1. Determining the Most Effective Growth Stage in Corn Production for Spectral Prediction of Grain Yield and Nitrogen Response Department of Plant and Soil Sciences Department of Biosystems and Agricultural Engineering R.K. Teal, K.W. Freeman, W.R. Raun, J. Mosali, K.L. Martin, G.V. Johnson, J.B. Solie, and H. Zhang Oklahoma State University With the escalation in environmental concern and cost of production, researchers have recently focused on investigating more efficient means of increasing grain yield while reducing fertilizer use. This study evaluated spectral reflectance, measuring the normalized difference vegetation index (NDVI) with a GreenSeeker® Hand Held optical reflectance sensor as a function of corn (Zea mays L.) hybrid, plant population, and fertilizer N rate. Initial investigation of these variables in 2002 and 2003 concluded that higher plant populations (>49,400 plants ha-1) caused early canopy closure, resulting in NDVI peaks at V10, where as NDVI did not peak at lower plant populations (35,568 plants ha-1) until R1. In the spring of 2004 with the addition of a third site and the availability of a green NDVI sensor, the trials were reconfigured removing one hybrid and imposing two more plant populations and the utilization of both green and red NDVI. Green NDVI values peaked between V7 and V8 when compared to red NDVI (peaked at V11) and green NDVI was not affected by plant population in the vegetative stages, as was red NDVI. Plant population increased NDVI measurements and reduced coinciding coefficient of variation (CV) measurements significantly as population increased from 37,050 to 66,690 plants ha -1, but no differences occurring between 66,690 and 81,510 plants ha -1. Green NDVI, Red NDVI, and CV were all highly correlated at V7, V8, and V9 growth stages. Coefficient of Variation data from V8 showed a relationship with measured plant population at sufficient N levels. Grain yield correlated well with both green and red NDVI at V8 and V9 growth stages. Vegetative response index (RI NDVI ) peaked between V8 and V9 at responsive locations, however correlation with final RI (RI HARVEST) was limited. Regression analysis indicated that early-season grain yield prediction and vegetative RI measurement was hybrid and site sensitive and needs further refining to improve accuracy. Nevertheless, this study revealed that N response could be determined at early growth stages using either Green or Red NDVI and that the potential exists to predict grain yield using either band. Abstract Three experimental sites were established in the spring of 2004 Eastern Oklahoma Research Station near Haskell, OK on Taloka silt loam soil (fine, mixed, thermic Mollic Albaqiustoll) Lake Carl Blackwell Agronomy Research Farm near Stillwater, OK on Pulaski fine sandy loam soil (course-loamy, mixed, nonacid, thermic Typic Ustifluvent) Greenlee Farm near Morris, OK on Taloka silt loam soil (fine, mixed, thermic Mollic Albaqiustoll) Ammonium Nitrate (34-0-0) was broadcast at 0, 84, and 168 kg N ha -1 by hand and incorporated in the soil shortly before planting Two Bacillus thuringiensis (bt) gene enhanced corn hybrids identified by their maturity date (99-day and 113-day) were planted at each site in 2004 Four seeding rates were evaluated in 76 cm rows 37,050, 51,870, 66,690, and 81,510 plants ha -1 Sensor readings were taken with a GreenSeeker Hand Held optical reflectance sensor (Ntech Industries, Ukiah, CA), measuring Red and Green, normalized difference vegetation index (NDVI) at different vegetative and reproductive growth stages Corn grain was harvested by hand, removing 2 rows x 9.14 m from the center of each plot Grain yield from each plot was determined and a sub-sample was taken for total N analysis Red NDVI = [(NIRref/NIRinc)-(Redref/Redinc)] / [(NIRref/NIRinc)+(Redref/Redinc)] Green NDVI=[(NIRref/NIRinc)-(Greenref/Greeninc)] / [(NIRref/NIRinc)+(Greenref/Greeninc)] Response indices (RI) Vegetative = calculated by dividing the highest N treated NDVI average by the check (0 N rate) average Harvest = highest N treated grain yield average divided by the check (0 N rate) average Materials and Methods Conclusions  Plant population can influence NDVI and grain yield prediction  CV can be used to predict plant population (improve yield prediction)  Green and Red NDVI from V8 and V9 growth stages was highly correlated with grain yield  Green and Red NDVI worked equally well for predicting grain yield from V7 to V9  Different yield prediction curves will be necessary for Green and Red NDVI  Vegetative response index (RI NDVI ) to N peaked between V8 and V9 at responsive locations  Need for added N can be determined early in season while the crop is small enough for side-dress N applications  Regression analysis indicated that early-season grain yield prediction and vegetative RI measurement was hybrid and site sensitive and needs further refining to improve accuracy Coefficient of determination (R 2 ) Greenlee FarmV7V8V9 Green NDVI 99-day0.3680.2580.399 113-day0.3440.3190.433 Red NDVI 99-day0.1790.2540.500 113-day0.4120.2210.467 HaskellV7V8V9 Green NDVI 99-dayNA0.7510.679 113-dayNA0.6440.673 Red NDVI 99-dayNA0.7450.671 113-dayNA0.5980.558 Lake Carl BlackwellV7V8V9 Green NDVI 99-day0.4290.5020.548 113-day0.1630.3220.250 Red NDVI 99-day0.5450.5490.529 113-day0.1430.2810.273 Exponential regression, NDVI and grain yield Linear regression, RI NDVI and RI HARVEST Coefficient of determination (R 2 ) Greenlee FarmV7V8V9 Green RI NDVI 99-day0.0390.0100.183 113-day0.4340.2550.614 Red RI NDVI 99-day0.0690.0030.086 113-day0.3250.5410.556 HaskellV7V8V9 Green RI NDVI 99-dayNA0.2350.056 113-dayNA0.0920.251 Red RI NDVI 99-dayNA0.1610.239 113-dayNA0.3960.529 Lake Carl BlackwellV7V8V9 Green RI NDVI 99-day0.7640.8190.757 113-day0.3810.8230.625 Red RI NDVI 99-day0.5930.6030.686 113-day0.6710.6810.817 Relationship between grain yield and NDVI at V8, 113-day hybrid over three locations Relationship between grain yield and NDVI at V8, 99-day hybrid over three locations Relationship between plant population and CV from Green and Red NDVI at V8 in the 99-day hybrid with sufficient N over three locations Influence of N rate on CV from Green and Red NDVI at V8 in the 99-day hybrid with high plant population, Haskell, OK Influence of plant population on CV from Green and Red NDVI at V8 in the 99-day hybrid with sufficient nitrogen, Haskell, OK Influence of plant population on Green and Red NDVI at V8 in the 99-day hybrid with sufficient N, Haskell, OK Influence of N rate on Green and Red NDVI at V8 in the 99-day hybrid with high plant population, Haskell, OK