Why is it that plants with a ‘poisonous to horses label’ are frequently sought out and eaten with some relish by horses. Don’t they know how that makes us feel!!
One answer is that some of the stronger and more toxic plants contain large molecule compounds with poor bioavailability. This means that the plant compounds remain in the stomach and do not and cannot cross the gut wall/membrane, which is useful because if they did liver/kidney damage would quickly follow.
One very useful plant chemical is tannin which is found in large quantities in the bark of trees and the woody stems of many herby shrubs. Trees with extremely high levels of tannin are oak (including the acorns), aspen and some willows.
The poor bioavailability of tannin in horses means that if ingested the effects will only be local ie along the gastrointestinal tract. As is the norm for plants, if eaten whole, they contain chemicals with many different effects and tannin is no exception, in the mouth whilst chewing, the action of tannin is as an astringent, tannins cause inflamed tissues to shrink and stop any bleeding.
In the stomach a tannin will act as an antacid, the cost of a quality antacid supplement is around £1.50 per daily recommended 64 g daily dose whilst the foraging horse requires only a daily dose of 75mg per day of a high tannin containing plant to achieve a similar effect.
The same low dose is also an anti -microbial and anti- inflammatory, relieving the pain from ulcers and whilst ridding the gut of it’s bad bacteria. Tannins are haemostatic meaning they will stop the bleeding of severe ulcers both in the stomach and hind gut.
As the tannins continue through the digestive tract they decompose and become other chemicals, their action is then as anti- oxidants which act to prevent diarrhoea and spasms.
The Irish Equine centre have identified an emerging problem of ergot alkaloid intoxication for those horses on a high perennial rye diet, symptoms include weight loss, poor condition and loss of performance. Tannins have the effect of binding to the alkaloids enabling rapid excretion from the body.
Horses ingesting a diet with a high ergot alkaloid or other mycotoxin content are likely to seek out and eat ivy together with oak/willow/aspen