Grazing muzzles are a staple in the tack room of many pony and horse owners. These muzzles fit over the mouth and nose of a horse and restrict grass intake while allowing access to pasture.
Grazing muzzles have been shown to reduce dry matter intake by between 30 – 80%.  These devices can help prevent laminitis and obesity in horses or ponies by decreasing calorie and sugar intake.
If your horse is over-conditioned and needs to lose weight, using a muzzle while on pasture will help you manage your horse’s weight without needing to isolate them to a stall or dry lot. 
Grazing muzzles allow the horse to be turned out in a herd, enabling social interaction and providing more space to move around. These devices are safe when used properly and have not been shown to cause psychological or physiological stress in horses or ponies.
Why Use A Grazing Muzzle?
The image of a horse spending all day grazing on a lush, green pasture may seem idyllic but, in reality, those lush grasses could seriously harm your horse.
Horses evolved as efficient grazing animals, capable of deriving all the energy they need from relatively poor-quality pastures. But the pastures used for grazing horses in domestic management settings tend to contain improved grass species with significantly higher nutrient density.
Lush pastures or paddocks contain high levels of nonstructural carbohydrates (NSCs), consisting of simple sugars, starch and fructans.
If your horse has unrestricted access to high-quality pasture, they could end up consuming too much sugar, contributing to a number of health problems including weight gain, metabolic syndrome and laminitis.
This is particularly problematic in the spring and summer when horses naturally have a higher appetite. Horses respond to the increased length of daylight by eating more in preparation for decreased availability in winter. 
Although free-choice access to pasture can be a health risk for many horses and ponies, turnout onto pasture has several benefits, including the ability to express foraging behaviours and socialization.
Fitting your horse with a grazing muzzle lets you give your horse freedom of movement and contact with their social group without letting them consume unhealthy amounts of sugar.
A grazing muzzle slows down the rate at which your horse can consume grass. You should also consider good pasture-management techniques to keep your horses safe while turned out on grass.
Carbohydrates in Grasses
How do you know whether your pasture is safe for your horse to graze on or whether a grazing muzzle is necessary?
Some horses can tolerate a higher level of carbohydrates in their grass, whereas others such as those with metabolic issues, may need to avoid all but the lowest quality pastures.
- Stress conditions such as overgrazing and frost
- Time of day
- Time of year
- Plant maturity
- Fertilization practices
The only way to determine the sugar content of your pasture is to take a grass sample and submit it for analysis.
Pasture samples should be frozen until analysis to avoid fermentation, which can lower the sugar level and lead to an inaccurate test result.
Note that there can be significant variation in sugar levels between samples taken from the same field. Submitting multiple samples can increase the reliability of the results.
Consult with a nutritionist for help with interpreting the results of your grass analysis.
Preventing Weight Gain
For metabolically healthy horses, lush pasture can be a rich source of protein and energy. Carefully managed turnout on pasture can support healthy weight gain in under-conditioned horses following potential weight loss during winter.
However, excess weight gain is possible with free-choice access to lush pasture. Horses can consume 1.6-3.2% of their body weight in grass over a 24-hour period, supplying a lot of calories and sugar. 
On a mixed pasture providing 20% crude protein and 2.5 Mcal/kg digestible energy, a 500 kg (1100 lb) horse at maintenance could obtain 150% of their daily energy requirement and over 300% of their required protein intake. 
Over-consumption of pasture can quickly lead to weight gain and increase the risk for metabolic dysfunction. Especially for easy keepers or horses with insulin resistance, rapid weight gain can predispose the horse to equine metabolic syndrome and laminitis.
Grazing muzzles help to restrict calorie intake by reducing forage consumption by 30% on average for horses grazing on different types of grasses.  This can prevent weight gain in healthy horses and help to maintain an appropriate body condition.
Laminitis is a major concern with ponies or overweight horses grazing on lush pasture. This painful inflammatory condition is one of the leading causes of euthanasia in horses.
Laminitis causes swelling of the sensitive hoof laminae, which support the coffin bone. In severe cases, the coffin bone can sink and/or rotate leading to grass founder.
While the exact cause of pasture-associated laminitis continues to be investigated, it is known that high insulin levels (hyperinsulinemia) contribute to the pathology. In most cases, laminitis can be prevented with dietary changes and the restriction of high-NSC forages. 
Horses at risk of laminitis should be turned out with a grazing muzzle during periods of the day when the sugar content in grass is lowest, usually the early morning hours. Turnout on a dry lot with appropriately selected hay or straw may also be recommended to reduce the risk of a laminitic emergency.
Obesity and Insulin Resistance
- Exercise intolerance
- Poor heat tolerance
- Increase in visceral fat (surrounding organs)
- Joint pain and orthopedic issues
- Unsoundness due to increased weight-bearing on their limbs
- Reproductive issues
- Respiratory and cardiovascular problems
Additionally, the increased fat accumulation can make a horse susceptible to insulin resistance. Horses with insulin resistance are more likely to develop laminitis and other metabolic complications, including hyperlipidemia and liver dysfunction. 
Insulin is a hormone released by the pancreas to regulate blood sugar (glucose) levels and transport glucose into cells. When a horse consumes soluble carbohydrates, blood sugar spikes and more insulin is released to promote uptake of sugar into cells.
Insulin-resistant horses are not as sensitive to the effects of this hormone, causing the pancreas to secrete more insulin. When blood sugar levels are elevated in IR horses, they are slow to return to baseline. 
Emaciated horses need to be re-introduced to forage and feed gradually to avoid potentially fatal consequences. Grazing muzzles slow down pasture consumption and prevent the horse from gorging.
Refeeding syndrome occurs when emaciated horses suddenly consume high levels of nutrients, such as carbohydrates, resulting in metabolic and electrolyte imbalances.
A sudden influx of carbohydrates causes insulin to spike, moving blood sugar into cells. Insulin also moves electrolyte minerals including magnesium, calcium, phosphorus and potassium out of the bloodstream