Before discussing breeding in more detail, it is important for you to know the meaning of three important terms. These are Heritability, Performance Testing and Progeny Testing.
|Progeny: a descendant or the descendants of a person, animal, or plant; offspring|
Some characteristics, or traits, are highly heritable due to genetic factors. This means that there is a good chance of passing on the trait to their offspring. Some traits that are highly heritable have little or no economic value, such as skin colour. However, other traits do have a considerable economic value, an example being the bodyweight of the mature animal. The number of muscle cells of an animal 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.
Traits that have a high heritability in beef animals are:
|Live-mass gain (post weaning)||50%|
|Thickness of backfat||30%|
A 30% heritability is low, 40 – 60% is high and above 60% is very high. By selecting bulls that have high heritability traits, and using them in a breeding programme, the whole herd can be changed in two or three generations. These changes may not be desirable. For example, selecting for size will increase the size of the animal, but in Africa the environment is suited to smaller cattle.
Examples of traits that have a low heritability are birth weights and weaning weights, because these depend to a large extent on feeding, like mother’s milk, and management factors; in other words they are affected by environmental factors. These traits cannot be improved by selection, but they can be affected by good management, and also by cross breeding.
This is done by measuring the performance of an animal while it is growing. Beef bulls are performance tested from birth to the time they reach maturity at about 2 – 2 ½ years old. They must have good levels of feeding and management so that their genetic capabilities are fully realised. A bull whose growth is stunted by poor feeding cannot show his genetic capacity for growth. Records are kept of food intake, daily live-mass gain, and the mature bull is judged by eye for his conformation, or appearance for muscling and fat cover. In this way the performance of one bull can be compared with that of another, and because of heritability, a bull that is a good performer will probably be a good breeder of calves like himself.
Conformation: the shape or structure of something, especially an animal.
Phenotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
Genotype: the genetic constitution of an individual organism, often contrasted with phenotype.
In any animal breeding there are three types of selection:
- Natural Selection, which is the survival of the fittest;
- Artificial Selection, which is based on the phenotype, or appearance, of the animals selected by the breeder; and
- Scientific Selection, which is based on the scientific principles of genetics and takes into account the genotype, or genetic makeup of the animals selected by the breeder.
This is the type of selection which takes place in herds of wild animals living under natural conditions. In nature, the fittest and strongest males serve the females so that their characteristics get passed on to their progeny. As they lose their strength through old age or disease they are replaced by younger males. As a result of a number of males serving the females, particularly in a large herd, breeding is by random mating, and this produces offspring which are close to the breed average; this is the best method of breeding animals for survival. In any herd of wild animals, such as buck, each member of the herd performs at about the same level; they eat the same food, run at the same speed, have the same amount of stamina, and the same resistance to ticks and other diseases and parasites. In other words, the group shows an average performance, which is the breed average.
In any breeding programme, this natural pull towards the breed average has to be overcome by the breeder if he is trying to produce exceptional animals, both as performers and breeders. This is why natural selection and random mating will never produce outstanding animals.
In this type of selection, the breeder is trying to improve the animals in his herd using the phenotype, or appearance, of the animal. He looks at a bull, judges his appearance, and decides that this is the type of animal he would like to breed in his herd. If all the animals in his herd had a similar conformation, or appearance, he would be well pleased. Therefore, he uses that particular bull on his cows in the hope that the offspring will be like their sire. This system will work well, provided that the characteristics selected have a high heritability.
If you look at the table on the table above you will see that most of the beef characteristics such as grading and muscling have high heritabilities, so that selection by phenotype will be fairly successful in a beef herd. If the bull has been performance tested as a young growing animal, there is a good chance that his live-mass gain will be passed on to his progeny, because growth rate does have a reasonable heritability.
This is the selection of animals for a breeding programme using both the phenotype, and appearance, of the animal, and the genotype or genetic make-up of the animal. A good knowledge of the science of genetics is necessary for this type of selection. The breeder has to know which traits have a high heritability, which have a low heritability, the effects of cross breeding, in breeding, line breeding and many other factors.
It is the best method of showing long term improvement in a herd, but it is a slow business and the breeder can suffer many setbacks – in fact many breeders never really succeed. In every case, the breeder is looking for a sire who not only has a good phenotype, but who also has a good genotype, and who will pass his good characteristics onto his offspring.
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. A Hereford Bull mated to a Hereford cow would be pure breeding. If the two animals were registered with the Hereford Breed Society, their mating would be pedigree breeding and the calf would be eligible for registration with the society as a fully pedigree animal.
Pure breeding depends on Selection and Heritability to be effective. The breeder must know the characteristics, or traits, that he wants in his cattle, and he must select for those traits. Animals with the desirable traits are kept in the herd, while those with undesirable traits are culled.
Points for selection in beef breeding are:
- Birth-weight of the calf;
- Daily live-mass gain after weaning; and
- General conformation of the animal, fat cover and beef characteristics of the animal.
If you look again at the trait heritability table above you will see that, apart from the first two traits above, the others have high heritabilities. The traits with high heritabilities can be improved by Artificial Selection based on the phenotype of the sire used, and those with low heritabilities can be improved by Scientific Selection based on the genotype of the sire, and also by good management and feeding.
A problem in beef breeding is that the milking capacity of the beef cow is the main factor in governing the weaning weight of the calf. However, milking capacity and beef characteristics are incompatible. Cows selected for their milk production tend to lose their beef characteristics and vice versa. In fact, some beef pure breeds have, in the past, lost much of their ability to produce milk, because of intensive selection for beef traits. This happened with the Aberdeen Angus breed in the U.K. to the extent that some pedigree cows were unable to produce enough milk to rear their calves.
Pure breeding is covered in more detail in the animal breeding course.
The mating of two animals of different breeds is called cross breeding. A Hereford bull mated to an Afrikaner cow is cross breeding and the resulting calf would be a cross bred calf. Cross breeding produces improvements in the cross bred animal due to a large number of fresh genetic combinations coming into existence. This condition is called Heterosis, or Hybrid Vigour, and it affects performance of the animal which in turn, improves the commercial value of the cross breed. The commercial farmer is looking for animals that perform well, whereas, the pure or pedigree breeder is looking for animals that 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 cross bred animal can combine the best qualities of each of its parents. If the parent pure breeds have been selected for good economic traits, these can be combined in the cross bred offspring; and
- The effect of hybrid vigour can improve the commercial performance of the crossbred animal. Hybrid vigour has most effect on those traits that have a low heritability, such as birth weight and weaning weight. A crossbred cow can often produce a bigger calf and more milk than a pure bred cow.
The disadvantage of cross breeding is that the crossbred animal will not pass on its improved performance to any offspring. Hybrid vigour is strongest in the first cross (the F1 generation) and it has much less effect on the second and subsequent generations of crossbreeds.
If a Hereford bull is mated to an Afrikaner cow, the offspring will show the effects of hybrid vigour. If two of these cross bred animals are then mated, the cross breeds produced by this mating, the F2 generation, will not have the performance of either of the parents, particularly in trait of milk production. However, if a cross bred cow from the F1 generation is mated to a bull from another pure breed, e.g. an Aberdeen Angus, the offspring of that mating will show hybrid vigour and perform well.
In beef breeding, the long-time interval between generations means that, except in very large herds, there is not much scope for anything more than a simple cross; e.g.
The farmer keeps a herd of pure Afrikaner cows and buys in the Hereford bulls that he needs for his breeding programme. However, because hybrid vigour has most effects on traits with a low heritability such as milk production, the bull benefits of cross breeding are obtained only if the F1 generation cows are used as mothers and themselves crossed with a third breed.
This type of cross breeding is easy to carry out with sheep, pigs or poultry, but it requires a very large unit in order to be effective with beef cattle.
SUMMARY OF BREEDING METHODS
Best of 2 breeds brought together + Hybrid Vigour
New genes coming together to give animals good phenotype but poor genotype.
Affects factors of low heritability
Figure 1: Hereford Bull