3Timsina J, Connor D J. Productivity and mauagenment of rice- wheat cropping system: Issues and challenges [ J ]. Field Crops Res, 2001, 69(2): 93-132.
4Yadav R L, Dwivedi B S, Pandey P S. Rice-wheat cropping sys- tems : Assessment of sustainability under green maturing and chemi- cal fertilizer inputs [ J ]. Field Crops Res, 2000, 65 ( 1 ) : 15 - 30.