A combination of various environmental factors this season tend to be aligning and can possibly create the perfect storm for Grass Tetany (hypomagnesaemia) in cattle over the next few months. These predisposing factors include:
High K and low Mg in young pasture
Following a dry spell, like the one experienced from late summer until early autumn this year, lush reestablishing pasture is commonly high in potassium (K) and low in magnesium (Mg). Low dietary Mg alone can lead to a deficiency large enough to cause Grass Tetany. However, high K levels can intensify the risk by interfering with Mg absorption sites in the rumen.
Cold, wet and windy conditions
Cold and wet conditions can significantly slow pasture growth and reduce dry matter intake, only for pasture to be flush with K when growth recommences on warmer days. Similar conditions, generally experienced from late autumn through to early spring in south eastern Australia, with the inclusion of wind can add stress to the equation, commonly associated with Grass Tetany. In fact, the NSW DPI (2009) reported that the first widespread cases of Grass Tetany in a season are often seen after the first spell of cold, wet windy weather or frosts.
Lactating, older and fatter cows are a high risk
Cows with calves at foot have a higher drain on Mg and minerals than other classes of cattle due to mineral requirements for milk production. At the same time, older and better conditioned cows have a lower ability to mobilise Mg around the body when required. Therefore, under the right environmental conditions, lactation can cause a deficit in Mg absorption versus Mg expenditure quickly leading to deficiency.
Impact of hypomagnesaemia on other classes of livestock
Although uncommon, Mg deficiency can still be fatal for other classes of livestock. In these livestock Mg deficiency is largely associated with a reduction in production. This generally goes unnoticed, however, studies have shown that in both cattle and sheep grazing green pastures and crops, effective mineral supplementation has resulted in increased liveweight gain of up to 60% (Dove H, et al 2011).
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