Bracken (Pteridium aquilinum); the effect of steaming on the nutritive value of bracken hay.

The bracken fern (Pteridium aquilinum) has been known for a long time to be poisonous to cattle (Penberthy, 1893) and horses (Muller, 1897) under farm conditions. Field cases of bracken poisoning in sheep are almost unknown; the more selective feeding habits of sheep may be responsible for their apparent immunity. Bracken poisoning has been experimentally produced in cattle (Almond, I 894), horses (Hadwen & Bruce, 1933), sheep (Moon & Raafat, 1951) and in rats (Weswig, Freed & Haag, 1946). The signs of bracken poisoning in simple-stomached animals, such as the horse and rat, are fundamentally different from those observed in ruminants. In the horse (Evans, Evans & Roberts, 1951) and the rat (Weswig et al. 1946; Evans & Evans, 1949) the signs are typical of thiamine deficiency and timely therapy with the vitamin produces complete recovery. It is now known that the bracken fern, in common with other pteridophytes, contains a thiaminase which converts thiamine into products which cannot replace the intact vitamin in mammalian nutrition (Evans, Jones & Evans, 1950). Bracken thiaminase is thermolabile (Evans & Jones, 1952) and steaming bracken for I h renders it harmless to rats (Evans et al. 1950) although bracken dried in a revolving-drum grass drier still contains thiaminase actively toxic to rats (Evans, Evans, Thomas, Watkins & Chamberlain, 1958). Bracken poisoning of cattle is characterized by haemorrhage and pyrexia with a severe leucopenia and thrombocytopenia due to depression of bone-marrow activity. The tissues of the affected animals are not deficient in thiamine. Thiamine therapy or even treatment with all known members of the vitamin-B group failed to cure the condition (Naftalin & Cushnie, 1954; Evans, Evans & Hughes, 1954a,b; Evans, Thomas, Evans & Edwards, 1958). The factor responsible for ruminant bracken poisoning has been proved to be different from thiaminase (Evans, Evans et al. 1958). Neither the steaming of fresh green bracken (Naftalin & Cushnie, 1954) nor the drying of fresh green bracken in a revolving-drum grass dryer (Evans, Evans et al. 1958) destroyed its toxicity to cattle. However, the steaming of bracken hay has been shown to lower its toxicity below a threshold level (Evans et al. 1954a, b) .