I wouldnt feed greens (if you mean brassicas) to horses due to toxicity. small amounts are probably ok but I wouldnt do it on purpose.
swede, turnips, carrots, parsnips, apples,pears, citrus, raspberries, rosehips are all fine in moderation. obviously these all have sugar and should be fed in small amounts if you are worried about their waistline. anything evergreen isn't good for them either.
potatoes arent good, neither is bread (although have used small quantities of stale bread in marmite sandwiches to disguise medication).
horses are very susceptible to toxins-far more than ruminants.