When horses are grazing, there is always a significant risk of poisoning by plants.  When there is good pasture management the risk is minimised, but even so there is always the possibility that plants will go undetected.  This is especially so when fields are surrounded by large hedgerows which are rarely inspected.  Overstocking poses a significant risk when there is a bareness of grass and animals consume plants that would normally be avoided. 

During the Autumn, acorns can pose a problem to the horse and a worry to the owner.  Consumption of acorns, especially when they are unripe, can be poisonous due to the high levels of tannin.  The signs are colic, constipation, weakness and passing blood in the urine.  Treatment is difficult.  Prevention should be centred on fencing off areas of oak trees where there are abundant falls of acorns, or manually removing them.  Obviously, this can be an almost impossible task, so the most important factor is to ensure that the animals have plenty of grazing or plenty supplies of hay. 

Within the New Forest, pigs are allowed to forage during the pannage season specifically to mop up the falls of acorns.