Hi, I have found if they are frustrated, they bite the hay bags and make bigger holes.  If he is losing weight, it may be the Metformin not the hay bags.  Did you start with 1 inch or 1 and 1/2 in hay bags?  

Best hay bags and my equestrian vet only uses Hay Burner hay bags.


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Kim R, Lafayette, CA  2019
https://ecir.groups.io/g/CaseHistory/files/Kim%20and%20Shomee