I've loaded the Dragon color swatches into Photoshop and there's only one entry for furrows. No altitude variations. I believe there's something wrong with them.
I've checked Dragon scripts and found no entries related to furrows (I think it's just me not searching in correct place)
Anyhow, I suggest you to try using CenterED to place the farmlands after you're done with mainland in Dragon. You don't need a lot of them (like, an island made of furrows), so it's not hard to place farmlands by hand there. And yes, you can tile them. CenterED is just too awesome to be ignored
Here's an example of farmland I've done in Centered withing 10 minutes:
http://www.picamatic.com/show/2008/05/2 ... 0x1160.PNG