It is often argued that there is a larger genetic variation within a particular breed than there is between breeds. Results from the US Meat Animal Research Centre show that both between and within breed, genetic variations are large and important for most traits of economic importance.
For example, Figure 1 shows that the lightest Charolais calfhas a heavier birthweight than the heaviest Jersey calf, demonstrating a real difference between breeds.
Similar results have been found for other important traits such as weight at weaning, yearling and mature ages; age of puberty; milk production; retail meat yield and marbling in carcases.
Rapid progress can be made by selecting breeds for a crossbreeding program for their desirable traits. After breed selection is completed, the herd can be further improved by selecting animals within this crossbred herd for the economically desirable traits.
Heritability comes close to 100% when selecting for traits between breeds but the heritability of differences within breeds for major economic traits varies from less than 10% to about 50% depending on the trait.
This is because estimates ofbreed differences are based on a large number of individuals of that breed type.
Estimates of differences within a breed are based on far fewer observa¬tions for a specific trait and so are not as accurate. As a result, selection between breeds is considerably more effective than selection within breeds.
Using crossbred bulls
Figure 1 Distribution of birthweight in the Charolais (C), Hereford x Angus (HA) and Jersey (I) populations
Crossbred bulls are being increasingly used in the US to combine desired characteristics more quickly than by using straight breeds.
Bull breeders can create crossbred bulls that represent intermediates in size, milk production, growth rate, etc, so that there are fewer variations in their daughters’ size, milk production, feed requirements, growth and carcase characteristics from year to year than might occur using purebred bulls.
An example would be using a Simmental x Hereford bull rather than a pure Simmental bull over Angus cows. The female offspring with 1/4 Simmental would be more suitable to retain as breeders than 1/2 Simmental heifers because of their smaller mature size and better fattening ability.
There will be extra hybrid vigour expressed because three breeds are used in only one cross rather than two.
A common myth about crossbred bulls is that they will produce unpredict-able offspring. The results from a Simmental x Hereford bull bred to an Angus cow are no different in terms of the range of calves produced than from a Simmental bull x (Angus x Hereford) cow.
Crossbred bulls tend to produce offspring somewhere in between the bull’s two purebred parent breeds. They also exhibit heterosis for reproductive characteristics, as do crossbred cows.
Crossbred bulls are most commonly used when developing a composite breed.
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