Milk thistle extract (Silybum marianum), also known as St. Mary’s thistle, is an herbal supplement primarily used to support liver function in horses.
Milk thistle seeds and leaves contain many bioactive compounds, collectively referred to as silymarin. Silymarin are flavonolignans with antioxidant, antiinflammatory and immunomodulating properties.
It can be used to provide nutritional support to horses prone to inflammatory issues like laminitis and allergies. The antioxidant properties of milk thistle can help horses stay healthy and recover faster from exercise and illness.
The liver is a crucial organ involved in detoxification and metabolism of carbohydrates, fats, and proteins. It breaks down toxins in the body for elimination and is therefore susceptible to injury or damage.
Older horses are likely to benefit from nutritional support of liver function because they have a longer period of lifetime exposure to toxins that could affect liver health.
Horses that tend to be overweight or those with equine metabolic syndrome (EMS) might have excess fat deposits in the liver which can negatively affect its function. These horses might benefit from milk thistle supplementation to promote normal detoxification function and metabolic health.
Mad Barn’s Milk thistle extract seed extract is a dried, powdered form of the seed of the Silybum marianum plant. The recommended dose is 4.5 grams per day for a 500 kg horse. This can be provided as dried powdered added to the feed or as brewed tea poured over the feed.
Benefits of Milk Thistle in Horses
Milk thistle is primarily known for its hepatoprotective effects, meaning it can protect liver tissue from damage and support the function of the liver.
Because the liver affects so many physiological processes in the horse’s body, boosting the function of this organ can have wide-ranging benefits for your horse’s health.
Below are the top seven benefits of Milk Thistle extract based on research in horses:
- Milk thistle helps liver cells make proteins that support cellular regeneration and repair from damage caused by exposure to toxins. This speeds up the replacement of damaged liver cells with new healthy cells – a process known as cellular turnover.
For horses that have liver issues or those at risk of developing liver problems, silymarin might promote normal repair processes to enhance overall liver function.
- The active ingredient silibinin found in this herb increases antioxidant protection in healthy horses. Horses given a high dose of silibinin for one week had higher levels of the antioxidant molecule glutathione in their blood.
Glutathione works alongside the antioxidant enzyme glutathione peroxidase, which combats reactive oxygen species (ROS) to prevent oxidative stress.
This improved antioxidant status helps maintain healthy cells and tissues, especially in the liver and intestine where silibinin will be most concentrated.
Increasing the availability of antioxidant enzymes in the liver helps to ensure the proper metabolization of nutrients like carbohydrates, proteins and fats, into energy without allowing oxidants (free radicals) to damage the cells. 
- By supporting antioxidant status in the liver, milk thistle extract may improve insulin sensitivity which might help horses with equine metabolic syndrome. It may regulate healthy blood sugar levels in these horses when exposed to high-sugar diets. Silymarin could support metabolic health in easy keepers or horses with abnormal lipid deposits like cresty neck.
- Silymarin has iron chelating effects which could enhance detoxication processes to protect your horse from the damaging effects of excess heavy metal exposure. These chelating compounds bind iron and help reduce serum ferritin levels and iron stores in tissues.
This might be especially beneficial to horses with iron overload. If you suspect your horse has high exposure to iron in their feed or water, you may want to consider supplementing their diet with milk thistle.
In humans that are genetically prone to iron overload, milk thistle seed was shown to decreased iron absorption. 
- Milk thistle extract has been shown to exhibit anti-inflammatory benefits in animals and humans, particularly when there is an immune challenge like an infection. It lowers pro-inflammatory cytokines which trigger immune responses.
In the short term, pro-inflammatory cytokines are beneficial because they help to clear out an infection, but if they are present at high levels for extended periods of time they can cause tissue damage. Milk thistle can help protect tissues by moderating the levels of pro-inflammatory cytokines. 
- Horses that have recurrent allergies like heaves or seasonal allergies related to insect bites, known as sweet itch, might benefit from the anti-inflammatory effects of this herb. Compounds found in milk thistle might inhibit immune over-reactions that are common in horses with allergies.
- Milk thistle might have benefits for healthy hooves in horses. Activation of neutrophils (immune cells in the blood) is part of the process that causes laminitis in horses. Silymarin compounds decreased activation of neutrophils isolated from horses in laboratory experiments.
More research is needed to understand whether this happens when horses are fed milk thistle extract.  
How Does Milk Thistle Extract Work?
Milk thistle seeds contain numerous bioactive compounds and phytonutrients that are thought to have beneficial effects. Some of the active ingredients identified in this plant include: 
- Silibinin (silybin)
Other compounds found that may promote health benefits for horses inlcude:
Silybin is the most biologically active compound in milk thistle extract. It is a flavonolignan (a type flavonoid) that acts primarily as an antioxidant to protect cells from dangerous by-products of metabolism, known as free radicals.
Silybin helps protect cell membranes and other structures from being damaged during the normal course of the cell’s life. Because these compounds can neutralize free radicals the cells use up less of the main antioxidants, such as glutathione, and therefore have higher levels of glutathione.
Most of what is known about how these compounds support liver health is based on studies in isolated cells or in animal models. More research in horses is needed to fully evaluate these claims.
Milk thistle may also work by supporting natural digestive function in horses. It has been shown to stimulate gastric enzyme secretion and could be beneficial for horses with digestive concerns.
Milk Thistle is generally well tolerated in horses. In healthy horses, concentrated extracts fed in the form of a silibinin phospholipid complex resulted in no adverse effects. . This product has not been studied in foals or breeding/pregnant animals so we do not know how it affects these horses.
How to Feed Milk Thistle Extract
When introducing a new supplement to your horse, start with a small amount.
Dosage guidelines vary depending on the form of the supplement used and whether it is provided as a raw ground powder or a concentrated extract. Follow the recommendations provided by the manufacturer of the product you are using or seek guidance from a veterinarian or qualified equine nutritionist.
It is recommended to begin feeding ground milk thistle seed powder at a rate of ~2 grams per day for a 500kg horse. You can gradually increase the feeding rate over a few weeks to the recommended maintenance dose of 4.5 grams per day for a 500kg horse.
This will help your horse become accustomed to it, especially if you have a picky eater. Bulk milk thistle powder can be given directly as a powder or brewed into a tea to pour over the feed.
Best Time to Use
The transition to fresh pasture in the springtime comes with many changes like a higher sugar intake and increased exercise that might strain liver function. Starting milk thistle extract a few weeks before the first turn out is anecdotally reported to help “prime” the liver in preparation for this transition.
Milk thistle has been shown to protect the liver from toxins in the environment, like those found in poisonous mushrooms. Liver damage in horses can occur if they consume poisonous plants such as ragwort or red maple trees. If these plants are prevalent in your area, consider adding this supplement to your horse’s diet.
Milk thistle extract can also be given at the same time as dewormers (anthelmintics) or certain antibiotics to help support liver health.
You should always consult a qualified nutritionist before altering your horse’s feeding regimen. Submit your horse’s diet for analysis online and one of our equine nutritionists will be happy to provide a complementary review .
Milk Thistle Side Effects
Milk Thistle supplementation is generally well tolerated in horses and unlikely to result in adverse effects. In humans, it has been shown to cause diarrhea and gastrointestinal upsets in a minority of people.
In healthy horses, concentrated milk thistle extract in the form of silibinin phospholipid resulted in no adverse effects when given up to 52 mg per kg of bodyweight (26 grams for a 500kg horse) for one week on four separate occasions. 
This herb may interact with some drugs that are used to treat laminitis and colitis. Speak with your veterinarian about treatment options for these conditions and whether this product or any other supplement should be given at the same time. 
Horses with laminitis caused by navicular disease might be treated with warfarin which has been shown to interact with milk thistle extract.
Milk thistle should not be given at the same time as the antibiotic metronidazole. This drug is commonly used in horses with colitis. It is metabolized by the pathways in the liver that are activated by compounds found in milk thistle extract; giving these at the same time might make metronidazole less effective.
It might speed up metabolism of opioids like morphine and buprenorphine which are as pain management during surgeries. It might also affect the metabolism of pethidine, an opioid drug that is used as a short-term treatment to manage pain in acute colitis.
Milk thistle extract should not be given to cattle or sheep because it can be toxic to these species due to its high nitrate content.
Signs that your Horse Might Benefit from Milk Thistle
Many horses can benefit from milk thistle due to improved antioxidant status and support of healthy liver function. The liver is one of the largest organs in the horse, accounting for up to 1% of their total body weight.
Elderly horses and those with metabolic issues like insulin resistance, equine metabolic syndrome, and Cushing’s disease are most likely to need nutritional support to help keep their liver healthy.
Horses being treated for Lyme disease or Equine Protozoal Myeloencephalitis (EPM) may also benefit from using this supplement. Some antibiotics used in the treatment of these conditions can have a negative impact on liver function.
The liver is the only organ in mammals that can regenerate itself when damaged, making it a very resilient tissue. It can perform adequately with as much as 80% of its function impaired. Despite this, horses that are otherwise healthy and young can show signs of liver damage.
The signs of equine liver disease are often subtle and non-specific. In horses with confirmed liver disease, the most common symptoms are: 
- Depression/lethargy – present in 68% of cases
- Anorexia – present in 56% of cases
- Weight loss – present in 50% of cases
- Colic – present in 50% of cases
- Diarrhea – present in 42% of cases
- Jaundice – present in 42% of cases
- Light sensitivity – present in 16% of cases
- Blood clotting disorders – present in 10% of cases
Horses that have chronic liver disease progressing towards outright liver failure are likely to exhibit signs of weight loss, sensitivity to light, and increased susceptibility to sunburns.
Severe cases of liver failure are rare, but are more likely to present with neurological issues like depression, yawning, blindness, disorientation, ataxia, compulsive wandering, head pressing, and circling.
The neurological signs of liver disease are caused by high levels of ammonia (hyperammonemia) circulating in the horse’s blood because the liver is no longer able to convert ammonia to urea. When ammonia is high in the brain it can affect the horse’s behaviour and movements.
Causes of Liver Disease in Horses
The most common cause of acute liver damage in horses is consumption of plants that contain the toxin pyrrolizidine alkaloid. This leads to liver necrosis (death of tissue) and fibrosis (hardening of tissue) that can not be recovered through normal cellular regeneration.
This toxin is present in ragworts and groundsels, fiddlenecks, rattlepods, and Hound’s tongue. Horses that have consumed these poisonous plants should be closely monitored for signs of liver damage. 
Equine liver disease can also develop slowly over time. Horses with equine metabolic syndrome or those that are overweight are at higher risk of liver problems. Many of these horses have fat accumulation in the liver which makes them more insulin resistant.
This sets off a vicious cycle in which fatty liver tissue causing more insulin resistance which makes the liver store more fat. Horses with excess fat deposits in the liver are also at risk of inadequate antioxidant levels within liver cells, which can impair detoxification function and cause cells to die off leading to liver damage.
Assessment of Liver Function in Horses
The best way to determine the health of your horse’s liver is though a liver function test, biopsy or ultrasound by your veterinarian.
A blood test might also show elevated levels of liver enzymes indicating liver damage including alkaline phosphatase (ALP), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), gamma-glutamyl transferase (GGT), glutamate dehydrogenase (GLDH) and sorbitol dehydrogenase (SDH).
Blood tests can also assess liver function by measuring the levels of bilirubin which is typically low in healthy horses. These tests can help your veterinarian determine the extent of liver damage and give clues to the underlying cause. 
Supporting Liver Health in Horses
If you suspect poor liver function in your horse or have it confirmed by a veterinarian, there are some things you can do to help support liver health in addition to feeding milk thistle. 
Breaking down protein is an important function for the liver but it is also a demanding process. Meeting the horse’s dietary protein requirement without exceeding it will help to not overburden the liver.
If possible, replace high protein forages like alfalfa and fresh spring grasses with first cut hay and fall pasture grazing.
Increase the number of meals per day to spread out the feed over multiple meals. This will also help minimize large spikes in nutrients reaching the liver after each meal, which will make it easier to metabolize these nutrients without overworking liver cells.
Avoid supplements that contain added iron. Iron is not easily excreted by the body and excess accumulations are stored in liver tissue. Chronically high iron intake from the diet or water is known to cause liver damage in horses.
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- Hackett, Eileen S. et al. Pharmacokinetics and safety of silibinin in horses . Am J Vet Res. 2013.
- Hackett, Eileen S. et al. Evaluation of antioxidant capacity and inflammatory cytokine gene expression in horses fed silibinin complexed with phospholipid. Am J Vet Res. 2013.
- Hutchinson, Carol et al. The iron-chelating potential of silybin in patients with hereditary haemochromatosis. Eur J Clin Nutr. 2012.
- Karimi, G. et al. “Silymarin”, a Promising Pharmacological Agent for Treatment of Diseases. I J Basic Med Sci. 2011.
- Reisinger, Nicole et al. Milk Thistle Extract and Silymarin Inhibit Lipopolysaccharide Induced Lamellar Separation of Hoof Explants in Vitro. Toxins. 2014.
- Zholobenko, A. et al. Polyphenols from Silybum marianum inhibit in vitro the oxidant response of equine neutrophils and myeloperoxidase activity. J Vet Pharm Thera. 2016.
- Bhattacharya, S. et al. Milk Thistle (Silybum marianum L. Gaert.) Seeds in Health. Nuts and Seeds in Health and Disease Prevention. 2011.
- Chin, A.C. et al. Effect of Herbal Supplement–Drug Interactions on Therapeutic Drug Monitoring. Therapeutic Drug Monitoring. 2012.
- Bergero,D et al. Hepatic Diseases in Horses. J An Phys An Nutr. 2008.