Question

Examples of outcrossing beef breeds

Answer 1

Answer:

here is the answer:

The Genetic Basis For Increased Productivity

Crossbreeding beef cattleConsiderable research by various State Agricultural Ex­periment Stations and the USDA has clearly demonstrated the potential for increasing beef cattle productivity through crossbreeding. Consequently, the use of crossbreeding for the commercial production of beef is rapidly increasing.

Crossbreeding (the mating of animals from different breeds) is similar in principle to a widely used mating system in straightbreeding programs known as outcrossing (the mating of unrelated and therefore genetically unlike animals within the same breed). Traditionally, breeders have used outcross­ing to increase performance levels within a breed by mating animals from distinctly different families or bloodlines. The results from crossbreeding are generally of larger magnitude than outcrossing, since, on the average, animals from different breeds will be more genetically dissimilar than animals from different families within the same breed. The genetic aspects of crossbreeding and outbreeding are similar, however.

There are two ways that crossbreeding can result in in­creased production levels. Crossbreeding provides the breeder the opportunity of combining the desirable characteristics of two or more breeds, thus achieving a higher overall perfor­mance level of desired traits among the crossbred animals than would generally be found within a given breed. This is frequently called breed complementarity, which refers to the strong points of one breed complementing or covering up the weak points of the other breed. The second way crossbreed­ing increases productivity is through the increased levels of performance for particular traits due to heterosis. Increased productivity can result through heterosis exhibited by both the crossbred calf and the crossbred cow.

Crossbreeding beef cattle

Although most crossbreds will exhibit some heterosis, their level of performance for every trait will not necessarily exceed that of the best straightbred. An example of this is growth rate in Charolais X British crosses. In such situations, the justification for making these kinds of crosses will be for combining the desirable characteristics of the two breeds rather than the heterosis exhibited for the trait.

Based on the accumulated experimental data, it appears that total production per cow in terms of pounds of calf weaned per cow in the breeding herd can be increased by 20 to 25 percent by systematic crossbreeding systems involving the three British breeds (Hereford, Angus and Shorthorn). About half of this increase in total production is dependent upon use of the crossbred cow to take advantage of heterosis for fertility and maternal performance.

Genetic Effects of Crossbreeding

The genetic effects of crossbreeding are the opposite of the genetic effects of inbreeding. Inbreeding results in depression with lowered rate of reproduction, reduced calf viability, rate of gain, delayed sexual maturity and delayed attainment of body maturity. In general, the same traits that exhibit the most inbreeding depression (low heritability traits like reproductive performance) are the same traits that exhibit the largest amount of heterosis under crossbreeding.

There are two basic genetic requirements for a trait to exhibit heterosis: (1) There must be genetic diversity between the breeds crossed and (2) there must be some non-addi­tive gene effects present for the particular trait involved. The failure of either one of these conditions being fulfilled for a particular cross for some trait would result in that trait exhibit­ing no heterosis. In such a case, the expected performance of the crossbred offsprings would simply be the average of the performance levels of the particular straightbred parents involved in the cross. For those traits that express heterosis, the magnitude of heterosis will be dependent upon how much genetic diversity exists between the two parent breeds.

Genetic diversity refers to the degree of genetic similarity or dissimilarity that exists between the two breeds. Breeds that have similar origins and that have been subjected to similar types of selection pressure during their development will be expected to be much more alike genetically (small amount of genetic diversity) than would breeds that have quite different origins and have been selected for different purposes during their development.