When I was WWOOFing the farm I was on had washers so I could of easily taken only one pair of work clothes and washed them every few days. For winter weather, depending on your level of what's adequate hygiene, I suggest 2 pairs of long johns and a thick layer of work strength clothing. Depending on the type of farm/s your visiting. Maybe a strong pair of knee reinforced khakis and a long sleeve well-stitched shirt. A good hat that covers your ears, a decent coat and buy a cheap pair of stretchy kids gloves and snip the finger tips off (you can pull thicker clothes over top if you need it but the finger tips being off help with gripping while keeping you hand warm).
Like Mike said don't feel the need to pack like a backpacker, working while traveling requires extra equipment! Good luck!