E-189 01/11 Texas Adapted Genetic Strategies for Beef Cattle IV: Breeding Systems Angus cow in a straightbred system. F1 Brahman-Hereford cow in a terminal system. Stephen P. Hammack* logical genetic strategy for a beef cow herd the breed, straightbreds generally have more uniform visi- should include four steps. First, determine your ble characteristics than most crossbreds. Straightbreeding production conditions (including climatic, for- is the simplest system to operate. Aage and marketing) and the levels of animal performance Inbreeding occurs over time in any breed, particularly that fit those conditions. Second, choose a breeding sys- in breeds closed to outside breeding stock. This can reduce tem. Third, identify the biological types and breeds within performance (called inbreeding depression), especially in those types that are compatible with the first two consid- traits such as fertility, livability, and longevity. One type of erations. And fourth, select for breeding the most useful inbreeding is linebreeding, which can, through carefully individuals within those breeds. planned matings, elevate the influence of a genetic line or There are about 75 breeds of cattle in the United States, individual while minimizing inbreeding over all. with 15 or so most common. Commercial cow/calf pro- *Professor and Extension Beef Cattle Specialist Emeritus, ducers who confine themselves to one breed have straight- The Texas A&M System. bred herds. If more than one breed is used, the herd is crossbred. Straightbreeding Straightbreeding simply means using the same breed for both sires and dams. Depending on the background of Crossbreeding If two F1s of the same cross are mated, the progeny, called F2, should have about half the Crossbreeding begins with the mating of heterosis of the F1. However, if F2s and subse- two purebreds of different breeds. This results quent generations of this cross are intermated, in first-cross progeny, termed F1. There are three their progeny average no additional loss of het- potential benefits of crossbreeding—heterosis, erosis except for any inbreeding that might favorable breed combinations, and complemen- develop over time. This is a factor in creating tarity. combination breeds or using a composite breed- Heterosis ing system as discussed later. Heterosis, also called hybrid vigor, occurs Characteristics express heterosis differently. when the performance of crossbred progeny Heterosis tends to be highest in fertility, livabil- is different (usually better) than the average of ity, and longevity; intermediate in milk produc- their parent types, as shown in the example in tion, weight gain, feed efficiency, and body size; Figure 1. This example illustrates direct heter- and lowest in carcass traits. osis, the effect of a crossbred individual’s gene Even with the benefit of heterosis the crosses combinations on its performance. There can from mediocre parents usually do not perform also be maternal heterosis, or the indirect effect as well as superior purebreds. So, the most pro- of a crossbred dam’s gene combinations that ductive crossbreds come from genetically supe- influence her calf’s performance through the rior parents. maternal environment she provides. Favorable Breed Combinations Even without heterosis there can be bene- fits merely from combining breeds. For exam- Breed A Average Breed B ple, breeds with high carcass quality are gen- 500 lb 525 lb 550 lb erally not well adapted to tropical conditions, and those that do have good tropical adaptabil- ity usually have relatively low carcass quality. Progeny Crossing breeds of these two types can produce 545 lb progeny with an acceptable intermediate level of (545 – 525 = 20 lb heterosis both traits. Such favorable breed combinations 20 ÷ 525 = 3.8% heterosis) can be the most important benefit of crossbreed- ing. They are the primary motivation for creat- Figure 1. Heterosis. ing new breeds by combining existing breeds, as discussed in the Combination Breeds section. Heterosis is the opposite of inbreeding Complementarity depression, so it is greatest in the progeny of Complementarity requires dissimilar sires least related parents. For instance, there is and dams. Complementarity results not only greater heterosis in crossing the less related from the favorable combination of different breeds Hereford (created in the British Isles) types but also from the manner in which they and Brahman (created from cattle originating are combined. As stated, the progeny of a breed in south central Asia) than in crossing the more with high carcass quality crossed with a breed closely related breeds Hereford and Angus (both with tropical adaptability have a blend of those originating in the British Isles). traits. But the most productive and efficient way Heterosis is reduced when both parents con- to conduct this cross is to use sires with high tain the same breed. There is no loss of heter- carcass quality and dams that are adapted to osis if the parents share no breed in common, tropical conditions. The reverse is not as effec- whether the parents are purebreds or crossbreds. tive because the dams are not well adapted and If an F1 is bred to either of its parent breeds it is the dams that must perform year-round (called a backcross), heterosis in its progeny is under prevailing conditions. about half of that expressed in the F1. If the F1 is Another example of complementarity is the not backcrossed but is bred to a third breed, or use of large sires on smaller dams. The result an unrelated crossbred, there should be no loss of is that dams produce a higher percentage of heterosis in its progeny because the sire and dam their weight, and do it more efficiently, than have no breed in common as in a backcross. if they were bred to sires of similar size. This 2 type of complementarity also can be realized does not guarantee complementarity; sires and in straightbreeding by using genetic variation dams must be different and they must possess within a breed. But it is done more effectively complementary traits. Specialized types can with crossbreeding. be used in terminal systems to take maximum advantage of complementarity by exploiting Types of Breeding Systems their strong points while minimizing or elimi- There are two basic breeding systems in nating weak points. Terminal systems are usu- commercial production. If replacement females ally crossbred but can be straightbred. are produced in the herd the system is continu- ous. If heifers are not put back in the herd, but Continuous Crossbreeding Systems are brought in from outside, the system is ter- True Rotations minal. True rotation systems use two or more Calves from continuous systems have two breeds and the same number of breeding functions: Some heifers are saved for replace- groups. The simplest true rotation uses two ments and go back into the herd, while the rest breeds and is sometimes called a crisscross (Fig. of the calves are grown and/or finished to pro- 2). A different breed of sire is used in each of duce beef. But all calves from terminal systems two breeding groups. Replacement heifers are have only one use—the production of beef. (A moved (rotated) from the group where they small segment of producers specialize in pro- were produced to the other group, where they ducing replacement females to be marketed to are mated to the breed that is not their breed of other producers.) sire to minimize loss of heterosis in the system. There can be combinations of continuous Females remain in that breeding group, with the and terminal systems. Producers should under- same breed of sire, for all of their lifetime mat- stand the differences in these systems to avoid ings. Over time, females in the two groups grav- inefficient and costly mistakes. itate toward a composition of two-thirds of the breed of their sire and one-third of the other Continuous breed. If a rotation has three or more breeds, Since a continuous system produces its a heifer is moved to the breeding group with replacement females, it requires only external the breed of sire to which she is least related to sires to avoid inbreeding. Because replacement ensure minimal loss of heterosis. females are retained, the cow herd has genetics from both sires and dams. If sires have genet- ics for traits that are undesirable in brood cows, Breed B ♂ those traits are perpetuated in the cow herd. Therefore, both sires and dams should have a A - sired ♀ group similar level of expression of important traits, (Replacement heifers moved to other group where they without any undesirable characteristics. Genetic remain for all matings.) extremes generally do not fit. Continuous sys- tems can be either straightbred or crossbred. B - sired ♀ group (System stabilizes with Terminal Breed A ♂ two groups of ⁄ sire breed In a terminal system, replacement females and ⁄ other breed.) come from outside the terminal-cross herd. Figure 2. True 2-breed rotation. Replacements can be either purchased or pro- duced in another herd. Since heifers are not retained for breeding there is more flexibility in Because they require multiple breeding choosing types of sires and dams. Genetics for groups, true rotations increase the complexity maternal ability are irrelevant in terminal-cross of a breeding program. (Artificial insemination sires because their female progeny are not saved can simplify the mechanics, but not the man- for replacements. However, genetics for mater- agement, of some crossbreeding systems.) Also, nal ability are important in sires that produce a compromise must be made between comple- the females used for a terminal cross. mentary matings and uniformity among groups. Terminal crossing is the only system in Because of these complexities and limitations, which complementarity can be realized. How- true rotations are uncommon, especially those ever, merely implementing a terminal system involving more than two breeds.