This study evaluates the impact of public agricultural research and development (R&D) and interstate knowledge spillovers on US agricultural productivity from 1971 to 2012. Using a statelevel production framework, it tests four spillover weighting schemes: USDA regions, geographic distance, production-mix clusters, and correlation-weighted clusters. Findings confirm that R&D significantly boosts total factor productivity, with national average social internal rates of return (19%-28%) substantially exceeding average local rates (9%-15%). Results are highly sensitive to spillover modeling, with agro-ecological similarity proving more predictive than geographic proximity. Public extension services further complement R&D, underscoring the need for coordinated investment to sustain productivity.
