Composite Breeds - What Does the Research Tell Us?

Heterosis achieved through continuous crossbreeding can be used to increase weight of calf weaned per cow exposed to breeding by 20%. Comprehensive programs of breed characterization have revealed large differences among breeds for most bioeconomic traits. About 55% of the U.S. beef breeding population involving 93% of the farmers and ranchers who produce beef cattle are in production units of 100 or fewer cows. Optimum crossbreeding systems are not easy to adapt in herds that use fewer than four bulls. Further, fluctuation in breed composition between generations in rotational crossbreeding systems can result in considerable variation among both cows and calves in level of performance for major bioeconomic traits unless breeds used in the rotation are similar in performance characteristics. Use of breeds with similar performance characteristics restricts the use that can be made of breed differences in average genetic merit to meet requirements for specific production and marketing situations. Retention of heterosis was shown to be approximately proportional to retention of heterozygosity in a four generation crossbreeding experiment. Retention of initial (F1) heterozygosity after crossing and subsequent random (inter sè) mating within the crosses is proportional to (n-1)/n where n breeds contribute equally to the foundation. When breeds used in the foundation of a composite breed do not contribute equally, percentage of mean F1 heterozygosity retained is proportional to 1 - ∑inPi2 where P1 is the fraction of each of n contributing breeds to the foundation of a composite breed. This loss of heterozygosity occurs between the F1 and F2 generations, and if inbreeding is avoided, further loss of heterozygosity in inter sè mated populations does not occur. The large differences that exist among breeds for most bioeconomic traits are the result of different selection goals in different breeds. Results from the Germplasm Evaluation Program at the U.S. Meat Animal Research Center provide evidence that genetic variation between breeds is similar in magnitude to genetic variation within breeds for many bioeconomic traits. The heritability of breed differences approaches 100%, whereas, the heritability of differences within breeds for major bioeconomic traits varies from less than 10% to about 50% depending on the trait. The heritability of breed differences approaches 100% because estimates of breed differences are based on the means of a large number of individuals from a representative sample. The large number of observations tends to average within breed genetic variation. Estimates of heritability of differences within breeds are generally based on single observations of individuals for a specific trait. Thus, selection among breeds is much more effective than selection within breeds, but is a one time gain in contrast to selection within breeds. The primary objectives of this experiment were to: 1) determine the percentage of initial heterosis (F1) that is retained in advanced generations of composite populations mated inter sè; i.e., to what extent is retention of heterosis proportional to retention of heterozygosity; 2) determine the feasibility of developing a more simple procedure for using heterosis through new populations of beef cattle based on a multi-breed (composite) foundation; and 3) evaluate composite populations as a procedure to use breed differences to achieve and maintain optimum performance levels for major bioeconomic traits