What is Bighead?

‘Bighead’ or Nutritional Secondary Hyperparathyroidism is a condition caused by grazing pasture rich in oxalate grasses, such as Setaria grass.

How does it work?

Sub-tropical pastures contain chemicals called oxalates, which bind to dietary calcium during digestion. This stops the calcium being absorbed into the body. Over time, horses become calcium deficient and start to use the calcium from their bones instead. Parathyroid hormone is overproduced and calcium and phosphorous become inbalanced. This leads to weak and brittle bones, that are easily deformed.

What to look for in your horse?

The non-weight bearing bones, such as the facial and skull bones, are affected first. Clinical signs usually take about 2 months to appear.

Swollen facial bones

Signs of bighead:

  • Stiff jointed gait
  • Joint tenderness
  • Loss of condition
  • Swollen jawbones
  • Loosened teeth
  • Depression/ lethargy
  • Bone fractures


The best prevention is to remove the horse from the pasture. However, many horse owners have no choice but to graze sub-tropical grasses. They are widespread in certain areas and popular for cattle. These horses need daily calcium supplementation.

Ideally, to ensure the best calcium absorption, the horse should be brought off the pasture daily to eat a hard feed containing the calcium. This helps reduce the interaction between grass and calcium so that more calcium is absorbed.

There are lots of calcium additives on the market. The important thing is to get one with chelated calcium that is easily absorbed in the small intestine. My favourite calcium formulation is http://drjenniferstewart.com/bone_formula.html


Setaria grass

If you think your horse is already affected with bighead, contact your vet for the best advice on treatment. It can be a long process to resolve any bone abnormalities.

Grasses that have high levels of oxalates:

  • Setaria Grass
  • Buffel Grass
  • Green Panic Grass
  • Kikuyu Grass
  • Pangola Grass
  • Para Grass
  • Signal Grass


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