Was that a fast idle? If it was that seems low to me. On your tractor is this a belt drive alternator? May want to check the belt tension. If you raise the engine speed to a fast idle it should rise above 14.0 volts when battery is fully charged.
13.6 volts will allow for a slow charge to your battery, if you can add the pto and lights at a fast idle and still maintain this you might be okay. I doubt from your measurements that the pto clutch is the problem and you should be able to mow with it without harm. I would think your alternator is not keeping up but a bad battery can drag down the voltage too. I think it would be wise to take the battery in and have it load tested to help eliminate it as a source of the problem. If the battery checks out good and is replaced fully charged then you should get readings near 14.0 volts when running. If not then I would remove the alternator and have it tested and or repaired.
Steve