Crop Rotation and another question or two!

Crop rotation - when is the "trigger" for the rotations to move "up the chain" - to "happen". I'm manually farming ATM. When they have harvested a field I select the next crop as soon as its not greyed out any more.

I've tried rotation, for example, Wheat, Fallow, Wheat. They harvest the wheat, and I'd expect it to immediately "roll up" to the next item on the list - i.e. Fallow, Wheat..(then what? blank) - but it doesn't.

Also, an observation - if you assign ALL your people to jobs (inc farming) leaving zero unnasigned villagers - the farmers hand plough. They dont' use the Oxen, no matter how many you have. Have a free couple of families and they do the ploughing. Seems its not a job for farmers! So at harvest time I now always makes ure there are 2 unnasigned families so they can ploug 4 fields at once, which they do.