Oak and acorns contain tannins which when ingested produce toxins which can be poisonous to horses. The risk is highest in the UK at the end of long dry summers, even so most horses are sensible enough not to eat large numbers. It seems that certain individual horses are more susceptible to acorn poisoning than others, some horses can tolerate small numbers of acorns but the toxicity of the acorns can vary from year to year. Acorns and oak contain tannic acid and other tannins which are toxic when consumed in sufficient quantities. These poisons can cause damage to the gut leading to problems such as diarrhea and colic signs, they can also cause damage to the liver and kidneys. There is no specific antedote and the prognosis is poor if kidney damage develops. In rare cases acorn poisoning can be fatal.

Prevention:

It is sensible to prevent access to large numbers of acorns during the autumn, particularly after strong winds when large numbers may have fallen off the trees. Acorns can be swept up or electric fencing used to keep horses away from areas around oak trees with large numbers of acorns. Some people control the acorns by using a roller to push them into the ground, but it is difficult to do practically on a regular basis. Alternatively a more novel approach is to allow pigs to graze the area since they can safely eat the acorns; this has the added advantage of training your horses to accept pigs!