Nutritional Secondary Hyperparathyroidism or ‘big head’:
Alcohols and Acids:
Introduced tropical grasses that contain soluble oxalates are associated with a condition in Australian horses known as nutritional secondary hyperparathyroidism or ‘big head’.
This condition is primarily a problem in the northern areas of Australia where introduced tropical grasses are most common.
Horses are most at risk of developing big head if the tropical grasses are the dominant species in the pasture.
Native grasses and many introduced temperate grasses are not associated with big head and are safe to use as pasture species for horses.
The soluble oxalates in introduced tropical grasses combine with calcium to form insoluble calcium oxalate crystals. The formation of these crystals reduces the absorption of calcium from ingested fodder and it alters the calcium to phosphorus ratio in the diet.
In effect, the horse suffers a calcium deficiency. This deficiency causes mobilisation of bone calcium to compensate for low blood calcium levels. Over time the bones lose so much calcium that they become soft and misshapen.
Some or all horses grazing the same pasture may be affected. Mares and foals have been reported to be more susceptible than stallions or geldings but all horses can be affected. The soluble oxalate content of tropical grasses is highest in periods of rapid pasture growth.
Signs of ‘big head’:
Signs of the condition usually develop after 6 - 8 months of grazing soluble oxalate pastures. Some cases have been reported to occur after only 2 months.
- Stiff and shortened gait
- Joint tenderness
- Loss in condition even when plenty of pasture is available
- Swollen jawbones - the upper, lower or both jawbones can be affected
Removal of the horse from the soluble oxalate pasture should see the resolution of lameness problems and the horse should regain condition. Facial swelling should also resolve, unless it is severe.
Grasses associated with nutritional secondary hyperparathyroidism
Identification note: distinguishing between the different grasses that grow in pastures can be extremely difficult and only a basic description is given here. See page 5 for more information on obtaining a precise identification of a plant species.
Buffel grass:
Cenchrus ciliaris [Poaceae]
Upright perennial that grows to 1 m. Tough rootstock. Fringe of hairs at inner junction of leaf sheath and leaf blade. Bristled, branched flower spike up to 15 cm long. 1 –3 spikelets per cluster.
Setaria (South Africa pigeon grass):
Compact tufted perennial that grows to 2 m. Fringe of dense hairs at inner junction of leaf sheath and leaf blade. Cylindrical flower spike up to 30 cm long. Fine, rigid bristles on flower spike. Spikelets in groups of 1 – 4. Under each spikelet group there are 6 – 10 rough hairs.
Pangola grass:
Digitaria eriantha ssp. pentzii (formerly known as Digitaria decumbens) [Poaceae]
Semi-erect perennial grass that grows to 1 m. Produces underground shoots. Fringe of short hairs at inner junction of leaf sheath and leaf blade. The plant produces a group of 3 – 10 flower spikes up to 20 cm long.
Guinea grass:
Densely tufted perennial that grows to 3 m. Produces short, stout underground shoots. The inner junction of leaf sheath and leaf blade is thin and translucent. Branched flower spike up to 50 cm long.
Kikuyu:
Perennial that grows to almost 1 m. Many underground (rhizomes) and aboveground (stolons), root-like stems that can take root and produce new growth. Fringe of dense hairs at inner junction of leaf sheath and leaf. Flower heads on short shoots. Spikelets in cluster of 2-4. Spikelets enclosed in a sheath.
Para grass:
Brachiaria mutica (also known as Urochloa mutica and Panicum muticum) [Poaceae] Perennial that has long stems (stolons) that can take root and produce new growth. The stolons can be up to 4 m long. Fringe of dense hairs at inner junction of leaf sheath and leaf. Produces numerous branched flower spikes up to 8 cm long.
Signal grass:
Brachiaria decumbens (also known as Urochloa decumbens) [Poaceae]
Perennial that has long stems (stolons) that can take root and produce new growth. The stolons can be up to 1.5 m long. Fringe of dense hairs at inner junction of leaf sheath and leaf. Produces 2 –3 flower spikes up to 8 cm long.
Purple pigeon grass:
Perennial that grows to 2 m. Produces underground shoots. The base of the inner junction of leaf sheath and leaf blade is thin, translucent and topped by hairs. Cylindrical, flower spike up to 18 cm long.
Management notes:
- Avoid grazing horses on pastures dominated by soluble oxalate grasses for periods longer than 1 month. This is particularly important when the grasses are rapidly growing.
- Use a mix of pasture grasses and legumes in horse paddocks. Some of the grasses NOT associated with ‘big head’ include: all native grasses, Rhodes grass, paspalums, couches, creeping blue grass, ryegrass and sorghums.
When there is no option but to graze horses on hazardous tropical grass pastures:
- Encourage the growth of the legume component of the pasture.
- Feed a calcium and phosphorus supplement. Seek veterinary advice to ensure that this is the right management technique for your horse. A supplement mix should be made available to each horse once a week for ad lib consumption, or the supplement could be split into portions and fed daily.
The supplement mix can consist of:
- 1 kg rock phosphate mixed with 1.5 kg molasses or,
- 1 kg mix consisting of 1/3 ground limestone and 2/3 dicalcium phosphate (DCP) mixed with 1.5 kg molasses.
- Use double these amounts under veterinary supervision as part of the management of horses showing signs of nutritional secondary hyperparathyroidism.
Information Sourced From:
