Grazing Muzzles (What does the research say?)

Equine obesity is an epidemic across the industry. From studies that illustrate judges scoring overweight horses higher (6), to equine obesity prevalence publications showing that about 50% of the evaluated populations are classified as overweight or obese (2, 7, 8, 9). It is a serious issue in the industry that is negatively impacting equine well-being.

For those that have been following Balanced Bay, you will know that part of the reason I became so keenly interested in equine nutrition was because I seriously struggled with keeping my mare Piper (the absolute love of my life) in a healthy body condition. My research during my MSc degree even focused on how horses gain and deposit weight. I often say that ‘easy’ keepers are actually the most difficult. So, for those that are caring for horses that struggle with obesity, I sympathize and understand what a challenge their management is.

Now with my experience working as an independent equine nutritionist, I unfortunately see horses on at least a monthly basis throughout the year, and on a weekly basis in the spring and summer months that cannot walk due to hyperinsulinemia associated laminitis. It is always a heart-breaking situation, but getting these horses on the right track is so rewarding.

It is all too easy to turn a blind eye to our overweight horses. Maybe they are happy in their lush pasture, maybe you don’t like the idea of restricting their intake with a slow feeder or grazing muzzle. Well, even if they haven’t had issues yet, it needs to be recognized that obesity is a serious welfare issue that must be addressed. As a horse owner, it is our responsibility to do what is right for them.

Frequently, part of the process to combatting equine obesity is implementing various management tactics to reduce dry matter intake. From slow feeders, track systems, and grazing muzzles, there are a plethora of options on the market, and each situation warrants a slightly different approach as it of course needs to be practical.

Like the dedicated horse owners I work with, we all want our horses to live a happy and healthy life, and with obesity on the rise, this means diving into the research on how we can best apply some of the management tools that are available to us.

Grazing muzzles are a tool that is increasing in popularity as they allow the horse to graze and reduce the need for that horse to be confined to a dry lot. The idea behind them is that they will reduce the horse’s bite size, and in turn reduce their pasture intake.

A crossover study published in 2016 by Longland et al. investigated the efficacy of grazing muzzles if the equids wore them for 10 hours per day, followed by 13 hours of free grazing. They concluded that muzzle use for 10 hours per day generally reduced the rate of weight gain, however, one of the ponies that began in the muzzle treatment gained a significant amount of weight and was not included in the free grazing treatment. This study also noted that the ponies did not show any aversion to muzzling. In the discussion it was mentioned that for animals kept on pasture 24/7 it is unknown if muzzling for 9-12 hours is effective in reducing their overall daily intake, or if the animal will compensate with increased intake during the hours that they are not wearing a muzzle.

Another consideration is if forage species may impact the efficacy of grazing muzzles. A 2024 study by Glunk et al. evaluated if a grazing muzzle would be effective with the horses grazing different plant species. They concluded that the effectiveness of the muzzle was not impacted by forage species, and that on average the forage consumed was decreased by about 30% when the horse was wearing the muzzle regardless of cool-season grass species.

A more recent study, published in 2020 by Davis et al. evaluated the effects of grazing muzzles on dry matter intake, as well as behaviour and physiological stress. The study had 3 treatment groups: no muzzle, a muzzle for 10 hours per day and a muzzle for 24 hours per day. They illustrated that horses that wore a grazing muzzle for 10 hours per day did partake in compensatory grazing behaviour, and therefore, for adequate weight control, muzzling for 24 hours was required. The horses that were muzzled for 24 hours lost weight, while both other treatment groups gained weight.

In addition to monitoring weight gain, behaviour and physiological stress parameters were also investigated. The authors scored the horse’s willingness to accept muzzle application, monitored their behaviour via 1 hour video recordings twice weekly, and tracked salivary cortisol levels along with heart rate and heart rate variability. On these parameters, treatment did not influence the willingness to accept muzzling, and there was no effect of treatment on behaviours associated with frustration (e.g., pawing and headshaking). Additionally, the salivary cortisol levels did not differ between treatment groups.

Despite this, the grazing muzzles did have an impact on the time budget of the horses. The horses wearing the muzzle for 24 hours spent the most time grazing, and horses that were muzzled for 10 hours spent the most time resting, whereas the unmuzzled horses walked the most. The results of the heart rate and heart rate variability evaluation was that horses that were muzzled for 24 hours had a lower heart rate than horses that were not muzzled at all.

This 2020 publication demonstrated that muzzling horses for 10 or 24 hours did alter their daily time budgets but did not induce physiological stress. Additionally, leaving the grazing muzzles on for 24 hours was necessary to prevent weight gain.

Overall, the research supports the use of grazing muzzles for horses that become over conditioned when housed on pasture. Depending on the publication, the research suggests that having a horse wear a grazing muzzle will decrease their pasture intake from 30% up to 80% depending on a few different variables such as hours worn and length of pasture as they tend to be more effective with long pasture (4).

To conclude, as a practicing equine nutritionist, I would like to stress the importance of knowing your horse and evaluating their unique situation. Horses need to be adapted to their muzzle over the period of at least a few days. Prior to leaving them in their muzzle you must observe them both eating and drinking while wearing the muzzle. Additionally, the muzzle should be removed at minimum once per day to ensure there are no rubs and that it is still fitting properly. In the study that had the grazing muzzles on 24/7, the animals had the muzzle removed for 30 minutes per day so that they could eat their vitamin and mineral supplements.

If you have any questions about grazing muzzles and how long your horse should wear theirs, or would like assistance with a weight management plan for your horse please email balancedbaynutrition@gmail.com

By: Madeline Boast, MSc. Equine Nutrition

References:

1. Davis, K. M., Iwaniuk, M. E., Dennis, R. L., Harris, P. A., & Burk, A. O. (2020). Effects of grazing muzzles on behavior and physiological stress of individually housed grazing miniature horses. Applied animal behaviour science, 231, 105067.
2. Giles, S. L., Rands, S. A., Nicol, C. J., & Harris, P. A. (2014). Obesity prevalence and associated risk factors in outdoor living domestic horses and ponies. PeerJ, 2, e299. 
3. Glunk, E. C., Sheaffer, C. C., Hathaway, M. R., & Martinson, K. L. (2014). Interaction of grazing muzzle use and grass species on forage intake of horses. Journal of Equine Veterinary Science, 34(7), 930-933.
4. Longland, A. C., Barfoot, C., & Harris, P. A. (2016). Effects of grazing muzzles on intakes of dry matter and water-soluble carbohydrates by ponies grazing spring, summer, and autumn swards, as well as autumn swards of different heights. Journal of Equine Veterinary Science, 40, 26-33.
5. Longland, A. C., Barfoot, C., & Harris, P. A. (2016). Efficacy of wearing grazing muzzles for 10 hours per day on controlling bodyweight in pastured ponies. Journal of equine veterinary science, 45, 22-27.
6. Pratt-Phillips, S., Munjizun, A., & Janicki, K. (2023). Visual Assessment of Adiposity in Elite Hunter Ponies. Journal of Equine Veterinary Science, 121, 104199.
7. Thatcher, C. D., Pleasant, R. S., Geor, R. J., & Elvinger, F. (2012). Prevalence of over conditioning in mature horses in southwest Virginia during the summer. Journal of Veterinary Internal Medicine, 26(6), 1413-1418.