I used a grazing muzzel on my horse as shes prone to lami but she became dehydrated as she couldn't drink very well. I would say you would need to monitor your shetland to check it is drinking ok.
I recently got a new horse who needed feeding up so i have changed her diet to mollichaf, alfa a and baileys 4. Her coat is starting to look great and she is slowly putting weight on but she is becoming fizzy, should i add a cool mix or is there somthing that is making her fizzy in her food...