Alas, the answer to this question is an unsatisfying…it depends. Grazing muzzles restrict 30% -70% of grass intake compared to grazing without a muzzle. Some factors that affect this are the horse's nature and the environment. Some horses will graze extra aggressively with a muzzle, possibly because they feel they aren't getting as much, so they work harder to get more grass. Other horses swing the opposite way; maybe they don't feel it's worth trying that hard. The good news is studies have shown that horses in grazing muzzles almost always get more exercise than horses in stalls or dry lots, so they're typically getting the one-two punch of diet and exercise versus just a diet.
Environmental factors that may contribute include the weather (which can affect how the grass tastes and how calorie-rich it is), the grass type, height, and whether or not it is evenly mowed, sparse & patchy, or tall and weedy. Typically, horses will be more motivated to graze if the grass is lush (usually early to mid spring). It is typically easiest for them to get more grass if it is mowed evenly somewhere between 6 to 10 inches. All of these factors are going to determine how much grass your horse gets with versus without a muzzle. Other factors that might contribute to your choice to modify their current grazing situation might be metabolic or other medical factors, which your vet might monitor with certain types of testing.
There are excellent ways to monitor your horse's health and weight to guide you on your muzzle journey. We always recommend using a weight tape to monitor your horse's weight gain/loss. Monitoring your horse's weight every two weeks to a month should give you a good idea of whether or not you are on the right track. For the other issues mentioned above, we recommend consulting with your vet/nutritionist to get specific weight goals or different ways to monitor their progress, as many horses with metabolic issues might not be overweight but will still need grass restriction.
Our muzzles and inserts offer various ways to change or modify how much grass your horse can get and we plan to offer more options in the future. Our diet inserts come in horse and pony size and make the holes about 50% smaller. Similarly, our leather inserts restrict an additional 30% and come in horse, pony, & mini size. We have ways to modify both options to fit horse and pony size muzzles purchased before the fall of 2022 without the slots for an insert. We also have some great ways to further customize the size of the holes for any of the muzzle sizes. The inserts can be a great way to taper muzzle use on and off throughout the year so you can maintain a healthy weight year-round. A great way to see how the muzzle works is to try it yourself by pressing it down into the grass and seeing how much you can get through the holes.
Reach out if you need any clarification or additional information on some of the above mentioned options. We are always here to help, constantly innovating, and always trying to improve the lives of our horsey friends.
Track weight here: GG Free Horse weight tracking program