Straightbreeding or purebreeding is the mating of sires and dams of the same breed, and is a widely used practice among seedstock producers with registered cattle to generate purebred lines. The subsequent mating of these purebred (straightbred) lines form the basis of crossbreeding programs frequently utilized by commercial producers, although straightbreeding is also practiced by commercial producers.
When purebreeding, there are two basic mating schemes which may be implemented: inbreeding or outcrossing. Inbreeding is the mating of parents more closely related than average of the population. Inbreeding results in an increase in the proportion of homozygous gene pairs and a decrease in the proportion of heterozygous gene pairs (opposite of crossbreeding). Linebreeding is a form of inbreeding where individuals which share a common ancestor(s) are intentionally mated to concentrate the inheritance of favorable genes from the common ancestor. Inbreeding and linebreeding may increase the prepotency for monogenic traits and highly heritable polygenic traits. However, inbreeding results in reduced performance, called inbreeding depression, due to increased homozygosity (opposite of crossbreeding), which is most pronounced in traits such as fertility, survivability, and longevity. As a result of the increase in homozygosity, inbreeding increases the probability of expression of genetic defects caused by undesirable recessive alleles. In contrast to linebreeding, outcrossing is the mating of parents of the same breed which are less related than the average of the population. Outcrossing serves to maintain a higher proportion of heterozygous gene pairs, and minimize the impact of inbreeding which may be inherent in some populations due to a high frequency of common ancestors among individuals in the population.
The advantages of straightbreeding systems include simplicity, as only one breed is utilized which may be contrasted to crossbreeding systems which utilize two or more breeds and therefore may require multiple breeding pastures and bulls. Simplicity of management for straightbreeding systems also appeals to small herds. Mating of parents of same breed also may result in progeny more uniform in appearance. Additionally, straightbreeding programs may be appropriate under certain environmental conditions where one breed is best adapted, or under specialty marketing systems which one breed best fits the desired product. In contrast, straightbreeding systems do not capture the advantages of heterosis and breed complimentarity realized through crossbreeding and composite breeding.
In seedstock herds, development of genetically superior purebred cattle designed to meet the breeding objectives of the operation and that of their customers is the primary goal. This is accomplished through use of available genetic tools and technologies such as genetic evaluations ( EPDs), embryo transfer, artificial insemination, and more recently genomics. Traits of importance for selection will be determined by the goals of the operation and that of their customer. For seedstock herds, revenue is generated through the value of their genetics to their clientele, as determined by the ability of those genetics to meet the needs of those producers utilizing their seedstock.
Simplicity of straightbreeding systems, coupled with market signals associated with branded beef programs, has resulted in many commercial operations with largely straightbred herds. Poorly designed crossbreeding systems that lack sustainability, value-based marketing factors associated with coat color and consistency, and in some regions limited accessibility to bulls of multiple breeds for natural service have also contributed to straightbreeding in commercial herds. Commercial producers should closely evaluate their herd goals and breeding objectives and implement the mating system which best fits their operation to balance inputs vs. outputs.
References and Additional Resources:
Breed and Composite Selection (NBCEC Sire Selection Manual) http://www.nbcec.org/producers/sire_selection/chapter6.pdf
Inbreeding in Cattle http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Version-6192/ANSI-3165web.pdf