Quick Answer: a grazing muzzle should not be left on 24.7, the vet advised time is 10 hours per day.
Testing 10 hours of horse wearing a muzzle:
Use of grazing muzzles for 10 hours followed by 13 hours of free grazing allowed ponies 23 hours of turnout in a large paddock with their social group. Overall, use of a grazing muzzle for 10 hours during the day resulted in significantly lower weight gain than when the same animals were allowed to graze freely, unhindered by a grazing muzzle. When viewed on a week-by-week basis, this effect was significant for weeks 2 and 3, but not week 1 as one animal lost weight during the first week of
Acknowledgments
The funding of this study by WALTHAM Centre for Pet Nutrition is gratefully acknowledged.
10 hours is said to be the maximum time for a horse to wear a muzzle. Great, but if you then leave the horse in its field it can and will eat a huge amount to make up. The only solutions are to stable the horse or bit better, create a dry lot and feed soaked hay if the horse is laminitis sensitive.
Creating a grassless track is the gold standard and if you can do this, you are lucky. My Skye and Tara live on such a track and cannot have grass, the smallest amount sends their pulses bounding so it's just not worth them having any.
Tip: from horse nutitionalist Donna, The Horse Guru is that if the horse has his head down eating, there is grass even if you think there is absolutely none. Beware.