Has the golden age of U.S. agricultural productivity growth ended? We analyze the detailed patterns of productivity growth spanning a century of profound changes in American agriculture. We document a substantial slowing of U.S. farm productivity growth, following a late mid-century surge—20 years after the surge and slowdown in U.S. industrial productivity growth. We posit and empirically probe three related explanations for this farm productivity surge-slowdown: the time path of agricultural R&D-driven knowledge stocks; a big wave of technological progress associated with great clusters of inventions; and dynamic aspects of the structural transformation of agriculture, largely completed by 1980.