Crop Age and Varietal Impact On Agronomic Nitrogen Use Efficiency of Sugarcane.

In Louisiana sugarcane production systems, crop cycle begins with planting between the months of July to September, harvesting the first crop (plant cane) about 14 months later, then harvesting the 2^nd (1^st ratoon) and 3^rd crop (2^nd ratoon) both on an 11-month cycle. Varietal diversification is essential to ensure success of sugar industry in Louisiana. Both crop age and variety are two determining factors of sugarcane nitrogen (N) requirement. Nitrogen recommendations have been based on crop age and soil texture, and with continuous adoption of more efficient cane varieties, current recommended N rates are lower than the recommendation few years ago. This study was conducted to document the changes on agronomic N use efficiency (ANUE) of different sugarcane varieties as affected by crop age. Multiple N x variety trials were established in 2004 in St. Gabriel and Jeanerette, LA. The varieties included CP 70-321, HoCP 91-555, LCP 85-384, Ho 95-988, L 97-128, HoCP 96-540, and L 99-226. Each trial consisted of four N rates (0, 45, 90, and 135 or 180 kg ha^-1) and three cane varieties arranged in a randomized complete block design with four replications. Plot yield and quality parameters (sucrose, Brix and theoretical recoverable sugar) were determined at harvest. Agronomic N use efficiency was expressed as the unit increased in sugar yield per unit applied N. The largest sugar yield difference among varieties across the trials was 2000 kg ha^-1. The sugar yield and ANUE of LCP 85-384 (reference variety for each trial) ranged from 6500 to 7500 kg ha^-1 and 12 to 26 kg sugar kg N^-1, respectively.  Plant cane generally attained higher sugar yield than the ratoon crops. Unlike variety, crop age showed evident impact on ANUE across trials with average sugar yields of 6, 28, and 29 kg kg N^-1 for plant cane, 1^st ratoon and 2^nd ratoon crops, respectively (P<0.05). The higher ANUE of ratoon crops was attributed to higher increase in cane tonnage due to N fertilization. Both Brix and sucrose content across N rates were comparable hence only minimal impact on ANUE was observed.