Beware selenium sales pitches

Words by: Andrew Swallow

Advice to supplement herds and young stock with extra selenium, based on overseas requirements, is not justified according to a comprehensive review paper published in the New Zealand Veterinary Journal.

Massey University vet Richard Laven undertook the study because of increasingly widespread reports of farmers being told the standard New Zealand recommended intake for selenium of 0.03mgSE/kg DM consumed, established by Grace et al in 1983, isn’t relevant for modern dairy cows.

The US recommended intake of 0.3mgSe/ kg DM is ten times New Zealand’s but is for cows in indoor systems on total mixed rations, Laven explains. Such diets lack fresh pasture. That’s crucial, because pasture is high in vitamin E that, as an antioxidant, reduces demand for selenium.

“Selenium is an important part of the antioxidant protection mechanisms within animal cells. Vitamin E can substitute for that,” he told the Dairy Exporter.

Another key difference between US and New Zealand systems is selenium loss from the animal in milk, faeces, and urine. US cows typically produce 12,000 litres of milk/year compared with New Zealand’s 6,000 litres, and because US cows are generally considerably larger and eating more they lose a lot more selenium in excrement.

The UK recommended intake of 0.1mgSe/kg DM reflects its systems, which typically see cows grazing in summer but indoors on a silage-based ration in winter – a halfway-house between the US indoor, largely grain-fed herds and New Zealand’s year-round pasture-based systems. Australia’s recommended intake is much closer to New Zealand’s, at 0.04mgSe/ kgDM.

Dietary content of polyunsaturated fatty acid (PUFA) also affects demand for selenium or vitamin E because PUFAs are prone to oxidising (“going-off”) in the body, adds Laven. If that’s not countered by antioxidants, such as vitamin E or those produced with selenium, dietary stress will reduce intake, production, and health of the animal.

Spring grass is high in PUFA but also high in vitamin E so it’s not a problem, he explains. In contrast, grain-based concentrate feeds result in high PUFA levels in the animal without the high vitamin E intake to compensate, hence the need to supplement with selenium and/ or vitamin E. Since selenium is a much cheaper supplement than vitamin E it tends to be preferred, which is another reason why the US recommended intake rate of selenium is so much higher: it’s cheaper than supplementing with vitamin E.

Low PUFA content in fodder beet probably means extra selenium supplementation isn’t needed for dry cows wintering on the root crop, even though it’s known the bulbs are low in selenium, because there’s no loss through milk production. The case for supplementation when cows are milking off fodder beet is more likely to be justified but there’s a lack of good data on vitamin E and fatty acid content of both fodder beet and brassicas, says Laven.

Similarly, there’s no evidence in New Zealand that retained fetal membranes, which are commonly linked to Se deficiency in other countries, are caused by Se deficiency.

“In New Zealand it is likely that the main impact of selenium deficiency is on milk production and mastitis risk.”

The only way to be sure if cows are genuinely deficient in selenium is to blood test, but even then there are some advisors using results to justify unnecessary supplementation, he warns.

A blood serum selenium of less than 85nmol (nanomoles) is deficient and 85-140 considered marginal. Anything above that is fine, yet Laven says he’s heard of farmers being told they should target 400nmol and that results of 150-200nmol are “close to marginal” so they should supplement to be on the safe side. He compares it to taking out insurance cover for a Lamborghini when you drive a Toyota

Richard Laven - “In New Zealand it is likely that the main impact of selenium deficiency is on milk production and mastitis risk.”

Corolla: it’s just not necessary.

“All that happens is the cows get rid of it in their urine or milk. The low end of the normal range is really where you want to be.”

Again US recommended figures are to blame, a blood serum selenium content of 440-890nmol being the target range there reflecting use of selenium to substitute for vitamin E. None of ten selenium trials with cows grazing pasture reviewed by Laven in the NZVJ support that range.

Laven notes the UK’s recommended blood content is in line with New Zealand’s.

A longer-term picture of how selenium status has changed can be obtained by having red blood cells analysed for an enzyme that correlates with selenium status, as well as checking serum. Low Se levels indicated by the blood cells but adequate levels in serum indicate the diet was probably deficient a couple of months previously, for example when the herd was on a winter feed, but intake in recent weeks has been adequate.

If bloods do show a genuine deficiency, and Laven suggests looking at serum and red blood cell content to gain a fuller picture, correction by injection, feed additive, or fertiliser prill are all effective, the latter being the cheapest and easiest in a grazed-herd situation.

SELENIUM SENSE

• Beware recommendations citing US levels. • Science review shows NZ recommendations robust. • Required minimum intake 0.03mg/

Se/kg DM. • Blood serum <85nmol Se is deficient, 85-140nmol marginal. • Just above marginal the place to be. • More does not = better if above marginal.