WRONG, WRONG, WRONG!!!
Please do not do this, grass cuttings will impact in a horse stomach much like they clump on the lawn when wet, they can then cause impaction colic, causing a big blockage in the intestines.
While I was working at a stud we had a horse come down with colic, for which we had no reason why. We almost lost her and when we went through her yard to try and find any clues we notices small amounts of clippings, the well meaning neighbour (non horsey) fed the horse his grass clippings when he mowed his lawn, of course being in a bare yard, she scoffed them down with glee and almost paid the price.