Identifying an In-Season Response Index and the Potential to Increase Wheat Yield with Nitrogen

season, while potentially costly, could significantly increase NUE. Current nitrogen use efficiency (NUE) of cereal crop production Recently, methods for estimating winter wheat N reis estimated to be near 33%, indicating that much of the applied quirements based on early season estimates of N uptake fertilizer N is not utilized by the plant and is susceptible to loss from and potential yield were developed (Lukina et al., 2001; the soil–plant system. Supplying fertilizer N only when a crop response is expected may improve use efficiency and profitability. A response Raun et al., 2002). Remote sensing collected by a modiindex using harvest data was recently proposed that indicates the fied daytime-lighting reflectance-sensor was used to esactual crop response to additional N within a given year. This response timate early season plant N uptake. The estimate was index, RIHarvest, is calculated by dividing the average grain yield of the based on a relationship between NDVI and plant N uphighest yielding treatment receiving N by the average yield of a check take between Feekes physiological stage 4 (leaf sheaths treatment (0 N). Although theoretically useful, RIHarvest does not allow lengthen) and 6 (first node of stem visible) (Large, 1954; for in-season adjustment of N application. The objective of this work Stone et al., 1996; Solie et al., 1996). The NDVI was was to determine the relationship between RIHarvest and the response calculated using the following equation: index measured in-season (RINDVI) using the normalized difference vegetative index (NDVI). Research was conducted in 23 existing field NDVI [(NIRref/NIRinc) experiments in Oklahoma. Each field experiment evaluated crop re(Redref/Redinc)]/[(NIRref/NIRinc) sponse to varying levels of preplant N. At Feekes growth stages 5, 9, and 10.5, RIHarvest was accurately predicted using RINDVI (r2 0.56). (Redref/Redinc)] [1] These results indicated that the in-season response index based on sensor readings is a viable method for identifying environments (i.e., where NIRref and Redref are the near-infrared and red fields) where the potential to respond to additional N exists. reflected radiance of the crop, respectively, and NIRinc and Redinc are the near-infrared and red incident radiance, respectively. Further analyses showed that a reliR and Johnson (1999) estimated current nitroable in-season estimate of yield (INSEY) could be obgen use efficiency (NUE) of worldwide cereal protained from dividing NDVI by the days from planting duction to be near 33%, which suggests that current N to sensing date (where growing degree days 0) (Raun strategies are extremely inefficient. Current Oklahoma et al., 2002). This INSEY was subsequently used to N recommendations are calculated using the equation, estimate N uptake in the grain based on a predicted Nrec Yield goal (kg ha 1) 0.033, where the yield yield level. Finally, using predicted wheat N uptake goal is based on the average wheat (Triticum aestivum (measured by NDVI) and projected grain N uptake L.) yield for the past 5 yr and, on average, 33 kg of N from estimated yield (INSEY), topdress fertilizer N is needed to produce 1000 kg of grain. Typically, all N is rates were determined (Grain N uptake Early season injected preplant as anhydrous ammonia between midplant N uptake) (Lukina et al., 2001). Recently, a response index (RIHarvest) was proposed August and mid-September. Avoiding excess applicathat indicates the actual crop response to applied N tion of N fertilizers in crop production is one way to (Johnson et al., 2000). The RIHarvest is calculated using increase NUE (Kanampiu et al., 1997). Application the following equation: methods that avoid applying large amounts of N at any one time can also increase NUE (Wuest and Cassman, RIHarvest (Highest mean yield N treatment)/ (Mean yield check treatment) [2] 1992). The soil–plant system is capable of loss via denitrification (Burford and Bremner, 1975; Olson et al., 1979, Burkart and James, 1999), runoff (Gascho et al., Increased nonfertilizer N contribution via mineraliza1998; Burkart and James, 1999), or leaching (Goss and tion or rainfall are the most likely reasons for low RIHarvest. Goorahoo, 1995; Paramasivam and Alva, 1997). Thus, The use of RIHarvest does not allow for in-season adjustthere is more N available for loss at any given time ment of N, thus its practical value to N management during the growing season if N is applied only once per is minimal. season. Multiple timely inputs of N during the growing In-season sensor measurements of NDVI as an indicator of wheat N uptake between plots receiving N and those not receiving N can be used in the same way using R.W. Mullen, K.W. Freeman, W.R. Raun, and G.V. Johnson, Dep. the following equation: of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078; and M.L. Stone and J.B. Solie, Dep. of Biosystems Eng., OklaRINDVI (Highest mean NDVI N treatment)/ (Mean NDVI check treatment) [3] homa State Univ., Stillwater, OK 74078. Contribution from the Oklahoma Agric. Exp. Stn. Received 7 Mar. 2002. *Corresponding author ([email protected]). Abbreviations: NDVI, normalized difference vegetative index; NUE, nitrogen use efficiency. Published in Agron. J. 95:347–351 (2003).