Nitrate loss in subsurface drainage and corn yield as affected by timing of sidedress nitrogen

Using chlorophyll meters, crop sensors, or aerial photography to fine-tune sidedress N application rates have been proposed for optimizing and perhaps reducing overall N fertilizer use on corn (Zea mays L.) and thereby improving water quality by reducing NO3 losses to surface and ground waters. However, numerous studies have shown that a range of sensors are unable to detect nitrogen deficiencies until at least the middle of and often late in the growing season. Sidedressing N early in the growing season has proven to be a sound strategy for optimizing yields and minimizing nitrate losses in tile drains. However, delaying sidedressing until mid-season (just before reproductive growth) has been shown to negate much of the yield and nitrate leaching loss benefits. For four years in an Iowa production field (2006–2009), we measured the crop yield and nitrate leaching losses to subsurface drain pipes in a corn–soybean [Glycine max (L.) Merr.] rotation when N fertilizer was sidedressed to corn at three different crop development stages. The first treatment had all of the N fertilizer applied when the corn was at the 2 leaf stage (V2). The other two treatments split the N fertilizer application equally between the V2 stage and when six corn leaves were fully extended (V6) or when 12 corn leaves were fully extended (V12). Waiting until the V12 stage to sidedress the remaining N is a compromise between when plant sensors may be sensitive to N deficiencies and when sidedressing may still provide a water quality benefit without a detrimental impact on corn yield. We found no consistent yield differences for corn among the three treatments with significantly decreasing corn yields in the order of V12>V2>V6 in 2008 but no differences in 2006 or when averaged over both years. Similarly, none of the N treatments affected soybean yields grown the following year. When averaged over all years, there were no significant differences in nitrate concentration or leaching losses in subsurface drains among the treatments. Thus, if crop sensors can provide N rate information for sidedressing by the V12 growth stage, optimization of N fertilizer rates for crop yield and minimization of nitrate leaching may be possible.