I'm taking it that OP is wanting a general guide, not that they're going to take it as a 'by the book' precise rule for their particular horse. I think FeedXL.com is a great program for that. You can use actual diet analyses if you have them, but can input different pasture types, brands of feed, etc, select different diseases or conditions(such as laminitis prone) that may apply to your horse & restrict options, etc. this will give you a very good 'general guideline' to go on.
Doesn't give amounts of water though as that should be a free choice thing. But in hot/dry environs, horses can drink up to about 90L per day.
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