The reason that the fields are clean, is most farmers use's a PP or PE (pre plant or Pre emerge) herbicde. Some do it in the fall of the year, most do it in the spring. What most of your pre-emerge herbicides consist of is some type of grass killer, and some type of broadleaf surpressent. There are very few one pass systems that work, so most generally they use a two pass herbide program. This means putting a good pre-emerge herbicide down, followed by a post emerege herbicide, with liberty and roundup being the favorites.
This also goes back to previous years. If a farmer has been running a good weed control program, there won't be many weeds in the field. Now on the other hand, if they have been trying to get by with minimum use of herbicides, alot of times they have a "flush" of weeds, which in turn gives them problems for many years to come.
