Animals have been known to die from grazing pastures recently top-dressed with superphosphate. The cause of death is in fact fluoride (not phosphate) toxicity as commonly thought.
Fluoride is found in all naturally occurring phosphate deposits, so ends up in all P fertilisers, including superphosphate (single, triple and dicalcic), DAP, MAP and RPR, but not products like lime, urea or potash. Sheep (especially pregnant and lactating ewes) are generally more prone to poisoning than cattle. Risky situations include:
• When fertiliser is applied to paddocks with stock on them.
• When stock are moved onto paddocks too soon after topdressing.
• When animals gain direct access to fertiliser and eat it.
• When stock feed (usually dairy cows) is contaminated with phosphate fertiliser; either accidentally, or when fertiliser products are deliberately added to boost dietary phosphorous and/or nitrogen levels.
The risk of poisoning from top-dressing is greater at higher rates of application and for dusty fertiliser that will more readily adhere to foliage (if applied when foliage is damp from frost or dew). Often the first sign of trouble is finding some animals dead. Others may be seen with vague metabolic signs suggestive of sleepy sickness (ewes), or milk fever (cows and ewes), but are unresponsive to treatment. Other symptoms can include depression, salivation, inflammation of the gut (bloating, gut pain, constipation or diarrhoea), nervous symptoms and damage to the lungs, liver and kidneys. Diagnosis can be made based on history, clinical symptoms, post-mortem results (kidney damage) and laboratory tests (blood, urine, liver, kidney and rumen contents).
To minimise the risk of fluorosis, stock should not be allowed to graze pasture top-dressed with phosphate fertiliser for at least 21 days or until at least 25 mm of rain has fallen. Note this will minimise, but not necessarily eliminate, the risk of poisoning. Where it is not possible to avoid grazing top-dressed pasture, applying well-granulated fertiliser to dry pasture is probably better than applying dusty product, especially to dewy or wet pasture. Avoid overgrazing this pasture to minimise direct uptake of fertiliser granules from the soil.
with Ravensdown Vet
Gavin Goble BVSc MRCVS