Unexplained weight loss in your horse is a cause for concern for any horse owner but is often straightforward to diagnose and address.
If your horse is losing body condition, it could indicate an undiagnosed health problem or it may be time to consider changes to your horse’s feeding and management.
Older horses and horses affected by chronic disease are more prone to weight loss than healthy horses.  Weight loss may also indicate a gut health issue, dental problem or concern with your horse’s social grouping.
Horses also lose weight when exposed to extreme weather or when fed a diet that does not provide sufficient dietary energy to match their needs.
In this article, we will review some of the top reasons why your horse may be losing weight and suggest management strategies to support healthy weight maintenance. We will also discuss how to feed a horse to promote weight gain.
Diagnosing Weight Loss in Horses
Numerous factors can result in weight loss in horses, a commonly encountered issue in equine veterinary practice.  Weight loss occurs when a greater amount of energy is expended than received through the diet.
Horses naturally see their bodyweight fluctuate over the course of a year, typically losing weight in the cold winter months and gaining it back during the summer.
A temporary loss of body condition may also occur during pregnancy, lactation, and when performance demands are increased during competition periods. 
But if your horse is losing weight for an unexplained reason, it may be time for veterinary intervention.
A thorough evaluation of diet, management strategies, and health status is necessary to investigate the cause of weight loss in your horse. Your veterinarian will help you diagnose causes of weight loss based on:
A clinical examination is needed to determine if weight loss is occurring due to malnutrition or disease.
During a clinical exam, your horse’s body condition score (BCS) will be assessed. BCS is a measurement of the amount of subcutaneous fat tissue a horse has.
A thorough dental examination is needed to determine whether the horse is physically capable of eating the feed provided to them.
Digestion begins in the mouth with mastication (chewing) of feed. Horses with poor dentition may not consume adequate amounts of feed or may not be able to derive enough energy from the feed they consume.
Blood and fecal testing may be completed to determine if your horse is losing weight due to an underlying health problem.
Your veterinarian may test for issues such as: 
- Liver or kidney malfunction
- Gastrointestinal abnormalities
- Parasitic infection
- Hormonal imbalances associated with conditions including pituitary pars intermedia dysfunction (PPID) or hyperthyroidism
Further diagnostic tests including endoscopy, ultrasound, biopsies, laparoscopy, and exploratory laparotomy may be used to determine the reason(s) for weight loss. 
Clinical Signs of Weight Loss
Common signs of weight loss include: 
- Visible ribs and backbone
- Bony projections present (in emaciated horses)
- No fatty tissues can be felt
- Lethargic behavior and poor performance in combination with weight loss
- Exercise intolerance
- Depression in combination with weight loss
Top 10 Causes of Weight Loss in Horses
1) Poor Quality Forage
Forage should be the basis of every equine feeding program and forage quality should match the needs of your horse.
If your horse’s forage is too low in quality, it may not provide sufficient energy and nutrients to meet her dietary needs. 
Forage quality is partly evaluated by looking at the digestible energy content. This is a calculation that takes into account the protein and fibre content of the forage.
Low-quality hay is low in protein (less than 8% crude protein) and high in fibre (NDF and lignin). This high fibre content can reduce your horse’s forage intake and make it harder to digest in the hindgut.
A typical 500 kg (1100 lb) horse at maintenance needs to consume 16,650 kilocalories (16.65 mcal) per day to maintain her bodyweight.
Energy requirements are higher in working, pregnant and lactating horses, ranging from 17,000 – 35,000 kilocalories (17 – 35 mcal) per day.
Horses consume approximately 2% of their body weight in hay or pasture each day. For a typical horse, this is approximately 10 kg (22 lb) of forage per day.
Low-quality hay might supply 1.4 – 1.7 mcal / kg in digestible energy.
Even if a heavily exercising horse were to maximize their expected hay intake and consume 2.5% of their body weight in low-quality forage, this would only supply 17,500 – 21,250 kilocalories per day.
For a heavily exercising horse needing 26,650 kilocalories per day, this could result in a significant calorie deficit or negative energy balance.
All horses should have consistent access to hay or pasture to support their behavioural need to forage for up to 16 hours per day. 
Hay quality varies depending on the age of maturity of the grass, when it was harvested and the environmental conditions in which it was grown. Have your horse’s hay analyzed to determine exactly how much caloric energy it provides.
Legume hays including alfalfa and clover typically provide more calories than grass hay.
Supplemental feeds, fat sources and nutritional products can be added to the diet to provide nutrients and calories that are missing from lower-quality hay or pasture.
Consider consulting with an equine nutritionist to determine your horse’s specific nutritional and energy needs.
Stress can contribute to weight loss in your horse.  Horses are keenly attuned to changes in their environment and often experience stress in unfamiliar settings or when their normal routine is disrupted.
While short-term exposure to minor stress is unlikely to impact their long-term well-being, major stressors or chronic stress can lead to weight loss and negative health outcomes.
Common causes of stress in horses include:
- Changes in training schedule or exercise level: Horses can experience stress if their training schedule becomes more intense or changes significantly. Inactivity or lack of adequate turnout can also lead to stress (i.e. in horses on extended stall rest).
- Changes in diet: Inconsistent feeding times and limited access to forage promotes stress. Ideally, your horse should have constant access to forage to minimize time spent with an empty stomach.
- Frequent transportation: Transport is often stressful for horses. Performance horses that travel frequently during show season may develop chronic stress as a result.
- Social grouping: Horses are very social creatures and can experience stress when companions join or leave their herd. Turn out with aggressive horses can also lead to stress.
- Housing conditions: Horses that are housed in crowded or noisy environments have higher stress levels, especially if their sleep is disrupted. Horses should ideally be stabled so that they can see other horses and in barns with adequate temperature control, ventilation and appropriate lighting.
- Pain and discomfort: Health conditions, injuries and diseases that cause pain can promote stress in your horse.
Investigate potential causes of stress in your horse and try to resolve them. Your horse may require medical treatment, relocation to a different housing environment, or a reduced workload.
3) Herd Hierarchy
Horses establish a natural pecking order when turned out in a group. Those that are low on the social hierarchy may experience bullying by other herd members and be chased away from their feed.
Hierarchy in the herd plays an important role in determining access to food and shelter.  Weight loss may occur in horses that are low on the pecking order if they are unable to consume an adequate amount of forage and grain to maintain their body weight.
Research shows that social hierarchy influences body condition.  Horses with a higher ranking within their herd tend to have a better body condition score compared to those that rank lower as they may have greater access to food and shelter.
Horses that have low social status may need to be separated from the herd to ensure they are consuming enough feed. If you are feeding your horses in a group setting, provide multiple access points for feed, water, and shelter to minimize competition.
Providing more feed access points than there are horses in the herd may help reduce “resource-guarding” by horses high in the social hierarchy and allow horses that are low in the hierarchy adequate access to food.
Reducing exposure to aggressive herd mates may also help to prevent weight loss caused by stress. 
4) Hindgut Issues
The horse relies on the microbial community in the hindgut to ferment fiber and produce end-products that the horse can absorb and utilize for energy. Disturbances to the normal microflora of the gut can decrease the fermentation capacity of the hindgut and can result in weight loss.
Disturbances to the hindgut microbes can be caused by:
- Meals containing high amounts of NSC (non-structural carbohydrates)
- Antibiotic administration
- Sudden change in feed
Meals containing high amounts of NSC can lead to starch and sugar passing through the intestine and reaching the hindgut. Microbes rapidly consume starch and sugar, quickly producing large amounts of acids, gasses, and more bacteria.
The acids reduce the pH of the hindgut, which can damage the intestinal lining and cause leaky gut syndrome. The bacteria that abound in the presence of starch and sugar can out-compete and reduce the number of bacteria that are helpful in fermenting fiber.
Feed fat and digestible fibre sources instead of feeds high in starch and sugar to increase calories. Fiber sources like beet pulp and soy hulls and fats like flaxseeds, whole soybeans, or oil are good options to boost calories without relying on starch and sugar. W-3 oil is an excellent option to increase calories and it is also high in DHA, an anti-inflammatory omega-3 fatty acid.
You can support the microbial community in the horse’s gut by feeding probiotics and prebiotics. Prebiotics and probiotics increase the number of beneficial bacteria in the gut so that they can efficiently ferment fibre and provide energy to the horse.
Optimum Digestive Health provides prebiotics, probiotics, and yeast to support the hindgut microbiome.
Stomach ulcers (squamous and glandular) and colonic (hindgut) ulcers can result in