The author is an associate professor of dairy cattle genetics at Penn State University.

Chad Dechow
I discussed the introduction of national health evaluations for Holsteins by the CDCB (Council on Dairy Cattle Breeding) in the December 2017 article “National health evaluations coming for Holsteins” on page 761. As part of that process, CDCB will release genetic evaluations for six different disease resistance traits in April of this year.

These are not the first genetic evaluations of disease resistance available to producers. We previously reported on Zoetis’ wellness traits in the April 25, 2016, issue on page 281 in the column “Genomics drives stepped-up health evaluations.”

We now have cow health genetic evaluations from several different sources, and each source presents their evaluations differently. CDCB presents their traits on a disease resistance scale. A +1 percent for mastitis resistance would indicate that 1 percent fewer of a bull’s daughters will have mastitis during a lactation. Zoetis and others present their evaluations on a standardized basis where that average bull is 100 and values higher than 100 indicate more disease resistance. Zoetis provides its wellness traits to producers through its genotyping service and some bull studs also present Zoetis wellness traits for their sires.

Individualized indexes

Other bull studs have developed their own proprietary health evaluations. Genex, for instance, has evaluations for subclinical ketosis, metritis, and foot health, which are all included in their Health $ index. They also present genomic mastitis resistance evaluations from the Canadian genetic evaluation system and calf survivability for Jerseys. Genex presents most of its traits on a standardized basis where 100 is average.

ABS Global has developed a genetic evaluation for cow health during the first 30 days of lactation, which it terms TransitionRight. ABS gives its sires a one-star to five-star indication of their genetic merit for transition cow health. Five stars indicates that daughters of that bull have $100 less disease costs during lactation than average for Holsteins and $50 less for Jerseys. One-star bulls are on the other side of the spectrum with -$100 and -$50 for Holsteins and Jersey, respectively.

CRV has Better Life Health traits, which include ketosis, claw health, and udder health. Their evaluations are also presented on a standardized basis with 100 as the average. They have also partnered with Neogen to provide Better Life Health evaluations to Igenity genotyping customers.

Genetic evaluations for immune response traits are also available and marketed by Semex under the Immunity+ banner. This is a different type of trait. Rather than obtaining records on health events from farms and using that information to derive a genetic evaluation, the immune response traits require administering a test to bulls and cows that determines how strong their immune systems respond to an antigen. This information is then included in a genetic evaluation.

Highly heritable

I am not an immunologist, but I have discussed the concept with colleagues who work in this area and they are comfortable with the general idea. As with any other health evaluation, there are pros and cons to the concept. On the plus side, immune response has a higher heritability -— close to that of milk yield — than disease resistance traits that are based on farm recorded health events.

That does not mean that immune response evaluations are more reliable because there are far fewer records available due to the intensive nature of obtaining the data. While there is evidence of a favorable relationship between immune response genetic merit and daughter health, more large validation studies conducted by independent parties are needed to truly quantify the effectiveness of this type of evaluation.

The multiple sources for cow health evaluations — and I apologize in advance if I overlooked any — demonstrate a challenge for breeders. They may get competing information on the same bull, and it will be difficult to keep track of the different scales used by different organizations.

Some of this challenge might be solved by the marketplace. If breeders begin to rely exclusively on a single source of evaluations because they are more widely available or prove more reliable, the marketing advantage of other evaluations will be lost and they may fade. That is not a certainty, however.

This would not be an issue if we had expectations of bulls ranking nearly the same for all the different genetic evaluations of health. Unfortunately, this is not my expectation. Consider two genetic evaluations that are conducted on entirely different groups of daughters — as appears to largely be the case thus far for Zoetis and CDCB evaluations — and with reliabilities for young bulls of 50 percent. Those two evaluations would have an expected correlation of 0.50, and many health resistance evaluations for young bulls will have reliability lower than 0.50.

A correlation of 0.50 will result in general agreement across a large population of bulls, but individual bulls will rank differently in some instances. What should our view of a bull be if there are two evaluations with large differences?

The optimist is likely to take the best evaluation if he likes the bull otherwise, whereas a pessimist will assume the worst. I’m more worried about the cynic, who will decide they must both be wrong and discount the whole idea of health evaluations as ridiculous.

Could re-rank

In all seriousness, we should expect that some bulls will re-rank across different evaluations. I have two pieces of advice. First, spread your risk by using a wider sampling of bulls, in which case you have some protection against a bull whose daughter health turns out to be poor. Second, pay attention to traits like productive life and livability. If there is conflicting health information on a bull, and he has poor productive life and livability, I would be cautious.

While companies have different approaches to how they have addressed health evaluations and will rank bulls differently, the methodologies they are using appear to be sound. In the end, companies with the ability to generate the largest databases should be expected to generate the most reliable and stable genomic evaluations for health traits. Not all companies report the reliability of their evaluations, which further complicates how to compare evaluations.

We have entered a new era where evaluations for a given trait on the same bull or cow are not all from the same source. Whether this becomes the new norm, or is something that happens for a short time period for this specific group of traits, remains to be determined. I think validation studies from independent sources, such as university researchers, will become more important if this does become the norm. If we are entering a new era, I don’t think it will be terrible. It will represent a change and breeders will adapt.

At the end of the day, you have to pick a company or companies you are comfortable working with and that you can trust. The different breeding companies market bulls with a different focus, but generally a breed’s population is moving in one general direction, and I expect you to have favorable outcomes so long as you are selecting bulls near the top of selection indexes such as Net Merit $.

I have to make one correction to last month’s column. At the time of writing, I expected the new CDCB health traits to be added to Net Merit $ in April. As of a December 8 meeting, CDCB has decided to delay adding health traits to the index in order to allow for more time to evaluate the new traits and educate breeders on these traits.