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.
Intermittent feeding, high-intensity exercise, stress, and certain drugs can promote ulcers.
Your horse may require treatment with medication such as omeprazole (the primary ingredient in GastroGard and UlcerGard), sucralfate (Carafate), antacids, and histamine-receptor antagonists to heal ulcers. 
Other strategies to help heal and prevent ulcers include providing your horse with consistent access to forage, reducing stress, and avoiding feeding a high grain diet.
Natural ingredients such as lecithin, glutamine, marshmallow root, and slippery elm are beneficial for supporting gut health in horses with ulcers. These ingredients are included in Mad Barn’s Visceral+ formula, which was specifically designed for horses with ulcers or those that are prone to developing them.
6) Intestinal Parasites
Parasites can cause weight loss with or without additional signs of illness. Health issues caused by parasites include poor performance, diarrhea, gastrointestinal irritation, and intestinal impactions that can result in colic. 
Types of parasites that most commonly require treatment in horses include: 
- Large strongyles
- Small strongyles
- Stomach bots
Small strongyles (cyathostomes) are known to cause gradual weight loss in affected horses by impairing gastrointestinal absorption and contributing to protein loss. 
Out of 60 horses assessed for weight loss at a referral hospital, 20 of them were diagnosed with a parasitic infection caused by small strongyles. 
Parasite load can be determined by directly assessing the feces, determining concentrations of worm eggs by flotation, and taking a culture of the feces to check for the infective larva. 
Horses with parasitic infections require treatment with anthelmintic medications. Classes of drugs available for parasite control in horses include ivermectins, benzimidazoles, pyrantels, and isoquinolines. 
7) Dental Problems
A range of dental issues including fractured and overgrown cheek teeth may be observed during oral examinations.  Sharp points may develop on the molars, which can cause pain and a reduced appetite. As horses age, the enamel ridges of the teeth wear down and may no longer be able to grind the long fibers in forage effectively. 
Gaps (diastemas) can form between the teeth and cause food to become trapped. When food lodges in the spaces between teeth it decomposes and promotes inflammation that can lead to periodontal disease. 
As the disease progresses, further infection and damage to the soft tissues of the gums and tooth sockets occurs. Periodontal disease can also lead to the loss of bone that contains the tooth sockets on the jaw bones (alveolar bone), mobile teeth, and loss of teeth. 
Clinical signs of dental disease include dropping food (quidding), food accumulations in gaps between the teeth, hypersalivation, facial swelling, bad breath (halitosis), weight loss, reduced appetite, and an increased incidence of choke and colic. 
Some horses (typically those that are older) are affected by a painful dental condition called Equine Odontoclastic Tooth Resorption and Hypercementosis (EOTRH). 
This condition affects the incisors and canine teeth (in some cases also the cheek teeth) and results in the dissolution of the tooth roots and excess production of cementum on the teeth as the body attempts to stabilize the affected teeth. 
Horses should have a dental examination at least once per year to check the health of their teeth. Most horses require dental floating annually or more often. 
During a dental examination, your veterinarian or equine dental specialist will check your horse’s mouth for sharp points on teeth, gaps between teeth, mobile teeth, and signs of dental disease.
Routine floating may improve minor dental conditions. If severe damage has occurred to the teeth and surrounding tissues, extraction may be necessary. 
Horses with dental issues that cannot be corrected with veterinary intervention may benefit from chopped hay, hay cubes or hay pellets. These forms of forage are typically easier to chew and digest than long-stem hay. Soaking these feeds will soften them even further to improve a horse’s ability to chew them.
8) Chronic Illness and Disease
Because weight loss can be caused by multiple factors, underweight horses should be assessed to determine if they have any underlying disease.
Diseases can cause weight loss by increasing the horse’s energy expenditure or impairing the body’s ability to properly metabolize energy and nutrients. Examples of conditions these conditions include: 
- Inflammatory or infectious disease
- Kidney disease
- Liver disease
- Equine asthma
- Cardiac failure
- Pituitary Pars Intermedia Dysfunction (PPID; Equine Cushing’s)
- Inflammatory bowel diseases
- Abdominal neoplasia or tumors
Other health issues that may result in weight loss. Chronic pain that causes stress may promote weight loss in horses by reducing appetite. 
Underlying illness and disease should be investigated and treated by a veterinarian.
9) Muscle Atrophy
Weight loss can be characterized by loss of fat mass, muscle mass, or both. Loss of muscle mass occurs when protein degradation outpaces protein synthesis, resulting in a net loss of muscle mass. This can occur in horses who are deficient in protein or receiving a low quality protein, older horses, and horses that are ill. Muscle atrophy is a common sign of horses with PPID.
Both plant proteins and muscle proteins are composed of amino acids. When your horse consumes protein, they break the protein down to amino acids and small peptides that get absorbed in the gut. Your horse then uses those amino acids to synthesize new proteins, including muscle protein.
Although horses can synthesize some amino acids (non-essential amino acids), other amino acids cannot be synthesized, and must be consumed in the diet in adequate amounts to support protein synthesis (essential amino acids). In most equine diets, lysine, threonine and methionine are thought to be the first three “limiting” amino acids.
Limiting amino acids are the essential amino acids in the diet that are consumed in the lowest amount relative to their requirement. In short, their consumption will limit protein synthesis until they are fed at the required rate.
To promote muscle development, most interventions are aimed at increasing protein synthesis. This can be achieved by:
- Stimulating muscle protein synthesis through exercise, and
- Ensuring your horse has an adequate intake of protein and limiting amino acids
To stimulate protein synthesis through exercise, consider adding some light exercise to your horse’s routine. Aim for at least 30 minutes of exercise, three to five days per week. Depending on your horse’s health status, soundness, and current exercise level, this could include hand walking, lunging or riding.
To ensure your horse is receiving an adequate protein and amino acid supply, you can test your hay to determine its protein, and even amino acid content.
If your horse needs additional protein based on your forage analysis, high protein feeds such as soybean meal or alfalfa can be added to the diet to increase their protein intake. Additionally, supplements like Three Amigos can provide additional limiting amino acid support.
10) Extreme Cold Weather
Horses can lose weight in extremely cold temperatures as they require more energy to maintain a stable body temperature.  if your horse is already underweight, they are at a higher risk of losing additional weight during cold weather as they have limited energy reserves.
Horses with arthritis-related neck pain that worsens in cold weather may have difficulty accessing water or feed when it is placed at ground level. 
Those with lameness or painful joint conditions may not be able to move effectively to obtain food and water in cold, snowy, and icy conditions. 
Underweight horses should have their body condition score measured often during the cold months of the year to determine if they are receiving an adequate amount of feed.
Supplementary feeds (pelleted concentrates/extra forage) may be required to provide sufficient energy for horses during the winter. Blanketing your horse in cold weather provides an insulating layer that helps to preserve body heat.
Read our guide to cold weather care for your horse for additional tips.
Monitoring Changes in Weight
Once you have identified and addressed the reasons your horse is losing weight, a nutritionist can help you design a feeding program that supports healthy weight gain.
Weekly or bi-weekly weight assessments are recommended to monitor progress and ensure your horse does not continue to lose weight. 
The best way to measure your horse’s body weight is with a weighbridge or scale. If this is not available, a weight tape can be used to conveniently estimate your horse’s weight by taking specific body measurements. 
Body condition scoring is also useful to track changes in fat mass.
Tips for Weight Gain
Follow these tips to maintain or safely increase your horse’s weight:
- Feed at frequent intervals
- Provide a forage-based diet
- Monitor your horse’s feed intake and eating habits
- Use an appropriate feed for your horse’s age and condition
- Feed a yeast supplement to improve feed efficiency
- Introduce any dietary changes slowly
- Consult with an equine nutritionist to optimize your horse’s nutrition
- Follow a regular parasite control program
- Have your veterinarian assess your horse’s teeth and overall health at least annually
To help formulate a weight gain diet for your horse, submit their information online and our nutritionists will provide a free diet evaluation and balancing.
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