I've pumped many a gallon of RV antifreeze (propylene glycol based) into tractor tires. It's non toxic, it actually helps keep the rubber fresh, as in not drying out, it won't rust the rims, it's non-flammable, and if you catch it on sale it's cheaper than automotive antifreeze.
If I had my druthers, I would choose liquid ballast over bolt on weights most any day. With the liquid, the tractor itself is not actually supporting the weight, which equates to less wear and tear in the long run.
