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a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011-1010
b Farm Services Agency, Elkader, IA
* Corresponding author (jsawyer{at}iastate.edu)
Received for publication November 6, 2006. Determining specific N fertilization rates to achieve optimal return is difficult. Crop N stress sensing uses the plant as an indicator of N fertilization need and has potential to improve N management. However, for making N rate decisions, a calibrated relationship between measured N stress and optimum N rate is required. Corn (Zea mays L.) plant N stress was determined with a chlorophyll meter (CM) at 102 site-years of N rate trials conducted from 1999–2005 with corn following soybean [Glycine max (L.) Merr.] (SC) and continuous corn (CC). Normalizing CM readings to relative chlorophyll meter (RCM) values reduced variation and improved the calibration of N stress with the nitrogen rate difference (ND) from the economic optimum nitrogen rate (EONR). With SC the adjusted R2 (adjR2) was 0.53 for CM readings and 0.73 for RCM values, and with CC the adjR2 was 0.57 for CM readings and 0.76 for RCM values. The same statistically significant (P < 0.001) relationship between RCM values and ND was found for both SC and CC, indicating RCM critical values of 0.97 and 0.98, respectively. This indicates the same calibration for N rate determination based on RCM values can be used for both rotations. Evaluation of RCM values at multiple corn growth stages indicated the same relationship to ND at the fifteenth leaf and silking growth stages, suggesting a period of time during mid-to-late vegetative growth to collect CM readings, and make in-season N rate decisions and applications. The calibration of RCM values to the rate differential from optimum N can be used by producers to determine in-season N applications for corn across varying production conditions.
Abbreviations: adjR2, adjusted R2 index • CC, continuous corn • CM, chlorophyll meter • EONR, economic optimum nitrogen rate • ND, nitrogen rate difference from EONR • RCM, relative chlorophyll meter • SC, corn following soybean
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