This paper uses panel data for the 1980-2004 period to estimate the contributions of public research to U.S. agricultural productivity growth. Local and social internal rates of return are estimated accounting for the effects of R&D spill-in, extension activities and road density. R&D spill-in proxies were constructed based on both geographic proximity and production profile to examine the sensitivity of the rates of return to these alternatives. We find that extension activities, road density, and R&D spill-ins, play an important role in enhancing the benefit of public R&D investments. We also find that the local internal rates of return, although high, have declined through time along with investments in extension, while the social rates have not. Yet, the social rates of return are not robust to the choice of spill-in proxy.