We have been having problems with our septic system again this winter.  After having the tank cleaners out, it appears that they think the horses walking over top of the drainfield pushes the frost down into the pipes. I'm not sure if any serious (expensive...) damage has occured yet, but thought I would mention it to help anyone else prevent this type of problem. 

We have a very small pasture area, so we try to take advantage of every inch - which means our pasture comes right up to the back of our deck.  I think in the future, I will fence off that area and make sure they stay off it during the winter months and possible only use it occasionally during the summer months.

Be sure to be careful if you have horses over top of any septic or waterlines during the winter months. Driving a vehicle over pipes can also push frost down into them. This can be a huge concern in cold weather states!