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Fluid Journal : Spring 2017
9 The Fluid Journal Spring 2017 preplant or at-planting application of the entire rate. Specific weather conditions can influence optimal N rate and corn response to applied N. Of interest, sites in Table 1 do not consistently have preplant or at-plant N with either lower yield or higher EONR compared to split/ sidedress. Only at one site did split/ sidedress application have either more than 5 bu/acre higher yield or 10 lb N/ acre lower EONR compared to preplant or at-planting N. On the other hand, one site had yield higher with preplant or at-planting N and at three sites the EONR was at least 10 lb N/acre higher with split/sidedress compared to preplant or at-planting N. Most frequently, the EONR and YEONR were the same. The mean EONR for both N application timings is nearly the same as the suggested MRTN rate for corn following soybean in Iowa at the 0.10 price ratio (CNRC, 2016). However, some individual sites have an EONR outside the most profitable range. One 2015 site at Chariton not included in Table 1 had a yield response still increasing at 250 lb N/acre, the highest N rate applied--with both preplant and split/sidedress timing. Soils at this site have poor internal drainage, no drainage tile, and there was high rainfall in 2015. The similar N response to rate with both preplant and sidedress highlights that wet conditions can cause N loss with any N application timing, and that sidedressing will not always avoid N loss conditions. Mid-to-Late Vegetative (In-Season). With high-clearance application equipment now commonly available, N application is now readily possible later into the corn vegetative growth period (in this report called “in-season”). Table 1 has several examples that could be considered “mid” vegetative timing. A summary of those sites was presented in the previous section, with no difference in yield of EONR between at-planting and mid-vegetative Table 2. Total fertilizer N applied and corn grain yield response to pre- and post-sensing N application in 30 fields with corn following soybean, 2004-2006 (Ruiz Diaz et al., 2008). N Application Treatment† Mean Total N Applied† Number of Sites with Post-Sensing N Applied Mean Yield‡ lb N/acre n bu/acre 0 0 - 141d 60 60 - 177c 60+ 115 28 185b 120 120 - 192a 120+ 131 9 193a 240 240 - 197a † Treatment is the pre-sensing N rate, with the “+” symbol indicating post-sensing based N. Mean total N applied is the sum of pre-sensing and post-sensing N rate across the 30 corn- soybean sites. ‡ Mean yields are not significantly different when followed by the same letter (P ≤ 0.10). Mean EONR (0.10 N:corn price ratio) for pre-N rates was 132 lb N/acre. Fig. 2. Corn yield with UAN solution or urea applied preplant (PP-N) and preplant plus V10 stage sensor-based (PP+S-N) UAN, 2009 and 2010 (Barker and Sawyer, 2012). Economic op- timum N rate (0.10 N:corn price ratio) in 2009 was 98 lb N/acre and in 2010 was 190 lb N/acre. Fig. 3. Corn yield with urea applied preplant (PP-N) and preplant plus V10 stage sensor-based (PP+S-N) urea, 2012 and 2013 with corn following soybean at eight sites (Barker and Sawyer, 2016). Economic optimum N rate (0.10 N:corn price ratio) across site-years for the PP-N ap- plication was 138 lb N/acre. “Timing of application is an important component of site- specific nutrient management.”