Bracken fern poisoning in horses

Bracken fern poisoning usually occurs in the fall when this plant is one of the few plants that is still green.

Photo by Tom Guthrie
Photo by Tom Guthrie

It is understood that well-fed horses are rarely at risk from consuming undesirable plants that may create health issues. However, as the grazing season progresses, desirable forage may become less available which increases the risk of a horse eating something they would otherwise avoid such as Bracken fern.

Bracken fern contains the enzyme thiaminase, which inactivates thiamine (vitamin B1). Inactivation of thiamine results in a deficiency that may cause neural dysfunctions in the horse. Horses will typically avoid eating Bracken fern since it is fairly unpalatable. However, it is possible that some horses may develop a taste for the young tender shoots and leaves.

Bracken fern poisoning is most commonly chronic in nature, having a cumulative effect on the horse which requires repeated exposure over time. Large amounts of this plant must be eaten for 30 to 60 days before a horse shows any signs of poisoning. Additionally, signs can appear two to six weeks after the horse has been moved from the pasture containing the fern. Furthermore, be aware that bracken fern found in hay can also be toxic.

Signs of toxicity include:

  • Weight loss

  • Progressive incoordination

  • Horse stands with legs wide apart and its back arched

  • Muscle tremors

Prevention and control measures

Bracken fern typically grows in shady moist areas and can be found in forests, wetlands and fields throughout Michigan. Bracken fern is a perennial plant that spreads by rhizomes and can be very difficult to control. Control measures may need to be repeated to achieve adequate success.

Options for control may include:

  • Regular cutting and mowing may eventually aid in control of bracken fern by weakening the plant.

  • Utilizing herbicides - If choosing to use an herbicide, always read the label to identify if there are any grazing restrictions associated with the product. However, reports show that using herbicides to control bracken fern in pastures has proven to be relatively ineffective.

  • Disking or plowing – best to do this in the summer to break up the rhizomes and bring them to the surface where they dry up and die.

  • Implementing an alternative grazing plan - For example, horses may be allowed to graze a respective pasture containing bracken fern for a period such as three weeks and then removed from that respective pasture for three weeks. The concept behind this alternative grazing plan is that the shorter time period of exposure, if bracken fern is consumed, doesn’t allow enough time for toxins from the plant to accumulate in the body.

Michigan State University Extension recommends walking your horse pasture(s) periodically throughout the grazing season. This creates the opportunity not only to identify potentially harmful plants but also allows you to observe your horse in an environment in which they spend a significant amount of time.

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