Dr Bruce Cottrill, author of this article and a ruminant nutrition expert with UK-based ADAS, has stressed the importance of selenium in ruminants, with practical considerations on their requirements, different sources of selenium and animal responses to supplementation.
Dr Cottrill also highlighted the difference between animal requirements, which are measured in experimental setups, and animal allowances, which are the amount needed to meet animal requirements in 'real life' conditions, hence including a safety margin to take into account variations in feed intake, feed composition and production level.
In dairy cows, Dr Cottrill thinks 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.
There are strong correlations between selenium status and immune or reproductive functions.
Sub-clinical mastitis, characterised by high somatic cells count (SCC) in milk, is considered a costly disease for the dairy industry. 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 reduce 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.
Over 20 scientific papers have been published on selenium supplementation and dairy cows' 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, vitamin E and organic selenium administration seems to represent an optimal strategy to optimise dairy cows' fertility.
In 2005, Dr Bill Weiss performed a trial at the Ohio State University, US, to assess the effect of selenium yeast (Alkosel R397) supplementation on dairy cows' milk selenium content and calves' selenium status (0.3 ppm selenium as sodium selenate versus 0.3 ppm selenium as Alkosel).
Results showed that when compared with inorganic selenium, selenium-enriched yeast Alkosel R397 significantly increased cows' serum, milk and colostrum selenium content, with a positive effect on selenium transfer to calves, both at birth and during lactation.
This result is explained by the higher bioavailability of selenium in Alkosel R397, compared with the inorganic selenium form. 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.
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 received increased levels of inorganic selenium, milk selenium content did not rise.
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Article made possible through the contribution of Lallemand Animal Nutrition.