14/06/2024
Big head disease
I have recently had a few questions regarding big head disease also known as Millersā disease or more correctly termed secondary hyperparathyroidism.
Secondary hyperparathyroidism is caused by a calcium deficiency which occurs when horses do not receive and absorb sufficient calcium from their diet. This can occur when horses eat forages with high oxalate content such as kikuyu, buffalo grass, setaria and panicum grasses, or when diets are high in phytates from large amounts of brans & grains without adequate calcium supplementation or a combination of these factors.
Available calcium and phosphorous in the total diet needs to meet the horseās daily requirements and they must also be in the correct ratio to one another to ensure adequate uptake. The ideal ratio of cal:phos is between 1.2 ā 2 parts calcium to 1 part phosphorous. While adult horses can do well with higher calcium intake it is not ideal for growing, hardworking and breeding horses where extra attention needs to be paid to this ratio.
An adult (>5yrs) 500 kg, horse needs between 38 & 70 g of calcium and 21 ā 44 g phosphorous per day depending on their work level. While NRC levels are lower than these amounts, the higher intakes have been shown to improve bone density especially in younger horses and their diets need special attention to maximise bone strength when they start to work before growth is finalised.
Obvious signs of a calcium deficiency take a long time to become evident as the body tries to keep blood calcium levels stable by absorbing more calcium from the diet, changing excretion patterns and eventually removing calcium from the bones, with those in the in the face being affected most making them spongy and enlarged, hence the name big head disease. Earlier signs of calcium deficiencies can include ā shifting lameness, splints, muscle pain and stiffness, saddle sourness and an unwillingness to move forward or jump happily.
What can you do about it?
1. Look at grasses and feeds to make sure there is nothing that will stop or reduce calcium absorption (oxalates & phytates)
2. Make sure calcium and phosphate intake meets your horses needs in the total diet (hay, feed, water and supplements)
3. Then supplement the diet to ensure your horses needs are met, ensuring to add sufficient calcium and phosphate to counter potential binding effects of other dietary ingredients.
4. Ensure a good cushion of safety using at least 200% NRC levels of available calcium and phosphorous ā the NRC themselves acknowledge mineral levels are the bare minimum needed to stop deficiency symptoms occurring.
5. Supplemental calcium can come from lucerne, feedlime and DCP/MCP but the latter also have phosphate in so check overall ratios to make sure they are ideal.
6. Phosphate is found in reasonable amounts in most grains & brans and can be supplemented using MCP or DCP.
It is also very important to note that just because your bagged feed MAY have a balanced Cal: Phos ratio it will not mean your horses diet has an adequate or balanced Cal: Phos ratio, as forages have a large impact on the total intake and balance.