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Standard treatments of U.S. agriculture assert that, before the 1930s, productivity growth was almost exclusively the result of mechanization rather than biological innovations. This article shows that U.S. wheat production witnessed wholesale changes in varieties and cultural practices during the nineteenth and early twentieth centuries. Without these changes, vast expanses of the wheat belt could not have sustained commercial production and yields everywhere would have plummeted due to the increasing severity of insects, diseases, and weeds. Revised estimates of Parker and Klein’s productivity calculations indicate that biological innovations contributed roughly half of labor-productivity growth between 1839 and 1909.