Hi Henry,
I've been told on several occasions that a battery won't freeze until the the charge gets low. I had 3 batteries in our last motor home and one of them developed severe corrosion on the terminals, and eventually lost its charge. Then when we got down below 0 for a week, the fluid froze and swelled up the battery until it looked like it was going to burst the case. The other 2 batteries held a good charge and were just fine. Keeping a trickle charge on it will keep it from freezing and as a bonus will whip that engine right over in the spring. One little bit of advice, don't get the cheapest charger you find, get one that will "cycle" the battery charge. That will have less tendancy to oxidize the plates.
(PS. If you take the battery out of the tractor, don't store it on concrete, if you have to put it on the floor set it up on a plank or a piece of non conductive material.)
I drive way too fast to worry about cholesterol.