Horses do not selectively graze because they need more Selenium or Vitamin B complex, for example: they just eat when they are hungry, and drink when they are thirsty.
Horses do prefer sweet and salty tastes but avoid foods that are bitter or sour. This is just as well, as many poisonous plants are bitter and a horse’s keen sense of smell can detect the chemical markers and remember the plant’s taste (see “Feed & Pasture").
If horses are out in a large pasture with a variety of grass species, they will selectively graze based on taste and texture, preferring young rather than mature plants and avoid most toxic plants.
Unfortunately, when a horse is confined in a paddock that offers little to eat except for poisonous weeds, its hunger will win and it will eat anything. This is also a problem when you let horses graze on pastures that have been sprayed with an herbicide: poisonous plants can become sweeter as they die and therefore become more palatable to a horse.
Once you understand that a horse has limited ‘nutritional wisdom’, you will realise that it is up to you to ensure that paddocks are free of dangerous weeds and that a good, nutritious pasture is provided.