breeds to introduce into their hybrid pig.
SYSTEMS OF CROSSBREEDING
Many systems of crossbreeding are employed by farmers and breeders, and to obtain the maximum benefit the cross-breeding programme must be carefully designed.
SINGLE CROSS
The simplest example of crossbreeding is to mate a boar from one breed, with a sow from another breed. The boar should be from a strain of the pure breed that is noted for its superior conformation, performance and lean meat production. The sow should be from a strain of the pure breed noted for its fertility, mothering ability as well as performance and carcass quality. An example is the Blue Pig produced by mating a Large White boar with a Wessex sow. The offspring show hybrid vigor, they are well-mothered, fairly uniform, and they inherit qualities for performance and meat production from their sire. The big disadvantage of this system is that two pure-bred herds must be kept in order to supply the parents for crossing, or the pure bred stock must be purchased. The system also fails to make full use of the hybrid vigor from the cross-bred sows, since these are slaughtered and not used for breeding.
BACKCROSSING OR CRISSCROSSING
In this system, two pure breeds are mated in a single cross to produce a crossbred pig.
| 1st Cross | LARGE WHITE BOAR | LANDRACE SOW | |
| LW | X | L | |
| Crossbred LWL |
The gilts from this cross are then mated back to a boar from either of the two original breeds.
| 2nd Cross | LW | X | LWL |
| Crossbred LWL |
The gilts from this cross are then mated back to a boar from the other breed used in the original cross.
| 3rd Cross | L | X | LWLWL |
| Crossbred LLWLWL |
This system results in about 2/3 of the inheritance coming from the breed of the boar last used, and 1/3 from the other breed. There is some variation from one generation to another and if the system is followed on a long term basis, some hybrid vigor reduces the need for the continual introduction of fresh, pure bred parents.
ROTATIONAL CROSSING OR CYCLICAL CROSSING
Pure bred boars from different breeds are used in rotation on selected cross-bred sows. The system attempts to retain the hybrid vigor which was attained when the first crossbred sows were used for breeding purposes. The degree of decline of hybrid vigor is much smaller than with Crisscrossing if pure bred boars are used each time. However, 8 large herds are needed to justify the different boars that are used.
| 1st Cross | LW | X | E (Essex) |
| Crossbred LWE |
| 2nd Cross | L | X | LWE |
| Crossbred LLWE |
| 3rd Cross | H (Hampshire) | X | LLWE |
| Crossbred HLLWE |
This is the system which the breeders of hybrid pigs use, and they control the qualities they require in their hybrid, by their selection of the various pure breeds used in their breeding programme. The hybrid vigor improves the fertility and mothering qualities of the hybrid sows, while the bacon qualities, length, lean meat etc., come from the boars used. Many pure breeds of pigs that were of minor importance are being improved and herd numbers increased, to provide the boars needed for the many different hybrid breeding programmes, which are being carried out in different parts of the world at present.
Whichever system of crossbreeding is used, it is essential that the best pure bred stock is used, particularly from the point of view of carcass duality and performance. Although cross-breeding brings about an immediate advantage in hybrid vigour, this advantage is not cumulative. The best method of improving the quality of crossbred animals is by improving the quality of the pure bred animals used in the crossbreeding programme. Pure breeds are improved by careful selection based on the heritability of traits and by performance and progeny testing.
4. HYBRID BREEDING
One of the first companies to carry out a hybrid pig breeding programme was the British Oil and Cake Millers (B.O.C.M.) in the U.K. Their pig improvement scheme began in 1963 because research had shown that many commercial strains of pigs were unable to make the best use of the company’s improved pig rations; they did not have the genetic potential to respond to better feeding. Work was carried out on the company’s farms in the U.K. with the aim of improving three breeds, the Landrace, Large White and Saddleback, and to produce a 3-way cross hybrid pig, with an overall performance 25% better than the average. The scheme started with the purchase of 1000 weaner pigs, of which 30 – 40% were acceptable genetically and visually. All the gilt pigs were performance tested, and the best 12.5% went into the foundation herd. The best sows were used subsequently to breed boars. The selection of boars was such that less than 1 in 250 male pigs was finally used. The final hybrid pigs are used for both pork and bacon production.
Most breeders of hybrid pigs work with large numbers, and use carefully selected and bred pure breeds. Two of these breeds are crossed to produce the hybrid sow with the benefits of improved litter numbers, milking ability and better health and vigor. The hybrid sow is then mated to an improved boar of another pure breed to produce the hybrid bacon or pork pig. The hybrid sows are produced by multiplying breeders who get their breeding stock from the hybrid breeding company, and these farmers usually work on a contract basis, being paid by the company for looking after the pigs. The commercial farmer buys his hybrid gilts from the company and the pure bred boars from the same company. A typical hybrid breeding programme is shown in the diagram overleaf.
There are a number of hybrid breeding programmes being carried out in the U.K. Europe and the U.S.A.
A Typical Hybrid Breeding Programme
