Heterosis Needs a Homozygous Recharge

Author explores cattle breeding, and challenges the notion of hybrid vigor.

Published on: May 26, 2010

The beef industry in this country runs on heterosis. It's perceived as the thing that fuels the production side of the equation by providing the only "free lunch" in the industry. At least that's the view held by the vast majority of industry participants for the past approximately 40 years.

There's been ample research conducted that on the surface appears to support this supposition, but careful and thoughtful evaluation reveals there may be some warts attached to this view. We'll explore those warts a little later, but first let's explore heterosis a bit.

Granted, heterosis occurs in the F-1 offspring when animals from two different breeds are mated. It's known that the more genetically pure each parent is within its respective breed, the greater is the degree of heterosis.

Genetic purity simply means purity for their characteristics. Genetic purity exists when an animal's characteristics are held in a homozygous state, i.e., their characteristics are controlled by dominant genetic combinations. In contrast, the characteristics of heterozygous animals are controlled by recessive genetic combinations. In this case genetic scattering occurs in reproduction, ensuring inconsistency and loss of uniformity in the offspring.

The available research suggests that the degree of heterosis is maximized in F-1 offspring of parents that are at the point of greatest divergence, one to another. Achieving the greatest degree of divergence is as simple as mating Bos taurus animals with Bos indicus animals, provided however, that each parent is the product of generations of linebreeding coupled to consistent and constant selection pressure. The more generations of such linebreeding that are behind the parents, the more genetically pure will be their characteristics, and the greater will be the degree of divergence.

Remember, genetic purity means all characteristics are controlled by homozygous (dominant) genetic combinations, and the homozygous state is created, developed and maintained via linebreeding coupled to consistent selection pressure constantly applied. No other breeding methodology has this capability.

Outcrossing, the practice of mating animals within a breed that are of little or no relationship, is the first step to breaking down the homozygous state, and crossbreeding results in its total destruction.

It logically follows that whenever that Bos taurus X Bos indicus mating previously mentioned occurs between parents who aren't from linebred populations, the gap called divergence shrinks to the degree that the parents are less pure genetically. The shrinkage continues at an accelerating rate in each succeeding generation because genetic purity quickly evaporates in all crossbreeding schemes. With this genetic collapse comes loss of control in all matings. The same thing occurs in any crossbred mating between parents of lesser divergence, i.e., Bos taurus X Bos taurus and Bos indicus X Bos indicus.

All crossbreeding, if very long continued, leads to a situation where the cowherd is virtually unable to contribute genetically because of radically diminished prepotency. Prepotency is defined as the ability of parents to stamp their characteristics uniformly onto their offspring, and it's surrendered very early in the process of crossbreeding. It doesn't take much crossbreeding to reduce a cowherd to being nothing more than an incubator for the next round of inconsistent and dis-uniform calves.

Continued crossbreeding ensures that outcome in matings is impossible to control; therefore, specification breeding is likewise impossible and everyday commodity standards look like mountains impossible to scale. It's why the industry hasn't been able to overcome the re-occurring problems of inconsistency and dis-uniformity identified in each and every USDA Beef Quality Audit ever conducted.

When we reach this point of failure, and according to those same USDA Beef Quality Audits we're there on a national scale, all that remains is for producers to awaken and realize they're left with a cowherd that's been reduced to a genetic garbage heap and that's totally incapable of responding to the industry's primary problems. One solution ought to spring to mind, and that's for producers to replace their cowherds with new ones possessing some genetic integrity, if such a thing is available in today's very heterozygous national cowherd, and then start over. This is doubly important if one intends to continue crossbreeding in pursuit of the perceived "free lunch."

Finding replacement stock possessing genetic integrity means finding commercial females that are all of one breed and sired by linebred bulls of the same breed. The biggest winners in the free-lunch crowd will be those whose new-found stock have been produced in this manner for multiple generations, and which are mated to linebred bulls from another breed. The increased level of genetic purity in the parents will act as a springboard in crossbred matings because it increases the degree of divergence between them.

Unfortunately breeding practices of the past forty years have ensured that commercial parent stock capable of meeting this standard must now be created and developed anew. It'll add measurably to the time required for the national beef cowherd to be extricated from the genetic bog wherein it now rests.

Why the negativity?

I don't know just when beef producers will awaken to the facts of life, but I do know a couple of major reasons they don't want to face the realities of their business. First is their learned bias, fear actually, of any kind of close breeding. There are many who boldly espouse the concept of working with nature, who'll shrink into their own skin at the thought of actually using a little closebreeding (linebreeding and some inbreeding) to regain some control of outcome. Nothing could be more natural than closebreeding. It's how all the wild species retain their species identification but we've been taught to fear it, actually to hate it.

Worse still is the industry's outright rejection of the only way to produce a suitable replacement animal for crossbreeding purposes, one capable of providing a place of renewal in the pursuit of heterosis. If closebreeding was used from time-to-time in the nations' many beef cattle herds, there would be a constant and natural point of renewal of heterosis within those herds.

The second major reason beef producers dodge the truth is because of their heavy investment in the status quo. We have somewhere between two and a half and three generations currently involved in the production side of the beef industry, who are invested to the maximum in the status quo, in every way that's possible. They're obviously financially invested, but perhaps more importantly, they have a heavy emotional investment that blocks the analytical and independent thought necessary to see through the fog of the status quo. In every endeavor of mankind, the status quo provides a natural hiding place for the fearful and those unwilling to think for themselves. It's the reason the trite old line about heterosis providing the only free lunch in the industry is so often parroted by people who fail to understand all its ramifications.

Rethinking heterosis

Heterosis has gotten far more than its due these past 40 years, partially because of research that on the surface appeared favorable to the notion that heterosis does provide a free lunch. But blind acceptance of any research result is unwise, regardless of the integrity of those in its charge. Everyone in the industry with a reasonable degree of intelligence and a rudimentary grasp of the language ought to seriously question every step of every research project we encounter. Logic says that if the right questions aren't asked, then the correct answers will never be found.

Perhaps that's the place to start with our questions. Was the research conducted in a way so that right answers could have been found? This is the area where the research apparently supporting the free lunch idea may have those warts mentioned in the first paragraph of this article.

So far as I can ascertain, all the research regarding heterosis that's been conducted since about 1970 has involved parent animals that were bigger in each generation than the previous one. Were any adjustments made for this phenomenon. Is it even possible to account for this with any degree of accuracy in research work?

Remember this was the era of the great frame race when breeders at both the seedstock and commercial levels were constantly in pursuit of bigger and bigger animals. Frame scores shot up in just a few years from frames three, four and five to the outer limits of frames seven, eight and nine. I've seen animals that, if the frame score chart were extended on the tall end, would have framed at thirteen.

The use of such sires in one generation, followed by even bigger sires in the next generation mated to the larger replacement daughters of the previous bull sent the cowherd spiraling upwards. This scenario has been repeated thousands of times in the past 40 years. Since this first occurred in the seedstock business, heterosis is not a possible answer for the extra size (pounds) being produced if the breeds were actually kept pure.


Jim Lents argues linebred cattle like these present the most consistent seed stock and the greatest benefits from crossbreeding.

With these new and larger cattle being used as research herds, the inability to adjust for constantly increasing size in incoming sires, and with the natural instinct on the part of some less scrupulous folks to discover the answer they sought, is it possible that what's been credited to heterosis actually is connected more to increasing animal size throughout the industry because of selection than it is to heterosis?

My gut says about 2-3% of the increases are actually attributable to heterosis, and the other 97% to the use of larger and larger stock in each successive generation for over three decades. That's a hard pill to swallow by someone steeped in the status quo, but it's a position standing on pretty solid ground.

Genetic jumbalaya

Consider what's been going on in the beef cattle industry for the past forty years. Wide outcrossing has been the order of the day within the older established British breeds -- Hereford, Angus and Shorthorn. This process of constant outcrossing has ensured that those breeds have moved towards a more heterozygous state for all characteristics, and at a progressive rate. On this account, they're less prepotent today than they were in former decades, and the USDA Beef Quality Audits bear it out.

There's wide speculation and in some cases actual knowledge of outside blood having infiltrated these older established breeds. Even a small amount of such activity means an inordinate amount of genetic damage has been done to the homozygous state necessary to hold together a breed's fabric. It's likely that only a handful of herds remain in any of these breeds to which suspicion and/or speculation cannot be successfully attached. They're the herds that appear to have seen past the fads and avoided the folly of genetic deterioration, and they're pretty rare in today's industry.

In the newer breeds, those that made their entries beginning around 1970, crossbreeding has prevailed, but in shrinking degrees. Here's why: As the first actual purebred representatives of each new breed arrived they joined the fray for market share. Essential to this was the existence of populations of stock that could each be identified as one the various "new breeds".

The quickest and cheapest way to accomplish this was through "breed up" programs typically called "percentage" programs. In these programs an actual purebred animal is mated to unrelated animals, often of unknown ancestry, with the offspring being designated as half bloods. When half bloods are in turn mated to purebreds the resulting offspring are called three-quarter bloods. This process is repeated, typically to the 7/8 or 15/16 level, with the stock then being declared full-blooded.

What's in a purebred?

The fundamental flaw in all percentage programs is the point at which the various percentage crosses were declared to be full bloods; a term obviously intended to imply genetic purebreds. Unfortunately the points were all set at arbitrary levels having everything to do with politics and promotion, and absolutely nothing to do with the realities of natural laws.

Breeds are by definition homozygous populations of stock descending from rootstock common to the entire population, with each member bearing a degree of kinship and likeness to every other member of the population. This isn't what percentage programs produce. They may look like a breed on the surface, however, genetically they're still crossbreds, but to a shrinking degree, and still many generations away from actual genetic purity.

Any knowledgeable geneticist can tell you that genetic purity, that point at which all characteristics are held in a homozygous (dominant) state, requires a population be closed to outside blood and linebred for fourteen consecutive generations. That hasn't happened in any of the percentage breeds based on continental breeds, and it hasn't happened in any of the homemade American composite breeds either. Accordingly, all percentage animals are heterozygous animals, many extremely so.

It's known that genetically pure parents from different breeds are the choicest candidates for crossbreeding. It logically follows that less-pure parents make less-than-choice candidates for crossbreeding because there will be less "pop and fizzle" in the offspring. Perhaps a more proper term would be less "heterotic boost" in the offspring.

The genetic condition of our industry is a serious matter. Because of the diverse breeding methodology broadly employed the past 35 to 40 years, the entire national beef cattle herd has been measurably weakened at the genetic level. The older seedstock, and actual purebred breeds of the country have been compromised, if only through reduced prepotency yielded up by decades of the wide use of outcrossing. They're left in a weakened genetic state. The newer composites and percentage continental breeds have never been genetically stabilized and purified via linebreeding, so contribute far less than is possible.

Weak link

The commercial sector desperately needs a genetic uplifting whether or not it continues the practice of crossbreeding, but the seedstock sector is genetically too weak from wide outcrossing or crossbreeding to respond. Seedstock breeders ought to be practicing linebreeding along with their selection. If it were so, the stock they deliver to the commercial sector would be far stronger genetically, and able to provide a greater heterotic boost in crossbreeding while at the same time providing greater control of outcome. The results of the USDA Beef Quality Audits suggest the latter is as important as the former, and consumers everywhere agree.

For the past three-plus decades our industry has been in decline. We've suffered the same fate other mature industries have suffered; that is, loss of market share, quality in decline and loss of ultimate consumer acceptance. Industries in this condition are ripe for a revolution, and I believe the beef industry is positioned for just such a revolution.

When it happens things will be different. The rules of the game will be permanently changed. Unless there is serious change in the beef breeds and their associations, breeds will become even more irrelevant than they are today and will lose value. Well-selected linebred genepools will increase in both importance and value. The commercial sector no doubt will still seek heterosis, but hopefully learn that all F-1 animals are for market purposes, not breeding purposes. When revolution comes to our industry, it isn't likely to be led by current industry leaders blinded by status quo thinking. It will be the work of the next generation, many from outside the industry, joined by a handful of visionaries from the present generation.

Jim Lents can be contacted by mail at 25398 SW Coombs Road, Indiahoma, OK 73552, by phone at 580-246-3560 or by e-mail at jhlents@sirinet.net