Date of this Version
Better Crops, Vol. 95 ( 2011, No. 4)
A team of University of Nebraska-Lincoln scientists, with partial funding from the Nebraska State Legislature, addressed this challenge. They conducted 32 irrigated trials across diverse production conditions of Nebraska from 2002 to 2004 to evaluate corn response to rates of split-applied N. The results were reported in two papers published in the January-February 2011 issue of Agronomy Journal.
The average maximum yield in these trials was 240 bu/A. When the previous crop was corn (CC) and soybean (CS), the respective mean yields with no N applied were 155 and 165 bu/A, and the mean grain yield increases to reach the yield plateau were 88 and 63 bu/A, respectively. The average economically optimal N rates (EONR) were 155 lb/A for CC and 110 lb/A for CS; this assumed the value of 1 bu of grain was equal to the cost of 8 lb of fertilizer N (Figure 1; Table 1). The mean yield at EONR was 233 bu/A.
An overall measure of N use efficiency is the amount of grain produced per unit of N applied. This averaged 1.5 and 2.1 bu grain per lb N applied for CC and CS, respectively (Table 1). The Nebraska average is about 1.1 bu of grain per lb of N applied. Therefore, N use efficiency at EONR was much higher in these high yield situations than is commonly achieved in Nebraska.
One component of N use efficiency is crop recovery of applied N (i.e. the difference in plant N uptake with and without N applied, divided by the N application rate). Mean fertilizer N recovery in above-ground biomass at EONR was 67% for CC and 76% for CS (Table 1). This is almost double the national mean recovery efficiency for corn, which is about 40%. There was a linear decline in recovery efficiency of 0.2% per lb/A of N for application rates in excess of EONR.
High recovery efficiency implies little loss of applied N, and little residual soil nitrate-N remaining after harvest. Residual soil nitrate-N may be of value to a subsequent cereal crop if not lost to leaching or denitrification, but will be of little or no value to a subsequent legume crop. Mean post-harvest residual soil nitrate-N at EONR was 49 lb/A (sampled to 48 in., analyzed using water extraction and cadmium reduction) and just 14 lb/A more than with no N applied (Figure 2). Residual soil nitrate-N increased greatly with N rates in excess of EONR.
Another component of N use efficiency is the conversion of plant N to grain N, or physiological efficiency. This is a function of N harvest index and grain N concentration. Grain contained 64% of the plant N. The average grain protein level at EONR was above 8%, as indicated by a grain N concentration of 1.3%, an increase from 7% protein with no N applied. Mean physiological efficiency at EONR was approximately 42 lb of grain per lb of N.