Obesity is a health concern in horses worldwide, with a prevalence estimated between 31 – 45% in some equine populations. 
Among ponies and easy keeper breeds, the prevalence is even higher with one study reporting that 72% of adult ponies were overweight or obese. 
Horses are generally considered obese when they have a body condition score of greater than 7 on the Henneke 9-point scoring system. 
Obesity is a welfare concern for horses because it increases the risk of other health conditions including arthritis, respiratory problems and poor fertility. Obesity may be a sign of metabolic syndrome, which predisposes horses to laminitis. It can also negatively impact your horse’s performance and soundness. 
While many factors can influence obesity, horses become overweight when they consume more energy than they expend on activity and maintenance over a long period of time. 
In this article, we’ll explore the causes, consequences, and risk factors for obesity in horses. If your horse is obese, speak with your veterinarian and your equine nutritionist to formulate a plan for healthy weight loss.
Is my Horse Obese?
Obesity is defined as the excessive accumulation of body fat resulting in a negative impact on the horse’s health. 
A horse is considered obese when fat accounts for more than 20% of its total bodyweight. Some very obese horses may have a body fat percentage of up to 40%. 
Judging whether your horse is overweight or obese can be challenging; research shows that horse owners frequently underestimate how fleshy their equine companions are. 
The 9-point Henneke Body Condition Scoring system can be used to evaluate whether your horse is overweight or obese.
This scale assesses your horse’s body condition by observing and palpating specific areas where fat accumulates in the body. These areas include the crest of the neck, the withers, behind the shoulders, over the rib cage, on the rump, and above the tailhead. 
You can determine your horse’s body condition by printing out Mad Barn’s Equine Body Condition Score Worksheet.
Risk Factors for Equine Obesity
Any horse can become overweight or obese; however, some horses are more prone to gaining weight than others.
- Feed provision
- Breed & Genetics
- Injury or stall rest
Providing more calories than the horse needs over a long period leads to fat accumulation, weight gain and obesity.
Overfeeding results in a positive energy balance and can occur if your horse:
- Is fed too much feed
- Is consuming energy-dense feed
- Has unrestricted access to lush pasture
- Is provided forage that is higher quality than required
Most of your horse’s diet should be fibre-rich forage. The quality of your horse’s forage should also match their energy needs.
Mature horses at maintenance (not exercising) can maintain a healthy body condition with relatively low-quality grass hay. These horses may consume too many calories if provided high-quality forages, such as alfalfa or immature grass hay.
In comparison, horses with higher energy needs such as performance horses or lactating mares require higher-quality forages to maintain their body condition.
Breed & Genetics
Some equine breeds are more prone to obesity than others. These breeds are generally identified as easy-keepers or good-doers because they tend to gain weight easily and require less feed to maintain body condition. 
Cob-type horses and native pony breeds are at the greatest risk of obesity. Cobs are 13.6 times and ponies are 2.3 times more likely to be obese than lightweight breeds. 
Drafts and Quarter Horses have the next highest risk of being overweight, followed by Tennessee Walking Horses. Lightweight breeds such as Standardbreds, working Arabians and Thoroughbreds have a lower risk of obesity. 
Certain breeds are more susceptible to becoming obese because of their genetics. Several traits linked to obesity such as cresty neck and insulin resistance have a genetic component in horses.
For example, a study in Welsh ponies and Morgan horses prone to obesity found a higher proportion of genetic variants linked to inflammation and dysregulated sugar and fat metabolism. 
The predisposition to obesity may be an evolutionary adaptation to poor-quality forages found in the wild.
When these breeds are fed improved high-quality forages, grains or commercial feeds they extract an abundance of calories, leading to weight gain.
Horses considered to be more dominant within the social hierarchy of their herd are at greater risk of obesity. 
Dominant horses have better access to communal food sources, such as hay feeders, and may consume more calories than their less dominant counterparts.
Horses that are higher in the herd hierarchy are also given more space to graze when turned out in a group. In comparison, low-ranking horses spend more time and energy being chased away or moving around dominant horses and may have a higher stress level.
Horses over 4 years old are at a greater risk of becoming overweight. 
Younger horses expend more energy growing and are more active than older horses, which could explain the difference in obesity risk between age groups. 
Historically, senior horses were likelier to be underweight than overweight. Horses over the age of 20 have more dental issues, exercise intolerance, and metabolic dysfunction that influences their ability to consume and expend calories. 
However, with modern management practices including regular deworming, dental and veterinary care, senior horses are better able to maintain a healthy body condition.
In recent surveys in North America, Europe, and Australia, senior horses are now almost equally likely to be overweight as underweight. 
Horses with a heavier workload have a lower risk of becoming overweight because of their increased energy expenditure. The more activity that your horse gets, the less likely they are to consume more calories than they are burning.
Horses ridden for pleasure have a lower risk of becoming overweight, and competition horses or horses in intense exercise routines have an even lower risk of obesity. 
Exercise also helps to improve insulin sensitivity, which contributes to healthy weight management. Seven days of light exercise improve insulin sensitivity in obese horses by as much as 60%. 
The management of horses encompasses everything from housing to daily routine, feeding, socialization, training, veterinary care, and more.
Management factors can impact your horse’s obesity risk by affecting feed intake, nutrient utilization and energy expenditure.
Several survey studies have identified management risk factors for obesity. One study found that larger, professional horse-keeping facilities had fewer overweight horses. 
Larger facilities are likely more knowledgeable about feeding, exercising, and maintaining an ideal weight on horses. Weight maintenance may also be more important to professional facilities with horses used in shows or competitions.
Another survey found that horses living on farms raising non-equine livestock for meat were more likely to be overweight.  This may be attributed to different feeding practices.
It is quite normal for horses to lose body condition throughout the winter because there is less available forage and the horse’s maintenance energy requirements increase due to thermoregulation. 
Sugar and starch levels in grass are highest in the spring when the grass is lush and growing quickly. This can contribute to weight gain in horses as summer approaches. 
Sugar and starch are non-structural carbohydrates (NSC), which are quickly digested in the foregut and provide the horse with rapidly available energy. In excess, non-structural carbohydrates contribute to weight gain.
Injured horses are prone to weight gain due to reduced activity and lower energy expenditure. 
If your horse is injured and put on stall rest or assigned a lower workload, consult with an equine nutritionist to adjust their feeding plan. Feed and pasture must be reduced in proportion to the decrease in exercise to prevent weight gain.
Negative Effects of Obesity in Horses
Horses become obese from consuming more energy than they expend. These extra calories contribute to fat accumulation and cause physiological changes that are generally detrimental to equine health.
Insulin Levels and Glucose Storage
When horses consume feed containing glucose (derived from starch or simple sugars), blood glucose levels rapidly rise. This triggers insulin secretion from the pancreas.
Insulin is a hormone that regulates blood sugar levels by promoting glucose uptake into cells. Insulin also tells the body to store glucose as