When is 'Big Head' a risk for horses?

Posted July 26, 2021

Nutritional Secondary Hyperparathyroidism or more commonly, ‘Big Head’, is a disease caused by a calcium deficiency in horses.


Animals store calcium in their bones. In times when there is a limited amount of calcium being absorbed, or when there is a severe calcium deficiency, horses will draw calcium from their bones, primarily from their face. It is then replaced by fibrous tissue giving them the impression of a ‘Big Head’.
This disease can develop in a timeframe of weeks, depending on what pasture the horse has access to, and owners are advised to carefully monitor their animals and supplement feed as necessary to prevent the onset of 'Big Head'.



Big Head occurs because of two main reasons:
  • A phosphorous calcium imbalance – too much Phosphorous can depress the absorption of Calcium. Always aim for a ratio of at least 1:1 ideally 2:1.
  • Oxalate rich pastures – Oxalates are molecules in the grass that bind to calcium in the feed, reducing its availability to the animal, which can cause a deficiency in horses.
Typically, tropical grasses Buffel, Setaria, Green Panic, and Kikuyu are high in oxalates especially during times of early growth or after fertilisation.



A ‘Big Head’ is the worst case scenario or the last stage of the disease and does not occur in all horses suffering from a calcium deficiency. Other symptoms include:
  • Lameness or stiffness in movement
  • Ill-thrift (growing at slower rates than expected) even with ample feed 
  • Rough coat
  • Enlarged or swollen facial bones
  • Tender joints
  • Loose teeth, difficulty chewing
  • Poor growth in young horses
  • Difficulty breathing



  • For all classes of horses in all grazing situations the complete diet needs to be balanced, and the phosphorous to calcium ratio needs to be at least 1:1.
  • Where horses have no alternative than to graze high oxalate pastures, it is important to increase the supplementation of phosphorous to calcium to 3:1.
  • Horses with higher calcium requirements – gestating or lactating mares and growing horses will also require extra calcium supplementation.



If symptoms are present a veterinarian should be consulted. Recovery time can vary depending on the severity of the symptoms and it can take many months for a full recovery.


Riverina manufacture a high calcium supplement Hi-Cal Horse & Pony Pellets.
For any additional information on balancing the feed, please contact our Riverina Nutrition Team.

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