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Fluid Journal : Fluid Journal 2008-2009
was again the best estimate of yield variability with an r-square of 0.74. At this corn growth stage the NDVI sensor r-squares increased for both sensors with the Crop CircleTM having an r-square of 0.59 and the GreenSeekerTM having an r-square of 0.66. The increase in r-square at this growth stage correlates to the increase in plant biomass for higher NDVI readings and the more linear leaf N content compared to the V8 growth stage. At the V10 growth stage, SPAD reading and plant height variables had similar r-squares with 0.59 and 0.62, respectively. Again, there were no significant multiple regression values and even with the insignificant results there were no r-squares higher than 0.74. These results suggest that at the V10 corn growth stage plant leaf N content is the best indicator of grain yield. V12. The primary difference between the V12 and V10 growth stages is in plant height where the r-square drops to 0.49 and the V12 growth stage has a significant multiple regression with a combination of soil N and leaf N yielding an r-square of 0.84. However, this value is curious considering soil N content did not correlate to N application rate of corn grain yield and the individual soil N r-square is only 0.10. This multiple regression most likely is an artifact of the data and we have to assume that leaf N content again best explains the corn grain yield variability. V14. Results at this growth stage show that the NDVI sensors best explain corn grain yield variability, with the Crop CircleTM having an r-square of 0.71 and the GreenSeekerTM having an r-square Mr. Shaver is a research associate/Ph.D. candidate, Dr. Khosla is an associate professor, and Dr. Westfall is a professor in the Department of Soil and Crop Sciences at Colorado State University. of 0.75. As the corn continues to get bigger more light is reflected to the sensor, resulting in more accurate NDVI calculations and higher r-square values when explaining yield variability. At this growth stage the leaf N content r-square drops to 0.50, suggesting that the levels of N in the leaf are now too low to accurately equate to yield as N is most likely being distributed to other parts of the plant to prepare for tasseling and subsequent pollination. Again, no multiple regressions were significant and all other measured variables had low r-squares. Overall Flag leaf N content best correlated to grain yield at the V8, V10, and V12 crop growth stages. At V14, NDVI had the highest correlation to grain yield and was similar for both sensors tested. No combination led to a higher correlation with grain yield than both leaf N content and NDVI did at the aforementioned growth stages. While we did not increase yield correlation in this study, the variables tested did help to increase our understanding of how these variables correlate with grain yield and provide valuable information for subsequent attempts to increase yield estimation accuracy. Figure 7. Corn grain yield by N rate. Figure 6. Corn plant height by N rate. N Rate p-value = <0.0001 LSD 0.05 = 8.1 Height (cm) 0 50 100 175 V8 V10 V12 V14 120 80 60 100 40 20 0 0 20 40 60 80 100 120 140 160 180 p-value = 0.0006 LSD 0.05 = 36.2 N Rate 0 50 100 175 Yield (bu/ac)
Fluid Journal 2005-2007