Webster dictionary defines “defect” as a physical problem that causes something to be less valuable, effective or healthy.
The cattle industry has taken an approach based on this definition since snorter dwarfism in the 1950s. Because defect carriers could have progeny with a defect, they need to be eliminated. This policy has been plausible and useful to the industry in the past because lethal or highly detrimental defects have been very rare occurrences. This thinking is beginning to undergo an evolution and will need more thought as technology has and continues to improve.
At the beginning of the industry’s initial policy making decisions and rule implementations regarding defects, and in the case of dwarfism and syndactyly (mule foot), carriers were found through experience. In order for a bull to be considered clean, he would be bred to at least 36 of his daughters. With defects like protoporphyria (proto) in Limousin, tibial hemimelia (TH) in Shorthorn and Galloway and pulmonary hypoplasia with anasarca (PHA) in Maine-Anjou, associations took the stance of control through testing. This philosophy applied even though required testing was restricted to AI sires and donor cows and testing of additional potential animals fell to the breeder. With proto, this has led to near eradication of the defect in the Limousin breed, while TH and PHA are still common and have become almost a selection criteria due to perceived related traits found in carriers. In these instances, testing was not a requirement for registration.
In 2008, the American Angus Association announced the discovery of Arthrogryposis Multiplex (AM) commonly known as curly calf syndrome. The improvements in DNA testing technology and improved computer system capabilities allowed for a policy that required animals to be tested free of the defect. This policy fit with the standard policy of defect rarity and eradication of lethal defects. Soon after AM came Neuropathic Hydrocephalus (NH) in 2009 with similar lethality and the same policy application. Following those policies, two non-lethal defects Contractural Arachnodactyly (CA) and Myostatin (M1) were treated with the same policy. This fall, the American Angus Association has changed its policy with the newest defect that has been discovered, Developmental Duplication (DD). The association will publish test results and owners of the animals will be notified of potential carriers in their herd. Testing is not required for registration and breeders can make their own decisions regarding the status of the animal.
This evolution in policy has proven wise as recent evidence has shown a wide variety of phenotypic expression among DD affected animals. Affected animals have ranged from nearly normal development while others display a variable set of duplications ranging from conjoined twins to polymelia to extraneous skin folds. In cattleman’s terms, this points to there being other gene affects associated with this defect that will take an indeterminate amount of time to assess. It will also be interesting once that is assessed, how carriers can be quantified in a system if it is not a simple recessive gene.
One of the stated reasons behind the policy shift is the improving technology needed to isolate these defects. The cost of sequencing an animal’s DNA has decreased dramatically in the last two years and this has led to a larger number of animals being sequenced. With all of the high density SNP genotypes (the basis for genomic enhanced EPDs) that have been collected in a number of breeds, and the ability to impute those to higher densities with some sequence information, there is a wealth of data for researchers to work with. Angus has been the first breed to really have their genetic sequence dug into by science due to sheer numbers, but that will not remain the case for long.
So the evolution of policy has started at Angus and I’d suggest the industry needs to start looking at defects differently in today’s environment for a number of practical reasons. First, we need to understand as an industry that there is no animal that is defect-free. As my judging coach used to say, “There is no such thing as a perfect one.”
According to sequence data at USMARC, the average animal has over 100 mistakes in its DNA that can cause any one of these simple recessives. Also, mutations in animal DNA happen every few generations. Some are good. Some are detrimental. These will continue to happen.
Second, the defects we are identifying have been around since before I was born. AM, CA, M1, DD have all had carriers found that were born in the 1970s and early 1980s (NH originated with GAR Precision 1680 who was born in 1990). Not only have they been around that long, but we’ve been selecting against them for just as long using normal cattle management techniques. You rarely see the affected calves with most of these defects as they cause early term abortions or reabsorptions. So by culling open cows, we’ve been getting rid of carriers since the beginning.
Our commercial customers were probably ahead of us on that front, as seedstock producers have been known to hold over or flush a cow that comes in open. The length of time from mutation to discovery also poses a real challenge for breed associations. We can track potential carriers all the way to the source via pedigrees, yielding some potential carriers that have less than one percent chance of being a carrier. So where do we draw the line? And how far do we go before members say enough is enough? How many registrations do we deny to carriers before there are none left?
Thirdly, there will be additional defects coming. There is currently a research project looking for deleterious defects that affect fertility in all major breeds. It might take some time to identify one in the Limousin breed, yet it is likely to happen. This is one of the reasons we took DNA testing in house, so that NALF has control over sample information and knowledge of any ongoing research involving Limousin.
Some researchers believe that in the future, there will be enough identified defects that breeders will use computers to mate their cows to minimize the chance of affected calves. While these things are interesting to look at from a research perspective, it’s important to remember that in the practical world the effect of these defects on fertility are small compared to the environmental effects. Simple recessive defects only yield an affected calf at most 25 percent of the time and only when two carriers are mated.
When it comes down to it, this is just another trait you will have to account for in your breeding program. Whether NALF will decide to ban carriers on DD or pass the decision onto members on what they want to do with carriers remains to be seen. I think that we can all agree that the biggest project we have in the seedstock industry is the education of the commercial producer on how to understand these defects.
The question will become whether genetically tested defects will continue as the kiss of death to carriers, or become just like phenotypic “defects” like low growth or low marbling. That question is one that seedstock breeders, commercial producers and the industry will have to answer as we move forward.