Trace minerals - Tiny but vital

Trace minerals play a major role in keeping a dairy cow's immune system functioning for better health

With the somatic cell count (SCC) penalty level being lowered to 400,000 from 500,000 in August 2012, take some time now to review trace minerals in your feeding program. They are one of the elements contributing to healthy udders and a low SCC.

The integration of nutrition with health and disease prevention is one of the biggest dairy management achievements in the last two decades. Good feeding management can help keep a dairy cow's immune system functioning properly and play a central role in keeping her healthy, including her udder.

Think of nutrition as one spoke in a large wheel when it comes to managing udder health and the various risk factors involved. Optimum feeding programs can't keep bacteria from entering the teat opening and causing an infection. Other spokes in the udder health wheel include providing the right environment for the cow, the right milking preparation and procedures, and practising an udder health program developed with your veterinarian.

Nutrition plays two crucial roles in supporting the cow's immune system. First is providing dietary energy. Early lactation is a significantly stressful time for a cow as she goes through negative energy balance, and dietary energy helps minimize negative effects on her immune system. Secondly, including optimum amounts of some trace minerals and vitamins in her diet-including vitamin E, selenium, vitamin A, manganese, copper and zinc-can help support her immune system.

When it comes to selenium, most of eastern Canada has a low soil selenium status, defined as less than 0.6 milligrams of selenium per kilogram in soil samples. This means forages and grains you grow on these lands don't naturally contain much of this essential nutrient. Conventional feeding practice supplements selenium to ensure the cow's requirement is met.

While herd-level selenium status hasn't been well documented in all provinces, most herds are thought to have adequate amounts. Producers generally supplement selenium in the diets of milking and dry cows from breeding age onwards and the use of selenium injections is also common.

However, sub-optimal selenium status can occur in some dairy herds. Summer-pastured dairy cattle that graze forages without proper attention to their trace mineral needs is one example of where this can occur. As well, producers sometimes reduce feeding rates to trim costs.

Some older research has found a negative relationship between selenium status and bulk tank SCC, but a previous Canadian study had not shown that association. Last year, Atlantic Veterinary College researchers reported on milk selenium status in relationship to udder health.

In their study, milk samples from individual cows at different lactation times, including pre-dry and early lactation, were taken from 18 dairy farms in Prince Edward Island, New Brunswick and Nova Scotia. Researchers also obtained bulk tank samples at four different times of the year to represent seasonal changes.

The study interpreted bulk tank selenium status results, expressed in µmol (a scientific unit of measurement) per litre, as herd status according to these thresholds:

  • less than 0.12 µmol per L, deficient herd status;
  • 0.12 to 0.20 µmol per L, low marginal herd status;
  • 0.20 to 0.28 µmol per L, high marginal herd status;
  • greater than 0.28 µmol per L, adequate herd status.

The herds' average bulk tank selenium concentration was 0.52 µmol per L, and only two samples from two herds had values below adequate status at less than 0.28 µmol per L. Similarly, the researchers observed adequate status for 97 per cent of milk samples taken from individual cows.

Recent evidence does not show an obvious health effect from supplementing organic selenium sources compared with inorganic sources, the researchers noted. While supplementation may yield a beneficial health response, they also indicated over-supplementation does not produce added benefit to animals that already have adequate selenium status.

It's generally recommended rations contain the maximum allowable amount of supplemental selenium. The current Canadian Food Inspection Agency limit is 0.3 parts per million in diet fed to dairy cows. While the maximum selenium level in the total diet of dairy cattle is set at 0.3 ppm, higher concentrations in the grain portion of the ration may be prepared by feed manufacturers so as to achieve the recommended intake of selenium. Total selenium intake for dry and lactating dairy cattle is set by CFIA according to body weight so that maximum selenium intake milligrams per day is (as-fed basis) 4.1, 4.99, 5.99, 6.38 for cattle weighing 400, 500, 600 and 700 kg respectively. Manufacturers can provide a feeding rate table for products containing added selenium.

Several other factors can impair selenium absorption and availability, including high iron, sulphate, calcium, copper or zinc levels in the diet or drinking water. These factors can reduce selenium absorption and increase an animal's apparent requirement.

There are a variety of approaches to ensuring adequate selenium status. Most common is to include a supplemental selenium source in the grain mix using inorganic selenium, an organic selenium source, or a blend of the two.

Selenium-enriched fertilizer applied to deficient land is another option to increase home-grown feed concentration. Trace mineral rumen boluses that contain selenium are also on the market.

There is a narrow range of supplementation you can safely provide before selenium can become toxic. Be sure to discuss the selenium status of your herd with your nutritionist.

Another mineral that plays an important part in immune function is zinc. It aids in building keratin, a protein in the teat canal that helps trap bacteria. Copper and manganese, other trace minerals, are important for many functions in the cow's body, including her white blood cells' ability to deal with bacteria.

As the recent Atlantic Canada research showed, herds with properly formulated and fed supplementation don't have sub-optimal selenium status. If you're not sure about your herd's status, however, talk to your nutritionist about supplementation options for your selenium and trace mineral nutrition program. This is particularly important if you have questions about mineral intake by pastured animals or absorption concerns.

References: O'Rourke, D. 2009. Nutrition and udder health in dairy cows: a review. Irish Veterinary Journal. 62(S): 15-20. Ceballos-Marquez, A., H. W. Barkema, H. Stryhn, I. R. Dohoo, G. P. Keefe, and J. J. Wichtel. 2010. Milk selenium concentration and its association with udder health in Atlantic Canadian dairy herds. J. Dairy Sci. 93: 4700-4709.

This article first appeared in the May 2011 Ruminations column of the Milk producer magazine.

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