Understanding and Controlling Grass Tetany

Winter and spring last year saw one of the most challenging grass tetany seasons on record, with some regions in southern Australia reporting cattle losses in the thousands. With rainfall and temperatures this winter looking very similar to 2021, producers have been extra vigilant this season and for the most part are taking proactive measures to reduce the risk. At ANP we believe that it is a good exercise to return to the basics, understand the disease and adopt proactive grass tetany risk mitigation measures when managing cattle grazing throughout winter and early spring.
Although grass tetany is caused by low concentrations of magnesium in the blood, it is important to understand what causes this to happen. There is a common belief that low levels of magnesium in feed is the cause. This is true to an extent, but it is missing a key part of the story. Low blood magnesium, in most cases, is caused by two similar interactions between magnesium and potassium, one within the rumen and the other in the soil, that limit the availability of magnesium to the animal.

Potassium has an antagonistic relationship with magnesium, where high concentrations of potassium in the diet can interfere with the major sites of magnesium absorption. Through this, high levels of potassium essentially reduce the amount of magnesium being absorbed in the rumen and therefore entering the bloodstream and used by the animal.

Studies of interactions between magnesium and potassium within the soil have revealed that high concentrations of potassium in the soil have a negative impact on magnesium absorption by the roots. The same impact can also be seen in balanced potassium soils on wet, cold days where pasture growth significantly slows. In these circumstances, the concentration of potassium builds around the roots of the plant replacing the magnesium. As the wet and cold days continue with limited plant growth, the concentration of potassium continues to build. It is the following warmer, sunny day where grass tetany issues are realised. This is because when the plant begins to grow, it absorbs a high concentration of potassium which then impacts magnesium absorption in the rumen.

Management strategies adopted should aim at preventing grass tetany from occurring rather than treating the disease. Acute forms of the disease are rapid, and death is often the first observed symptom. Successful prevention of grass tetany involves an integrated approach that aims at:
1. Allowing animals access to continuous supplies of adequate levels of supplementary magnesium
2. Maximising absorption of magnesium in the gastrointestinal tract by delivering available sources of magnesium
3. Offering an available magnesium supplement well ahead of the danger period to ensure that magnesium levels don’t reach critically low levels leading into winter
4. Identifying agronomic practises that can reduce absorption or interfere with normal magnesium metabolism, e.g., application of potash or urea and feeding dual-purpose forage cereals.

Magnesium supplementation should be delivered daily as the body has limited sources to pool from and has daily obligatory losses associated with maintenance (3mg/kg LW) and lactation (120mg/kg of milk). Supplying a supplement high in available magnesium is the most common and efficient method used to meet daily requirements.
The source of magnesium is important for maximising the availability of magnesium to the animal.

Although MgO (Causmag) has a high concentration of magnesium, it is less soluble and therefore less available to the animal than other highly available sources of magnesium, especially if the granular type is used. StockGro-HiMag contains multiple sources of soluble magnesium to maximise availability to the animal.

StockGro-HiMag and StockGro-HiMag + Bovatec liquid feed supplements have repeatedly shown to be highly effective against grass tetany in cattle. These supplements do not only deliver a continuous supply of available magnesium in the form of soluble salts, to increase absorption across the whole gastro-intestinal tract, but are also fortified with acid salts to encourage magnesium mobilisation and availability, and a complete blend of trace elements. StockGro-HiMag + Bovatec is particularly ideal for lactating cows and growing weaners, as Bovatec has been shown to boost milk production, feed conversion efficiency and growth of cattle on pasture.

For more information about ANP's HiMag range see product summaries below:
StockGro-HiMag - Liquid Feed Supplement. Ideal for dry cows grazing winter pastures. https://ausfarmnutrition.com/products/stockgro-himag
StockGro-HiMag + Bovatec - Liquid Feed Supplement. Ideal for lactating cows or growing weaners grazing winter pastures and crops. https://ausfarmnutrition.com/products/stockgro-himag
StockMins-HiMag - Granular Mineral Supplement. Ideal for sheep grazing winter pastures and crops, or cattle where the risk of grass tetany is low. https://ausfarmnutrition.com/products/stockmins-himag