Improving Nitrogen Use Efficiency in Cereal Grain Production with Optical Sensing and Variable Rate Application

and 10% in corn (Hilton et al., 1994). Fertilizer N losses due to surface runoff range between 1 and 13% In 2001, N fertilizer prices nearly doubled as a result of increased (Blevins et al., 1996; Chichester and Richardson, 1992). natural gas prices. This was further troubling when considering that the world N use efficiency (NUE) in cereal grain production averages Urea fertilizers applied to the surface without incorpoonly 33%. Methods to improve NUE in winter wheat (Triticum aestiration can result in NH3 volatilization losses in excess vum L.) have not included high spatial-resolution management based of 40% (Fowler and Brydon, 1989; Hargrove et al., on sensed plant growth properties nor on midseason prediction of 1977). In cooler temperate climates, NO3 losses through grain yield. Our objective was to determine the validity of using intile drainage have approached 26 kg N ha 1 yr 1, or season estimates of grain yield (INSEY) and a response index (RI) 23% of the total N applied (Drury et al., 1996). In to modulate N at 1-m2 spatial resolution. Four winter wheat field general, loss of N only occurs when mineral N (NH4 experiments were conducted that evaluated prescribed midseason and NO3 ) are present in excess of plant needs (Johnson N applications compared with uniform rates that simulated farmer and Raun, 1995). practices. Our methods recognize that each 1-m2 area in wheat fields needs to be sensed and managed independently and that the need for fertilizer N is temporally dependent. Averaged over locations, Spatial Scale of Nitrogen Availability NUE was improved by 15% when N fertilization was based on Conventional N fertilization practices apply a single optically sensed INSEY, determined for each 1-m2 area, and a RI rate over areas of tens to hundreds of hectares before compared with traditional practices at uniform N rates. the crop is planted. Following extensive soil sampling, optical sensor measurements of plants, and geostatistical analyses, we determined that the spatial scale of N availW consumption of fertilizer N was 85 529 551 ability was at 1 m2 and that each square meter needed Mg in 1999 (FAO, 2001). Of the total fertilizer to be treated independently (Raun et al., 1998; Solie et N consumed, cereal production accounts for 60%, or al., 1999). This contrasts with the 1-ha-grid soil sampling 51 317 730 Mg (FAO, 1995). Only 33% of the total N currently promoted in precision agriculture. At a typical applied for cereal production in the world is actually cost of $10.00 per sample for soil analyses, soil sampling removed in the grain (Raun and Johnson, 1999), much to manage at the meter level is impractical. less than that generally reported (Hardy and Havelka, 1975). In 1999, the unaccounted 67% represented a Response Index $15.9 billion annual loss of N fertilizer (Raun and Johnson, 1999). With the increasing costs of N fertilizer due Evaluation of grain yield response to N fertilization to natural gas shortages, the unaccounted 67% is now in 15-yr corn and 30-yr wheat experiments has shown estimated to be worth more than $20 billion annually. that check plots where no N has been applied exhibit Considering these poor use efficiencies and the associwide variation in the supply of soil N from year to year ated costs of improper management, technological ad(Johnson and Raun, unpublished, 2002). This temporal vances are needed to reduce excess nutrient applicadependence of N availability reinforces the need for tions. midseason measurements that account for N supplied through mineralization. Raun et al. (2001) developed Low Nitrogen Use Efficiency an index to predict potential grain yields with no added fertilization (YP0 ). However, it was necessary to deterNitrogen use efficiency (NUE) in cereal grain producmine the potential yield increase that could be achieved tion is low for a variety of reasons. Plant N losses as from in-season applications of fertilizer N. This work NH3 have accounted for 52 to 73% of labeled N (15N) led to the development of a fertilizer response index in corn (Zea mays L.) (Francis et al., 1993) and 21% (RI) that was calculated by dividing average normalized in winter wheat (Harper et al., 1987; Daigger et al., difference vegetation index (NDVI) from a non-N-lim1976). Fertilizer N losses via denitrification have been iting strip (created in each field by fertilizing a strip at estimated at 9.5% in winter wheat (Aulakh et al., 1982), a rate where N would not be limiting throughout the 10% in rice (Oryza sativa L.) (DeDatta et al., 1991), season) by the average NDVI in a parallel strip that is W.R. Raun, G.V. Johnson, R.W. Mullen, K.W. Freeman, W.E. ThomAbbreviations: GDD, growing degree days; INSEY, in-season estiason, and E.V. Lukina, Dep. of Plant and Soil Sci., and J.B. Solie and mated grain yield; NDVI, normalized difference vegetation index; M.L. Stone, Dep. of Biosyst. and Agric. Eng., Oklahoma State Univ., NFOA, nitrogen fertilization optimization algorithm; NUE, nitrogen Stillwater, OK 74078. Contribution from the Oklahoma Agric. Exp. use efficiency; RI, response index; RINDVI, in-season response index; Stn. Received 7 Aug. 2001. *Corresponding author ([email protected]. RIHARVEST, response index at harvest; YP0, potential yield with no okstate.edu). added fertilization; YPMAX, maximum obtainable yield; YPN, potential yield with added N fertilization. Published in Agron. J. 94:815–820 (2002).