Response of aerobic rice to sunhemp seed rates and nitrogen levels under brown manuring: Nutrient uptake, nutrient use efficiency and economic returns

A field experiment was conducted during the summer and kharif seasons of 2023 at the Agronomy Field Unit, Zonal Agricultural Research Station, GKVK, UAS, Bangalore to evaluate the effect of different sunhemp seed rates and nitrogen management practices on nutrient uptake, nutrient use efficiency and economics of aerobic rice. The experiment consisted of three sunhemp seed rates (0, 50 and 75 kg ha⁻¹) and four nitrogen levels (0, 75, 100 and 125% of the recommended dose of nitrogen—RDN). Results revealed that sunhemp incorporation significantly enhanced nitrogen, phosphorus and potassium uptake in rice. Among the seed rates, 50 kg ha⁻¹ of sunhemp recorded higher nutrient uptake (104.39, 28.24 and 97.16 kg ha⁻¹ of N, P₂O₅ and K₂O, respectively), which was statistically on par with 75 kg ha⁻¹. Application of 125 per cent RDN resulted in significantly higher uptake (113.68, 30.75 and 105.63 kg ha⁻¹ of N, P₂O₅ and K₂O, respectively). The interaction between sunhemp and nitrogen levels was significant, with the combination of 75 kg ha⁻¹ sunhemp + 125 per cent RDN recording the higher nutrient uptake (116.34, 31.46 and 108.17 kg ha⁻¹ of N, P₂O₅ and K₂O, respectively). Rice + sunhemp at 50 kg ha⁻¹ recorded significantly higher phosphorus and potassium use efficiency (89.07 kg kg⁻¹ each) and was on par with rice + sunhemp at 75 kg ha⁻¹. Among nitrogen levels, 125 per cent RDN produced the higher phosphorus and potassium use efficiency (102.61 kg kg⁻¹ each). The combination of rice + sunhemp at 50 kg ha⁻¹ with 125 per cent RDN recorded the maximum phosphorus and potassium use efficiency (106.08 kg kg⁻¹ each). Economic analysis showed that the same combination of 50 kg ha⁻¹ sunhemp + 125 per cent RDN yielded the higher gross returns (₹147,725 ha⁻¹), net returns (₹89,063 ha⁻¹) and benefit-cost ratio (2.52), confirming its superiority for aerobic rice cultivation.