Usually as mathematicians when we see a problem like this we assume some things are ideal until the problem is simple enough that we can solve it.
For example in this problem we might assume that the bale is perfectly cylindrical, that there is no friction that the ground is perfectly flat etc. The trouble with this problem is that if make all of those simplifications we find that you can use any gear ratio you like and the bale will move (speed may vary), yet we know that in practice this is not true.
Probably the one simplification that we really cannot make is that the bale is circular. Instead it will have a flat side on the ground and this means that as it starts to turn it will tend to rise going over the corner between the flat "side" and the circular circumference. It will also deform so that the flat side moves around the circumference and this will consume energy.
Now I think this is a practical problem that you actually want to build, so I suggest you need to find a way of measuring (in the paddock!) the torque required to turn the bale. Then you can easily find the required gearing ratio.
With a strong arm somehow attached to your male and a spring balance (perhaps fisherman's scale?) you should be able to find a way to calculate the torque.
You will probably find that the torque required to start the bale turning will be considerably bigger than the torque required to keep it turning. And of course you may find that the cordless drill simply is not powerful enough.