Unfamiliar with the toxic effects of New Zealand’s plants, the early British settlers suffered major stock losses when their cattle, sheep and horses first browsed on tree tutu (Coriaria arborea) and ngaio (Myoporum laetum). Although both species were quickly identified as being poisonous to livestock, they continue to poison animals that stray into unfenced bush.
Pinātoro or Strathmore weed (Pimelea prostrata), a ground-hugging, open-country shrub, is poisonous to horses and cattle but seems to have little effect on sheep.
Bracken poisoning of cattle often occurred in North Island hill country between 1950 and 1970, when cattle were used to trample bracken fronds on land being developed for pasture. After a few weeks of feeding largely on bracken, cattle begin haemorrhaging. Horses experience muscle and nerve disorders if they graze on bracken for long periods.
Tutu and elephants
On at least three occasions circus elephants have been poisoned by tutu. In the late 19th century there was no cure, and a stricken animal died a few hours after feeding on young shoots. In the 1960s two poisoned elephants recovered after being injected with barbiturates. Veterinarian David Marshall recalled that three elephants, poisoned (but not fatally) while travelling through the tutu-infested Buller Gorge, produced spectacular waist-high piles of vomit.
British settlers brought many of their familiar plants, some of which are poisonous. The acorns of all oaks (Quercus) are poisonous, especially to cattle and sheep. Yew trees regularly kill browsing cattle and sheep. The toxins are soon absorbed and cause heart failure. Pregnant cows are likely to abort if they eat macrocarpa (Cupressus macrocarpa) leaves late in pregnancy.
Weeds also arrived with the new settlers. Ragwort (Senecio jacobaea) flourishes in pastures in high rainfall areas. It is highly toxic and causes liver damage in cattle and horses. High-country sheep are susceptible to poisoning from St John’s wort (Hypericum perforatum), a pretty, yellow-flowered weed. Sheep become sensitive to sunlight after eating the leaves: unpigmented parts of their body redden, swell and start oozing fluid.
Nitrates in plants
Nitrogen is an essential element for growth, but some plants growing in fertile soils accumulate so much nitrogen that animals grazing on them become poisoned. Green feed-crops such as turnips, and fast-growing pasture grasses are the main culprits. Cattle, sheep, horses and deer are at risk as they readily convert nitrate to nitrite, and in this form the nitrogen interferes with oxygen transport in the blood system. Affected animals may die a few hours after eating nitrogen-rich fodder.