Predicting Sugarcane Response to Fertilizer Nitrogen Using An In-Season Vegetative Index.

Sugarcane is the most important row-crop in Louisiana.  Currently, determining recommended N-rate for sugarcane in Louisiana is usually based on crop age and soil type.  In-season environmental conditions not a factor in determining recommended N-rates.  Therefore, over or under application of N fertilizers is an ever-present potential risk.  The objective of this study was to determine the relationship between an in-season vegetative response index (RI) using normalized difference vegetative index (NDVI) and the response of sugarcane yield.  The study was evaluated on six site years between 2008 and 2010, in southern Louisiana.  Each trial received varying amounts of fertilizer N and was evaluated over four different varieties.  Sensor readings were taken two weeks after N fertilization using a GreenSeeker® handheld sensor for four sampling times with two weeks interval,.  Traditional means to determine RI, i.e. comparing only a non-limiting N plot to a check plot, posed a limitation in terms of number of data points in the model.  A modified RI calculation was used to determine both in-season and harvest RI.  This modified RI compared all N rates to the check plot to obtain a RI for all applied N rates.  The modified in-season RI shows potential at predicting sugarcane yield response to fertilizer N; with coefficient of determination value of 0.91 and 0.86 for cane yield and sugar yield, respectively.  In summary, NDVI collected early in the season can be used to predict the sugarcane yield response to fertilizer N.