Hi:

This is not an easy question to answer as I don't think direct comparisons among counties are part of the data collected by the National Agricultural Statistics Service any more. USDA-NASS Crop County Data is in fixed field format for crop years 1972 through 1998. The data items are planted and harvested acreage, yield per harvested acre, and total production by county and Crop Reporting District. Files include data by cropping practice (such as irrigated, non irrigated, summer-fallow and continuous crop), depending on the crop and state.

Beginning in 2000, NASS no longer publishes Crops County Data files. These data (and much more) can now be found using QuickStats.  Quick Stats allows you to find information based on commodity, location and time period. It also allows you to visualize data on a map, export results, and save links for future use.

Here is the URL for that link:

http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1270

The Census of Agriculture for 2012 can be found here:

https://www.agcensus.usda.gov/Publications/2012/

I hope this helps!