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The study sites are located at the University of Nebraska Agricultural Research and Development Center near Mead, NE (Figs. 1 and 2). These sites are large production fields (49 to 65 ha) and provide sufficient upwind fetch of uniform cover required for adequately measuring mass and energy fluxes using tower eddy covariance systems. Two sites (41 9' 54.2''N, 96 28' 35.9''W, 361 m and 41 9' 53.5''N, 96 28' 12.3''W, 362 m) are equipped with center-pivot irrigation systems while the third site (41 10' 46.8''N, 96 26' 22.7''W, 362 m) relies on rainfall. The three sites are within 1.6 km of each other. Prior to initiation of the study, the irrigated sites (1 and 2) had a 10-year history of maize-soybean rotation under no-till. The rainfed site (3) had a variable cropping history of primarily wheat, soybean, oats, and maize grown in 2 to 4 ha plots with tillage. All three sites were uniformly tilled by disking prior to initiation of the study to homogenize the top 0.1 m of soil and incorporate P and K fertilizers, as well as previously accumulated surface residues. The soils are deep silty clay loams consisting of four soil series at all three sites: Yutan (fine-silty, mixed, superactive, mesic Mollic Hapludalfs), Tomek (fine, smectitic, mesic Pachic Argialbolls), Filbert (fine, smectitic, mesic Vertic Argialbolls), and Filmore (fine, smectitic, mesic Vertic Argialbolls). Geomorphology is shown in Figure 3.

Figure 1. Schematic of the study sites near Mead, Nebraska.

Figure 2. Field Service Agency photograph of the study area shown in Figure 1.

Figure 3. Geomorphology of the study area near Mead, Nebraska.

Since initiation of the study in 2001, all sites have been under no-till. Crop management practices (i.e., plant populations, herbicide and pesticide applications, irrigation) have been employed in accordance with standard best management practices (BMPs) prescribed for production-scale maize systems. Table 1 summarizes major crop management information (including the dates of planting and harvest, cultivars planted, and average crop yields) for the 2001 to 2003 period. To account for differences in water-limited attainable yield, plant densities were lower in rainfed crops at Site 3 than in irrigated crops at Sites 1 and 2, which follows best management practices. Nitrogen (N) was applied as urea ammonium nitrate solution. Under irrigation, N was applied in three applications (2/3 pre-plant and 1/3 as two fertigations through the sprinkler system) to improve N use efficiency. In contrast, a single N fertilizer application was made to maize in the rainfed system. Total N fertilizer rates for both the irrigated and rainfed sites were adjusted for residual nitrate measured in soil samples taken each spring before planting following recommended guidelines.

Site / Year   Crop / Cultivar   Plant Population (plants ha-1)   Planting Date   Harvest Date   Applied N (kg N ha-1)   Grain Yield (Mg ha-1)
Irrigated continuous maize (48.7 ha; Site 1)
2001   M Pioneer 33P67   82,000   May 10   October 18   196   13.51
2002   M Pioneer 33P67   81,000   May 9   November 4   214   12.97
2003   M Pioneer 33B51   77,000   May 15   October 27   233   12.12
Irrigated maize-soybean rotation (52.4 ha; Site 2)
2001   M: Pioneer 33P67   81,000   May 11   October 22   196   13.41
2002   S: Asgrow 2703   153,000   May 20   October 7   0   3.99
2003   M: Pioneer 33B51   78,000   May 14   October 23   169   14.00
Rainfed maize-soybean rotation (65.4 ha; Site 3)
2001   M: Pioneer 33B51   53,000   May 14   October 29   128   8.72
2002   S: Asgrow 2703   156,000   May 20   October 9   0   3.32
2003   M: Pioneer 33B51   58,000   May 13   October 13   90   7.72

Table 1. Crop management details and grain yield for the three sites during 2001 to 2003 (M - maize; S - soybean. Maize grain yield: adjusted to 15.5% moisture content; Soybean grain yield: adjusted to 13% moisture content).

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