Grass tetany

Grass tetany is most common in lactating cows. Clinical signs develop when serum magnesium levels fall below a critical level (hypomagnesemia). 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.
  • Older 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 high potassium or nitrogen content (or application of potassium or nitrogenous fertiliser).
  • Short pasture (less than 1,000kg DM/ha) at the one to two leaf stage (as they are more likely to have elevated potassium:magnesium ratios with higher grass tetany risk).
  • Pasture potassium greater than 35g/kg dry matter.
  • Pastures with low sodium intake (less than 2g/kg dry matter).
  • Pastures with low roughage intake.
  • Cold, wet weather that reduces feed intake.
  • Lactating cows held off feed (for example to mark calves) or cows in oestrus.
  • Very fat cows (ie: a fat score of 3.5 or more) or very thin cows (ie: a fat score of 2 or less).
  • Increasing fertiliser application and pasture quality.

Identifying and diagnosing grass tetany

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

  • Cows found dead, with ground rubbed from convulsions prior to death.
  • Excitement and convulsions.

Pasture assessment (availability, quality, mineral leaf test to evaluate mineral balance) and stock fat score assessment help determine risk. A veterinarian may use blood samples to determine magnesium and calcium levels and urine samples to test for magnesium.

Prevention strategies for grass tetany

  • Increase the magnesium intake of at risk groups of cows:
    • Feed hay treated with magnesium oxide during at-risk periods.
    • Use intra-ruminal bullets containing magnesium.
    • Feed hay (source of calcium).
    • Provide loose licks containing limestone, salt and magnesium oxide mixed with molasses.
    • Use the Health Cost Benefit Calculator to determine the cost-benefit of the various options to increase magnesium intake.
  • Avoid high-risk pastures (high potassium due to potassium or nitrogen application or short pasture in the one to two leaf stage) or pastures where cows have a history of grass tetany. Potassium intake levels are minimised when plants are grazed at the three to four leaf stage.
  • Avoid sudden changes of feed or feed quality.
  • Lower the herd age structure as older cows are higher risk.
  • Change 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.
  • Keep mustering to a minimum and avoid other stressors.

More information

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