Plants containing more than 2% soluble oxalate (DM) have the potential to cause acute oxalate poisoning in ruminants, however poisoning is normally associated with much higher concentrations (e.g. >10%).
The effect on stock depends on ‘pre-conditioning’, i.e. whether stock have had previous exposure to plants containing oxalate, and whether the stock are hungry or not when given access to the plant.
The soluble oxalate is absorbed and combines readily with calcium to form insoluble calcium oxalate crystals within the body. Death is usually due to hypocalcaemia but can also result from damage caused by the calcium oxalate crystals in the kidneys, rumen wall and lungs.
Clinical signs include difficulty in breathing, staggering, collapse and a quiet death. Perennial pastures and shrubs reported to occasionally cause acute oxalate poisoning include saltbush, small leaf bluebush, setaria, buffel grass and panic grasses.
Ensuring these species are not the sole feed source and that pasture with a low oxalate content is also available to stock will minimise the risk.
Big head in horses:
If most of the calcium in plants is in the form of insoluble calcium oxalate, then horses grazing such plants will be at risk of developing calcium deficiency. This does not occur with ruminants as the rumen bacteria breakdown the calcium oxalate, releasing the calcium for absorption.
As a result, pasture that may be excellent for ruminants can be dangerous for horses. Plants that have a calcium to total oxalate ratio less than 0.5 and more than 0.5% total oxalate content (DM) are considered hazardous.
Clinical signs include weight loss, lameness, fracturing of long bones and swelling of the bones of the face and jaw; hence the term ‘big head’. Perennial pastures and shrubs reported to cause big head in horses include: buffel grass, setaria, panic grass, kikuyu, signal grass, small leaf bluebush and saltbush.
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