Controlled-release urea for use in sugarcane ratoons

Urea is the main solid N fertilizer used worldwide. Among its advantages are the high nutrient content, ease of handling and application, and low cost. Urea is subject to high losses through NH₃ volatilization if surface-applied on soils. Therefore, ammonium nitrate is the preferred N source, especially in the ratoon cycles. Urea treated with the urease inhibitor NBPT and controlled-release fertilizers are options to replace ammonium nitrate. Controlled-release urea (CRU) is produced by coating the urea granules with resin or synthetic polymers so that the urea in the nucleus is not in contact with soil moisture. This study aimed to test CRU in sugarcane ratoons compared with ammonium nitrate (AN) and urea treated with urease inhibitor (UR-NBPT) at three rates: 60, 120 and 180 kg ha^-1. Efficiency was also compared with two control treatments, the N control and the NPK control. Two field experiments were established and conducted in 2020 and 2021 in Sao Paulo state, Brazil. Sugarcane yield was evaluated in both experiments. N losses as NH₃ volatilized were measured in the first experiment. All sources were equally efficient in promoting increases in sugarcane ratoon yield in both experiments. The relative yield obtained with N fertilizers and the control without N showed yield increases in the first experiment of 40, 54 and 68% for 60, 120 and 180 kg N ha^-1 as AN, respectively; 29, 61 and 61% for 60, 120 and 180 kg N ha^-1 as urea-NBPT, respectively; and 32, 62 and 67% for 60, 120 and 180 kg N ha^-1 as CRU, respectively. Yields were higher in the second experiment, but increases in yield were lower compared to the first. Increases of 7, 10 and 16% for AN in three doses, 5, 12, 15% for UR-NBPT and 7, 9 and 14% for CRU were observed, respectively. N losses as NH₃ volatilized were relatively low; CRU fertilizer showed good protection against NH₃ losses by volatilization, protecting urea for about 30 days. This protection, however, did not affect yield. NH₃ losses reached 13% of the N applied in the treatment with urea (about 16 kg N ha^-1), 11% for Urea-NBPT (about 13 kg N ha^-1) and 9% for CRU (about 11 kg N ha^-1). Analyzing the results of both experiments CRU did not show an advantage in efficiency compared to AN and Urea-NBPT.