Ragwort:

Can cause cirrhosis of the liver, and is particularly dangerous to horses, pigs and cattle.

Yew: Toxic leaves can cause convulsions, breathing issues and heart failure, especially in horses, cattle, and pigs.

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Foxgloves (digitalis): Overdose causes nausea, vomiting and diarrhoea, and in extreme cases death. Toxic to poultry, cattle, cats and dogs.

Sycamore seeds: Cause 'equine atypical myopathy', which weakens horse muscles so they struggle to breathe and then die from cardiac arrest.

Hemlock water dropwort (poison parsnip): Possibly the deadliest indigenous plant in Britain. Used in ancient Europe to execute people.