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 ¾ one breed and ¼ 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 ¼ 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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