Water productivity of irrigated corn in Nebraska “This cornfield, and the sorghum patch behind the barn, were the only broken land in sight. Everywhere, as far the eye could reach, there was nothing but rough, shaggy, red grass” Willa Cather (My Antonia, 1918), novelist from Red Cloud NE Patricio Grassini Research Assistant Professor University of Nebraska-Lincoln Water roundtable meeting Oct 9, 2013

Water and stability average yields and coefficient of variation by county for maize and soybean in Nebraska (USDA-NASS) Assured water supply greatly increases yield and reduces year-to- year variation in yield. Irrigated agriculture attracted investment in livestock feeding operations, biofuel refineries, and manufacturing of irrigation equipment. Grassini, unpublished Grassini et al., In Press

Water-food nexus We need water to produce high and stable grain yields Appropriate metrics that account for both crop production and water use are needed in the discussion about water & agriculture. Water Productivity (WP, kg grain per inch of water supply) provides a good framework for the discussion

Developing a WP benchmark for corn in NE Yields were simulated over 20-y for 18 locations in western Corn Belt using Hybrid-Maize model. Crops assumed to grow under optimal conditions (no nutrient deficiencies and no incidence of pests, diseases, weeds). Model inputs based on actual sowing date, plant population, weather, and soil properties at each of the 18 locations. Rainfed Irrigated Grassini et al. (2009) *Available soil water (0-5 ft) at planting + planting-to-maturity rainfall + applied irrigation

Validation of Mean Water Productivity Function Crops grown with adequate nutrient supply and without loss from diseases, insect pests, and weeds Grassini et al. (2011)

Framework to diagnose and identify options to improve water productivity in farmers’ fields Seasonal water supply (in) Grain yield (bu ac -1 ) 1) Higher yields, same water supply with better crop mgmt 2) Less water, same yield with improved irrigation mgmt 3) Higher yields with less irrigation water Mean WP function slope = 8 bu ac-in -1 WP boundary slope =11 bu ac-in bu/ac 225 bu/ac 33 in 24 in

777 field-year observations from irrigated maize fields in central Nebraska ( ) Tri-Basin Natural Resources District Each circle represents a producer field Stars indicate weather stations ( ) or rain gauges ( ) Grassini et al. (2011)

Water productivity (WP) in the Tri-Basin NRD Producer-reported yields in Tri-Basin NRD, Each data point corresponds to an irrigated corn field. Grain yield (bu ac -1 ) Seasonal water supply (in) WP boundary 11 bu ac-in -1 Mean-WP function slope = 8 bu ac-in -1 n = 777 Maximum yields ~245 bu ac -1 Average farmer’s WP = 5.8 bu ac-in -1 Water requirement for maximum yield ~ 36 in

● Simulated yield under limited- irrigation management (75% of fully-irrigation except during the interval around silking when the crop was fully-irrigated) ■ Simulated yield under fully- irrigated conditions (irrigation based on ET O and phenology) Opportunities to reduce applied irrigation water substantially without reducing productivity Reported yield and actual water supply under pivot (  ) and gravity ( Δ ) irrigation systems Seasonal water supply (in) Grain yield (bu ac -1 ) 11 bu ac-in -1 8 bu ac-in -1 Actual Pivot Actual Surface 37,819 ac-ft yr -1 Optimal irrigation 20,639 ac-ft yr -1 Limited irrigation 33,252 ac-ft yr -1 Total saving: 91,710 ac-ft y -1 (~32% of current water use in corn!) Energy saving equivalent to annual electrical use of 4,300 houses in NE! Large scope to save irrigation water, without hurting yield, through replacement of existing surface systems by pivots and fine tuning adjustment of irrigation schedule Grassini et al. (2011)

Benchmarking yield and efficiency of corn & soybean cropping systems in Nebraska Patricio Grassini, Jessica A. Torrion, Kenneth G. Cassman, James E. Specht Collaborators: Jenny Rees (UNL Extension Educator) & Daryl Andersen (Little Blue NRD)

Data on yield, N fertilizer rate, and irrigation water annually reported from 10,000+ fields since of 23 NRDs collaborating on this project Nebraska Natural Resources Districts (NRD) data

On-farm data survey Data from 1030 dryland and irrigated fields in NE planted with corn and soybean in 2010, 2011, and 2012 Collected data include: field coordinates, yield, applied NPK fertilizer, lime and manure and time of application, irrigation, type of irrigation system, tillage system, crop rotation, planting date, crop maturity, plant density, pesticide rates and time of application, incidence of diseases and insects.

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