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Maintaining calcium balance of horses grazing Australian pastures

Bones play an important role in mineral balance; storing a reserve of calcium and phosphorus at a ratio of approximately 2:1 within their matrix.

When horses graze sub-tropical pastures, their ability to absorb calcium during digestion is significantly reduced, altering the balance of calcium and phosphorus in their diet. Prolonged grazing without calcium supplementation on these pastures causes a condition known as Nutritional Secondary Hyperparathyroidism (or Big Head) because dietary calcium becomes deficient.

Why is calcium unavailable in sub-tropical grasses?

Sub-tropical pastures contain chemicals called oxalates, which bind to calcium during digestion in the small intestine. The calcium-oxalate compound renders the calcium insoluble in the horse's small intestine, where the majority of calcium absorption takes place. The calcium-oxalate compound passes from the small intestine to the large intestine, where it can be broken down during microbial digestion, however by this stage of the digestive tract, very little calcium is actually absorbed.

Sub-tropical grasses that contain high levels of oxalates:

  • Buffel Grass
  • Green Panic Grass
  • Kikuyu Grass
  • Pangola Grass
  • Para Grass
  • Setaria Grass
  • Signal Grass

What happens to horses grazing sub-tropical grasses?

Horses grazing on fast growing pasture high in oxalates, over a prolonged period, usually 2 months or more, without calcium supplementation, are at risk of suffering from Nutritional Secondary Hyperparathyroidism.

A complex system exists within the horse's body to monitor the blood calcium level. As the horse continues to graze pasture high in oxalates and consequently receive deficient levels of calcium in their diet, parathyroid hormone is released from the horse's pituitary gland. Parathyroid hormone stimulates the process of resorption of calcium stored in the horse's bones, particularly non weight bearing bones, predominantly the facial and skull bones.

As calcium is removed from the bones, they become weakened and are deformed by everyday activities such as grazing. The facial bones soften and begin to protrude and swell outwards, hence the term Big Head.

Signs of Nutritional Secondary Hyperparathyroidism or Big Head:

  • Stiff-jointed gait
  • Joint tenderness
  • Loss of condition
  • Swollen jawbones
  • Loosened teeth

How do you balance diets of horses grazing sub-tropical pastures?

In many pastures throughout Australia, horses are left with no option than to graze sub-tropical grasses. It is important to provide these horses with a calcium supplement, like CAL-PLUS® with BIOTIN, to prevent bone disorders from occurring.

Ideally, to ensure optimum absorption of calcium, the horse should be brought into a yard to consume a hard feed containing a suitable calcium supplement on a daily basis. This may help to reduce the interaction of calcium and oxalates during digestion and ensure the calcium is free of oxalates in the small intestine, where it is available for absorption and utilisation by the horse.

If you suspect your horse may be suffering from Nutritional Secondary Hyperparathyroidism, contact your veterinarian for advice.

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