Just had another thought. Pumping air is more energy efficient than pumping water.

How great are the cattle's daily water needs? Could you put a diffuser in the tank and use aeration to keep one section ice free?

You also might consider some insulation. If you stapled a black plastic drop cloth onto some foam insulation and covered 80% of the tank surface, you might get a significant improvement. That might work well for your upcoming weather with highs of 38F and lows of 20F.

I do NOT think that would work for the stretches of nights with lows of -4F, but every rancher I know has to put in heroic measures for their cattle for those brief periods.