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Grass tetany

Grass tetany is most common in lactating cows. Clinical signs develop when serum magnesium levels fall below a critical level (hypomagnesemia). Hypomagnesemia occurs when magnesium inputs fall below magnesium outputs. Affected animals develop muscular spasms and convulsions and can die of respiratory failure.

Conditions when grass tetany is likely to occur

  • properties with a history of grass tetany
  • cows greater than five years old in first three to four months of lactation
  • grass dominant pastures from late autumn to early spring
  • pastures with a high potassium or nitrogen content. This can occur naturally or through the application of potassium or nitrogenous fertiliser
  • short pasture of less than 1,000kg DM/ha, at the one to two leaf stage. They are more likely to have elevated potassium to magnesium ratios with higher grass tetany risk at this stage of growth
  • pasture with potassium levels greater than 35g/kg dry matter. High potassium intake reduces magnesium absorption
  • pastures providing sodium intake less than 2g/kg dry matter
  • pastures providing low roughage intake
  • cold, wet weather that reduces feed intake
  • lactating cows held off feed (e.g. to mark calves) or cows in oestrus
  • very fat cows with a fat score of 3.5 or more
  • very thin cows with a fat score of 2 or less
  • increasing fertiliser application and pasture quality.

Identification and diagnosis

Clinical signs that would lead you to suspect grass tetany include:

  • cows found dead, with ground rubbed from convulsions prior to death
  • excitement and convulsions
  • assessment to determine risk through:
    • pasture assessment of availability and quality and mineral leaf test to evaluate mineral balance
    • stock fat score
  • a veterinarian may use blood samples to determine magnesium and calcium levels and urine samples to test for magnesium.


Increase the magnesium intake of at-risk cow herds by:

  • feeding hay treated with magnesium oxide during at-risk periods
  • using intra-ruminal bullets containing magnesium
  • feeding hay as a source of calcium
  • providing loose licks containing limestone, salt and magnesium oxide mixed with molasses
  • using the Health Cost Benefit Calculatorto determine the cost-benefit of the various options to increase magnesium intake
  • avoiding high-risk pastures (high potassium due to potassium or nitrogen application or short pasture in the one to two leaf stage). Potassium intake levels are minimised when plants are grazed at the three to four leaf stage
  • avoiding pastures where cows have a history of grass tetany
  • avoiding sudden changes of feed or feed quality
  • lowering the herd age structure as older cows are at higher risk
  • changing the time of calving from autumn to spring to reduce the period of risk. This will not eliminate the risk of grass tetany but will reduce the period when prevention is required. This decision should be considered with other management procedures
  • keeping mustering to a minimum and avoiding other stressors.