The harsh reality: there are probably several toxic plants in the same environment as your horse. In North America alone, thousands of poisonous plants are more common than you would ever think. So, how does my horse know to avoid these plants?
The good news is that horses are more likely to prefer quality forage over weeds and other plants. Horses are also large animals, and it would take a significant volume of poisonous plants to disrupt their well-being. However, this does not mean toxic plants should be ignored.
It is always a great idea to walk your horse's paddock a few times a year to not only look for poisonous plants but anything that can harm your horse. Don’t know what type of plants to look for? No need to worry. We put together a list of the most common poisonous plants for horses so you can remove these plants from your pasture and know the signs of toxicity in case of an emergency.
Also referred to as the “crazy weed, Locoweed is a leafy perennial with a short stem and most commonly accompanied by white or purple flowers found in the West and Southwest regions of the United States. While there are a few different Locoweed species, all contain an alkaloid called swainsonine. Swainsonine is known to interrupt the production enzymes necessary for saccharide metabolism. The resulting buildup of sugar in the horse’s body significantly impacts the horse’s neurological system. 
The most common clinical signs of a horse intoxicated from Locoweed include strange behavior such as exaggerated gaits, excessive head bobbing, or any erratic movement. Unfortunately, there is no current treatment for horses suffering from locoism, and the neurotoxic effects of swainsonine make affected horses dangerous to handle.
Red Maple Leaves
Red Maple Trees are best known for carrying green leaves in the spring and summer and bright red leaves in the fall. These medium-sized trees can be found all around the country but are most commonly spread out between Canada to Florida and from Minnesota to Texas. When red maple tree leaves begin to wither, they become toxic to horses and promote the breakdown of red blood cells, leading to organ damage. 
Excess consumption of red maple leaves can be fatal to horses. Symptoms of toxicity include extreme fatigue, dehydration, discolored urine, increased heart and respiratory rate, and lack of hunger. Currently, there is only one treatment for red maple leaf poisoning, intravenous fluids. In some circumstances, a veterinarian may even conduct a blood transfusion.
Yew is an evergreen shrub with clustered, thin leaves and red or yellow berries. The yew plant can be found in the Eastern and Central United States and on the West Coast. Every part of the yew plant contains an alkaloid named taxine, except for the skin of the berries.
The toxin taxine is known to cause serious respiratory issues and cardiac collapse in horses. Therefore, the yew plant can be fatal to horses. Signs of toxicity include breathing issues, colic, and anxious behavior.  Unfortunately, there is no treatment for yew plant poisoning, so prevention is vital.
Also referred to as spotted water hemlock, this perennial weed with hairless stems and leaves with small white flowers grows throughout the United States, especially in wet, marshy lands. Considered one of the most toxic plants for horses in the United States, the plant contains a cicutoxin alkaloid that affects neurons in the brain. 
Water hemlock poisoning can cause difficulty breathing, extreme salivation, seizures, dilation of the pupils, and ultimately death, usually occurring within two to three hours of ingestion. While little treatment is available, if you catch the signs before it progresses, supportive care can be of great help. However, horses are often left with skeletal muscle and heart damage that may be permanent.
Oleander, an evergreen shrub with thick, dense leaves and flowers that can be white, pink, or red. Oleander can be found in usually only hotter climates, such as in the southern United States region.
The plant contains a toxin in its leaves known as oleandrin. These toxin alters the beating of the horse's heart and can result in difficulty breathing, irregular heart rate, and colic.  However, if caught early, horses can survive with supportive care. Some of these treatments include the administration of activated charcoal, which disrupts the absorption of the toxin, and anti-arrhythmic drugs.