I confess that analyzing a horse's diet and the results of forage testing etc makes my head spin. I'm working on understanding it better by taking every nutrition webinar I can fit into my schedule. But, what I can tell you is that unless you have tested all of the sources of your horse's nutritional intake and have had it analyzed (either by you having taken a course or by a nutritionist), you really have no idea what your horse is eating much less if it is meeting their needs.
Having a child has really brought this into perspective for me. I never thought I would be so careful about what my daughter eats but after careful observation, her body is very clear about what her nutritional needs are! She is growing so she needs a different set of nutrients from what I need or what a 80+ year old grandma needs. So we cannot feed all different breeds and ages of horses that have such varying amounts of exercise the same thing either.
I am very much a "do-it-yourself" advocate but in this case, if I had a horse of my own, I would trust someone who specializes in this field to at least point me in the right direction. For Vermonters, there is Nicole Sicely of Custom Equine Nutrition who can help you work out a nutrition plan for your horse. She also sells a supplement that has been developed specifically for our area to help meet the nutritional needs of horses being fed forage from Vermont!
Rather than me watering a very complicated topic that I'm just starting to learn more about myself, I suggest you get out there and start getting some testing done. It's not hard, expensive or even time consuming especially when you weigh it against all of the added cost and time it takes to deal with a sick or injured horse. Spring is the time when many horse owners get their horses vaccinated, dewormed, checked for dental problems, etc. Add getting your horse's diet on the right track to the list!
Here are some links to get you started:
www.CustomEquineNutrition.com - Nicole Sicely 802-730-2977