Your horse is YOUR horse, and that means the final decision on her care is always up to you. If your request is something that the boarding stable can do (and less feed certainly falls into that category) they should accommodate you. That being said, since you acknowledge that the barn owner is an experienced horsewoman, she probably has a good reason for doing things the way she does. I'd approach it as a potential learning experience. Ask her to explain how she formulates the feeding plan. If it turns out that she hasn't given it a lot of thought, that would be the time to tell her you'd like your horse to have less grain. But give her the benefit of the doubt first.
I agree with Martha. She's your horse so you have to make the decision. But if you're unsure find out why she's giving her the amount of grain she is before making your decision. I personally think most horses do not need grain.
If the owner does not want to talk to her, one on one, perhaps, she could have her vet talk and explain about too much grain in the diet.
I'd tell her I want to learn more about horse-keeping and ask her how she determines how much grain to feed each horse, then discuss the specifics of your horse. Based on her answer, you may agree with her decision. If not, tell her your concern and ask her to adjust the feeding for a trial period so you both can evaluate. Remember, she trying to be helpful and considerate by ensuring your horse has plenty to eat.
I agree with both Claire and Martha if feel like you don't have enough experience ask your vet and have the manager explain why she feeds the way she feeds owning horses is always a learning experience
If I were you, I would go up to her and ask politely why she does things a certain way. For example, if you wanted to confront her about the amount of grain she is giving, you could just walk up to her and say "I don't know how much grain my horse needs, but that looks like a lot that you are feeding my horse. Could you tell me why you feed that amount?". Of course, don't sound like a demanding know it all when it comes to talking to her, but act genuinely interested in hearing her reasoning of why she does things her way. You might even get some really good horsey advice from her that you can use in the future! Most people are happy to help the younger generation of horse lovers grow in their knowledge as long as they are ready to learn!
It's a careful balance. The owner's job is, ultimately, to care for your horse, which means she has to know what she's doing. You pay her to do this, which means that she works for you. That means that you have to take the responsibility in making sure she is doing her job. If you feel she isn't feeding your horse correctly, go and find out from as many sources as possible everything to do with feeding horses. Once you've done that, write down exactly why you have a problem with the way your horse is being fed and what needs to change. Then you can go to her, ask her why she feeds the way does, and tell her the reasons why you'd like to feed differently. From my experience, if you're able to give clear knowledgeable reasons, most owners don't have a problem working out any problem you may have.
why do you think she is feeding her too much grain? Have you had horses before and do you know what she was being fed where you bought her from? I am a farm manager and I usually go over the feed program with new boarders before the horse even comes and I usually ask what the horse was previously on and at what amount to stay consistent. I also ask about the turnout/exercise program the horse was on.