Avocados, Oleander, lawn clippings & inappropriate amounts of sugar, pellets, grain and grass are just some of the things that can be poisonous to horses.
There are many garden plants that can be toxic to horses and other species. Below are just a few examples. Beware of prunings and waste, especially when they are the only green thing in the paddock.
Avocado plants and fruit contain a toxin that can cause severe respiratory difficulty and death by exposure to only very small amounts of the plant. Avoid access to the plant, its cuttings and prunings, and its fruit.
Oleander is a common garden ornamental plant in which ALL parts of the plant are HIGHLY toxic to most species, including horses. They should not be given access to either the growing plant or to cuttings or waste from pruning, as there is evidence that illness and death can result from ingestion of only a few leaves. This may be more important when there is little green feed at the end of the dry period.
Horses love sweet things and a sugar cube can get you a long way, but too much carbohydrate is a significant cause of founder (laminitis). Breaking into the feed shed and eating a heap of grain or pellets can cause acute founder which can be life-threatening within 48hrs, as can fresh green grass in large amounts. Ingestion of large amounts of grain or pellets can also cause acid-related damage to the gut itself due to fermentation, and resulting in a colic which is often fatal.
Grass clippings (from mowing) should NOT be given to horses, as the small particle size can cause choke or obstruction, and can also cause fermentation and acid production which can lead to colic, toxaemia, laminitis (founder) and death.
Here's a great article by the RIRDC on poisonous substances for horses, and it's a very extensive piece. Access it here.
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