Superphosphate poisoning of cattle has been reported, but in NZ it is more typically a condition affecting pregnant and lactating ewes when exposed to topdressed pasture. Goats during kidding have also been affected as have deer exposed to recently topdressed paddocks. It is now believed that the fluoride in superphosphate plays a dominant role in the poisoning process. The role of season and stress of lactation are also important predisposing factors.
Occasional outbreaks of superphosphate poisoning have been recorded when sheep gained access to fertilizer bins and ingested raw material.
Between 1963 and 1975 there were 37 recognised outbreaks of superphosphate poisoning, but this is probably only the tip of the iceberg and it is likely that over the years many stock have been accidentally poisoned from phosphate ingestion.
There is now evidence that suggests that fluoride plays an important role in superphosphate poisoning and is probably the major toxic component. Experimental work on sheep poisoned with sodium fluoride (NaF) when compared to sheep poisoned with superphosphate containing 1.5% fluoride, showed that although there were differences in the clinical and pathological manifestations of the two forms of poisoning, an equivalent toxic dose of NaF and of fluoride in a toxic dose of superphosphate produced similar lesions in the same target organs. It has been concluded that phosphate probably plays a contributory role in the poisoning but fluoride plays the dominant role, and there may be an interaction between the two, the nature of which has yet to be established. Superphosphate is approximately six times more toxic to ewes than serpentine superphosphate.
Phosphate rock materials vary in their fluoride content. Christmas Island and Nauru rocks, which have been used in New Zealand contain about 2.2% and 2.7% of fluoride respectively. Some fluoride is lost during the manufacturing process and superphosphate made from roughly equal proportions of the above phosphates contains between 1% and 1.5% fluoride.
Reactive phosphate rock materials, which can be used in direct application fertilizers on pasture, contain variable but higher amounts of fluoride than superphosphate. Phosphate rock from North Carolina commonly contains about 3.5% fluoride, where as Sechura phosphate rock from Peru contains 2.5% to just over 3%. Because these materials are not processed, the amounts of fluoride added to pasture from a reactive phosphate rock could potentially be greater. However to offset this risk the application rates of raw rock are usually lower than superphosphate.
Another complicating factor is the form of occurrence of fluoride in superphosphate. This varies with the proportions of Nauru and Christmas Island phosphate rock used in the manufacturing process and it is very different from that in reactive phosphate rocks.
The phosphate in reactive rock has very low water solubility in contrast to superphosphate which suggests that much less soluble phosphate would be taken in by the ingestion of reactive rock phosphate than of superphosphate. However rock phosphate particles could dissolve in the more acidic parts of the animals digestive system thus offsetting its lower solubility. Hence the role of both types of phosphate is still important in phosphate poisoning.
Dry stock seem to be less at risk than pregnant and lactating animals (ewes). Most episodes of poisoning have been in the spring period and when rain has not fallen following topdressing (20mm to 30mm of rain is usually sufficient to reduce leaf and topsoil contamination). Short pastures and high stocking rates have been significant factors in toxicosis.
Clinical signs usually appear about five days after exposure. When sheep have access to superphosphate bins clinical signs and death may be more sudden.
Sheep show a severe yellow brown diarrhoea, anorexia, marked thirst, weakness and staggering gait. The gait and recumbency with nasal discharge, are similar to the clinical picture seen in hypocalcaemia of the ewe.
The gross lesions seen include abomasal ulceration, enteritis, pale blotchy liver and renal cortical swelling and pallor. Histologically there is evidence of renal tubular obstruction and nephritis.
The main biochemical changes recorded are hyperphosphataemia, oliguria, uraemia and azotaemia. The analytical confirmation of superphosphate poisoning is based on the analysis of serum, or urine, or rumen contents for fluorine.
The animals develop a severe terminal uraemia and death usually occurs in 48 hours from the onset of symptoms. Mortality rates of up to 10% have been recorded.
No treatment has proved successful. Calcium borogluconate has been suggested because of the similarity of the clinical condition to hypocalcaemia in the ewe.
It has been suggested that topdressing only part of a property at one time, particularly if feed is short could be a useful precautionary measure. This would seldom however fit in with most topdressing operations. Withholding stock from recently topdressed areas until after rain has fallen and avoiding too close a grazing with pregnant and lactating animals are all fairly obvious precautionary actions.
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O'Hara, P.J. and Cordes, D.O. (1982). Superphosphate poisoning of sheep: a study of natural outbreaks. N Z vet J. 30:153 155.
O'Hara, P.J., Fraser, A.J. and James, M.P. (1982). Superphosphate poisoning in sheep: the role of fluoride. N Z vet J. 30:199 201.
O'Hara, P.J., McCausland, I.P. and Coup, M.R. (1982). Phosphatic fertilizer poisoning of sheep: experimental studies. N Z vet J. 30:165 169.
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Swan, J.B. and McIntosh, I.G. (1952). The Toxicity of North African phosphate and superphosphate to milking cows. Proc N Z Soc Anim Prod. 12:83 85.
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Surveillance (1974) 1: 25 26 Superphosphate poisoning in cattle.
Surveillance (1975) 4: 11 Superphosphate Toxicity (sheep)
Surveillance (1979) 5: 15 Superphosphate poisoning (cattle).
Surveillance (1981) 3: 22 Superphosphate poisoning (sheep).
Surveillance (1981) 3: 24 Suspected superphosphate poisoning of sheep.
Surveillance (1984) 1: 41 Superphosphate poisoning in red deer.
Surveillance (1984) 3: 17 Does superphosphate ingestion predispose to polioencephalomalacia?
Surveillance (1986) 2: 27 Superphosphate poisoning.
Surveillance (1998) 25(1):13 Superphosphate poisoning of sheep.