Demands on dairy stock are high during spring and selenium (Se) can make a real difference to performance.
Selenium is essential for animal health but is not readily available in many New Zealand soils. South Island sedimentary soils, coastal sands and the pumice soils of the central North Island plateau are particularly vulnerable to selenium deficiency.
Pasture species also influence the amount of selenium in the diet of grazing animals. For example, browntop naturally contains more selenium than ryegrass, which in turn has higher concentrations than clover.
Ruminants tend to be more susceptible to selenium deficiencies than other animals, possibly due to insoluble selenium compounds forming in the rumen. Increasing the sulphur content of the diet may reduce selenium availability and substances found in some legumes may cause selenium to be lost via urine.
Selenium helps keep cell membranes intact and supports the immune system and thyroid. A diet lacking in selenium can decrease milk production and increase instances of infertility and retained placentas in cows. Selenium is transferred across the placenta and through milk, so the selenium status of a cow influences selenium levels of her developing calf. Insufficient selenium levels can cause calves to be stillborn, born weak or premature or suffer from poor growth or succumb to white muscle disease.
Selenium is relatively easy to incorporate into your fertiliser programme. Its uptake by pasture is not generally influenced by levels of other macro or micro-nutrients or pH in New Zealand soils. Selenium can be applied with maintenance fertiliser or with spring or autumn nitrogen.
Mixed pasture herbage testing is the first step. If levels are below 0.03 micrograms (mg) of Se per kilogram (kg) of drymatter (DM) (below 30 micrograms per gram in your mixed pasture test results) you may need to supply selenium. Ideally, levels should be benchmarked in autumn and re-checked in spring. However, if you missed the autumn window, spring testing is still beneficial and will allow you to take action in advance of mating and/or to support peak milking demands. Analysing selenium levels in tissue and blood samples can complement herbage testing. The cost of testing and addressing selenium levels is minimal relative to potential benefits.
Two forms of selenium are commonly used as fertiliser: slow-release barium selenate and/or fast-release sodium selenate. Ballance Selenium (one percent Se) combines both forms to give pasture an initial lift and then sustain selenium levels over time. This makes it effective for spring or autumn application at a recommended rate of one kg per hectare (ha) for dairy pasture.
Raising pasture selenium levels above 0.03 mg Se/kg DM is not likely to increase milk production. However, animal performance improvements may be seen in lactating stock if dietary selenium levels are between 0.1 and 0.3 mg Se/kg DM. Young stock also benefit from keeping selenium intakes within this higher range. After taking herbage tests and considering selenium inputs from feed and other sources, an appropriate supplement can be provided via drinking water, lick blocks, injections or boluses.
Prolonged ingestion of feed containing more than five mg Se/kg DM can lead to selenium toxicity or selenosis. This intake is 16 times higher than the upper end of the 0.1 and 0.3 mg Se/kg DM range. To risk toxicity through fertiliser, you would need to apply 50 times the recommended rate. To cause toxicity through stock dosing, you would need to deliver 5 to 10 times the approved rate.
For help with herbage testing and advice on selenium fertiliser, talk to your Ballance Nutrient Specialist or your local Fonterra Farm Sourceteam.
Article supplied by Ballance Agri-Nutrients