Crop rotation, tillage and crop residue management for wheat and maize in the sub-humid tropical highlands II. Maize and system performance

Cropping systems in the sub-humid tropical highlands are characterised by continuous cultivated cereal monoculture, leading to serious erosion and fertility decline. There is a lack of information on improved agronomy, in particular crop rotation, the use of legumes, reduced tillage and crop residue retention, designed to counter these problems. Over the period 1991-1995 an experiment was carried out in the central highlands of Mexico, at El Batan (latitude 19degrees31'N, longitude 98degrees50'W, altitude 2250 m) to test the effect of improved agronomy. Under rainfed conditions (mean annual rainfall 603 turn, one crop per year), binary rotations of maize-wheat, maize-vetch, wheat-vetch and wheat-medic pasture were compared with continuous wheat and maize. Most rotations were tested under the four combinations of tillage (zero versus cultivated) and crop residue (retained versus harvested). All plots were split for nitrogen fertilizer, and appropriate herbicides were used for weed control. Maize after wheat outyielded continuous maize under all conditions of tillage, residue and nitrogen fertilization. Within the maize-wheat rotation, zero tillage with residue retention was clearly superior (average yield across N levels of 5025 kg/ha at 10% moisture) to the other tillage-residue combinations (average 4249 kg/ha), and during dry periods, showed less wilting. With continuous maize, yield was especially poor with zero tillage regardless of residue (average 3113 kg/ha), and this was associated with poor early growth and variable stunting even under wet conditions and for which there is no clear explanation. Maize after vetch yielded well with zero tillage (4372 kg/ha), but poorly with cultivation (3128 kg/ha), possibly due to less soil water at sowing in the latter. The main treatment yield variation was associated with either wilting score or radiation interception or both, all measured before tasseling. Observations of ponding and runoff during rain events indicated that runoff was negligible where crop residue was retained on the surface with zero tillage, but significant in all the other tillage-residue combinations. Without nitrogen, maize after vetch outyielded the other rotations, but these latter yielded more with N, having a much greater response to fertilizer nitrogen (18.4 kg/kg versus 4.5 kg/kg). The average net economic benefit, calculated for each cropping system by partial budgeting, was best for farmer practice with fertilizer, although vetch-maize with zero tillage and residue removal, and wheat-maize with cultivation and residue removal, both with nitrogen fertilizer, were close behind this treatment. Residue retention was disadvantaged by the high value of residues as fodder, and Wheat rotations by the lower yield relative to maize, for no grain price advantage in the Mexican market. (C) 2002 Elsevier Science B.V. All rights reserved.