Irrigated & Rainfed Corn/Soybean Agro-Ecosystems In the Central US Great Plains: A Perspective On Productivity & Input-Use Efficiency.

Nebraska is the third and fifth largest corn and soybean-producing state, respectively, with 60 and 45% of total acreage of these two crops under irrigation. The State’s counties are subdivided into 23 Natural Resources Districts (NRDs), with boundaries defined by watersheds. Each NRD is a political entity that has responsibility for implementing policies and regulations related to protection of its natural resources. In most NRDs, crop producers must submit annual forms documenting, for their fields, geospatial locations, crop yields, and applied nitrogen (N) fertilizer and irrigation. These data, collected from ~10,000 fields, have been routinely gathered since 2004, and represent a useful database for benchmarking geospatial variation and change over time in productivity and efficiency of corn and soybean-based agro-ecosystems in Nebraska. Preliminary data analysis indicates: (i) large variability in yields, applied inputs, and efficiencies for N fertilizer (NUE) and irrigation among and within NRDs and years; (ii) higher corn yields in irrigated vs. rainfed systems, but with similar or often higher NUE in the former vs. the latter, despite ~80% higher N fertilizer rates in the former (especially in NRDs where pre-plant N applications are not allowed and total N dose is split into several in-season applications); (iii) a yield plateau in irrigated corn yields from 2004 to 2011, though upward yield trends were apparent in rainfed corn and in irrigated and rainfed soybean; and (iv) a yield benefit of 0.5 Mg/ha of corn following soybean (versus corn after corn) – a benefit that was consistent in all NRDs, but not across years. Statistical analysis of spatial and time trends in yields, applied inputs, and efficiencies, complemented with analysis of additional management data collected in an on-going independent survey and generated with simulation models, will help identify weather, soil, and management factors that account for the observed variation.