Ashfalls may be poisonous to livestock and result in clinical diseases, including hypocalcaemia, fluorosis, forestomach and intestinal damage, and secondary metabolic disorders.
Fluorine aerosols in the eruption column and cloud that become attached to fine ash particles pose a potentially significant threat to livestock. As smaller ash particles have large surface areas relative to their mass, the fine particles can transport significant amounts of soluble fluorine onto pastures far downwind from an erupting volcano. The smallest ash particles travel the greatest distance from a volcano; thus a thin layer of fine ash only 1 mm thick can contain potentially toxic amounts of fluorine. Livestock ingest fluorine directly as ash is consumed along with pasture feed and soil.
Fluorine poisoning has occurred in several Icelandic eruptions, and was partially attributed to the deaths of over 2,000 grazing animals following the 11 October 1995 eruption of Ruapehu, New Zealand (Cronin et al., 2003). An immediate toxic dietary intake of Fluorine is >100 µg/g for grazing animals, but a lower concentration may cause sickness. Cattle can tolerate around 40 µg/g, and sheep up to 60 µg/g. Under normal winter conditions in New Zealand sheep ingest 260-275 g of soil per day, and sheep foraging for feed covered with ash are likely to take in even more (Cronin et al., 2003).
Chronic fluorosis may occur when livestock are exposed to elevated levels of fluorine over the medium to long term (months to years) causing a range of symptoms which impact on livestock welfare and performance. Poisoning may cause lesions in the nose and mouth, and hair to fall out around the mouth. Other symptoms include nutritional and stress related diseases, convulsive seizures, pulmonary odema, and kidney and liver changes. A tooth condition known as 'spiking' may also occur, causing outgrowths to develop on molars and making chewing difficult.
When toxic levels of fluorine on pastures are identified, it is recommended that livestock be removed from the affected areas until sufficient rainfall has leached the fluorine from the ash and/or use supplementary feed and clean water supplies..
A high sulfur concentration adhered to the ash may induce copper and cobalt deficiencies in the long term.