Nitrogen fertilization of Bt cotton (Gossypium hirsutum)–wheat (Triticum aestivum) cropping system

A field experiment was conducted during the rainy (kharif) and winter (rabi) seasons of 2006–07 and 2007–08 at New Delhi, to assess the performance of 3 cotton (Gossypium hirsutum L.)–wheat [Triticum aestivum (L.) emend. Fiori & Paol.] systems as affected by nitrogen (N) fertilization. In cotton, treatments formed by combination of 3 cotton hybrids [2 Bt (‘MRC 6304’ and ‘RCH 317’) and 1 non-Bt hybrid (‘LHH 144’)] in main plot and 4 levels of N fertilization (0, 60, 120 and 180 kg/ha) in sub-plots were tested. In succeeding wheat, cotton treatments were further sub-divided to accommodate 3 levels of N fertilization (0, 60, and 120 kg N/ha). Pooled data revealed that 2 Bt cotton hybrids being at par gave 55.4% (2.99 tonnes/ha) higher seed-cotton yield (SCY) than non Bt cotton hybrid (1.93 tonnes/ha) and the cotton hybrids did not markedly alter the performance of succeeding wheat. However, the system productivity (cotton-equivalent yield, CEY) of Bt cotton–wheat was 24.2% (4.72 tonnes/ha) higher than the non-Bt cotton–wheat system (3.80 tonnes/ha). The per cent contribution of Bt cotton to CEY of system (69.7) far exceeded that of non-Bt cotton (49.6). Both agronomic nitrogen-use efficiency (ANUE) and physiological nitrogen-use efficiency (PNUE) of Bt cotton and ANUE of succeeding wheat were higher than the non-Bt cotton and following wheat. Yield, N uptake of cotton, wheat and system increased with each successive increase of 60 kg N dose from 0 to 180 kg/ha. The N response of cotton was found to be quadratic with optimum dose of 173.0 kg/ha. ANUE and PNUE of cotton were the highest with 60 (5.73) and 120 kg N fertilization (17.05) and decreased thereafter with increasing N dose. Wheat crop responded to direct application of N up to 120 kg/ha. The currently recommended N input (300 kg/ha) of the non-Bt cotton (180 kg/ha)–wheat (120 kg/ha) system can be reduced by 27.8 kg with adoption of ‘MRC 6304’ Bt cotton.