Crossbreeding Systems for Beef Cattle (E2701)
Crossbreeding can increase levels of production in livestock in two ways: 1. Complementarity and 2. Heterosis
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Reasons for Crossbreeding
Crossbreeding can increase levels of production in livestock in two ways:
1. Complementarity utilizes the desirable characteristics of two or more breeds to achieve a higher frequency of desired genes among the crossbreds than could be found within a single breed. In other words, the strong points of one or more breeds can be used to compensate for the weak points of another breed. Geneticists refer to this as additive gene effects.
2. Heterosis (hybrid vigor) results from non-additive gene effects. It is defined as the percent of superiority expressed in a trait by crossbred progeny over the average of the parent breeds in the cross. Heterosis is calculated by the following formula:
% heterosis = crossbred avg. - straightbred avg/straightbred avg.x 100
As an example, assume that the two parent breeds in a cross had weaning weight averages of 575 and 475 pounds and their crossbred progeny averaged 550 lb. The percent of heterosis would be:
550-525/525 x 100 = 4.8%
The basic objective of crossbreeding systems is to optimize simultaneously the use of heterosis and breed differences within a given production and marketing environment. The production environment includes feed resources as well as climatic conditions.
Heterosis and Its Effects
The level of heterosis tends to be inversely proportional to heritability. In moderately to highly heritable traits, such as carcass characteristics, the level of heterosis is low. On the other hand, in traits having low heritability, such as fertility and Hvability, heterosis is high. In general, heterosis is expressed to a greater degree in reproduction and in traits expressed up to weaning time.
Heterosis is classified as either individual or maternal. Individual heterosis is that expressed by the crossbred calf; maternal heterosis is that expressed by the crossbred dam. Table 1 illustrates the relationship between heritability and heterosis in various traits. It is important to note that reproduction traits, which are low in heritability and for that reason cannot be changed readily through selection within a pure breed, can be markedly improved through the effects of heterosis. Conversely, carcass traits exhibit little or no heterosis but respond well to selection within a breed. As animals in a cross become geneticallv more divergent or unlike, heterosis is usually higher. As an example, when Bos taurus (European) breeds are crossed with Bos indicus (Brahman) breeds, the effects of heterosis are greater than those shown in Table 1. On the other hand, when bloodlines within a breed of cattle are crossed, little, if any, heterosis is expressed.
The cumulative effects of individual and maternal heterosis on calf weight weaned per cow exposed can be very dramatic. This is shown in Table 2, which summarizes a long-term crossbreeding study conducted in two phases at the Fort Robinson Research Station in Nebraska. Phase I measured individual heterosis by comparing crossbred calves against straightbred calves, both of which were raised by straightbred dams. Weaning percentage was 3 percent higher for crossbred calves because of 3 percent higher livability. This, coupled with a 4.6 percent heavier weaning weight, resulted in 8.5 percent more calf weight weaned per cow exposed in favor of the crossbred calves.
The effects of maternal heterosis were measured in Phase II by comparing crossbred against straightbred cows, both of which were raising crossbred calves. Weaning percentage was 6.4 percent greater for the crossbred cows because of a higher conception rate; there was no difference in calf livability. Because the crossbred cows milked heavier, their calves weighed 4.3 percent more at weaning time. These two factors together resulted in 14.8 percent more calf weight weaned per cow exposed for the crossbred dams. In the third column of Table 2, the combined effects of individual and maternal heterosis are summarized. Crossbred cows raising crossbred calves weaned 23.3 percent more calf weight per cow exposed than straightbred cows raising straightbred calves. About two-thirds (14.8 percent) of this advantage was due to maternal heterosis and one-third (8.5 percent) to individual heterosis. Experiments involving Brahman x European crosses have shown even greater cumulative increases over the average of the straightbred parents.
If your goal is to maximize heterosis, the following requirements should be met (assuming natural service and raising your own replacements):
1) Avoid backcrossing and subsequent loss of heterosis by making the most divergent matings possible.
2) Two or more breeding pastures are needed.
3) Two or more breeds of bulls are needed.
4) All females must be identified by breed of sire and year of birth.
Certain crossbreeding systems can help mitigate some or all of the above requirements.