After a winter of dry hay, last years grass from the horse's point of view, anything green can be very tempting for a horse. During certain seasons or under certain pasture conditions some plants can be more dangerous to horses than at other time. A time of high risk can be the early spring. Many toxic weeds sprout out before the grasses do. During a dry period without rainfall, late fall as the grass is dying off, even an overgrazed pasture can cause your pasture conditions to be at high risk for poisonous plants.

Allowing these conditions in a pasture can give weeds a chance to grow and flourish. Horses may eat them under such conditions when normally they would leave them alone. Ryegrass that is intended for lawns may carry fungi (endophytes) that grow inside the plants. Tall fescue can be harmful to pregnant mares in their last trimester of pregnancy because it can harbor an endophyte. Toxins found in the leaves from the red maple tree that are normally at low levels after wilting become very strong making them a danger to horses. In the spring oak buds and young leaves can be toxic for horses. We often think our horses know instinctively to stay away from poisonous plants. NOT TRUE. If there is not plenty of good forage available, if a horse is bored, hungry, or thirsty they may be tempted to nibble on a poisonous plant.

Many poisonous plants taste bad to horses and they will avoid them, but not all do. It can be difficult to diagnose plant poisoning in your horse. The signs can mimic many other problems and they can vary. Symptoms may appear in hours of contact with the plant, or days, or even weeks after contact with the offending plant. If the poisoning is gradual the signs may be subtle such as weight loss or intolerance to exercise. Some symptoms that can be indications of plant poisoning in horses are: gastrointestinal upset (diarrhea or colic),

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