Plants for discussion in the Summer Horse Nutrition Course
Mature blueweed (Echium vulgare) which was the causative plant for acute hepatitis in a group of horses in Ontario.
Horse owners and veterinarians need to differentiate between Echium sp. and other causes of hepatitis including plants which cause pyrrolizidine alkaloid toxicity.
Photos 2 a
Immature giant hogweed (Heracleum mantegazzianum).
Exposure to the sap can result in severe burns and blistering to the affected areas, It is found in Wellington County, Ontario, Canada. It is related to Queen Anne's Lace (wild carrot) and Angelica
Photo 3 Horsetail (Equisetium arvense ) Both the sterile (green pine tree-like) form and the brownish fertile form of the plant are seen in the photo. All parts, green and dried, can be toxic. If consumed in large quantities e.g., 2 kg per day for a 454 kg horse, for 1-2 weeks it can have detrimental effects on horses. Horsertail contains thiaminase an enzyme that inhibits the production of vitamin B1. Vitamin B1 is responsible for extracting energy from carbohydrates, fats and proteins. Clinical signs of Equisetium poisoning are seen primarily in young, rapidly-growing horses when large amounts are incorporated into hay.
Photo 4 Leaves from 3 commonly grown maple trees. From left to right, red maple (acer rubrum), sugar maple (acer saccharum) and silver maple (acer saccharinum). The ingestion of red maple leaves are known to be toxic.
The red maple tree is a tall hardwood with green leaves that have three large "fingers" or points and five prominent veins in the leaves. The edges of the leaves are serrated. The leaves turn a brilliant crimson and sometimes yellow in the fall. The toxin is gallic acid.
Sugar maple, silver maple and red maple are capable of hybridizing. Only red maple is known to be toxic. However the hybridization issue makes it difficult to answer definitively whether the other species are toxic or not.
Photo 5 a
Bracken fern is a commonly seen plant in wooded areas and along the side of fields. As with Equisetium, bracken containsthiaminase an enzyme that inhibits the production of vitamin B1. Poisoning is primarily seen in young, rapidly-growing horses when large amounts are incorporated into hay and fed for extended periods.
Photo 5 b
Bracken fern naturalizing into a horse pasture. It is late in the fall and the years growth has been killed by frost.
Tansy ragwort which causes liver damage associated with pyrrolizadine alkaloid toxicity.
Climbing nightshade or bittersweet (solanum dulcamara) Stems and leaves are poisonous.
Clovers and Related Problems
Red clover left and alsike clover on the right
Fungal growth on the underside of a clover leaf - Sooty blotch (Cymodothea trifolii) causative agent of alsike clover poisoning
Photosensitization in a horse. Can be caused by a number of plants
Slobbers in a horse