We have had marmots. Similar situation. I walk those fields in summer months, they are only active in the summer. The hubby shot a lot of them, on the worst year we had. He apologizes as he pulls the trigger, but there were just too many to not do this. We have neighbours who like to kill coyotes. Killing coyotes removes the marmot’s natural population control. Then they eased off a bit on the coyotes, so there were a few more of those around. Then we had two badgers who took up residence in a bank hill. Marmot population decreased substantially, between the gun, the coyotes, and the badgers. And I still walk those pastures regularly, fill in any holes with rocks, and sometimes top that off with a bit of topsoil in the bucket of the tractor.
Yes, horses are smart, and look for the holes, as long as they are thinking about it, and keep a map in their heads as to the location of holes. But if they forget and run without looking or without thinking, these can be “leg-breaker” holes. So it is worthwhile keeping up on the patrols.
We still have a few marmots, but not often out in the pastures.
We had a dandy huge marmot hole at the side of a hayfield (they also eat alfalfa- damage our crop). This must have been a huge colony of families. But they go to bed for the winter, and only the hole is left. I turned horses out for winter on this field without realizing about the hole. The horses got together, and made a job out of it. Several of them, grouped around the hole, digging. They dug that hole OUT. They worked in teams, spelled off each other as one would lose interest, another would move in, and keep at the digging. They worked at it for several weeks That colony is gone now. “Way to go, guys!”.
