1. In-Season Nitrogen Fertilization Based on Sensor-Estimated Potential Yield
E.V. Lukina, K.W. Freeman,K.J. Wynn, W.E. Thomason, G.V. Johnson,
M.L. Stone, J.B. Solie, W.R. Raun
Oklahoma State University
Department of Plant and Soil Sciences

2. Introduction Low nitrogen use efficiency for cereal production (33%)
In-season reflectance readings from wheat at high resolutions (1m2) have been shown to be highly correlated with biomass
Our present focus has been to apply N rates to each 1m2 area based on predicted potential yield
Potential grain yield: yield predicted for a given year and site, based on the assumption that the level of growth factors responsible for early stages of development of the crop will be maintained (limitations that existed at early stages of growth will continue to similarly influence development to maturity, e.g., N deficiency)
Currently the nitrogen use efficiency for cereal production is estimated at 33% worldwide, which results both in economic loss and environmental risk. In OSU, we’ve been working to increase NUE using sensor-based systems. We found that in-season reflectance readings from wheat at high resolution as 1m2 was highly correlated with biomass. Our present focus has been to apply N rates to each 1m2 area based on predicted yield potential. Here yield potential is defined as the theoretical maximum dry grain yield that could be produced per unit area when manageable yield factors are non-limiting in a specific year.

3. Objective
To investigate the potential for N fertilization using in-season estimates of potential yield for every 1m2 based on plant reflectance readings
The objective of this study is to investigate the potential for N fertilization using in-season estimates of yield potential for every 1m2 based on plant reflectance readings

4. Materials and Methods Winter wheat Coker, Custer, and 7853
Experimental design
Randomized complete block
Four replications
4m x 7m plots
1m2 subplots for variable rate treatments
Spectral reflectance readings at 671nm (red) and 780nm (near infrared) taken at Feekes physiological growth stages 4 and 5
We conducted two experiments in different locations in OK. One field was planted with winter wheat coker, the other with 7853.
We used a randomized complete block experimental design at both locations, with four replications. The plot size was 4m x 7m. For treatments with variable N rates the plot was divided into 28 1m2 subplots.
We took spectral reflectance readings at Feekes physiological growth stages 4, which was in mid-Feb and Feekes 5, which was in early March. The wavelengths we measured were 671nm in the red range and 780nm in the near infrared range

5. Optical sensor developed by OSU
Here is the optical sensor we used. It’s mounted on a John Deere tractor. It measures both solar irradiance and plant surface reflectance.

6. Materials and Methods
NDVI (Normalized Difference Vegetation Index) calculated for each subplot (1m2) by the equation (NIR-Red)/(NIR+Red)
For two of the treatments, N rates were determined based on tissue N need at F4 and F5, respectively
For the yield potential treatment, N rates were determined based on EY (estimated yield) index:
EY=(NDVIF4+NDVIF5)/GDD
GDD=(Tmin+Tmax)/ °C
After the readings were collected, a Normalized Difference Vegetation Index was calculated for each subplot by the equation: NIR minus Red over the sum of NIR and Red.
For fixed rate treatments, we used N rates of 40 and 80lb/ac respectively.
For two of the other treatments, N rates were determined for each subplot based on tissue N need at F4 and F5 respectively
For the yield potential treatment, N rates were determined for each subplot based on INSEY index, that is in-season estimated yield. INSEY was computed using the sum of NDVI at Feekes 4 and 5, divided by the growing degree days between the two readings. Growing degree days was determined by the average of the highest and the lowest temperatures less 4.4 centigrade.

7. Materials and Methods
Ammonium nitrate applied after sensing in early March (March 6-13)
Plots harvested by treatment (4m x 7m) in mid June
Grain weight and percent moisture automatically recorded
Total N in grain samples analyzed by dry combustion (Schepers et al., 1989)
Immediately after the second reading in early march, ammonium nitrate fertilizer was applied
Plots were harvested by treatment (4m x 7m) in mid June
Grain weight and percent moisture were automatically recorded
Total N in grain samples were analyzed by dry combustion

8. Covington Feekes5 NDVI Contour Map
Spatial variability
0.24
0.28
0.32
0.36
0.40
0.44
0.48
0.52
0.56
0.60
0.64
0.68
0.72
0.76
0.80
Covington Feekes5 NDVI Contour Map
This is the contour map of NDVI at Feekes 5 for the field in Covington, OK. It reflected the significant microvariability in the field. NDVI values not only varied along the slope and among treatments in the same replication, but also among subplots in one treatment. It indicated the need to treat each subplot differently, and so we did.

9. This is the field in Morrison, OK, before the fertilization.

10. This is the field at Covington, OK
This is the field at Covington, OK. We were applying ammonium nitrate to each 1m2 area by hand.

11. Results
Due to weed problems and high soil test N, one field experiment in did not respond to N fertilizer
Grain yield and N fertilizer requirement were highly correlated with yield potential-based N fertilization for both years at both locations.
Grain yield, N uptake and NUE were increased using EY compared to fixed rates in 1999, while the next year results were not consistent with the first year of study.
Because of serious weed problem and high residual soil test N, the field in Morrison did not respond to N fertilizer. So the results and conclusions we drew were based on data from the field in Covington.
By analyzing the data, we found that grain yield and N fertilizer requirement were highly correlated with yield potential-based N fertilization.
We also found that Grain yield, N uptake and NUE were increased using INSEY compared to fixed rates

12. Results
The highest grain yield and N uptake during the second year of experiment was obtained on the plots with fixed N rate of 90 kg N per ha.
The highest NUE’s were observed on the yield potential-based N fertilization plots at Morrison location, and 45 kg of N per ha (fixed rate) at Covington.

13. Grain yield response to N rates, Covington, 1999
First, let’s look at the response of grain yield to N rates. The graph on the left showed yield response to fixed N rates. The one on the right, yield response to N rates based on predicted yield potential. By comparing the two graphs, we can see that grain yield was highly correlated with yield potential-based N rates, with an r2 of

14. Grain yield response to N rates, Covington, 2000

15. Grain yield response to N rates, Morrison, 2000

16. N fertilizer requirement as influenced by N rates, Covington, 1999
These are the graphs of N fertilizer requirement as influenced by N rates. The y axis is the amount of N in kg required to grow a ton of wheat. It’s also highly correlated with yield potential based N rates, with an r2 of

17. N fertilizer requirement as influenced by N rates, Covington, 2000

18. N fertilizer requirement as influenced by N rates, Morrison, 2000

19. Grain yield response to Treatment
In this graph, we compared grain yield and N rates among treatments. 40 and 80 are treatments with fixed N rates of 40 lb/ac and 80lb/ac, respectively. YP stands for yield potential treatment. F4 and F5 are treatments based on tissue N need, computed using NDVI at Feekes 4 and 5, respectively.
The yield potential treatment produced an average yield significantly higher than the other treatments. About 2400kg/ha of wheat was produced by the yield potential treatment with less than 80 kg N/ha, while the fixed rate treatment, with 90 kg N /ha, only produced less than 2000kg/ha.

20. Grain yield response to Treatment
YP1 (based on 98, 99, 00 forage N uptake data) YP2 (based on 00 N uptake data only)

21. N uptake response to Treatment
In this graph we compared N uptake in the grain among treatments. As we can see, N uptake for the yield potential treatment was the highest

22. N uptake response to Treatment

23. NUE response to Treatment
Here is the response of NUE to treatment. Treatments fertilized based on yield potential reached an average NUE of 38%, compared to 17% with the fixed rate of 90 kg N/ha.

24. NUE response to Treatment

25. Conclusion
EY may be used as a predictor of potential yield and for adjustment of in-season N fertilization rates
Yield potential-based in-season N fertilization may increase grain yield, N uptake and NUE
These results suggest that INSEY may be used as a reliable predictor of yield potential and for potential adjustment of in-season N fertilization rates. We expect that yield potential-based in-season N fertilization may increase grain yield, N uptake and NUE