Pasture management is an important part of looking after your horse.  You should regularly check your horse's grazing for poisonous plants.  Many plants that are poisonous to horses cause neurological and liver damage which can be life-threatening.

Unless you have a good general knowledge of poisonous plants it is unlikely that you will be able to easily identify which plants are poisonous to your horse or not.  There are some plants that you will be familiar with, eg ragwort, but others that you may not be familiar with, eg charlock.  It is useful to familiarise yourself with the most common plants poisonous to horses so that if you see some in your horse's field you will know that it needs removing.