In some areas this plant is called pigweed or lamb's quarters. The plant I've photographed here is what we call lamb's quarters. It has smooth, light colored leaves and a woody red stem. The 'flower' looks rather like small pale green cauliflower cluster.
A horse would have to eat a large quantity of lamb's quarters for the toxin to take effect. Unless there is no other feed available it is unlikely a horse will eat this plant.
If a horse consumes a large quantity of lamb's quarters symptoms may include:
- weakness
- lack coordination
- respiratory distress
- coma
- kidney failure
Resources: Indiana Plants Poisonous to Livestock and Pets Cooperative Extension Service, Purdue University
If you suspect poisoning from any plant or substance call your veterinarian immediately.