Crop rotation, tillage and crop residue management for wheat and maize in the sub-humid tropical highlands I. Wheat and legume 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 the impact on crop performance of improved agronomy designed to counter these problems, in particular crop rotation, the use of legumes, reduced tillage and crop residue retention. Over the period 1991-1995 an experiment to test improved agronomy was carried out in the central highlands of Mexico, at El Batan (latitude 19degreesN, longitude 99degreesW, altitude 2250 m). Under rainfed conditions (mean annual rainfall 603 mm, one crop per year), binary rotations (with phased entry) of wheat-maize, wheat-vetch and wheat-medic pasture were compared with continuous wheat and maize; most rotations were tested under the four combinations of tillage (cultivated versus zero) and crop residue (retained versus harvested). All plots were split for nitrogen fertiliser, and appropriate herbicides were used for weed control. Wheat grain yields are reported here for the first two complete cycles of the binary rotations (1992-1995), which comprised two dry years and two wet ones. Continuous wheat yielded slightly better than wheat after maize (3679 kg/ha versus 3441 kg/ha at 10% moisture), while wheat after vetch fluctuated, being superior in the wetter years and inferior in drier ones, the latter effect associated with lower soil water and greater plant water stress. Wheat after medic was only tested under zero tillage, and was superior most years (4123 kg/ha), compared to vetch-wheat (3925 kg/ha), wheat-wheat (3658 kg/ha) and maize-wheat (3356 kg/ha). Part of the yield benefit of wheat after legumes could be attributed to a lower responsiveness of yield to nitrogen fertiliser. Overall yield was unaffected by tillage, although zero tillage was significantly higher in the drier 2 years. Besides, for wheat after vetch, zero tillage was consistently superior, especially in the drier years. Overall yield was unaffected by straw residue retention, but there was a significant increase in the driest year. Yield components, disease levels and weed counts are reported. Of significance was the greater level of tan spot in zero till residue-retained wheat after wheat and after maize, the greater number of weeds in zero till wheat after legume, and the suppressive effect of cereal residue on grass weeds. Maize yields and overall economic performance of the different systems are presented in a subsequent paper (Fischer et al., 2002). (C) 2002 Elsevier Science B.V. All rights reserved.