This paper uses panel data for the 1980-2004 period to estimate the contributions
of public research to US 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
