I think you have to be careful about what you call 'underweight'
A super fit well muscled horse can actually look thin compared to how it would look when it came in off grass prior to getting in shape for a seasons work
You have to look at the body score really, muscle tone, general health - are the eyes dull, is the coat lacking a healthy shine, is the skin dry and dehydrated. What are the horses energy levels like
We have 2 OTTB's at our barn. They are both EXTREMELY tough keepers. We have to give them lots of grain, and they are both still ribby. But if they don't get worked, they are so hot-headed you can't handle them. Sooo, it's a tough one with them.
I got my horse severely underweight and his owner had been riding him hard (running barrels ect). He had horrible back pain that took months of chiro and rest to fix, is incredibly cinchy and will rear occasionally when cinched, and has a reluctant attitude about being ridden now. It's taken months of chiro, vet and now finally working with a trainer to try to get him back to being a working member of equine society. Posted via Mobile Device