Professional users can access this tool which will allow you to average a selection of information across serveral years.
Averaging can be done based on either crop type, variety, holding farm, or field owner. You can also limit the sections within each of these to quicken the calculation. Select the starting season and how many years previous (maximum 20) to calculate the average. The results will display the average within each year, as well as a total average for the complete time span.