Purpose: This paper is concerned with the impact of the University of California Cooperative Extension (UCCE) on regional productivity in California agriculture. UCCE is responsible for agricultural research and development (R&D), and dissemination of agricultural know-how in the state. Method/methodology/approach: We estimate the effect of UCCE on county-level agricultural productivity for the years 1992-2012, using an agricultural production function with measures of agricultural extension inputs alongside the traditional agricultural production inputs at the county level. Findings: Results show a positive impact of UCCE through its stock of depreciated expenditures. For an additional dollar spent on UCCE expenditures stock, agricultural productivity, measured as value of sales at the county level, improves by $1-9 per acre of farmland for knowledge/expenditure depreciation rates between 0 and 20 percent. Practical implications: Results suggest that county differences in productivity could affect extension expenditures. The high level of contribution found in the results would be especially useful during a period of political pressure to reduce public spending for agricultural extension in the state. Theoretical Implications: Theoretical implications suggest that agricultural systems with higher level of knowledge depreciation are associated with higher resulting incremental agricultural productivity per an additional dollar spent on UCCE expenditures stock. This suggests that extension policy should consider also the agricultural system (crop mix). Originality: We use original budgetary data that was collected especially for answering our research questions from archives of UCCE. We estimate impact of extension at the county level in California, on the value of agricultural sales (of crops and livestock). We developed an extension expenditure stock, using current and past expenditures data, and different depreciation rates, following the theory of Knowledge Production Function.