Kelp is a species of seaweed that is a nutritionally-dense source of protein and trace minerals. It contains the essential mineral iodine, which is important for thyroid function, growth and metabolism.
Horses with digestive health issues, low appetite, poor tolerance to cold weather or a dull, dry coat may benefit from kelp supplementation.
Kelp also contains prebiotics that promote a healthy gut microbiome. It is sometimes fed to horses with diarrhea and leaky gut.
This nutritional seaweed has also been shown to benefit horses with gastric ulcers by promoting the production of compounds that protect the stomach lining.
Kelp meal is safe for horses, but it should not be over-supplemented. Chronically high intake of iodine can affect thyroid function.
Kelp for Horses
Seaweed has been consumed by humans for thousands of years. There are over 1,700 different species of seaweed, divided into three major groups: green, red and brown seaweeds.
Kelp is a type of brown seaweed of the genus Laminaria. It is naturally found in cold ocean waters and can be grown in controlled environments. 
This is the type of seaweed most commonly fed to horses and other livestock. Kelp grows along coastlines and is naturally grazed by many herbivorous animals, including sheep and cattle in Scotland, Iceland, Norway, and Scandinavia.
Feral horses located near large bodies of water consume kelp, particularly during snowy weather when pasture is less accessible.
Kelp has also been fed to horses since the Roman period as a vitamin and mineral supplement. It is typically fed in a dried, powdered form to domesticated horses. 
Iodine levels vary by seaweed species, age at harvest, and the harvesting and drying method used.
In one study, iodine content was measured in seven samples of laminaria seaweed harvested in Maine, British Columbia and Washington. Levels of this trace mineral ranged between 796 to 2984 mg per kg. 
Because kelp is such a concentrated source of iodine, over-supplementation could result in iodine toxicity. Always follow manufacturer feeding guidelines closely.
If you are adding kelp to your horse’s diet, we recommend using commercial supplements that provide a standardized amount of iodine per serving. Reputable supplement manufacturers should perform quality control analysis to verify the levels of minerals in the product.
Kelp is also a source of other nutrients including essential minerals, amino acids and fatty acids. It contains two limiting amino acids – lysine and methionine – that are commonly lacking in the horse’s diet.
Below are the other macrominerals and trace minerals that are most abundant in this genus of seaweed: 
- Calcium: 0.9%
- Magnesium: 0.74%
- Phosphorus: 0.14%
- Iron: 120 ppm
- Manganese: 6.5 ppm
- Zinc: 22 ppm
- Copper: 1.7 ppm
Kelp meal is typically fed to horses in relatively low amounts and is unlikely to meet dietary requirements for the minerals listed above. It is recommended to feed kelp alongside a balanced equine vitamin and mineral supplement.
How to Feed Kelp to Horses
Horses are typically fed kelp at a rate of 5 – 20 grams per day, depending on iodine needs and the concentration of the product used.
Because both iodine deficiency and toxicity pose a health concern for horses, care should be taken to measure serving sizes carefully and to feed based on instructions from the manufacturer or your nutritionist.
A nutritionist can help you calculate how much iodine is in your horse’s diet and how that compares to individual requirements. Commercial feeds often include iodine in the form of calcium iodate or ethylenediamine dihydriodide (EDDI).
You can submit your horse’s diet for analysis to determine whether your horse is meeting their iodine requirement.
Generally, it is recommended to use kelp in the form of a finished supplement product rather than feeding kelp on its own. This will reduce the risk of over-supplementation and possible adverse effects.
Kelp Benefits for Gut Health
In addition to its use as a nutritional supplement, Kelp is also fed to horses to support gut health and the microbiome.
Research suggests there are benefits for the horse’s natural defense against ulcers as well as for hindgut function. Supporting gut microbial health can also have the following benefits: 
- Improved resistance to stressors such as transportation, toxins, and heat
- Enhanced immune function
- Improved muscle development
- Improved lactation performance
Evidence in humans and other mammals suggests that seaweed can help reduce the incidence and severity of gastric ulcers.
Studies on red seaweed in rats found gastroprotective benefits with potential anti-ulcer effects linked to reduced inflammation and reduced gastric secretions. 
Red seaweed also reduced gastric ulcer severity in horses. In one study, 10 horses were supplemented with red seaweed at 40 g per day for 30 days.
All horses exhibited a decrease in gastric ulcer severity at the end of 30 days.
Kelp stimulates the release of prostaglandins, which are fat-based compounds that are involved in the response to injury and illness. Prostaglandins help to protect the gastric mucosa, which is the the mucous barrier of the stomach.
- Stimulating production of mucous to provide a protective barrier in the stomach
- Increasing bicarbonate production to buffer stomach acids
- Supports growth of new intestinal tissue
In addition to increasing prostaglandin production, the high calcium content of kelp can temporarily buffer the stomach acid. 
Kelp is a great source of prebiotics, which are compounds that support the beneficial microbes in the gut. The prebiotic fucoidan found in kelp has anti-oxidant and anti-inflammatory benefits.
In animals and humans, fucoidan has been shown to interfere with the attachment of the pathogenic bacteria H. pylori to cells lining the stomach. 
Changes in gastric microbial populations are associated with incidence of gastric ulcers in horses. Protecting the stomach from damage by pathogenic microbes could help reduce the incidence and/or severity of gastric lesions. 
Mad Barn’s Visceral+ is a gut health supplement that contains 2 grams of kelp meal per serving. It supports gastric and hindgut health as well as immune function.
Kelp contains polysaccharides, which are large compounds made of many sugar molecules bound together. Polysaccharides have prebiotic effects that can support hindgut health and digestive function.
The most abundant prebiotics in brown kelp are: 
These are broken down by beneficial microbes in the gut, particularly those of the genus Bacteriodes. When these microbes break down the polysaccharides they generate beneficial products including short-chain fatty acids that are absorbed and used by the horse for energy.
The prebiotic benefits of kelp have been observed in several species, including ruminants and monogastric animals. Kelp supplementation decreases pathogenic microbes such as Clostridia, Salmonella and Campylobacter. 
By improving the microbial profile of the hindgut, kelp can help with dysbiosis a condition in which the hindgut is overpopulated with undesirable microbes that produce toxins and create inflammatory issues in the gut.
In horses, an imbalanced microbiome has been linked to:
Mad Barn’s Optimum Digestive Health contains kelp meal as a source of prebiotics. This product is designed to support hindgut health.
Pregnant and Lactating Mares
Iodine is important for fertility, gestation and growth because it is required to make thyroid hormones T3 and T4. Thyroid hormones affect metabolism in all cells of the body.
Iodine deficiency can lead to fertility issues in mares and result in stillbirths. Foals born to broodmares that had low iodine intake during pregnancy may display the following designs of deficiency:
- Goitre – an enlarged thyroid gland
- Poor bone development
- Contracted tendons
- Inability to stand at birth
The iodine content of gestation and lactation diets should be carefully assessed before adding kelp meal as a supplement.
Too Much Iodine
Feeding too much kelp can can cause health issues for the foal. Foals are extremely sensitive to iodine – this trace mineral crosses the placenta and can also pass into the milk of lactating mares.
In one report, mares fed 50 mg of iodine per day during pregnancy had foals with severe iodine toxicity. In foals, signs of toxicity include: 
- Leg weakness
- Poor skeletal development
- Poor muscle development
Consult with your veterinarian if you have an ill-thriving foal. A physical examination and blood test for thyroid hormones can be used to assess iodine status.
Kelp meal is used to add iodine to the equine diet and to support gastric and hindgut health.
It is important to carefully assess the iodine supply of your horse’s diet before supplementing with kelp meal as a standalone ingredient. Both iodine deficiency and toxicity can occur in horses and they result in similar symptoms.
To avoid concerns about feeding too much iodine, look for commercial supplements that contain kelp as an ingredient or consult with a nutritionist to determine the correct feeding rate for your horse.
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