Do you wear gloves? If not, then try using some. Make sure they don't fit too tight.

You say you aren't using toe straps, so I am assuming you are not using clipless pedals and shoes either? What kind of shoes are you using? If they have soft soles then they can be putting too much pressure on the ball of your foot, causing numbness. Try using shoes with stiffer soles, or get a pair of mountain bike shoes.