Crossbreeding

Hybrid vigour, or heterosis, cannot be fully explained, but there are various theories as to its cause. However, the result is that the efficiency of the animal is increased. It has the same influence on the animal as if the environmental conditions were much improved, with the result that an increase in mass gain, reproductive ability and general vitality is measured.

Hybrid vigour is the increased production of certain traits from the crossing of genetically different individuals. The offspring exceeds the average performance of their parents for traits for which hybrid vigour is expressed.

For example:

• Breed A averages 610 pounds at weaning
• Breed B averages 590 pounds at weaning
• When crossed, the A x B calves average 625 pounds at weaning

The hybrid vigour from this mating can be calculated with the following equation:

 

The hybrid vigour for this cross is 4 percent above the average of the parent breeds for weaning weights. This is known as individual heterosis. Individual heterosis is the increase in production seen in the crossbred offspring. This type of heterosis is generally seen in the growth traits of the crossbred offspring. Throughout this module, the % heterosis will be a reference to an F1 (first-generation cross) with 100 percent heterosis.

It has also been found that the greater the difference between two crossed breeds, the more heterosis occurs. It is also important that the parents used for crossbreeding should be as pure as possible.

The characteristics influenced most by cross-breeding are those with a low heritability for example fertility, milk production and growth. These are the traits influenced most by environmental conditions and are difficult to improve through breeding.

The aim of crossbreeding

1. To utilise heterosis in obtaining a more productive slaughter animal
2. To combine the favourable properties of two (or more) breeds for a specific purpose and so to obtain animals that are better adapted and mature faster, with higher fertility and better milk production

The Purpose of crossbreeding

A breeder must set himself a specific goal with crossbreeding. Crossbreeding without a goal can lead to chaotic results.

Goal 1. To attain a more productive slaughter animal

This aspect is clearly illustrated by the crossbreeding work performed at Vaalharts many years ago with Jerseys.  It is generally known that the Jersey does not produce the ideal slaughter animal, since it was bred for milk production. Jersey cows were then crossed with Afrikaner, Charolaise and Hereford bulls.

Some of the offspring were slaughtered at an age of 18 months after a finishing period of 118 days. Carcass mass in the Afrikaner, Charolaise and Hereford crosses were 206,4 kg, 227 kg, and 210 kg respectively. Carcass masses in the three crosses were good, and much better than would be expected in a pure Jersey. The Charolaise crosses produced the highest carcass mass with a good meat conformation.

The desired carcass type will therefore determine what sire breed should be used in the cross.  Thus, if fat covering is a prerequisite for good grading, the Hereford will be the obvious choice as the sire breed. This is also the reason for the British beef breeds being so popular for crossbreeding in South Africa.

However, grading requirements are in the process of changing in favour of more muscle and relatively less fat. As a result of this, large-frame lean meat breeds and dual-purpose breeds are becoming more popular. Since less fat is produced these breeds and their crosses are usually also more efficient as regards feed utilisation.

Similar results were obtained with Afrikaner cows crossed with different beef and dual-purpose breeds.  Brahman-Afrikaner crosses also performed very well under veld conditions. Under these conditions, they maintained an excellent growth rate and produced a carcass of above-average quality. However, under feedlot conditions, these crossbred oxen did not perform better than pure Afrikaners. Clearly, the ultimate aim of crossbreeding must be defined.

Goal 2. To attain more productive breeding animals

The characteristics that derive the most advantage from crossbreeding are those with low heritability, i.e. those traits which are difficult to improve by breeding.  Milk productions and notably fertility are two such traits. The F1 heifers can thus gain much with regard to these traits.

It is known that generally Hereford is characterised by low milk production.  Crossbreeding with the Hereford cow will have as a result that female offspring will show considerable improvement as regards this trait.  When the sire is a dual-purpose animal, it can be expected that milk production will be much improved.

Generally, milk production in the Afrikaner cow is not sufficient to maintain a high growth rate in her calf.  Crossing of Afrikaner cows with a dual-purpose bull such as the Simmentaler has also had good results.  The F1 heifers attain sexual maturity at an earlier age as a result of their faster growth and are capable of maintaining a high reproductive rate.  These animals also produce large quantities of milk and consequently, their calves grow fast.  In addition, the contribution of the Afrikaner ensures that the heifers are hardier and better adapted to extensive conditions than the pure Simmentaler.  On the other hand, the Simmentaler contributes to the growth potential and the conformation of the offspring.  

The question is often asked whether the use of F1 bulls also holds the advantage.  Most results prove that the use of F1 bulls exert a negative influence on the offspring.  The explanation for this phenomenon is that the advantage of F1 females is not so much associated with their breeding ability, but with their ability to make better use of the environmental conditions.  This ability is not very heritable and thus cannot be transmitted to the offspring by the bull.  The recommendation, therefore, is not to use crossbred bulls.  Earlier in the chapter mention was made of the fact that maximum heterosis is obtained when parents used in cross-breeding are as pure-bred as possible.  Fertility, milk production and growth of F1 heifers are thus the expression of the heterosis caused by the mating of pure animals and must not be confused with the above-mentioned statement. 

The choice of suitable breeds for crossbreeding

The purpose of crossbreeding will to a large extent determine which breeds are chosen.  In South Africa, there is little justification for crossing exotic breeds, except when grading up animals to a specific breed.  The reason for this is that the most exotic breeds are efficient producers under favourable conditions and in addition, their numbers are relatively small.  It is thus difficult to obtain sufficient good mothers for crossbreeding.  The crossing for example of the Hereford with the Friesland is applied in dairy herds where offspring are not retained or where young heifers are mated with a beef bull so that their potential can first be gauged.

In South Africa, the Brahman is usually used as the dam breed given their large numbers and the hardiness and mothering ability of the cow.

The following crosses can be considered for specific conditions and purposes: 

1. Zebu breeds (Afrikaner and/or Brahman) crossed with:

• Zebu for breeding animals or store oxen under extensive production conditions.
• New breeds (Bonsmara, etc.) for grading up to these new breeds.
• Small-frame beef breeds (Hereford, Sussex, etc.) for slaughter animals under both intensive and extensive conditions.
• Small-frame dual-purpose breeds (South Devon) for breeding animals and slaughter animals.
• Large-frame dual-purpose breeds (Simmentaler, Pinzgauer, etc.) for breeding animals – watch out for calving problems in certain breeds.
• Crossing with large-frame beef breeds is not recommended because of calving

2. New breeds (Bonsmara) crossed with:

• Small-frame beef breeds for slaughter animals under intensive and extensive conditions.
• Other crosses are not recommended.

Large frame dual-purpose (Simmentaler etc.) with:

• Zebu breeds for breeding animals in any system
• New breeds for slaughter animals and breeding animals in any system.
• Small-frame beef breeds for slaughter animals under intensive conditions.

1. Small-frame beef and small-frame dual-purpose breeds can be crossed with Zebu and new breed bulls for slaughter and breeding animals in any system, but this is not strongly advocated.
2. Large-frame beef breeds (Charolaise etc) are not recommended for cross-breeding purposes since the smaller bull breeds will not be able to maintain the production potential of the pure breeds.
3. Dairy breeds are often crossed with bulls of Zebu breeds, small-frame beef breeds, and sometimes also new breeds. Good slaughter animals can be obtained. Where Zebu blood is brought in, the female offspring can also be used as breeding animals in any system.

Problems encountered with crossbreeding

The best result with crossbreeding is obtained when well-bred animals representing two divergent pure breeds are mated with each other.  The cowherd used for crossbreeding must either be bred by the breeder himself or brought in.  A herd of purebred animals must thus be available to produce a sufficient number of good purebred breeding animals for crossbreeding purposes and to supply replacement heifers.  However, depending on the standard of the pure herds, it is difficult, or even impossible to do this. Under favourable conditions, only 10% of good pure heifers are available for crossbreeding.  It would be sufficient to keep the crossbreeding herd constant, but not to increase the size of the herd, or to improve the herd.  best replacement heifers are used for the pure herd.

Cross-bred animals are usually much more productive than their dams and require sufficient feed to fully express their potential.  More and better-quality grazing is thus a prerequisite for efficient production with this type of animal.  It can easily be stated that 20% more grazing per animal will be required since these bigger animals eat more.  If the same number of crossbred animals is kept on the same limited area of land as their pure-bred mothers, the veld will undergo damage and retrogression.  The advantages of hybrid vigour will then be changed to a disadvantage.

The heterotic effect is maximised when the first crosses are produced.  Over time this effect will become diluted when second and third crosses are produced.  Nothing permanent is built up for posterity. With the correct attitude towards pure-breeding, animals can be bred over a course of time that will maintain optimal and stable production in a specific environment.

Crossbreeding with small cow breeds and large bull breeds often result in calving problems because of the birth of large calves.  It was established that for every 0.45 kg increase in birth mass the percentage of difficult calving increased by 3, 2% in 2-year-old Hereford and Angus heifers.

The F1 generation often has a livelier nature than their parents.  The crossing of two temperamental breeds and consequent handling problems make this type of farming challenging.

Results obtained with crossbreeding in one location and with one herd are not necessarily repeatable in another herd, since the foundation material can vary to a large extent. Results of trials conducted at Mara Research Station showed that half Hereford and half Afrikaner animals had a hybrid vigour of + 28% at 24 months of age, whereas at Neudamm Research Station a hybrid vigour of + 9,8% was recorded. The hybrid vigour of a half Aberdeen Angus half Afrikaner at 24 months of age was – 45%, and at Neudamm + 3,5% at 20 months of age.  These results are probably extreme but serve to indicate that figures for hybrid vigour must be interpreted with care.  Both of these previous examples were for growth on veld.  The hybrid vigour for other traits does not necessarily follow the same pattern.