“It's rusty too,” intones the Dungeonmaster, “and pieces of it keep breaking off. Look, you're not supposed to be farming. You're supposed to go into the forest and find the dark elves.

This is off-topic, but that anecdote should go right on top of the list of things every GM should avoid doing. Regardless of anyone's gender.

If your players want to plow the field, let them plow the field. If your players want to sit in the tavern getting drunk all day, let them sit there for a bit. When the inevitable dark elves attack and burn the fields for the tenth time (after stealing all the mead from the tavern), the combat you (the GM) crave will develop naturally.

The 3rd edition WFRP takes a more structured approach to the problem. The minions of Chaos (let's face it, it's always Chaos) get a track, with a pointer on it. Each time the players make a mistake, waste time, or bicker amongst themselves, the pointer moves up a notch. There are markers along the track; once the pointer passes the marker, certain events are set in motion, and the situation grows worse for our heroes; the exact details depend on the scenario. When the pointer reaches the end of the track, all hell breaks loose and the PCs get to make one desperate last stand against the forces of Chaos whom they failed to stop.