Enhance milk quality through breeding

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At an estimated $2 billion annual cost, the economic effect of mastitis is staggering. The number of cows affected is also astonishing. Researchers estimate that mastitis affects approximately 38 percent of the nation’s dairy cows. And no farm is spared by this complicated disease caused by multiple pathogens. Data from nine herds in a study by Ohio State University show that even well-managed herds in this trial had more than 45 cases of environmental mastitis per 100 cows per 305-day lactation.

click image to zoom While progress has definitely been made in the control and prevention of certain types of mastitis over the years, the disease remains a serious management issue. Heightening the challenge is the fact that milk quality and on-farm antibiotic usage are under greater and greater scrutiny these days.

So, as you work to prevent the disease as much as possible and deal with individual cases, researchers work to learn more about cow-based factors that contribute to mastitis susceptibility and enhance an animal’s immunity and innate mastitis resistance.

These solutions are a mix of tools currently available and those still in development. Here’s what you need to know about how these strategies work.

Sire contribution
Thanks to renewed interest in recent years to “functional” traits, you can work to lower mastitis incidence on your farm via sire selection. Bull proofs include helpful data on a host of health parameters. And, when it comes to mastitis, that means using predicted transmitting ability (PTA) for somatic cell score (SCS) as you evaluate potential herd sires.

For example, one recommendation would be to identify potential herd sires as usual, based on your herd’s genetic goals. Identify sires on that list with PTASCS 3.2 or higher. Use sires with scores lower than 3.2 first. Then, use bulls with higher PTA-SCS sparingly and only when they are truly exceptional for other economically important traits. (For more hints on how to use this tool, go to: http://nmconline.org/articles/SireSelection.htm)

Keep in mind that decreasing mastitis incidence via sire selection is a long-term project that requires constant vigilance, but it is one tool that’s available to you immediately. In the future, it may also be possible to evaluate bulls based on breeding values of their immune response capacity.

Cow factors
Genetic discoveries may also change how you deal with mastitis when it comes to cows. These factors focus on a cow’s natural immunity to fight off mastitis pathogens. Since commercial tests are not yet available, it is impossible to offer step-by-step instructions as to how to incorporate them into your mastitis-management and breeding programs. However, here is a glimpse into the theory behind how such tests would work.

click image to zoom To learn more about an animal’s innate immune response to mastitis challenges, David Kerr, associate professor of animal science at the University of Vermont, and his colleagues are testing cow skin cell-response to mastitis-causing pathogens. They are also testing these cells from animals of several different ages to determine whether the magnitude of the response changes over time.

Ultimately, this could lead to the identification of specific alleles of genes that offer superior disease resistance via individual cow tests. In turn, these results could be used in the selection of future breeding stock. Or, it may lead to the development of a fibroblast (see the glossary sidebar on page 18) test that examines animal responses at a young age as part of a diagnostic program to decide which animals to select as herd replacements.

New diagnostic tools and heritability
Meanwhile, researchers at the University of Guelph have developed a patented test system for skin and blood samples to quickly identify individual cows with both high antibody and cell-mediated immune responses. These are critical traits for protection against extra- and intra-cellular pathogens, respectively. The technology is currently undergoing market assessment and beta testing.

“It uses quantitative genetic methods to identify and select individuals with higher breeding values for immune response traits,” says Bonnie Mallard, professor in the University of Guelph’s Ontario Veterinary College.

The system features two simple lab tests to identify high, average and low immune responders. High responders have two-to-four-times less disease, particularly mastitis, metritis, ketosis and retained fetal membranes compared to low or average responders. ‘This has important implications for improving animal health, decreasing the use of antibiotics and improving overall food safety,” says Mallard.

click image to zoom In addition, a bovine immune-endocrine microarray chip may be used to assay skin and blood samples to identify genes associated with immune responsiveness and disease resistance. However, current selection is based on breeding values and not genetic markers.

The test was also recently used on 690 cows from 58 herds across Canada and found that 15 percent of cows were high immune responders, 70 percent were average responders and the remaining 15 percent were low responders. Preliminary results show that among all cases of clinical mastitis for cows tested, cows classified as high immune responders had the lowest occurrence of coagulase-negative staphylococci.

Study results indicate that the heritability for antibody and cell-mediate response traits are moderate to high, says Mallard. Between 14 and 56 percent of the variation in immune response can be explained by genetic variation.

“Since a significant genetic component has been identified in these immune response traits, it is possible to include immune response in breeding programs to make genetic gains in overall dairy health,” she concludes.



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