INBREEDING REVISITED
[Published December 1, 2008, pp.24-26]
[Download printable PDF file]
[Return to Table of Contents]
In line with the theme of this month’s journal, the year in review, I want to revisit a topic that we addressed earlier in this year. The topic was inbreeding. Specifically the breeder wanted to know at what percentage of inbreeding would inbreeding depression begin to manifest itself. The breeder also asked how much inbreeding is "probably tolerable" and what amount is too much. I pointed out that inbreeding, while potentially risky, has its place in seedstock breeding programs. Inbreeding can result in animals with superior breeding values (i.e. EPD) and seedstock breeders know that it is those superior breeding values that get passed on to offspring. As you all know, however, inbreeding brings with it the potential for inbreeding depression—reductions in performance for many traits, especially fertility and survival. As the discussion on the topic of inbreeding ended I made a few comments on how inbreeding will reduce heterosis inherent in Brangus. I want to make a few more points relative to the reduction in heterosis when inbreeding is practiced.
Brangus are a composite cattle breed and composites provide the opportunity to reap the benefits associated with the two primary reasons for crossbreeding—complementarity and heterosis. We all know what heterosis brings to the table—improved performance, especially in survivability and fertility, as well as improvements to growth rates. Complementarity is a separate “plus” to composites. It is the production of more desirable offspring by crossing breeds that have complementary characteristics. Specifically, Brangus take advantage of hybrid seedstock complementarity, meaning they have taken advantage of the strengths associated with Angus and Red Angus combined with the strengths of the Brahman breed to create a new breed with the ability to perform well under harsh environmental conditions.
The concern with increasing levels of inbreeding is the effect it has on one of these two primary reasons for crossbreeding. First, let’s be clear, inbreeding will not decrease the improved performance that Brangus sees from complementarity. Complementarity is a product of the breeds chosen in the development of the composite and is completely separate from heterosis. In essence complementarity is due to the genes that the parents from each breed pass on to their offspring in the process of developing the composite. This is separate from the effects of heterosis so inbreeding will not adversely impact Brangus complementarity.
Efforts to maintain heterosis, however are a different story. Remember that heterosis is how the genes are combined in the offspring of the sire and the dam, and cannot be passed from one animal to the next. As such heterosis is completely determined by how we choose which sires to mate to which dams. For example, the offspring of Angus dams crossed with Brahman bulls will perform better than the average of their parents. That increase in individual performance is a product of the maximum possible heterosis. Brangus retain about 47% of that maximum level of individual heterosis. In Brangus there is also improved performance because the dams are all crossbred and as such we anticipate better reproductive performance and higher percent calf crops weaned. This is maternal heterosis. Irrespective of the type of heterosis (individual or maternal), research shows that increases in the level of inbreeding negate the improved performance resulting from heterosis. As inbreeding increases the benefits of heterosis decrease. For commercial breeders this is a big concern because the advantages of crossbreeding can have a large impact on profitability of the commercial operation. For seedstock breeders, decreased levels of heterosis are not as great of a concern because seedstock are sold for their ability to produce high performing progeny. Even so, the seedstock cowherd still reaps the benefits of higher levels of heterosis. Either way, increasing inbreeding levels reduce the improved performance resulting from heterosis.
Potential reductions in heterosis have implications for how seedstock breeders market their bulls to their commercial customers. Therefore, seedstock breeders should be cognizant of opportunities to help their commercial customers maintain heterosis in their cow herd. Helping the Brangus commercial customer retain as much heterosis in their cow herd will reap economic benefits to their commercial customers and hopefully in turn result in increased demand for animals from the seedstock breeding program.
The founders of the IBBA realized that loss of heterosis and a small genetic pool would be detrimental to the breed. The IBBA allows for creation of “new” Brangus animals starting with the use of registered Angus/Red Angus and Brahman cattle to eventually produce the 3/8 Brahman 5/8 Angus required to be a registered Brangus individual. So even though there are breeders that utilize inbreeding to produce better seedstock for their customers, there is substantial opportunity to combine new genetics to maintain heterosis in the breed and to maintain the high levels of performance seen in the Brangus breed.
[Download printable PDF file]
[Return to Table of Contents]
[Top of Page]