Autumn Fertiliser Choices – What kind of Spring will you have?

A client posed a question this week when we were discussing fertiliser options. He said, “I can buy a heck of a lot of magnesium oxide to supplement the cows in spring with what you are proposing we spend on magnesium fertiliser this autumn why not just supplement the cows directly?” It’s a good question.

Magnesium deficiency in Taranaki is famous. There are even some old timer names for it – “Taranaki anaemia” and “leather bag”. Magnesium is a requirement of just about every biochemical process. Deficiencies then, even marginal ones, have the potential to limit animal performance in many different ways other than grass staggers and milk fever. All dairy farmers know this. Cows are drenched, bulleted and injected with the stuff. Tonnes of it is dusted over pastures and poured into water troughs. The amounts used are often not rational – more is better! There are a few problems though and important ones.

I call the problem “supplementation risk”. It is rooted in the way magnesium deficiency works in the cow. Cows have a pump mechanism in the rumen that can suck magnesium out of the rumen fluid and into the bloodstream. As such, a cow can function on low dietary magnesium. However, increasing potassium content degrades the effectiveness of this pump. At 3% potassium in feed the pump function is reduced by 30%. At 3.5%, magnesium absorption by this mechanism is ineffective.

The other method of magnesium absorption relies on maintaining a magnesium concentration in the rumen so that it just floods into the bloodstream. With potassium above 3% (and just about every dairy pasture is) the herd becomes increasingly reliant on a magnesium intake high enough to influence blood levels i.e. a total reliance on the efficiency of the supplementation programme.

Additionally, magnesium supplements generally taste terrible and cows will go to some lengths to avoid it. Dusting is hugely inefficient and worse in wet weather and water treatment is similarly inefficient in wet weather. Which is why calvings and metabolics are so much more severe in wet springs. At the other end of the scale magnesium toxicity is a real problem notwithstanding the sheer waste and cost of magnesium through indiscriminate use.

Planning on achieving the lowest potassium content and highest magnesium content in pastures over calving takes all the sting out of supplementation risk and consequently a major source of risk for milk fever and the long list of related issues is also mitigated.

Controlling magnesium deficiency in the spring begins with considered fertiliser choices in the Autumn. Simply pumping more magnesium into cows in the spring just does not work overly well.

Related: Why Pasture Sampling is a "Must Do" this Spring

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