Page 102 - July 2019

“Horses typically avoid poisonous plants, but hunger, curiosity, or boredom may lead them to eat things they shouldn’t.”
by Heather Smith Thomas
Most poisons that affect horses are found in toxic plants that contain dangerous alkaloids. Some of these are so deadly that they can kill a horse
quickly even if a very small amount is ingested, whereas others are lethal only after many small doses accumulate to dangerous levels in the body.
Most poisonous plants, trees, shrubs, and grasses vary in level of toxicity and the conditions in which poisoning occurs, but others are extremely poison- ous at all times. Some are harmless in small amounts or during certain seasons, but become dangerous
if eaten in large amounts or if they are chemically altered by freezing or enzyme activity such as when cyanogenic glycosides in chokecherry leaves turn into hydrogen cyanide. Certain plants absorb sub- stances from the soil that accumulate to toxic levels, such as selenium does in locoweed.
Horses typically avoid poisonous plants, but hunger, curiosity, or boredom may lead them to
eat things they shouldn’t. They may nibble shrubs through the paddock fence or eat plants along the trail when you ride if you aren’t paying attention. Horses may also eat harmful plants inadvertently baled in hay or raked into lawn or hedge clippings. Some of the plants and shrubs used for landscaping in the yard, garden, barnyard or fairgrounds, or along a fence row, road, or trail may be poisonous. Most poisonous plants that might be eaten by horses are “forbs” — broad-leafed flowering plants. Only a few grasses are dangerous to horses.
If you suspect a horse may have eaten something toxic or is showing behavioral changes or digestive upset, get veterinary help immediately. Some types of
poisoning are irreversible and fatal, while others can be resolved with prompt emergency treatment.
Plant poisoning should be suspected if horses experience sudden illness or death after being moved to a new pasture or fed different hay. Poisonous weeds in hay can be a serious threat because hay-fed horses typically have no other food options, unlike
a horse at pasture. Some horses sort out strange weeds in hay, but greedy eaters may eat them despite their taste or smell. Even a not-so-hungry horse may eat harmful hay because the strange and unpleas- ant taste or smell may be diminished in the drying process before baling.
There are hundreds of plants that can be toxic to horses, but some only grow in certain regions of the country. Becoming familiar with those that might be a threat to horses in your own geographic region and climate is important. Some grow wild and might be in fields or pastures, while others might be found in your own yard or garden.
Garden flowers and plants poisonous to horses include, but are not limited to, buttercup, narcissus (daffodil), lily of the valley, and delphinium. Wild delphinium is called larkspur. Tall larkspur is highly poisonous to cattle, but not as deadly to horses. However, if a horse exerts after eating delphinium, he may die from impaired muscle activity and impaired breathing. Rhubarb leaves contain soluble oxalates that crystallize in the kidneys causing kid- ney failure and death. Don’t let a horse eat in your yard or garden and never let them eat any plant that is not a part of his regular diet, especially if you are not familiar with it.
PLANTS THAT CAN BE TOXIC TO HORSES
Heather Smith Thomas
100 SPEEDHORSE, July 2019
EQUINE HEALTH