The SCImago Journal and Country Rank (SJR) is a portal that includes the journals and country indicators developed from the information contained in the Scopus® database.These indicators are used to access and analyse scientific domains. 

Agriculture and Biological Sciences country rankings

A search on the SJR portal on agricultural domain in the period 1996 to 2012 provides the following top ten countries by documents cited in their order of 1-10 rankings -

The SCImago Journal and Country Rank is an attempt to improve journal metrics and it was designed (developed from the widely known algorithm Google PageRank®) to weight the citations according to the prestige of the citing journal, also taking into account the thematic closeness to the citing and the cited journals.

To learn more about the SCImago methodology read this paper.

Visit the portal at http://www.scimagojr.com/index.php