A dairy cow’s immune system, along with reproduction, is an important fitness trait. It’s a trait that’s of major consideration for producers because the ability to find and identify cows with a superior immune response can reduce disease incidence and improve milk quality and farm profitability.

Producers may also expect healthier animals to demonstrate improvements in other traits, including reproductive fitness.

To help find cows with superior immunity, researchers at the University of Guelph have developed patented high immune response (HIR) technology that makes it possible to classify animals as high, average or low immune responders based on their estimated breeding value (EBV).

Repeated results show these high responders have the ability to produce more balanced and robust immune responses compared with average or low responders. High responders essentially have about half the disease occurrence of low responders.

Plus, they can pass their superior immune response genes on to future generations, thereby accumulating health benefits within the dairy herd.

Select for disease resistance

Genetic selection for enhanced disease resistance is one novel approach that may meet this goal.

Research at the University of Guelph focuses on developing genetic and genomic (DNA sequence information), as well as epigenetic (DNA structural information) methods to improve animal health that take advantage of the animal’s ability to make appropriate immune responses.

Genetic approaches also can work well in combination with other preventive approaches, including vaccination, and may in fact enhance other traits such as reproduction, feed efficiency and growth.

Fortunately, the heritability of many immune response traits is high enough to allow for improvement using genetic selection. In dairy cattle, applying this approach resulted in reduced mastitis incidence in high immune responders as well as improved response to vaccination and colostrum quality.

In general, high responders have about one-half the disease occurrence of low responders.

Testing in action

This ability can be demonstrated with on-farm results.

The Semex Alliance, Canada’s largest dairy genetics company, obtained an exclusive license from the University of Guelph to utilize the HIR procedure to identify sires with the high immune response classification. These sires are designated as Immunity+, marking their enhanced capacity to make protective immune responses.

The immune response traits used in establishing HIR EBVs are moderately to highly heritable, with heritability estimates of approximately 0.25 to 0.35. This is in the same range as those for milk production traits and well above those for most reproductive traits.

To date, more than 1,000 Holstein sires and dams have been tested and have not shown any substantial negative impact of selecting for enhanced immune response on production traits and few negative associations with reproductive traits.

Similarly, daughters of HIR sires have lower disease and higher productive indices than other sires. For example, data from the company’s recent sire proofs showed these sires had a production index score of 186 points greater than non-HIR bulls. They also demonstrated net merit scores that were $165 greater than their counterparts.

Tie to important traits

Multiple studies over many years have shown that breeding for enhanced disease resistance based on breeding values of immune response does not negatively affect production traits. In fact, this strategy may improve overall herd life and certain reproductive traits.

In a Canada-wide study, no differences were detected in 305-day milk yield, protein yield, fat yield or overall lifetime profitability in HIR cows compared with low- or average-responder cows.

In addition, when relative immune response breeding values of sires were estimated from this study, beneficial associations were noted between immune response and herd life and some reproductive traits, as well as between high cell-mediated immune response and milk yield.

These results suggest these traits may improve by breeding for enhanced immune response.

Effects on reproductive traits

Furthermore, high immune-response animals show no substantial difference in production traits such as yield of milk, milkfat or milk protein.

However, correlations of the antibody-mediated immune response and reproductive traits showed a negative correlation with calving ease (-0.19).

On the other hand, beneficial correlations have been noted between:

• Immune response and 56-day non-return rate (0.16)
• Numbers of services to conception (0.20)
• First service to conception (0.18)

In addition, a positive beneficial association exists between cell-mediated immune responses and gestation length. More importantly, daughters of HIR sires have beneficial correlations with pregnancy rate and calving ease compared with non-HIR sires.

As a result, dairy producers can selectively breed for improved immune response using the HIR technology without necessarily reducing genetic gain in other important traits such as pregnancy rate and calving ease.

It is important to keep in mind that in order to obtain maximum health benefits, both antibody- and cell-mediated immune responses should be included and kept in balance when selecting for enhanced disease resistance.

Genomic profiles of immune response

The use of genomics has brought advancement in genetic accuracy and improvement in production traits. Many novel applications for improving fitness traits using genomic information, however, still need to be explored.

In one study, 186 genetic markers that are part of 11 genetic pathways were shown to differ between the low and high groups of cows based on antibody responses, and 21 genetic markers were associated with cellular responses.

Results of this work also were validated in the HIR sires and make it possible to estimate genomic breeding values for immune response to improve health in subsequent generations.

Research also is underway to establish a large reference population of Holstein sires and dams with immune response phenotypes and genotypes. This is part of a larger Canada-wide 10,000-cow project that aims to obtain genomic information on various traits including milk spectral data, feed efficiency, health and more.

If successful, it may be possible in the future to identify high or low immune responders from a simple DNA sample.

Take-home message

As the dairy industry continues to increase its focus on health traits, this includes distribution of sire proofs to improve mastitis based on physical udder characteristics and somatic cell score.

The goal of research at the University of Guelph and the HIR test is to enhance broad-based disease resistance by improving both antibody- and cell-mediated immune responses. In keeping with this objective, high immune-responding cows have improved resistance to a number of diseases, including mastitis, metritis, pneumonia and Johne’s disease.

In addition, daughters of HIR sires have improved pregnancy rates and daughter calving ease.

Ultimately, no adverse associations with production traits have been noted. These results demonstrate that it is possible to genetically improve animal health without compromising other economically important traits.

Therefore, given current knowledge, it’s accurate to say that fertility is not compromised when dairy producers select for improved immunity. PD

This information was presented at the 2014 Dairy Cattle Reproduction Council Annual Meeting in Salt Lake City, Utah, last fall.

References omitted due to space but are available upon request. Click here to email an editor.