Please refer to the following IL Dept. of Revenue Publication for property tax assessment questions:
http://tax.illinois.gov/publications/pubs/pub-122.pdf
Depending on your situation (is this land enrolled in CRP, CREP, or IFDA; land use history; and acreage), you may need to develop a forest management plan in order to guarantee the "other farmland" assessment, which is 1/6 the equalized assessed value as cropland.