Relation Between Early Season-Measured Agronomic Variables and Sugar Yield Responses to Nitrogen.

Since sugarcane is a semi-perennial crop, its nitrogen (N) recommendation is commonly refined based on crop age and soil texture in Louisiana production systems. However, this method can over- or underestimate cane N rate requirements due to temporal and spatial variability. Several researchers have demonstrated the use of mid-season plant N response to refine N rate recommendation. The objective of this study was to evaluate and relate the early-season response to N fertilization (RI) of select agronomic variables (dry biomass, tiller number, and canopy height) to RI of measured sugar yield at harvest of three sugarcane varieties (HoCP 96-540, L 01-283, and L 99-226).  A variety x N (0, 45, 90 and 135 kg N ha^-1) trial was established at the LSU AgCenter Sugar Research Station in St. Gabriel and New Iberia Research Station in Jeanerette, LA from 2010 to 2012. Biomass, tiller number, and %N were measured once a week for four consecutive weeks from three weeks after N fertilization (WKN). At harvest, sugar yield was collected and its N response was compared with mid-season measured agronomic parameters. Nitrogen response of sugar yield was highly variable and high N supply did not necessarily translate to high sugar yield and vice versa.  The positive correlations between N response of agronomic variables (sugar, r = 0.7, and cane, r = 0.69) collected at 5 WKN and N response of sugar and cane yield at harvest were considered notable. Sugar and stalk yield increase due to N was also correlated to RI_N% at 4WKN. The findings of this study demonstrated the importance of projecting  N response in sugar yield and the potential of using N responses of agronomic variables measured early in the season for refining N rate recommendation.