Use of crossbreeding to make production gains
by Janice M. Rumph, assistant professor of genetics, Montana State University College of Agriculture
Crossbreeding is a relatively easy and inexpensive way to increase performance in your cow herd. By doing something as simple as buying a bull (or semen) that is a different breed than your cow herd, you can increase the performance of your herd by as much as 20%, depending on the trait and the breeds involved. The increase in performance is due to the phenomenon of heterosis, or hybrid vigor, and it can occur in almost every performance trait that is measured in beef cattle.
Heterosis occurs whenever two animals are bred together that have different breed make-ups. Breeds are breeds because they have been selected for certain traits over the years. Although all cattle possess genes for the same traits (i.e., genes for birth weight, weaning weight, marbling, etc.), as breeds have evolved, some breeds have been selected for different copies of those genes than others. These different copies are known as alleles and each animal has two alleles one from its sire and one from its dam. For instance, all cattle have genes for coat color, but Angus have evolved to have alleles that code for black coat color while Herefords have evolved to have alleles that code for red coat color.
The same is true with production traits, such as weight, fertility, or carcass traits. Individual breeds may have a higher frequency of certain alleles than other breeds. Because of this, when we cross animals of different breeds together, alleles that may not normally come together in purebred animals come together in crossbred animals and this is what causes heterosis.
Types of crossbreeding systems
There are three main types of crossbreeding systems: rotational, terminal, and composite, but there are an endless number of variations for each these systems. Many times, beef producers are told that if they want to utilize heterosis, the best way do so is by using some sort of rotational cross that is what has traditionally been considered to be the best crossbreeding scheme for beef cattle. This involves breeding cows of breed A to bulls of breed and then breeding the BxA daughters of that mating back to breed A. Cows are always bred to the same breed of bull as their maternal grandsire and never the breed of bull as their own sire.
The advantage of this system is that it allows the producer to retain some of the heifer calves as replacements, eliminating the need to purchase replacements. The disadvantage of a pure rotational system is that it discourages the use of large, growthy breeds that are traditionally used for terminal crosses because the resulting heifers may not be perceived as being suitable replacement females. Because calves are not completely crossbred (the females are not a completely different breed than the bulls they are bred to (they are 1/3 the breed they are bred and 2/3 the other breed in the rotation), 100% of the possible heterosis is not available. A traditional two breed rotational cross would result in a level of 67% individual heterosis as well as 67% maternal heterosis in all calves. Maternal heterosis will affect traits such as weaning weight because crossbred females produce more milk than their purebred contemporaries. If a third breed is introduced into the system, heterosis levels, both for individual and maternal, will increase to 86%.
If, instead of introducing a third breed into the rotation, a producer decided to convert from a two- breed rotation to a three-breed rotaterminal, he would not have to change his rotation scheme at all All that would be required would be an additional breed of bull to be used as a terminal sire. Ideally, third breed would be a large, growthy breed and be used to breed the older cows to minimize any potential calving difficulty, while heifers and younger cows would still be bred to bulls from the rotation. calves from the final cross to the terminal sire included) would be fed out and slaughtered. The replacement heifers for the rotation would come from the calves of the heifers and young cows that were bred to the rotation sires. All calves in this system would still express 67% maternal heterosis, the same as in the two breed rotation, but the rotaterminal system has the advantage that the terminal sired calves experience 100% heterosis because their sire was a completely different breed from their dams. Steers born from the rotation matings and sold to slaughter would still have 67% individual heterosis, like in a two-breed rotation. Just by adding this final terminal cross, research has shown that this type of crossbreeding system could utilize 75% of the available heterosis overall, an 8% advantage over the traditional two-breed rotational system.
Additionally, if the breeds used in the rotation are small to medium sized breeds, there is an even greater advantage to using a large terminal sire. The amount of feed required to maintain the cow herd increases as the mature size of the cow herd increases. If small to medium sized cows are used, maintenance requirements can be minimized. If these cows, once they have become old enough to handle calving larger calves, are then bred to a large terminal sire breed, the resulting calves will still exhibit the growth of the sire breed and require fewer days on feed.
One disadvantage to both rotation and rotaterminal crosses is that they require more bookkeeping than other crossbreeding systems because if you operate a strict rotation, it is important to keep track of the sire breeds of each female so that she is not mated to the same breed as her sire. If natural service is used, this also requires the use of two or three different breeds of bulls on hand and therefore, multiple breeding pastures are necessary in order for cows to be bred to the correct breed of sire.
Although retention of replacement heifers is a big advantage of the rotational system, if an inexpensive source of replacement heifers can be found, a purely terminal system is the crossbreeding scheme that best utilizes heterosis. A bull bred to purebred cows of a different breed will result in 100% individual heterosis in the F1 calves, but they will not have the advantage of any maternal heterosis. Maternal heterosis is important, especially in milk production, which will ultimately affect weaning weight. This can easily be remedied, if instead of purebred cows, the bull is bred to crossbred females. This results in 100% individual and maternal heterosis because the cows are completely crossbred as well as the calves. This best utilizes heterosis, but requires a ready supply of inexpensive replacements to be economically feasible because all heifer calves would be fed out and sold to slaughter.
beige = Individual Heterosis
purple = Maternal Heterosis
Percent Change in Performance due to Heterosis
Milk = Milk Production
Gr = Carcass Grade
Cut = Carcass Cutability
FC = Feed Conversion, Postweaning
MW = Mature Weight
YW = Yearling Weight
ADG = Average Daily Gain, Postweaning
WWPC = Weaning Weight (per cow exposed)
WW = Weaning Weight
BW = Birth Weight
CI = Calving Interval
WR = Weaning Rate (per cow exposed)
CR = Calving Rate (per cow exposed)
FSC = First Service Conception Percentage
Purely terminal systems require less bookkeeping because all cows will be bred to the same breed so it is not necessary to make note of each cow's of sire. Additionally, because only one breed is used as the sire, one breeding pasture can be more readily used.
An obvious disadvantage of this system is replacement heifers. The issue of a source of replacement heifers could be remedied by having supporting populations. In other words, for a three breed terminal cross, a producer would have to three separate purebred breeding populations, one for each of the three breeds in the cross. Additionally, another population would have to be formed that takes the purebreds from two of these populations and breeds them to create the F1 females that will ultimately be used in the final terminal cross. This requires a great deal of excess animals and resources to ultimately create calves in the final cross that experience 100% individual or maternal heterosis, but even with the purebred progeny being produced from the supporting populations, the average individual heterosis in this system is estimated to be 65% based on research from Texas A&M University.
Another crossbreeding system that is easy to implement is that of composites. Heterosis levels vary depending on the number of breeds and the proportion of each breed in the composite. Like a rotation, individual and maternal heterosis levels will be the same because the calves will have the same proportions of breeds represented as their dams. Composite animals are nothing more than crossbred animals that are bred together so that each calf has the exact same breed makeup as its sire and dam. The more breeds used in a composite, the more available heterosis that is utilized, also the more equal the breeds are represented in the composite (example, of each of 4 breeds) the higher the level of heterosis obtained.
Like rotations, composites allow you to retain replacement females and like terminal crosses, they also allow utilization of multiple sire pastures because all females are being bred to the same "breed" of
Summary
In conclusion, crossbreeding can increase the performance of any herd with little to no additional costs to the producer. The options for crossbreeding systems are limitless and the type of system implemented is largely dependent on the resources that are available to the producer. When deciding on which system to use, care must be taken to insure the breeds chosen compliment each other in order avoid potential problems that could be brought on crossing to incompatible breeds together.
% of Available Heterosis Utilized in the Final Cross
PB Purebred
2BR Two-Breed Rotation
3BR Three-Breed Rotation
3BRT Three-Breed Rotaterminal
F1 Purebred Bulls on Purebred Cows of another breed
F1D Purebred Bulls on Crossbred Cows
3BC Three-Breed Composite (1/2 of one breed, 1/4 of
each of the other two)
4BC Four-Breed Composite (1/4 of each breed)