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From Dr. Bruce Cottrill (ADAS, UK) presentation at Lallemand’s International Selenium yeast seminar (Grenaa, Denmark, 2007)

Selenium in ruminants is a topic well-known of Dr Bruce Cottrill, a ruminant nutrition expert who works for ADAS, a UK independent environmental and agricultural consultancy and service provider. In 2000, he was commissioned by the UK Ministry of Agriculture to conduct a survey on how to improve cow milk selenium status.

In Grenaa, Dr Cottrill exposed the importance of selenium in ruminants, with practical considerations on their requirements, the different sources of selenium and the animal responses to supplementation. Finally, experimental data confirmed the benefits of selenium yeast supplementation in ruminants: increased selenium status in blood, muscles and milk, increased immunity (reduced somatic cells in milk), better immune transfer to the calf…


How much selenium do ruminants need?

Starting with a few definitions, Dr Cottrill highlighted the difference between the animal requirements, which are measured in experimental setups, and animal allowances, which are the amount needed to meet the requirements of the animals in “real life” conditions, thus including a safety margin to take into account variations in feed intake, feed composition and production level. Dr Cottrill recommends following the guidelines of the Canadian National Research Council (NRC):

For dairy cows: 0.3 mg/kg DM (NRC Nutrient Requirements of Dairy Cows, 7th revised edition, 2001)
For beef cattle: 0.1 mg /kg DM (NRC Nutrient Requirements of Beef cattle, 7th Revised Edition, 2000)
For sheep: 0.1-0.2 mg/kg DM (NRC Nutrient Requirements of Sheep, 6th Revised Edition, 1985)


Based on these guidelines, we recommend the following inclusion rate when using Alkosel® R397 as a selenium source:

- Dairy cattle: 150g/ton feed (0.3 ppm)
- Beef cattle: 100g/ton feed (0.2 ppm)

In dairy cows, Dr Bruce Cottrill reckons that the herd selenium status is best estimated by measuring bulk milk selenium content, which is correlated to the animals plasma contents, while measuring blood selenium content is a more expensive, and tedious method.


How to increase selenium intake?

Selenium enters the food chain through plants, which absorb selenium from the soil and convert it into organic selenium. In fact, only plants and yeast have this ability, animals are incapable of incorporating selenium into aminoacids. However, as we are now aware, the soil in most parts of the world is deficient in selenium, with consequences on animal and human selenium status. For this reason, several strategies have been developed to increase animals’ selenium intake:

• The use of selenium enriched fertilizers (New Zealand, Finland…), which outcome depends on various external parameters
• Feed supplementation with sodium selenite or selenate (inorganic form), which, until now, was the only option under EC regulation
• Feed supplementation with selenium yeast, which contains selenium as highly bioavailable organic form (selenomethionine, selenocysteine).
  

Fig. 1: Selenium status of forage crops in Europe. (Map constructed from Oldfield, 1999).

Health benefits of selenium supplementation

There are strong correlations between selenium status and immune or reproductive functions.


A- Improved immunity: reduced mastitis risks

Sub-clinical mastitis, characterized by high somatic cells count (SCC) in milk, is considered as the costlier disease for the dairy industry. It has been estimated to cost the US dairy industry over  $ 1 billion.  Among the factors influencing SCC, it has been shown that high plasma selenium content reduces SCC in milk (Weiss, 1990).

Supplementation with bioavailable selenium yeast can help reducing SCC in milk, as shown in a study by Malbe et al. (1995). Smith et al. have linked this protective effect to the influence of the antioxidant status on neutrophil functions, the circulating immune cells implicated in the early response against pathogens.




Fig. 2: Somatic cells count is directly linked to the cow selenium status (Weiss, 1990).




Fig. 3: Selenium yeast, which is more effective than inorganic selenium to raise blood selenium level, has the ability to reduce SCC in milk (Malbe, 1995).



B- Reducing the incidence of post-partum retained placenta

Retained placenta affects 9-20 % of all calving in the US, costing millions to the dairy industry. While a multi-factorial condition, the link between retained placenta and Se/vitamin E deficiency was established as early as 1969. To date, over 20 peer-reviewed papers evaluate the benefits of Se, vitamin E or both on retained placenta, with 2/3 reporting a positive response in retained placenta prevention with Se/vitamin E supplementation.


C- Selenium and dairy cows fertility

Over 20 scientific papers have been published on selenium supplementation and dairy cows’ fertility, with a variety of protocols, diverse forms of selenium and parameters used to measure fertility. Dr Cottrill concludes that the weight of evidence makes a strong case that selenium and Vitamin E deficiencies negatively impact on reproductive health and performance. To date, further studies are still needed to identify optimum levels of dietary selenium and the best administration periods. However, the administration of Vitamin E and organic selenium seems to represent an optimal strategy to optimize dairy cows’ fertility.


D- Improving calves selenium status

In 2005, a trial was performed by Dr Bill Weiss, at the Ohio State University, USA, to assess the effect of selenium yeast (Alkosel®R397) supplementation on dairy cows milk selenium content and calves’ selenium status (0.3 ppm Se as sodium selenate vs. 0.3 ppm Se as Alkosel). When compared to inorganic selenium, selenium enriched yeast Alkosel®R397 significantly increased the cows’ serum, milk and colostrum selenium content, with a positive effect on selenium transfer to calves, both at birth and during lactation, with subsequent advantages for calf health. This result is explained by the higher bioavailability of the selenium in Alkosel®R397, compared to the inorganic selenium form. In fact, most of the selenium that is transferred via the placenta is organic, and in the same way, inorganic selenium is unable to get transferred to the colostrum and milk.




Fig. 4: Alkosel significantly increases colostrum and milk selenium levels post-calving (Weiss, 2005).





Fig. 5: Alkosel offers better selenium transfer to the calves, first through the placenta and then the colustrum (Weiss, 2005).



Increased milk and meat selenium content: addressing a public health issue?

In order to increase selenium concentration in milk, many investigators have shown the importance of the form of selenium. While blood selenium level increases with selenium intake in both inorganic and organic (yeast) form, milk selenium content does not follow the same pattern. As mentioned before, only selenium yeast allowed milk selenium content to increase accordingly. When cows receive increased levels of inorganic selenium, milk selenium content did not raise.




Fig. 6: Effect of the Selenium source on cow’s milk Selenium content (Ortman and Pehrson, 1999).

Thus, due to its high bioavailability, selenium yeast is a good source of selenium to improve selenium content in ruminant milk and meat. But is this sufficient to help restoring adequate selenium status in human? Dr Cottrill estimates that, in the UK for example, where the average selenium intake is about 35 µg/day , increasing milk selenium content from about 10 to 30 µg/ liter  could provide an extra 6 µg/day, representing about 20% of the current deficit.


Key points:

• Selenium is an essential nutrient for ruminant livestock.
• Organic selenium (selenium yeast) is more bioavailable than inorganic forms (sodium selenite/selenate).
• Selenium and Vitamin E deficiencies negatively impact reproductive health and performance. Supplementation with both anti-oxidants is beneficial.
• Selenium yeast supplementation reduces somatic cells in milk.
• Selenium yeast supplementation can increase the selenium content of milk and meat, with positive effects on maternal selenium transfer and consumer benefits.

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