Nitrogen stress sensing and in-season application for corn production

Determining the most economical N application rate for producing corn (Zea mays L.) is one of the more difficult challenges facing producers. Since N is a large input cost, and fertilizer costs have increased dramatically, producers are looking for methods to improve N rate management. Nitrogen rate trials were conducted across Iowa from 1999-2005 with corn following soybean (Glycine max L. Merr.) and continuous corn. Multiple rates of N were applied, with corn plant N stress sensing at the Rl growth stage with a Minolta SPAD 502 chlorophyll meter (CM). Results show a significant quadratic-plateau calibration between relative chlorophyll meter (RCM) values and differential from economic optimum N rate (EONR) for corn following soybean, with a similar relationship for continuous corn. Similarity in RCM values at VI5 and Rl stages suggest there is a period of time, rather than one critical stage, during late vegetative growth to collect CM readings. Earlier (VI5) CM sensing of plant N stress should provide adequate time for making in-season N rate decisions and N applications before Rl growth stage. To study the in-season approach to N management, trials were conducted at 22 sites across Iowa in 2004 and 2005. The treatment structure consisted of field length strips, replicated three times, with six different N treatments; 0 (control), 67 (reduced rate), 67+ (plus in-season N), 134 (agronomic rate), 134+ (plus in-season N), and 268 kg N ha⁻¹ (reference). Corn plants were sensed for N stress. The previously developed calibration that related RCM values to N rate differential from EONR was used to determine in-season N application rates. In-season N was applied with high clearance equipment before or at the Rl growth stage. Results demonstrated that N deficiency stress sensing was reasonably successfully with the 67+ and 134+ N rate strategies (68% and 82% correct N deficiency stress detection, respectively). Economic return calculations indicated the PRE application rate of 134 kg N ha⁻¹, with affirmation of N stress and determination of additional N need through plant sensing, to be a more cost effective strategy than using a lower PRE N rate