CROSSBREEDING (HETEROSIS)

Carefully planned and well managed crossbreeding systems are a tool which have the potential to deliver significant benefits to beef producers. Travel anywhere in regional Australia and you are bound to see numerous herds of crossbred cattle, with many commercial producers  taking advantage of the performance “boost” created by mixing two or more purebreds.

The main benefits of crossbreeding result from:

• Additional performance as a result of hybrid vigour (also known as heterosis).
• The ability to combine the benefits of different breeds (also known as breed complementarity).

Hybrid Vigour (or Heterosis)

Hybrid vigour is the amount by which the performance of the crossbred animals exceed (or differ from) the average performance of the purebred parents that are used in the cross. An example of this for weaning weight is demonstrated in Figure 1.

This was further illustrated in a crossbreeding research trial undertaken by the Queensland Department of Primary Industries which highlighted the benefits achieved through a structured crossbreeding program for weaning weight. This trial included crosses of Hereford, Angus and Shorthorn cattle in Southern Queensland.

The results of this trial are demonstrated in Figure 2. Compared to the straight bred calves, the F1 crossbred calves showed on average an 8.5% increase in weaning weight per cow mated. While significant, a larger increase of 23.3% was observed is the F2 calves, being those calves bred from F1 cows. The additional “boost” was obtained from maternal heterosis.

Importantly, heterosis is not just observed for weaning weight but in many economically important beef cattle production traits, especially in traits of “low” heritability such as reproduction and adaptability traits.

Table1 below illustrates the relationship between heritability and heterosis for different categories of beef cattle traits.

Reproduction and maternal traits have low heritability and the traditional response to selection in breeding programs will generally be slower compared to high heritability traits. At the same time however, significant improvement in these traits can be made through programs that maximize heterosis.

The inverse is true with carcase traits. Significant and rapid progress can be made through selection for carcase traits in a breeding program, while crossbreeding has little or no heterosis effect. Growth traits are moderate for both heritability and heterosis, making progress possible through both selection and crossbreeding.

The amount of hybrid vigour achieved will depend on the type of crossbreeding or composite system implemented. A composite breeding program is a crossbreeding system that is stabilised (inter-mating the crossbreds).

Table 2 lists the types of crossbreeding systems, the levels hybrid vigour (both individual and Maternal) retained and estimates of increases in weaning weight per cow mated.

To re-iterate, to fully benefit from hybrid vigour the cow herd should also be crossbred to also take advantage of maternal heterosis. Crossbred cows when compared to purebred females will generally have:

• increased conception rates
• improved calving ease
• increased percentage of calves weaned
• a longer production life (i.e. longevity)

Breed Complementarity

Breed Complementarity results when combining the strong traits of one or more breeds to compensate for the weak traits of another breed.

For example, a British breed female (eg. Angus, Hereford, Shorthorn) mated to a European breed sire (eg. Charolais, Simmental, Limousin) complement each other exceptionally well.  In general terms, the British breed female contributes early maturity, easy finishing and calving ease while the European breed sire contributes high growth and muscle.

Crossbreeding Considerations

There some considerations that need to be taken into account regarding the implementation of a crossbreeding program.

These include:

• Managing and/or sourcing replacement females   (depending on crossbreeding system implemented).

• Crossbreeding programs may require more joining groups than a purebred system. This could be eased by purchasing replacement heifers although locating a source of heifers available on a regular basis with suitable health status and known breeding may be difficult.

• Hybrid vigour may result in higher birth weights when sires of a high growth rate breed are joined to heifers of a smaller breed. Selection of suitable genetics is essential to counteract this negative.

• Crossbred females may be larger and therefore consume more feed than purebred females. Research has indicated that productivity per hectare still favours the crossbred female.

Selection & Crossbreeding

Crossbreeding should not be seen as an excuse for using “low” performing genetics (i.e. bulls) within a breeding program. Regardless of hybrid vigour, the performance of the crossbred herd will depend largely on the genetics of the parent, the management level and the environment that is used.

Figure 3 illustrates the benefits of combining selection with crossbreeding.