~DMSION OF AGRICULTURE U~l_}J RESEARCH &: EXTENSION Agriculture and Natural Resources University of Arkansas System FSA3055 Crossbreeding Systems for Small Beef Herds Bryan Kutz For most livestock species, Hybrid Vigor Instructor/Youth crossbreeding is an important aspect of production. Intelligent crossbreed- Generating hybrid vigor is one of Extension Specialist - the most important, if not the most ing generates hybrid vigor and breed Animal Science important, reasons for crossbreeding. complementarity, which are very important to production efficiency. Any worthwhile crossbreeding sys- Cattle breeders can obtain hybrid tem should provide adequate levels vigor and complementarity simply by of hybrid vigor. The highest level of crossing appropriate breeds. However, hybrid vigor is obtained from F1s, sustaining acceptable levels of hybrid the first cross of unrelated popula- vigor and breed complementarity in tions. To sustain F1 vigor in a herd, a a manageable way over the long term producer must avoid backcrossing – requires a well-planned crossbreed- not always an easy or a practical thing ing system. Given this, finding a way to do. Most crossbreeding systems do to evaluate different crossbreeding not achieve 100 percent hybrid vigor, systems is important. The following is but they do maintain acceptable levels a list of seven useful criteria for evalu- of hybrid vigor by limiting backcross- ating different crossbreeding systems: ing in a way that is manageable and economical. Table 1 (inside) lists 1. Merit of component breeds expected levels of hybrid vigor or het- erosis for several crossbreeding sys- 2. Hybrid vigor tems. 3. Breed complementarity 4. Consistency of performance Definitions 5. Replacement considerations hybrid vigor – an increase in 6. Simplicity the performance of crossbred 7. Accuracy of genetic prediction animals over that of purebreds, also known as heterosis. breed complementarity – an Merit of Component improvement in the overall Breeds performance of crossbred offspring resulting from the For any crossbreeding system to crossing of breeds of different be effective, the breeds in the sys- but complementary biological tem must be well chosen. Each breed types. included in a crossbreeding system backcrossing – the mating of an Arkansas Is must bring favorable attributes to individual (purebred or hybrid) to Our Campus the cross. Determining the appropri- any other individual with which ate breeds to use in a crossbreeding it has one or more ancestral system can be challenging. Another breeds or lines in common. Visit our web site at: challenge is the availability of animals https://www.uaex.uada.edu of those breeds. University of Arkansas, United States Department of Agriculture, and County Governments Cooperating Table 1. Expected Heterosis Levels and Breed Complementarity Attributes of Several Crossbreeding Systems Expected Heterosis Crossbreeding System Offspring Dam Breed Complementarity Two-breed terminal cross 100 0 maximum Three-breed terminal cross 100 100 maximum (using F1 females) Two-breed rotation 72 56 some Three-breed rotation 91 70 minimal Breed Complementarity that require an unrealistically high level of man- agement are unlikely to remain in place for very Breed complementarity refers to the production long. More complex breeding systems often conflict of a more desirable offspring by crossing breeds with important management practices unrelated that are genetically different from each other but to breeding. For example, beef cattle crossbreeding have complementary attributes. In beef cattle systems that require many breeding pastures make breeding, it is often stated as “big bull 5 small cow” grazing management difficult. It is important that comple-mentarity. The big bull contributes growth cross-breeding systems fit with other aspects of cattle and leanness to the offspring, and the small cow production. This means that crossbreeding systems requires less feed to maintain herself. The result is a should be kept simple. desirable market animal economically produced. Consistency of Performance Accuracy of Genetic Prediction The higher the accuracy of genetic prediction, A crossbreeding system should ideally produce the lower selection risk and more predictable the a consistent product. It is much easier to market a offspring. Because relatively little performance uniform set of animals than a diverse one. It is also information on commercial animals is recorded and much easier to manage a female population that even less is reported for analysis, accuracy of predic- is essentially one type than one made up of many tion in a commercial operation refers to accuracy of types, each with its own requirements. Crossbreed- prediction for seed stock inputs to the crossbreeding ing systems vary in their ability to provide this kind system – typically sires. In many cases, accurate of consistency. EPDs are available for purebred sires, and cross- breeding systems using purebred sires benefit as a result. Replacement Considerations In terms of hybrid vigor, the ultimate female is an F1. Commercial producers would like to have entire herds of F1 females. How can you produce a Definitions continuous supply of F1s? One way is to maintain hybrid – an animal that is a cross of breeds within purebred parent populations to cross to produce F1s. a species. A second way is to purchase all the replacements EPD – expected progeny difference. needed from a third party. Neither of these methods is optimum for most producers. A number of cross- breeding systems manage to overcome the replace- ment female dilemma by allowing breeders to produce Examples of Crossbreeding Systems replacement heifers from their own hybrid popula- tions. However, this convenience comes at a price, Terminal Cross a price typically paid in loss of hybrid vigor, breed complementarity and simplicity. The simplest form of crossbreeding is a termi- nal cross. In this system, all offspring are marketed, making it necessary to purchase replacement heif- Simplicity ers. If F1 replacement heifers (females that have Crossbreeding systems should be relative- 100 percent hybrid vigor for maternal traits) are ly simple. Expensive systems or complex systems purchased and are bred to bulls of a different breed, Purchased Breed C terminal F1 A B maternal F1 C (A B) market offspring Figure 1. Example of a terminal crossbreeding system using purchased F1 females. both cows and calves take advantage of maximum requires multiple mating pastures, one for each sire heterosis. This system also allows the most flexibility breed. In a two-breed rotation (see Figure 2 for an in choosing breeds to use. Replacement heifers can be example), two breeding pastures will be needed. purchased that are comprised of “maternal” breeds A three-breed rotation would need three breeding and bred to terminal or high-growth breed bulls. This pastures. This system is designed to produce replace- type system is optimal for many cow-calf producers. ments. Replacements leave the group into which This system is illustrated in Figure 1. they were born to join the other breeding group as a replacement. As seen in Figure 2, replacements out An even simpler form of this system just uses of sire breed A move to the group that is to be bred two breeds. Bulls of breed A are bred to females to sire breed B and replacements out of sire group of breed B to produce F1 A 5 B offspring. These B move to the group to be bred to sire breed A. The offspring will exhibit maximum heterosis, but since more breeds that are included in the rotation, the the females that produced these calves were not greater amount of heterosis. Each breed added also crossbreeds, the offspring were not able to take increases the level of management needed to keep the advantage of any maternal heterosis. system operational. Rotational Cross Rotation in Time – Another commonly used form of rotational crossbreeding is rotating sire breeds across time. In this system, only one breed of Spatial Rotations – The classic form of a rotational crossbreeding system is a spatial rota- sires is used at one time. Typically, sire breeds are tion. In spatial rotations, all breeds are used at the rotated every one or two breeding cycles. This system same time but are separated spatially. This system is simpler to manage than a spatial rotation, but the Breed A Replacement Breed B a higher proportion a higher proportion Breed B Breed A Replacement Figure 2. Example of a spatial rotation using two sire breeds. level of observed heterosis is somewhat less, due to increased backcrossing. This system is illustrated in Figure 3. The major problem with utilizing this system is that over time the groups of breeding BreedPurchased B Breed A ~ F1 A B females become very inconsistent in their breed -+O makeup and performance. This introduces inconsis- maternal tency in their offspring. This variation in calf per- formance can be a hindrance during marketing of the offspring. -+O ~ offspring sold Excess Summary In comparing crossbreeding systems, you will notice that each system excels in some criteria, Breed C ~ A often at the expense of other criteria. Inevitably, B Time there are trade-offs to be considered. Some systems sustain very high levels of hybrid vigor but are a management nightmare. Some take advantage of breed complementarity but cannot produce their own -+O ~ offspring sold offspring replacement females. A planned mating system is the nucleus of a successful crossbreeding program. The mating sys- tem should maintain heterosis at an optimal level and permit uninterrupted production of a uniform Breed D ~ A B C product from generation to generation. Matching -+O the crossbreeding system to the facilities and envi- ron-ment is of utmost importance. Likewise, the choice of breeds is important. The use of a cross- breeding system that produces offspring efficiently will play a large part in the profitability of cow-calf producers. For more information or to receive assistance Continue rotation with another breed or breed back in setting up a crossbreeding system for your herd, to Breed A and start the rotation again.