Well-placed to weather conflicts
Shipping disruption caused by Houthi rebels in the Red Sea has so far not impacted fertiliser prices or supply on farm.
This is due in part to scepticism that the low minimum recommended intakes do not reflect the needs of the modern dairy cow.
To address the confusion, Ravensdown commissioned Stacey Hendricks and Associate Professor Richard Laven from the Institute of Veterinary, Animal and Biomedical Sciences to review all available national and international scientific literature to determine whether current reference ranges for selenium are still relevant today. But first, more about selenium.
Symptoms of deficiency
Selenium deficiency is known to cause a variety of syndromes in cattle, sheep, goats and horses on grazing diets. In cattle, the principal clinical problem associated with selenium deficiency is white muscle disease (Nutritional Muscular Dystrophy), which causes cardiac muscle degeneration, but it is also associated with a range of sub-clinical conditions such as ill-thrift, production losses and reduced fertility.
Selenium supplementation has been shown to cause subtle improvements in animal performance, for instance enhanced immunity, although unambiguous methods to measure such benefits have yet to be determined.
How selenium deficiency occurs
Contrary to most trace elements, selenium shows a good soil to plant to animal relationship. However, it is estimated that 30% of New Zealand grassland provides insufficient selenium for grazing livestock. Granite rock formed by extreme temperatures, and rhyolite ash from volcanoes are low in trace elements and weather slowly, giving rise to soils low in selenium (and cobalt).
The selenium content of NZ pastures ranges from 0.005 to 0.070 mg/kg DM and those concentrations tend to be lowest in spring when pasture growth is greatest. Selenium concentration in brown top is greater than that of ryegrass or white clover grown on the same soil, and deep rooted plants such as dock and lucerne accumulate greater amounts of selenium than shallow rooted plants.
What is the right level for NZ?
NZ-specific reference ranges for Se in pasture and animal tissues have been established for nearly 20 years based on supplementation trials and quantative modelling. These studies resulted in a dietary recommendation of 0.03 mg Se/kg DM for dairy cows grazing NZ pastures. The confusion over higher recommendations is partly driven by the belief that the intensive dairy systems of today require a higher level of supplementation of selenium.
The research commissioned by Ravensdown draws from 57 publications written between 1960 and 2015. The review found that minimum dietary recommendations differ between countries, being 0.03; 0.04; 0.1; and 0.3mg Se/kg DM in NZ, Australia, UK and US, respectively. The difference between the NZ and US recommendations are in part due to the high intake of vitamin E from grazed pasture (in NZ) versus total mixed ration (TMR) housed dairy systems . Vitamin E is a strong anti-oxidant, which acts to spare selenoproteins.
The review showed that most of the data reviewed is consistent with the current NZ recommendations of 0.03mg Se/kg DM. Several studies in herds with marginal or inadequate selenium status – based on NZ recommended levels – showed no effect on either productivity or health after selenium supplementation.
One cautionary finding from the review was that while selenium has been the best researched trace mineral, there is still uncertainty about a definitive upper threshold of the marginal range. While it is possible that the upper threshold of the marginal serum selenium range is too low, this is not justification to match the US recommendation of >890 nmol/L.
Based on the outcome of the review there is no evidence to support changing the current dietary recommendation of 0.03mg /Se/kg DM for dairy cows grazing NZ pastures.
Research findings
1. The nutritional importance of adequate levels of Se in dairy cows is well established.
2. In cattle, the principal clinical problem associated with Se deficiency is white muscle disease but it is also associated with a range of sub-clinical conditions.
3. Only milk production and reproductive losses have been reported in grazing dairy cows in NZ.
4. Minimum dietary recommendations differ between countries, being 0.03; 0.04; 0.1 and 0.3 mg Se/kg DM in NZ, Australia, UK and US, respectively.
5. It has been suggested that the difference between the NZ and US recommendations are in part due to the high intake of vitamin E from grazed pasture versus total mixed rations. Vitamin E is a strong anti-oxidant which acts to spare selenoproteins.
6. Reference ranges for animal tissue samples for diagnosing Se deficiency differ markedly between countries (see table above extracted from review).
7. Most of the published data reviewed are consistent with the current NZ recommendations rather than the US ones. Several studies in herds with marginal or inadequate Se status, based on the NZ recommendations but severely deficient based on US recommendations, showed no effect on either productivity or health after Se supplementation.
8. One cautionary aspect of the review was that while Se has been the best researched mineral, we still lack a definitive upper threshold of the marginal range as one study in the 1990s showed an increase in milkfat production with Se supplementation in cattle with whole blood Se greater than 400 nmol/L (serum >220 nmol/L). While it is possible that the upper threshold of the marginal serum Se range is too low, this is not justification for matching the US recommendation of >890 nmol/L.
Supplementation
If selenium supplementation is required, methods include an addition of selenium prills to fertiliser, oral drenching, selenium addition to supplementary feed and drinking water, or direct treatment to the animal via controlled release boluses, injection and pour-on.ω
