NO! Feeding horses on lawn mower clippings can be very dangerous for several reasons. When lawn mower clippings are fresh they are fermenting (this is why they are warm or even hot when you put your hand inside a fresh pile of clippings). If a horse is given a pile of fresh clippings to eat he/she can gorge on them. As the clippings have been chopped up small (by the mower) the horse does not need to chew them and therefore swallows the clippings without mixing them with saliva (which is what happens when a horse chews its food normally). This means that the clippings arrive in the stomach already fermenting and without the benefit of saliva to ‘dilute’ them (in the normal situation grasses that are eaten by the horse do not start to ferment to this extent until they are much further along in the gut). The gases given off by the fermenting clippings can expand to the point that they rupture the stomach (which is fatal). If the clippings do not cause rupture of the stomach, they can result in colic (abdominal pain) due to complications further down the intestinal tract.
Horses are particularly susceptible to poisonous plants, and in this situation to a high level of gas build up in the stomach, because they have a one way valve on the stomach that prevents vomiting (or even burping). If a human or a dog were to find itself in this situation they would be able to bring the offending food back up or at least bring the gas back up and relieve the pressure. A horse cannot do this.
When a horse eats lawn mower clippings they may also eat plants that they would otherwise avoid (because they are poisonous etc.) but because they have been chopped up with other more palatable plants the horse cannot detect them. Additionally the lawn may have been sprayed with chemicals which the horse will then ingest along with the clippings.
Likewise garden waste (such as prunings from bushes and trees) can be dangerous unless you know for certain that they are not poisonous.
If your horse lives in a paddock that has suburban properties adjoining then be aware that the property owners may be dropping clippings/garden waste over the fence on a regular basis. It is usually a good idea to ‘double fence’ in such an area. A simple electric fence several feet to the inside of the perimeter fence may be enough. Post notices asking people to not feed your horse/s. Chat to neighbours about the risks and ask them not to drop any garden waste over the fence. In many cases people think they are doing your horse a favour and once the dangers are explained to them they will stop doing it.
For more information please see: https://www.equiculture.net/responsiblehorsecare