Agronomy Advice

Trial Results: Effect of Selenium Enriched Fertilizer on Grazing Cows

Yara undertook detailed trials last year to investigate the impact of cows grazing on pasture that had been fertilised with a selenium enriched fertilizer

Image of a cow grazing in a field
Image of a cow grazing in a field

Yara’s Booster range was formulated to not only promote healthy grass growth but by adding selenium into the mix, support healthier livestock as well!

To test the validity of this approach Yara undertook trials to investigate the impact of selenium levels in grazing in-calf dairy heifers.

Taking a herd that had received selenium supplementation throughout winter housing via concentrates, they were split into two groups. The first group grazed in fields fertilised with YaraMila EXTRAGRASS which is an NPK fertilizer with sulphur, but contains no selenium (27-5-5+6%SO3). The second group grazed on grass that had been fertilised with YaraMila STOCK BOOSTER S (25-5-5+5%SO3+Na+Se).

To monitor selenium levels, blood tests were taken from a random number of heifers involved in the trial and an average was calculated for each group.

At the beginning of the trial, the heifers had an average level of selenium in their blood of 846 U/g Hb for group 1 and a similar level of 865 U/g Hb for group 2. Once heifers start grazing, you would expect to see a drop in selenium levels as it’s much harder to add selenium to their diet when they’re at grass receiving no supplements. However, the hypothesis is that by enriching the grass with selenium, you therefore increase dietary selenium which will support healthier levels of the nutrient in the animal’s blood. This form of selenium is much more available to the animal, so you need much less of it. Compared to this, inorganic selenium sources found in concentrates, licks and boluses deliver a less readily available form of selenium and so you need much more of it to make a difference and this can become an expensive option.

The heifers were left to graze on their respective fields for 3 months (from May until August) and the relevant fertiliser was applied three times during that time. At the end of the trial, blood tests were taken from the heifers again to identify selenium levels.

The results showed that the average level of selenium found in the cows’ blood in group 1 (who grazed on grass treated with YaraMila EXTRAGRASS) dropped to 439, a reduction of around half their starting level of selenium.

For group 2, who grazed on grass treated with YaraMila STOCK BOOSTER S, the drop was much lower - on average, the level was 658, a much healthier level which should maintain better herd health for this group of heifers as they calve and enter the milking herd.