Trace Elements and the Immune System

When I look back at the advice I offered farmers on trace mineral management twenty to thirty years ago, I cringe in embarrassment. Like many veterinarians of the period, my understanding of the role of trace elements was somewhat rudimentary. At that time, it was not uncommon to encounter the clinical symptoms of selenium and copper deficiency in the South Waikato; ill-thrift, white muscle disease, and fading coat colouration. So my main concern was with managing gross deficiencies of the essential elements, copper, selenium, cobalt and iodine.

I recognised that milk production improved where certain farms established regular supplementation programmes, especially selenium, but did little in the way of monitoring the trace element status of particular herds. Apart from its use to prevent facial eczema, I would never have believed that supplementing with zinc might have been beneficial in New Zealand. In fact, I didn’t really appreciate the significance of seasonal fluctuations in the availability and demands of cattle for copper.

Today it’s a different story. With greater understanding of how and why these trace minerals act to maintain the health status, immune competence, and production capabilities of dairy cattle, much more attention is paid to monitoring and fine-tuning the mineral needs of individual herds.

We now recognise the importance of copper, selenium, and zinc for maintaining the body’s immune system and natural defence mechanisms. When the levels of these trace minerals fall, immune system function is compromised well before any clinical signs of deficiency occur. The body’s requirements for these minerals varies from day to day depending on the challenges that confront the immune system.

Zinc alone is essential to over 200 enzyme systems, many of which are associated with immune responses. Adequate copper is also needed if the immune system is to function effectively to repel invading micro-organisms, while adequate selenium is necessary for enzymes that protect against free radical damage.

If reserves of these minerals are not available when infection strikes or some other stress occurs, the body cannot defend and repair itself efficiently. In the milking dairy cow, the udder is under daily attack from bacteria that enter by the teat ducts. Natural defence mechanisms that are reliant on these minerals control most of these invaders. Somatic cell counts (SCC’s) reflect both the degree of challenge and the effectiveness of the natural defence that is mounted.

The most stressful time for dairy cows is at calving. Unfortunately, its just at that time that their immune system is naturally at its lowest ebb and pasture levels of these minerals tend to be at their lowest. Cows with an inadequate copper, selenium, and zinc status at calving are less capable of mounting efficient immune system defences. This can result in a higher incidence of mastitis, retained foetal membranes, or various infectious conditions such as salmonellosis.

Only copper can be held in significant reserves in the liver. Zinc can be drawn from small reserves in the skin and hooves, but selenium is stored hardly at all. The daily dietary intake continually replenishes these minerals, but in soil deficient areas this intake may not be adequate enough for optimal functioning of the defence systems.

So as cows approach drying-off, it is worth considering their current trace mineral status, and what it is likely to be at calving. If liver copper reserves are adequate, this will see cows through winter and into calving. But there may be a need to consider giving supplemental selenium and zinc during the dry period, especially during the last three weeks, to ensure their defence systems are able to operate as best possible at calving time.

The significance of trace minerals in maintaining a competent immune system is now well recognised. I used not to think about providing trace minerals to ensure that immune systems and natural defence mechanisms were functioning optimally, but its important if you want cows to be able to handle the stresses and challenges of calving as best possible.