National Agricultural Library Digital Repository (NALDR) "provides access to primarily historical USDA publications either digitized by NAL or through NAL’s partnerships with other institutions", including:
Annual Reports and Yearbooks of Agriculture
Agriculture Handbooks
Bulletins
Economic Research Service - Agricultural Economic Report
Journal of Agricultural Research
"The Census of Agriculture is the leading source of facts and figures about American agriculture. Conducted every five years, the Census provides a detailed picture of U.S. farms and ranches and the people who operate them. It is the only source of uniform, comprehensive agricultural data for every state and county in the United States." - from USDA website