Acorns contain pyrogallic acids, up to 8%, although the amounts vary according to the age of the acorn. Unripe, green acorns contain the highest levels of pyrogallic acids, and certain species of oak are more toxic than others. Youngest oak trees having the greatest levels of pyrogallic acids and tannins.

Horses seem to become addicted to the acorns, seeking them out over other forage and feed.

Toxic signs become apparent after horses have been eating acorns for between one and four weeks. Acorn poisoning is usually fatal in horses.

Management: fence off the area the acrons are falling into; rake up the acorns; move fields.

Pigs are resistant to pyrogallic and tannic acids. In an ancient right of common, called 'Panage', in the New Forest and the Forest of Dean pigs are used to clear up the acorns.