by: Heather Smith Thomas
Many commercial stockmen take advantage of heterosis (hybrid vigor) by crossbreeding. Research in the early 1900's demonstrated the benefits, which include breed complementation (combining some of the desired traits from two or more breeds, to gain characteristics that were lacking in one of the parent breeds) and heterosis. No breed has all the production characteristics you might desire in your herd, and by careful crossbreeding you can add the traits you want from a different breed, and overcome some of the weaknesses of each breed, producing animals that exhibit more desired qualities and less of the weaknesses. The crossbred animal will benefit from heterosis�the performance advantage that enables the crossbred to exceed expectations of the parent breeds. Heterosis beneficially influences many traits that are important for increased beef production, including fertility and reproduction, calf survival (hardier calves), maternal ability, growth rate of young animals, efficiency and longevity.
Heterosis is maximized when mating animals with very different genetics, as when using animals of breeds that are completely unrelated�such as a British breed and a breed containing Bos indicus (zebu) bloodlines, like Brahman. Heterosis is also great when breeding British breeds to continental breeds, even though all of these animals are Bos taurus. The British breeds are less closely related to most continental breeds than the British breeds are to each other (or the continental breeds to one another).
Breeding animals within the same breed always limits genetic potential to some degree because all modern breeds were created by using a certain amount of inbreeding to �fix� the desired characteristics so there would be uniformity in the offspring. The gene pool in any given breed is limited. A certain amount of production potential is always sacrificed in order to gain the uniformity desired in a breed, since the most dependable way to gain uniformity was by using inbreeding and linebreeding in the early history of the breed. A breed is essentially a closed group of cattle (not allowing any infusion of other genetics) and thus accumulates some inbreeding over time, even if it's not done deliberately.
A crossbred animal is created by mating two straightbred animals of different breeds or a crossbred animal to an animal of a third breed, or two crossbred animals of different mixes of breeds. Crossbreeding is the opposite of inbreeding. It opens the door for much wider genetic variation, and results in heterosis. As explained by Dr. Jim Gosey (retired University of Nebraska Extension Beef Specialist), heterosis is actually the recovery of accumulated inbreeding depression. In just one generation, the offspring exhibit the maximum of what was lost through generations of �pure� breeding within a closed gene pool.
Many stockmen feel that heterosis is most easily maximized with a three breed crossing system, mating a crossbred cow with a bull of a third breed. It is often easier to buy (or use semen from) a purebred bull than a crossbred bull, unless the bull is a composite. Composites are gaining in popularity because they simplify the breeding program; the animals are already mixed in a desired combination.
A composite is an animal created by mating two animals that have crossbred parents of similar breeding; in other words the breed �mix� is the same in sire and dam and has been standardized into a predictable blend over several generations of breeding crossbred to crossbred. The animals are all the same percentage of certain breeds, such as half and half of two breeds, or 3/8 of one breed and 5/8 of another, or a certain blend of three or even four or more breeds. Some of the �breeds� in use today like Brangus (a stabilized percentage of Angus and Brahman), Santa Gertrudis (Shorthorn and Brahman) and Beefmaster (Brahman, Shorthorn and Hereford) were some of the first composites. There are a growing number of popular composites today, such as Angus and Gelbvieh, Angus and Salers, Angus and Chianina, several combinations of British and continental breeds, etc.
Crossbred Cows Provide The Ultimate Benefit
The greatest amount of benefit gained by crossbreeding is with crossbred cows. Even though many stockmen use crossbreeding of straightbred parents to produce exceptional market calves (calves that gain faster than straightbreds and do well in the feedlot), the crossbred cow is the key to maximum beef production and profitability in a cow-calf operation, since hybrid vigor in the cow produces phenomenal maternal advantages. Research has shown that a crossbred cow is eight percent more efficient than a purebred cow, lives 38 percent longer and has 25 percent more lifetime production (pounds of calf weaned). This is partly due to the fact that crossbreeding has the biggest impact on traits that are not highly heritable (and hence more difficult to improve through selective breeding within a breed), such as fertility, age at puberty, and longevity.
Crossbred cows live longer and are also less apt to be culled for being late or open, due to increased fertility. Any cow that can calve at two years of age, never miss a year of calving, and stay in your herd another year or two beyond average culling age makes you money. When you consider all the benefits of a crossbred cow, you can see why animal scientists call this the �only free lunch� in the cattle business. As pointed out by Dr. Larry Cundiff (U.S. Meat Animal Research Center, Clay Center, Nebraska), data from their heterosis studies showed that breakeven costs of production were reduced about 10 percent by using crossbred cows. Another study at Montana State University compared effects of breed and heterosis on heifer pregnancy using purebred and crossbred females of several breeds. Results showed that a higher percent of crossbred heifers calved at two years of age (reaching puberty and becoming pregnant earlier) than purebreds.
And if a cow is healthier, with a stronger immune system due to hybrid vigor, she develops better immunity when vaccinated, imparts better colostrum to her calf, which in turn keeps him healthier through the risky days of early calfhood. Genetics plays a big role in an animal's immunity and immune response; the crossbred animal is hardier than a straightbred animal partly just because genes control the process of recognizing disease agents and inbreeding doubles up more of the undesirable immune-response genes. Every pure breed is inbred to some degree. Crossbreeding ensures more genetic diversity and optimal immune response. Thus a crossbred cow tends to have more optimum immune system function than a straightbred cow, and hence not only stays healthier herself but may also produce more protective colostrum.
When all factors are weighed, the crossbred cow gives you the most benefit. By contrast, the stockman who is merely trying to take advantage of hybrid vigor in the calves (using straightbred cows and bulls of another breed) gains less impact on profitability. Calf weaning weights for crossbred calves are five percent more (and yearling weights four percent more) than straightbred calves. The research study in the 1990's that came up with these figures showed that a straightbred cow with a crossbred calf earned an average of $23.37 more than if she had a straightbred calf. But a crossbred cow with a crossbred calf netted $116.88 more than a straightbred cow with a straightbred calf. This is one reason a number of producers went to crossbred cow herds.
In the past decade, however, with increasing popularity of �black� cattle and the drive toward more uniformity and marbling, many of America's commercial cow herds have lost most of the heterosis they once had. Due to market pressures for beef calves, many stockmen have been using bulls of just one breed, and the replacement heifers become more and more straightbred with each generation. According to Dr. Jim Gosey (retired Beef Extension Specialist, University of Nebraska), the loss of heterosis in these herds shows up most quickly in the traits that are least heritable (and most associated with inbreeding depression), namely reproduction (fertility), hardiness and longevity. The price paid for loss of heterosis is cumulative�as a number of very small losses add up and amount to a substantial sacrifice in lifetime productivity.
As one cattle buyer observed following a cold slow spring during which feed supplies were short, most of the cows in several herds he worked with were thin, and there was a high rate of open cows after the breeding season. Interesting to note, the cows that bred back the best, and on time (in spite of the tough conditions), were the old crossbred cows that were still in the herds. The younger females that were a high percentage of straight breeding didn't do so well.
How Can You Keep The Positive Effects Of Heterosis?
The maximum benefit from heterosis is in the first generation (F1), producing a crossbred animal from two different parent breeds. The next generation (F2) loses some of that vigor if the F1 female was bred back to a bull of one of the parent breeds. Some stockmen therefore use a bull of a third breed in order to produce calves with maximum heterosis from the crossbred cow. Maximum benefit can also be obtained by using a crossbred bull, of different breeds than the crossbred cow.
There are various degrees of hybrid vigor in calves produced from various crossbreeding systems, such as a two or three breed rotational cross. To get an idea of the range of difference, we can assume that breeding a purebred to a purebred of the same breed produces zero percent hybrid vigor and breeding a purebred to a purebred of a different breed (especially if the two breeds are very genetically different) results in 100 percent heterosis in the offspring.
In a traditional two breed rotational system, crossbred cows of breed A and B are bred back to bulls of breed A (creating calves that are � one breed and � the other breed). Then those daughters are bred to bulls of breed B. The bull breed is continually switched back and forth. After a couple of generations the heterosis obtained stabilizes at about 67 percent, according to Michael MacNeil (research geneticist at the USDA Agricultural Research Service at Miles City, Montana, in 1998). Adding another breed to the rotation (switching the sires for each generation between bulls of breed A, B, and C) extends and expands the effects, resulting in 86 percent heterosis in each generation. Adding a fourth breed to the rotation results in an increase to 93 percent heterosis, which will be continued indefinitely in each crop of calves from this mix (a sire from breed A, B, C or D bred to crossbred cows that embody the four-way cross).
One disadvantage of any rotational crossbreeding system, however, is that the breed makeup of the calves swings heavily (slightly more than half) toward the breed of the sire in each generation. As pointed out by MacNeil, unless the breeds used are similar in certain traits and performance level, there can be a lot of variability in the calves produced, from one year to the next. Another disadvantage is that rotation systems require a stockman to have more than one breeding pasture (since there will always be two or more sire groups), and sorting of cows into the proper group so their calves will be sired by the proper breed of bull to make the system work. This can be difficult on some ranches that use community range pastures, or ranches that utilize intensive grazing management with rotation of pastures.
One way around this is to use crossbred bulls. Then the breed mix in the calves can be kept more consistent, without swinging so heavily toward one breed or another. If crossbred bulls of different breeds than the crossbred cows are used, heterosis is maximized once again, or if crossbred bulls of the same (or one of the same) breed are used, heterosis is somewhat reduced but the breed mix can be kept at a more acceptable level�for instance if you want to create calves that are only � continental blood rather than half.
Another way some ranchers minimize the breeding pasture problems is to use bulls of one breed for about three years and then change to a bull of the second breed, and then to a third, or back again, and so on. The rotation of bull breeds over time will sacrifice some heterosis but this loss can be minimized if you use bulls of three or four different breeds, for only two years each.
A more recent answer to some of these problems has been the development of composite blends of breeds. Using a composite bull on composite cows reduces the need for separate breeding pastures or rotating breeds of sire.
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