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A robust immune response is key to better animal health

Steven Larmer for Progressive Dairyman Published on 18 April 2016

Health remains one of the primary reasons for animals leaving the herd. Why do some cows get sick more often than others when they’re living in the same environment, eating the same ration and being exposed to the same pathogens?

It all comes down to the genetics of the immune system. Immunity is an animal’s natural ability to fight off disease, and it is clear that some cows are better fighters than others.

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Trying to select for cows that can better resist disease is a goal of every dairy producer, but figuring out how to do so has proven difficult in the past. Traits associated with health and longevity are often measuring a variety of factors, only a small portion of which are related to immunity.

Even looking at individual disease incidence contains a large variety of environmental and management factors, limiting the ability to determine which animals have the right genes to fight off disease. This is largely why longevity and disease traits have low heritability.

Greater progress with higher heritability

Heritability is the proportion of total variance in a trait that can be explained by genetics. This means traits with low heritability are more difficult to make genetic progress in relation to the total amount of variation in the population.

High-heritability traits are usually easily and directly measured, and genetics have a larger impact on the difference observed between animals.

Production traits are the prime example of this, and it is clear that genetic selection for production has made an enormous impact. About half of the 369 percent increase in production efficiency of dairy cattle since the dawn of A.I. can be attributed to genetics. Conversely, it is much more difficult to see these kinds of impacts on lower-heritability traits.

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Selecting the right animals is increasingly difficult with lower heritability, and the amount of response expected in a generation of selection relative to the overall variation of the trait is much lower.

The use of genomics has helped identify the genetics for lower-heritable traits such as longevity and disease; however, the heritability of the traits themselves does not change and so the response to genetic selection for these traits is still limited.

High immune response technology

Measuring a trait directly, rather than an outcome such as disease incidence or longevity, can greatly increase the heritability and ability to make genetic progress. This is the principle that led to the development of high immune response (HIR) technology by Bonnie Mallard at the University of Guelph.

By directly measuring an animal’s response to a non-specific disease challenge, it is possible to determine how well that animal will fight off any future disease. This ability to fight off any disease is the ultimate goal in producing healthier cows. Mallard’s research has shown that the heritability of immune response is substantial; at 30 percent, it is the same as the heritability of milk yield.

Using the HIR test to identify sires with higher immune response allows producers to select bulls that will pass on that immunity to their daughters, knowing that it is highly heritable and provides resistance to nearly any disease challenge that may come their way.

Mallard’s group has also shown that HIR animals have a greater response to vaccinations and produce higher-quality colostrum for their offspring. There is a large amount of variation in the immune response of cattle, and since a higher proportion of that variation is genetic in nature, genetic selection for HIR animals can result in a significant reduction of most major diseases.

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Verification in farm data

Producers have had the ability to select HIR sires since 2012, also known as Immunity+ sires. The results of disease incidence of the first crop of daughters are now available. Health events were extracted from the herd management software for a number of herds to compare disease incidence.

Table 1 shows the differences in disease incidence of daughters of HIR sires versus herd average disease incidence for the same age and lactation groups (results from up to 12 large herds depending on trait analyzed).

Health events in HIR-aired cows compared to herd average

Reductions in disease from using HIR sires have been tremendous to date. Not only have diseases directly caused by a pathogen challenge invoking the immune system been reduced (such as mastitis), there have also been significant decreases in metabolic and reproductive disorders.

This is attributed to the animal being able to better utilize energy to deal with metabolic or reproductive function rather than having to use energy to try to maintain health.

The drastic reduction in persistent mastitis, defined as two or more separate cases of mastitis in the same lactation, is expected as cows with stronger immune systems are better able to adapt and prepare for future disease challenges and are less likely to get the same disease twice.

Another interesting dataset was analyzed from a large dairy collecting colostrum quality measurements on all fresh cows. Cows sired by HIR sires had, on average, higher colostrum quality.

A correlation of immune response status of their sire and colostrum quality of +0.55 was observed. More importantly, no HIR sires had daughters who had average colostrum quality below the 22 percent threshold recommended for use, compared to 27 percent of herd average sires having daughters whose colostrum, on average, was too poor to feed to their calves.

Immune responses to common diseases in dairy cattle are important for breeding a long-lasting, profitable herd of cattle. HIR technology is able to provide these benefits quickly and effectively due to a high heritability and a broad-based resistance to a host of disease challenges.  PD

Steven Larmer
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