Im not sure about your vet but mine offers nutritional consultations, and its not just lip service. They take samples of hay and grain and get a full breakdown analyisis, they either do a blood pannel. I usually do it with our yearly check up and they compare her overall health and give me a report on what her deal is and make suggestions as needed, both specific and general (Example: Hooves need bioten, but b/c of her past injury try Recovery EQ). You might see if your vet offers something similar.
Also I have a yearly subscription to FeedXL.com
(1 year Pro, for 1 horse is $60, but you can try for a month for $15 or $20 depending on the level of info/ break down you want.) My vet recomeneded it and said that one of his clients was over supplimenting her gelding and that the $20 bucks for 1 month of the website saved her $30-something every month for a suppliment she really didnt need to feed.
Basically you put in your horses info, height weight, breed, health issues, excersize levels, etc (it gets pretty specific) and then you inputwhat you're currently feeding and supplimenting, and it has damn near everything, I tried to fool it and it couldn't find a couple local things, but if your email then a link to the product or put the nutritional info in the mail to them they put it right up.
Anyways after you put in all of your diet information, it generates a report on:
1 your horses needs based all the info you entered and
2 how your current diet is meeting those needs. Are you supplimenting too much or too little should you switch feeds b/c of the startch levels.
And then you can go in and create as many diets as you want. Play with switching grain mixes, what happens when you add a multivitamin or switch amount of forage. It will tell you everything. And then goes on to break down where the nutrition is coming from. There are some features I would like to see but heres hopeing to see them in future versions.
I love it and swear by it!