What is 'Big Head' in horses?

With Jessica Weeks


Nutritional secondary hyperparathyroidism, more commonly known as ‘Big Head’ in horses, is a disease that is caused by calcium deficiency. The deficiency can be due to a lack of calcium intake, an excess of phosphorous intake or an imbalance in the calcium to phosphorous ratio, which is usually caused by an unbalanced diet or from the horse grazing oxalate pastures.

Deficiencies in the diet often arise when there is an increased demand for calcium, such as a lactating or pregnant mare and in young horses when there is a high growth demand. Imbalances in the diet occur when feeds are mixed incorrectly or the levels of nutrients in roughage vary. When horses are grazing oxalate pastures, the oxalates bind the calcium in the hind gut making it unavailable to be absorbed by the horse. The grass itself may contain high levels of calcium, however the oxalates prevent the calcium from being available for the horse. Grasses that contain high levels of oxalates are Buffel, Setaria, Kikuyu and Panic grass.

Adequate blood calcium levels are vital for survival. When the calcium is not available for absorption into the blood stream from their dietary intake, the horse will start redirecting calcium from the bones to maintain adequate levels in the blood. Big Head is named after the ‘big head’ appearance which occurs when horses facial bones become brittle and swollen, due to prolonged calcium deficiency causing high levels of calcium to be drawn from their bones. Initial symptoms of Big Head include shifting lameness, weight loss, a rough coat and they can be sore to touch.

​To prevent Big Head, calcium supplementation is required. The calcium to oxalate ratio must be kept above 0.5parts calcium to 1part oxalate, so the more oxalates in the pasture the more calcium supplementation is needed. When providing calcium supplementation you must keep in mind the calcium to phosporous (2:1) and calcium to magnesium (3:1) ratios and ensure these are balanced to avoid further issues. 

​Lucerne hay is an extremely valuable feed source when trying to combat Big Head as it adds calcium and also reduces the amount of oxalate pasture intake. Calcium phosphates are a good source of supplementation as they contain high levels of calcium and phosphorous, and they are relatievly inexpensive. Organic and chelated calcium are also good sources as they cannot be bound by the oxalates, although they are quite expensive and contain low concentrations of calcium, so they require larger doses. 

​If a horse already has Big Head, they can recover with correct supplementation. Calcium needs to be fed at 200-400% of requirements for the first 12months. Removing the horse from the oxalate pasture, calcium supplementation and making up their roughage requirement with a combination of non-oxalate grass & lucerne hay, is the best way to beat Big Head. Young horses will respond to this better than older horses and some older horses may never lose the swelling of their facial bones if they have had it for an extended period of time. 

​Keep in mind that each oxalate grass type contains different levels of oxalates. A rough guide is Kikuyu and Buffel 15g/kg, Narok Setaria 45g/kg and Kazungula Setaria 70g/kg of oxalates. You can use these rates to calculate the amount of calcium needed for supplementation using the ratio of 0.5calcium to 1oxalate. For example, on Buffel grass you will need to feed 7.5g of calcium per kg of grass eaten. 

Feel free to contact myself or one of our merchandise team on (07) 4660 0400 or email me at; jess@dalbyruralsupplies.com.au for more information about Big Head ofrcalcium deficiences in horses.