Improving Nitrogen Use Efficiency in Cereal Grain Production with Optical Sensing and Variable Rate Application (Agron. J. 94:815-820)
W.R. Raun, J.B. Solie, G.V. Johnson, M.L. Stone, R.W.
Mullen, K.W. Freeman, W.E. Thomason, and E.V. Lukina
Department of Plant and Soil Sciences, Department of Biosystems and Agricultural Engineering, Oklahoma State University, Stillwater, OK 74078, USA. Contribution from the Okla. Agric. Exp. Sta. * - corresponding author, E-mail: email@example.com
In 2001, N fertilizer prices nearly doubled as a result of increased natural gas prices. This was further troubling when considering that the world N use efficiency (NUE) in cereal grain production averages only 33%. Methods to improve NUE in winter wheat have not included high-spatial-resolution management based on sensed plant growth properties, nor on mid-season prediction of grain yield. Our objective was to determine the validity of using in-season estimates of grain yield (INSEY) and a response index (RI) to modulate N at 1m2 spatial resolution. Four winter wheat field experiments were conducted that evaluated prescribed mid-season N applications compared to uniform rates that simulated farmer practices. Our methods recognize that each 1m2 area in wheat fields needs to be sensed and managed independently and that the need for fertilizer N is temporally dependent. Averaged over locations, NUE was improved by more than 15% when N fertilization was based on optically sensed INSEY, determined for each 1m2 area, and a response index (RI) when compared to traditional practices at uniform N rates.
Low N Use Efficiency
Spatial Scale of N Availability
Response Index (RI)
N Fertilization Optimization Algorithm
Materials and Methods
At Lahoma and Perkins, wheat was planted late due to dry fall conditions. At all sites, severe cold was encountered in December and January, thus restricting winter growth. Spring growing conditions were good, characterized by adequate and timely rainfall, limited disease, and no frost damage.
Large differences in forage N uptake (accurately predicted using NDVI, Lukina et al., 2001) were noted at all sites, and these differences in N uptake produced large disparity in the minimum and maximum N rates applied determined using the NFOA (treatments 6, 7, and 8, Table 4). For treatment 6 (all fertilizer applied mid-season, variable rate), at Covington, the minimum was 32.4 and the maximum 102.8 kg N ha-1. This is a broad range considering that it comes from 96 1m2 sub plots (4 reps, 24m2 plot size). Similarly, a wide range was noted at the other sites, indicative of large spatial variability within relatively small areas.
At three of the four sites, (exception was Perkins), a significant response in grain yield was observed as a result of applying N (Table 3). The importance of applying preplant fertilizer in order to maximize yields was evident when comparing results from the 45 kg N ha-1 preplant + mid-season-NFOA (treatment 8) to those where all N was applied mid-season (treatments 2 and 3, Table 3).
Results from the four sites confirmed previous work showing that yield potential could be accurately predicted (Raun et al., 2001). At Chickasha, low yield potential (YP0), and a limited response to N were projected. As a result, NFOA predicted that yields would be maximized at low mid-season N rates, which was in fact observed (Table 3). Yields were maximized for treatment 8 (45 kg N ha-1 preplant + mid-season N, variable applied, 1784 kg ha-1) compared to treatment 4 (45 kg N ha-1 preplant + 45 kg N ha-1 mid-season, 1677 kg ha-1), where an additional 29 kg N ha-1 was applied with no associated yield increase. Similarly, comparing the yields obtained from mid-season-only treatments, it is apparent that treatment 6 (all fertilizer applied mid-season, variable rate average of 19.8 kg N ha-1) was equal in yield to that obtained when either 45 or 90 kg N ha-1 as a fixed rate was applied mid-season (treatments 2 and 3).
At the Perkins site, the sandy loam soil dries out quickly without timely rain, and lower soil moisture storage becomes more yield limiting than the silt loam soils at the other sites, thus, measured grain yields were lower than predicted. This anomaly has been confirmed by other studies at this site (Raun et al., 2001). In addition, predicted response to applied N from in-season NDVI measurements was overestimated by RINDVI at this site, likely due to limiting moisture at anthesis that restricted response to other adequately supplied growth factors. Because no yield response to N was noted, it was not included in the average estimates of revenue and NUE in Table 3.
Higher yields and response to mid-season N were predicted and observed at Covington. At this site, a higher N need was calculated (104.3 kg N ha-1, treatment 8) than what would normally be applied mid-season by farmers. It was therefore encouraging to find that this added N resulted in increased grain yield (3269 kg ha-1, treatment 8, versus 2744 kg ha-1, treatment 4). Projecting whether or not a response to applied N could be achieved is critical to this work. Excluding Perkins, the predicted response to applied N using optical sensor measurements (RINDVI) in early spring was positively correlated with grain yield response that could be attributed to applied N in the harvested grain (RIHARVEST).
For the four sites evaluated, the largest difference in plant growth due to preplant N nutrition was predicted to take place at Lahoma from in-season NDVI measurements and that was confirmed at harvest, two months later (0 N versus 90 N preplant). Wheat growth in treatments 2, 3, 6, and 7 was similar, and notably poor in early April when yield potential was sensed, since none of these treatments received preplant N. The response index predicted the magnitude of an achievable N response, since nearly double the yields were produced from mid-season applied N (RINDVI of 2.22 and an RIHARVEST of 2.19). Having the ability to predict that yields can be doubled if mid-season N is applied is in itself a powerful tool. Furthermore, it is equally important to know how much N to apply to achieve that doubling of yields. At the Lahoma site, 50.9 kg N ha-1 (spatially applied) was needed to produce yields projected with RINDVI, equal to 90 kg N ha-1 applied mid-season (treatment 6 versus treatment 3, Table 3). Applying the NFOA enables the determination of yield increases possible via mid-season application of N and it allows us to estimate how much N is needed to obtain that projected yield. Although applying all of the N preplant (treatment 5) produced maximum yields at this site, this management practice requires that farmers take more risk. Once a good plant stand is secured (dryland wheat production is highly dependent upon rainfall soon after planting), added fertilizer inputs can be tailored to what is made possible by the growing environment.
Averaged over the 3 sites with N response, when all N was applied mid-season based on NFOA (treatment 6), grain yields were increased (+273 kg ha-1) compared to a similar single rate, using similar fertilizer N rates (43.1 versus 45 kg N ha-1, treatment 2). At $0.10 per kg of wheat grain, this would have a value of $27.30 per hectare. When comparing treatment 6 (all fertilizer applied mid-season, variable rate) to a much higher single N rate of 90 kg N ha-1 applied mid-season (treatment 3), the same amount of grain with the variable rates was produced, but with 46.9 kg less N ha-1. At $0.55 per kg N, the savings in fertilizer N would have a value of $25.79 per hectare. Similar results were noted when ½ of the N rate (22.6 kg N ha-1) predicted using NFOA was applied, producing 1619 kg grain ha-1, contrasted with a grain yield of 1562 kg ha-1 and 45 kg N ha-1 applied at a single rate (treatment 7 versus 2).
Simple estimates of revenue (averaged over the three sites where significant differences due to treatment were observed) for all treatments are reported in Table 3 (grain revenue minus fertilizer costs). Using the same values for grain and fertilizer previously reported, treatment 8 (45 kg N ha-1 preplant + mid-season N variably applied) increased revenue by more than $9.00 over all other treatments, but required 17.5 kg N ha-1 (45 + 62.5=107.5) more N when compared to an average N rate of 90 (applied preplant, split, or all mid-season). Similar benefits of treatment 6 which used NFOA can be seen over both the 45 and 90 kg N ha-1 mid-season single rates (treatments 2 and 3), increasing revenue by more than $28.00 ha-1 while using less fertilizer N. Treatments 2, 3, and 6 received all N mid-season, the only difference being that treatment 6 received N spatially applied to each 1 m2. In either scenario this increased income will more than cover the increased technology costs, expected to be somewhere between $4.00 and $5.00 ha-1. We expect the greatest economic benefit for this practice to occur under conditions of high and spatially varying N stress.
Estimates of NUE were determined by subtracting N removed (grain yield times total N) in the grain of 0-N plots from that found in plots receiving added N, divided by the rate of N applied. Averaged over locations, NUE was improved by more than 15% when comparing treatment 2 with treatment 6 where similar rates were applied. All of the treatments that employed NFOA (treatments 6, 7, and 8) resulted in equal or increased NUE when compared to any of the single rate combinations (treatments 2-5). The environmental benefit of this increased NUE cannot be determined, but is considered important.
The Response Index (RINDVI) accounts for both the likelihood of obtaining a response to in-season applied N, and the magnitude of the response to applied N at a given level of potential yield with no additional fertilizer (YP0). The predicted yield that can be achieved with added N fertilization or YPN = (YP0)*RINDVI, will not generally be more than double YP0. Because it would be unlikely to double yields (YP0) from in-season applied N (YPN), we placed a limit of 2.0 on RINDVI. In this regard, YPMAX is needed to place limits on YPN in those cases where YPN may exceed the biological limits previously documented for specific environments. An exception to the RI limit of 2 would be expected in environments conducive to high N immobilization (e.g., no-till) or small contributions from N mineralization (e.g., irrigated desert soils).
A prototype of a commercial scale variable N rate applicator that employs the concepts discussed in this paper has been developed (www.ntechindustries.com). Implementation of the NFOA concept requires collection of mid-season NDVI measurements from optical sensors mounted ahead of each fertilization nozzle, and prescribing fertilizer rates computed on-the-go for each 1m2 area. The optical sensor based N fertilizer applicator is equipped with a GPS receiver for post processed geo-referencing of all optical sensor data. For each field, farmers will provide the date of planting in order to compute INSEY (NDVI/days from planting where GDD>0) on-the-go. Growing degree day data is available to growers through various means. Just prior to planting, a non-N-limiting strip (NLS) will be applied in each field. If farmers apply preplant N at a lower rate or if they do not apply fertilizer at all, the NLS will be used to later establish a field specific Response Index (RINDVI). Prior to applying mid-season fertilizer, the non-N-limiting strip will be optically sensed adjacent to the farmer practice in order to determine the field specific RINDVI.
Improvement in fertilizer N use efficiency beyond the promising results of these experiments may be possible from foliar applications of urea ammonium nitrate solutions (common liquid N fertilizer used for in-season applications) and by variable N rate application. Granular ammonium nitrate fertilizer applied to each 1m2 reported here likely would have decreased NUE, since, unlike foliar applied N it would be subject to surface runoff, microbial immobilization, volatilization, and denitrification prior to being absorbed by plant roots.
This study demonstrates that crop reflectance measurements using optical sensors can be used to set more efficient and profitable fertilization levels. The techniques that have been developed are appropriately applied at spatial scales of 1 m2 and will require optical sensor-equipped variable rate applicators. The techniques rely on non-N limiting test strips in fields which allow an in-season estimate of fertilizer response. The use of NFOA may eventually replace N fertilization rates determined using production history (yield goals), provided that the production system allows for in-season application of fertilizer N. Fertilizing each 1m2 area based on mid-season estimates of grain yield and the likelihood of achieving a response to added fertilizer could lead to improved NUE in cereal grain crops.
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