<p>19th Nitrogen Workshop. 2016. Skara. Sweden. </p><p>ESTIMATING THE NITROGEN FERTILIZER REQUIREMENTS OF HIGH- YIELDING HYBRID GRAIN CORN VARIETIES </p><p>VILLAR J.M., RUFAT J., BELGUERRI H., VILLAR P., PASCUAL M. </p><p>TOPIC: KNOWLEDGE TRANSFER AND IMPLEMENTATION</p><p>1. Objectives</p><p>High-yielding hybrid corn varieties (Zea mays L. ssp. May) are widely cultivated in the Ebro Valley (Spain), where corn growers tend to apply N in large amounts. Current guidelines for grain corn N fertilization indicate that a maximum of 28-30 kg N per ton of grain yield are necessary [1], although the usual recommendations are around 25 kg N t-1. While grain N concentration is not a very useful indicator for characterizing the N status of corn [2], it has been widely used to compute the N fertilizer requirements of corn. In the Ebro Valley, 78% of farmers apply more than 300 kg N ha-1 [3]. The objectives of this field study were to review and improve the guidelines for the use of N fertilizer for high-yielding hybrid grain corn varieties grown in irrigated calcareous soils in the Ebro Valley. </p><p>2. Materials and methods</p><p>We used experimental data from eight field experiments (316 experimental plots) carried out over four growing seasons (2012 to 2015). All the trials were done on commercial farms located in the Ebro Valley (North-East Spain) and received either sprinkler (seven fields) or surface (one field) irrigation. The plot area was 9 x 9 m. All the soils were calcareous and had medium textures and distinct organic matter contents (1.8-3%), but the soil depths were different. Eight hybrid cultivars (600-700 FAO cycles and one FAO 280) were planted with a row spacing of 70 cm. The planting dates were between 15 April and 1 May, except for the FAO 280 (on 1 July). N was applied at the V6 stage. The N sources were fertilizers such as urea and calcium ammonium nitrate, some of which included urease or nitrification inhibitors. The treatments also included a non-fertilized plot. The amounts of N applied ranged from 0 to 375 kg N ha-1. Soil samples were taken from all plots before planting and/or applying N fertilizer (at V6 stage) and then at harvest to measure NO3-N content. N concentration was determined by Kjeldahl digestion. A stalk nitrate test was performed on all the plots at harvest [4]. The experimental design was a randomized complete block with four replications. 3. Results and discussion</p><p>The mean percentage of N in the grain was 1.36% (±0.17, CV 12.5%), while the percentage of protein was 8.3%. The highest grain yields (>16 tha-1) were obtained with grain N contents of between 1.2 and 1.4%. These results were consistent with [5] and [6].</p><p>Our results showed that there was no relationship between yield and grain N content. There was also no correlation between applied N and the N content in the grain or stalk. However, a positive correlation was observed between applied N and the concentration of nitrate in the cornstalk at harvest. A linear plus plateau model was adjusted between available N (Soil NO3-N plus applied N) and grain yield. The critical level of available N at corn stage V6 was 370 kg N ha-1. Above this critical level of available N, no increase in yield was obtained. On average, the harvest index was around 0.5. An appropriate concentration of N in the stalk is around 0.58%. An excess of available N tends to increase the concentration of N in the stalk at physiological maturity, causing a luxury consumption of N. The N fertilizer recovery never exceeded 50%. Under appropriate irrigation management, very high yields of corn can be achieved (>16 t ha-1) in the Ebro Valley when the amount of N applied does not exceed 250 kg N ha-1. When preparing a fertilization plan for corn, the previous crop [7][8], soil organic matter content, nitrate content in irrigation water, and soil nitrate test [9] should all be considered, in addition to the yield goal.</p><p>4. Conclusions</p><p>Our results indicate that the total amount of N fertilizer applied must not exceed 275 kg N ha-1 even when there is a high yield goal (more than 16 t ha-1). In the Ebro Valley, we recommend that the amount of N fertilizer to be applied to properly managed, irrigated corn (weeds and irrigation) should range from 200 and 275 kg N ha-1 for yields of between 12 and 20 t ha-1.</p><p>References</p><p>[1] Betrán J. 2010. In: Guía práctica de la fertilización racional de los cultivos en España. 135-141. Ministerio de Medio ambiente y Medio Rural y Marino. [2] Cerrato and Blackmer. 1990. Agron J. 82:744-749 [3] Lloveras et al. 2012. Vida Rural, February. 24-30. [4] Wilheim et al. 2005. Agron. J. 97: 1502-1507 [5] Gabriel and Quemada. 2011. Europ J. Agronomy 34, 133-143. [6] Ciampitti and Vyn. 2012. Field Crops Research 133, 48-67. [7] Bundy. 2004. Univ. of Wisconsin-Extension. Madison. [8] Cela et al. 2011. Agron J. 103: 520-528. [9] Ferrer et al. 2003. Agronomie, 23, 561-570. Acknowledgements</p><p>The authors wish to thank the fertilizer companies and farmers for their cooperation.</p>