Deadly nightshade or belladonna as it is sometimes called is a toxic plant that grows easily in many areas of north America. This relative of potatoes and tomatoes acts through solanine, saponins or atropine like chemicals. Leaves and stems are toxic with the green berries also being poisonous.
Signs can range from dilated pupils to erratic changes in heart rate and rhythm to death. Horses may show neurologic effects including excitability, incoordination and seizures. Definitely not a plant you want your horses eating!
Nightshade does not appeal to most horses and in fact is often found as an incidental when people walk their pastures. Given options, horses seem to naturally avoid eating nightshade. Unfortunately nightshade can be incorporated into hay if it has infiltrated hayfields. Also, if your pastures are overgrazed or dry up in drought years, even nightshade may look palatable to a hungry horse.
This plant has white or lavender flowers with berries that range from green to yellow/orange to black. (Common nightshade is another species and not as toxic but still not a desirable diet for a horse). It will grow alone or more commonly as a vine twining in and around your fences – this makes it harder to pull up and clear.
If you suspect your horse has eaten nightshade you need to contact your veterinarian as soon as possible. Early and mild cases can be treated successfully using drugs to reverse the atropine type symptoms.
It makes sense to walk your fields and turn out areas monthly looking for poisonous plants. Nightshade can be pulled up, cut back or dug up to remove it. If you cut it back, you can treat the root areas with herbicides or salt. Make sure the pulled up plants are removed to an area safely away from any livestock (or pets).