Nitrogen Fertilizer Effects On Canopy Reflectance and Yield In Surface Irrigated Lesquerella.

Lesquerella (Lesquerella fendleri (Gray) Wats.) is a potential biofuel feedstock crop well adapted to desert growing conditions in the southwestern United States. The objectives of this study were to investigate nitrogen (N) fertilizer effects on active sensor (Crop Circle ACS-470) canopy reflectance (1m height) and to establish relationships between vegetation indices, in-season biomass, plant N and yield. We conducted a completely randomized block experiment with four replications using five urea-N fertilizer rates (0, 56, 112,168, 224, 336 kg ha^-1). There were two irrigation levels. Fertilizer was applied in five equal splits at pre-plant, 5-6 leaves stage, first flower, 4 weeks after first flower, and 8 weeks after first flower. Seed yields responded quardratically to N rates in both seasons, with the highest yields being 2200 kg ha^-1. We calculated amber (590 nm) and red (670 nm) normalized difference vegetation indices (NDVIA and NDVIR, respectively) throughout the crop season. Results show that NDVIA and NDVIR increased with N rates and appear to reach a plateau at rates higher than 168 kg N ha^-1. In both irrigation levels, NDVIA and NDVIR were able to differentiate fertilizer rates beginning approximately from one week prior to first flower. Starting from two weeks after first flower, NDVIA and NDVIR showed high correlations with plant N and seed yield.