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. 
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. 
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. 
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.
Horses with low-protein diets can’t maintain topline muscle. Microorganisms in the equine gut also rely on protein for nitrogen. 
Without enough fermentable fibre from soft, high-quality hay, populations of good bacteria recede, and other species flourish. 
Other Causes of Abdomen Distention in Horses
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.