Predicting Sugarcane Response to Nitrogen Using a Canopy Reflectance‐Based Response Index Value

In Louisiana, sugarcane ( Saccharum officinarum L.) N rate recommendations are established based on N response trials and further refined for specific crop age and soil type. Without accounting for current growing conditions and soil N levels, these recommendations can potentially lead to under‐ or over‐application of N fertilizers. The objective of this study was to determine if N response index at harvest (RI Harvest ) can be predicted using normalized difference vegetative index (NDVI) response index value (RI NDVI ). Sensor and yield data were collected from different N field trials from 2008 to 2010 in St. Gabriel and Jeanerette, LA. Nitrogen fertilization treatments ranged between 0 to 201 kg N ha −1 . A GreenSeeker Hand Held Optical Active Sensor (Trimble Navigation, Ltd., Sunnyvale, CA) was used to obtain NDVI readings for each of three consecutive weeks beginning 3 wk after fertilization. There was a strong relationship between RI NDVI and RI Harvest using the traditional method of determining RI, comparing plots that received high N rates to check plots, with coefficient of determination ( r 2 ) values of 0.92 for cane tonnage and 0.81 for sugar yield ( P < 0.05). When using a modified RI value, which compared all N rates to the check plot, relationships between RI NDVI and RI Harvest were comparable, with r 2 values of 0.85 and 0.81 for cane tonnage and sugar yields, respectively ( P < 0.05). Our results suggest that NDVI collected 4 wk after N fertilization can be used to predict sugarcane yield response to fertilizer N using the relationships established by either the traditional or modified RI methods.