Not so much about the alfalfa (I know nothing about that other than horses eat it, but it doesn't sound like they did much damage) but about the irresponsibility towards his horses the guy is showing. They had no water, as you said. Horses can't go without water! If it'd been me I'd probably have brought the poor creatures a couple buckets, even if they weren't mine; not their fault their owner is a moron. (Tash is right about not jumping to conclusions but even if the owner had extenuating circumstances for why his horses were in someone else's field for five days, there is *no excuse in the world* good enough to explain why he couldn't bring them a bucket of water).

Also, what if something had happened to one of them? If one had got sick? He wouldn't know, they were in your field. Or if one had spooked and run into your fencing/machinery/whatever you have there; horses can do amazingly stupid things to themselves sometimes. If you're a horse owner you should be aware of that and you cannot just drop it on someone else.

If it happens again I'd report him to animal control straight away.