You might also like to get the AI Extention mod. It is a helper mod that is really helpful in single player. Once you get it just use 'ctrl + 'H' to bring up the interface.

I have had that happen on several occations. It is probably the spread overlapping the other field on the last pass of the first field. Therfore the AI helper thinks there is more to go.

Also take note that you only have to plant once. It will regrow unless you cultivate or plow the field again.

I am having the same problem with the A8800. I tried almost every trailer and dollys, but with no luck. They need to fix it and put an update in.