Technology and Productivity in US Corn and Soybean

The Agricultural Policy Review is primarily an online publication. This printed copy is produced in limited numbers as a convenience only. For more information please visit the Agricultural Policy Review website at: www.card.iastate.edu/ag_policy_review. C OMPETING DEMANDS from food, feed, energy, and environmental uses are placing stress on global land resources. To deal with these challenges, much hope rests on sustaining the trend of past productivity growth by developing and adopting new technologies. In this context, there is much to learn from the US experience of tremendous yield gains achieved thanks to improved crop varieties and management practices. Research at CARD has reexamined the statistical evidence concerning corn and soybean yields. The data used are county-level average yields from the USDA for the period 1964– 2010 for non-irrigated agriculture in all US counties with significant production of these two crops. The main objective was to isolate the specific contribution of the adoption of genetically engineered (GE) varieties from other key determinants, including germplasm improvement attributable to traditional breeding, and weather conditions. To measure weather impacts, daily temperatures from the nearest weather station were used to construct monthly growing degree days variables (useful temperatures in the range of 50–86 degrees), and also excess heat degree days variables (harmful temperatures in excess of 90 degrees). We also accounted for the impact of water stress via a monthly Palmer index (which measures soil moisture relative to normal conditions). The model also included the changing pattern of nitrogen application over the period studied. Our results confirm the importance of weather effects on yield, a reminder of the uncertainties and risks associated with the prospect of climate change. For both corn and soybeans, we found a positive response of yields to growing degree days and a strong negative response to excess heat. For moisture, the results show that production benefits from a dry spring and a dry harvest season, other things equal, and ample moisture in the summer months enhances yield. Increased nitrogen fertilization has also contributed significantly to yield increases, particularly for corn—the US average nitrogen application rate increased from 49 lbs/acre in 1964 to its peak of 136 lbs/ acre in 1985 (it has leveled off since then). Once weather, fertilization, and county-specific differences in soil productivity are accounted for, the remaining systematic trend in yield can be attributed to the role of improved varieties. The assumption that underlying germplasm improvement due to traditional …