Can you stop grain?? Of course! If the horse isn't an athlete, 'poor doer' or otherwise needing 'high octane fuel', there's absolutely no need for it. Even if they do need the extra energy, there are other healthier alternatives anyway. Grain is generally not the best for horses anyway IMO, due to it being high starch & low fibre & hard to digest. Corn is VERY high starch - somewhere around 70%! - so should be particularly avoided.
Oats are a bit of an exception, but generally all grains fed should be well processed - cooked, micronised, etc - so that it's easier for the horse to digest and especially starchy feed should also be fed little & often(if it's going to be fed at all that is), which will maximise the amount of energy the horse gets from it and reduces the 'side effects' feeding this type of diet causes. So rather than cutting down amount of meals, cut the amount per meal.
I'd personally forget the grain - with the good pasture you speak of, you'll probably have enough probs managing their weight without it - and start feeding them a good complete nutritional supp/grain free ration balancer or such, to help their diets be better balanced.
I suppose you're not comfortable removing the grain completely because it's something that you've always done, not to mention I think you're wise if you're taking any advice here with a... lump of rocksalt!
I don't advise you take our words for it, but I do suggest you do some study into equine nutrition & feeding, to come to a more informed decision.