Big Head in Horses and Donkeys


Big head is a calcium deficiency of horses and donkeys grazing introduced tropical pasture grasses.  It is caused by crystals of calcium oxalate in the grass blades that prevent the horse or donkey from absorbing calcium from the grass during digestion. It is especially prevalent in spring and summer when these grasses are growing the fastest.   Cattle and sheep are not affected.


How can Big Head be prevented?


Use native grasses where possible. Avoid grazing horses and donkeys on introduced tropical grasses for more than a month at a time.  These grasses are Setaria, Buffel grass,  green panic, kikuyu, guinea grass, papa grass, pangola grass and signal grass.  If only hazardous grasses are available, encourage the growth of a legume component in the pasture to provide a source of feed free of oxalate, and feed a calcium and phosphorus supplement.


Mineral and supplement mixtures which will provide the required amount of calcium and phosphorus include:-


  1. 1 kg rock phosphate mixed with 1.5kg molasses

  2. 1 kg of a mixture of 1/3 ground limestone and 2/3 dicalcium phosphate (DCP) mixed with 1.5kg molasses.


This amount of supplement should be consumed per week and is the preventative dose not the treamtent dose for a horse with clinical bighead disease. Ideally it is preferable to supplement your horse on a daily basis, so divide the amount by seven and feed this much daily i.e approx 140 gms/day. Calcium uptake is more effective this way than feeding once weekly. Either of these preparations  should be fed to each horse while grazing hazardous pastures.  The molasses is used as a carrier and to make the supplement attractive.  It can be omitted if the animal can be persuaded to eat their supplement on a daily basis.  A balanced calcium supplement block can be purchased from Mullumbimby Rural  Co-op and placed in the paddock for horses to consume as they require. Ask Tony which block will best suit your needs. 


Because of their smaller size, feed donkeys a proportionately smaller ration.  Other mineral supplements which provide calcium:phosphorus ratio of 1:1 can be used but are likely to be more expensive than the mixtures listed above.


Can big Head be cured?


The lameness and illthrift can be cured.  The swellings of the jaws may not fully disappear if the animal was severely affected.  Double the amount of mineral supplement listed above should be fed to affected animals for at least 6 months to replace the mineral lost from their bones. Of course removal from the offending paddock is essential and transfer to a paddock with no offending pasture.