A large, round belly doesn’t always mean your horse is overweight. Some horses have a hay belly that makes them appear pregnant, yet they may struggle to maintain enough body condition to cover their ribs.
While multiple factors contribute to abdominal distention in horses, poor hindgut fermentation of high-fibre, low-quality forage is the primary culprit.
These horses may not be getting enough energy and protein from their diet, leading to poor topline and body condition. Dietary changes or gut support are necessary to get rid of the hay belly.
This article will review the causes of hay belly in horses and discuss how a balanced feeding program can prevent it.
Hay Belly in Horses
If your horse has a pendulous midsection but limited muscling or fat on the rest of his frame, he may have a hay belly.
This bloated appearance is a sign of an internal problem that could affect your horse’s health and ability to extract nutrients from their feed. 
The equine digestive system thrives on consistent forage intake, and healthy horses can eat large amounts of hay without appearing bloated.
But poor quality forage takes longer to digest and therefore lingers in the hindgut, leading to abdominal distention. This occurs due to increased gut fill, gas production in the gut and impaired digestive function.  Low-quality hay is characterized by high fibre and low protein. 
Ironically, these horses are typically underweight due to poor nutrient absorption. To meet energy requirements, horses need to eat a larger volume of this less nutritious hay. 
Signs of Hay Belly
A distended gut is the telltale sign that your horse has a hay belly. Your horse’s abdomen increases in size and hangs significantly lower than usual.
However, a large belly can indicate other health concerns that need to be ruled out, such as parasites, colic, ulcers, or PPID (Cushing’s disease).  Broodmares also have a low-hanging abdomen during pregnancy.
Horses with hay bellies lose muscle and fat due to poor absorption of energy from the diet and low protein content. Horses may appear skinny or malnourished with visible ribs and reduced muscling over the withers, back and hindquarters. 
Inadequate nutrient intake also causes poor coat condition; hay-belly horses often have a dull coat to accompany their pot-belly look. 
Some horses only show signs of hay belly in their physical appearance. However, other horses have additional symptoms, such as low-grade colic, excessive stretching, restlessness, and sweating. 
Hay Belly vs. Overweight Horse
How can you tell if your horse has a hay belly and isn’t just fat? While overweight horses can have round guts, horses with hay bellies are usually underweight.
You can determine if your horse has a hay belly by evaluating his body condition score (BCS). The body condition scale gives horses a score from 1 (emaciated) to 9 (extremely obese) based on fat deposits in six body areas. 
Horses deposit fat on their neck, withers, ribs, tail head, spinous processes, and behind the shoulder. Belly size is not a good indicator of body condition. 
Horses with round bellies and fleshy fat deposits throughout their body are likely overweight. But if your horse also has easily discernable ribs, spinous processes, and hip joints, he likely has a hay belly. 
Equine Gut Microbiota
Horses have a large population of bacteria, yeast, fungi, and protozoa in their hindgut. In healthy horses, good microbes that support digestion and gut function dominate the microbiome and protect against harmful bacteria. 
Beneficial hindgut bacteria ferment fibrous forage to create volatile fatty acids (VFAs), one of the main energy sources for the horse.
The equine microbiome is also responsible for synthesizing several important nutrients, including B vitamins. 
A balanced hindgut microbiome supports optimal digestion and nutrient absorption. But diet changes, stress, and poor-quality forage can disrupt the gut microbiota and cause dysbiosis. 
Gas is an inevitable by-product of bacterial fermentation. But overgrowth of certain bacteria can lead to excessive gas production and hay bellies. 
Causes of Hay Belly in Horses
Consuming a high volume of low-quality forage is the primary cause of hay belly in horses.
However, there are several reasons your horse may appear bloated. 
Horse owners can feed large amounts of mature hay and still struggle to put weight on their horses.  This is because grass loses nutritional value as it matures.
Mature hay is usually very coarse and stemmy. A high stem-to-leaf ratio indicates that the forage contains more lignin which is an indigestible, infermentable fibre. 
In comparison, leafy hay has lower levels of lignin. 
A forage analysis will help you determine the fibre content of your horse’s hay by measuring structural carbohydrates.
Structural carbohydrates are fibre components that add rigidity to plant cell walls. This includes lignin, cellulose, and hemicellulose. 
Hemicellulose is more fermentable than cellulose while lignin is indigestible by the horse and can not be broken down by microbes. Cellulose digestibility also decreases with increased lignin content. 
These values are reported on a hay analysis as neutral detergent fibre (NDF), which measures cellulose, lignin, and hemicellulose, and acid detergent fibre (ADF), which measures only cellulose and lignin.  Lignin content is also presented as a separate value in a forage analysis.
Hay with high lignin content takes longer to digest and increases gut fill. ADF negatively correlates with overall digestibility, so a higher ADF value indicates lower-quality hay. 
Poor-quality hay also has low protein content. While high-quality young grass and legume hays (i.e. alfalfa) are excellent protein sources, overly mature forage tends to be low in protein. 
Your horse needs protein to support hoof strength, coat quality, organ function, bone health and all other functions of the body.  Adequate protein is also essential for performance, muscle building, exercise recovery and maintaining body condition.
Amino acids are the building blocks of protein that assist with tissue growth and repair.  Essential amino acids for horses include lysine, methionine, and threonine. 
Horses with low-protein diets can’t maintain topline muscle. Microorganisms in the equine gut also rely on protein for nitrogen. 
High-fibre, mature hay can disrupt the equine microbiome and lead to the overgrowth of gas-producing bacteria.  Excessive gas production contributes to abdominal distention. 
Without enough fermentable fibre from soft, high-quality hay, populations of good bacteria recede, and other species flourish. 
Hindgut dysbiosis reduces digestive efficiency and increases the risk of colitis, colic, ulcers, and other health conditions. 
High-starch feeds, stress and medications can also shift microbial populations, leading to abdominal distention and increased colic risk. 
Other Causes of Abdomen Distention in Horses
Young horses often appear to have a hay belly due to high intestinal parasite load. Parasites disrupt the gut microbiome and interfere with digestive function. 
Horses with high worm loads can appear underweight and bloated. A fecal egg count can determine if worms are the root cause. 
In older horses, uncontrolled Cushing’s disease (PPID) weakens the abdominal wall muscles and gives them a sagging belly. A lack of exercise can also reduce muscle tone and slow digestive function. 
Hay Belly Treatment
If you suspect your horse has a hay belly, consult your veterinarian to rule out other health conditions that can cause abdominal distention in horses. These conditions may require medical intervention. 
If hay belly is confirmed, work with an equine nutritionist to make dietary changes, including selecting appropriate forage.
If you cannot switch your horse’s forage immediately, consider feeding a digestive health supplement to support gut function. Yeast supplements can also promote better nutrient absorption in horses. 
Fat or protein supplementation might be necessary to regain body condition if your horse is underweight.
The first step to eliminating a hay belly is transitioning your horse to higher-quality hay with lower fibre and lignin content. Always make feed changes gradually to avoid colic.
Look for high-quality hay that is green, leafy, and fine-textured. Grasses harvested at optimal maturity will have higher nutrient concentrations and better digestibility.
Avoid overly mature hay. Lignin content increases as the plant matures, making grass coarse, stemmy, and less palatable. 
While visual evaluation can give you some cues regarding forage quality, confirming nutrient content with a forage analysis is recommended.  The following general ranges can help you interpret the results of your hay analysis:
- High-quality forage: 40-50% NDF, 30-35% ADF
- Moderate-quality forage: 50-60% NDF, 35-40% ADF
- Low-quality forage: >60% NDF, >40% ADF
Forages with low ADF levels are more efficiently digested and contribute less to gut fill. You should notice a decrease in your horse’s abdominal distention and an increase in muscle mass within several weeks of switching to higher-quality hay. 
If you don’t see improvement, your horse may need extra gut support or added protein sources. Horses that struggle to keep weight on while consuming high-quality forage may have an underlying health condition. 
Protein for Hay Belly
Feeding legume hay is a good way to increase your horse’s protein intake. Alfalfa and other legumes contain more protein than grass hays and can help horses with hay bellies rebuild toplines. 
Adding protein sources to your horse’s diet can compensate for low-protein forage. Recommended protein sources include:
- Alfalfa cubes or pellets
- Ground flax
- Soybean meal
- Canola meal
- Copra meal
- Dried distiller’s grains
- Whey protein powder
- Amino acid supplements
Not every horse needs grain. But well-formulated concentrates can help fill nutritional gaps in forage-based diets. 
If choosing a ration balancer or complete feed, ensure that you select one with low levels of starch and added sugar. High-starch diets can disrupt hindgut bacteria and cause hindgut acidosis. 
Horses that require more energy in their diet benefit from easily digestible, high-fibre ingredients such as beet pulp to support hindgut function. 
Probiotics for Hay Belly
Probiotics are direct-fed microbials that can support the equine microbiome and help to re-establish healthy bacteria populations. 
Probiotic supplements typically contain several strains of naturally-occurring microbes that support digestive function, feed efficiency, and the immune system. 
Resident populations of beneficial hindgut microbes decline without adequate digestible fibre in the diet. Probiotics can help to restore balance to the gastrointestinal tract in horses recovering from hay belly and out-compete harmful bacteria associated with excessive gas production. 
Mad Barn’s Optimum Digestive Health is a complete gut health supplement with 20 billion CFUs of probiotics in each serving. This supplement also contains yeast, prebiotics, digestive enzymes, toxin binders, and immune nucleotides to address optimal hindgut health.
- Horses with hay belly have distended abdomens and may have poor body condition.
- Low-quality forage with high levels of fibre and lignin is the primary cause of hay belly in horses.
- Diets high in indigestible fibre disrupt the microbiome, leading to excessive gas production and gut fill.
- Poor nutrient absorption prevent horses from getting the nutrition they need to maintain muscle mass and overall health.
- Protein supplementation and probiotics support horses with hay bellies as they transition to a higher-quality forage.
Is Your Horse's Diet Missing Anything?
Identify gaps in your horse's nutrition program to optimize their well-being.
- Hillyer, M. et al. Case control study to identify risk factors for simple colonic obstruction and distension colic in horses. Equine Vet J. 2010.
- Santos, A. et al. Understanding the equine cecum-colon ecosystem: current knowledge and future perspectives. Animal. 2011.
- Edouard, N. et al. Voluntary intake and digestibility in horses: effect of forage quality with emphasis on individual variability. Animal. 2008.
- Kohn, R. et al. Effect of plant maturity and preservation method on in vitro protein degradation of forages. J Dairy Sci. 1995
- Carroll, C. et al. Body condition scoring and weight estimation of horses. Equine Vet J. 1988.
- Ralston, S. Evidence-based equine nutrition. Vet Clin North Am Equine Pract. 2007.
- Costa, M. et al. The equine intestinal microbiome. Anim Health Res Rev. 2012.
- Garber, A. et al. Factors Influencing Equine Gut Microbiota: Current Knowledge. J Equine Vet Sci. 2020.
- Di Pietro, R. et al. Species-Level Gut Microbiota Analysis after Antibiotic-Induced Dysbiosis in Horses. Animals (Basel). 2021.
- Palomari, A. et al. Influence of maturity on alfalfa hay nutritional fractions and indigestible fiber content. J Dairy Sci. 2014.
- Linn, J. et al. Forage quality analyses and interpretation. Vet Clin North Am Food Anim Pract. 1991.
- Traxler, M. et al. Predicting forage indigestible NDF from lignin concentration. J Anim Sci. 1998.
- Latham, C. et al. Effects of dietary amino acid supplementation on measures of whole-body and muscle protein metabolism in aged horses. J Anim Physiol Anim Nutr (Berl). 2019
- Gomez, A. The Horse Gut Microbiome Responds in a Highly Individualized Manner to Forage Lignification. J Equine Vet Sci. 2021.
- Park, T. et al. Comparison of the Fecal Microbiota of Horses with Intestinal Disease and Their Healthy Counterparts. Vet Sci. 2021.
- Debeffe, L. et al. Negative covariance between parasite load and body condition in a population of feral horses. Parasitology. 2016.
- McCue, P. Equine Cushing’s Disease. Vet Clin North Am Equine Pract. 2002.
- Berreta, A. et al. Equine Probiotics-What Are They, Where Are We and Where Do We Need To Go? J Equine Vet Sci. 2022.
- Pearson, W. et al. Exploring relationships between body condition score, body fat, activity level and inflammatory biomarkers. J Anim Physiol Anim Nutr (Berl). 2018
- Coverdale, J. Can the microbiome of the horse be altered to improve digestion? J Anim Sci. 2016.
- Broderick, G.et al. Desirable characteristics of forage legumes for improving protein utilization in ruminants. J Anim Sci. 1995.
- Muhonen, S. et al. Effects of Differences in Fibre Composition and Maturity of Forage-Based Diets on the Microbial Ecosystem and Its Activity in Equine Caecum and Colon Digesta and Faeces. Animals (Basel). 2021.
- Bulmer, L. et al. High-starch diets alter equine faecal microbiota and increase behavioral reactivity. Sci Rep. 2019.
- Collinet, A. et al. Biomarkers for monitoring the equine large intestinal inflammatory response to stress-induced dysbiosis and probiotic supplementation. J Anim Sci. 2022.
Leave A Comment