‘Big Head’ in horses – When are they at risk?

Photo courtesy of Pixabay.com

Nutritional Secondary Hyperparathyroidism or 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’.


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.
Commonly 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, stiff
• Ill thrift 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.

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.

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.