Subtropical highlands of the world have been densely populated and intensively cropped. Agricultural sustainability problems resulting from soil erosion and fertility decline have arisen throughout this agro-ecological zone. This article considers practices that would sustain higher and stable yields for wheat and maize in such region. A long-term field experiment under rainfed conditions was started at El Batan, Mexico (2240 m a.s.l.; 19.31 degrees N, 98.50 degrees W;fine, mixed, thermic, Cumulic Haplustoll) in 1991. It included treatments varying in: (1) rotation (continuous maize (Zea mays) or wheat (Triticum aestivum) and the rotation of both); (2) tillage (conventional, zero and permanent beds); (3) crop residue management (full, partial or no retention). Small-scale maize and wheat farmers may expect yield improvements through zero tillage, appropriate rotations and retention of sufficient residues (average maize and wheat yield of 5285 and 5591 kg ha(-1)), compared to the common practices of heavy tillage before seeding, monocropping and crop residue removal (average maize and wheat yield of 3570 and 4414 kg ha(-1)). Leaving residue on the field is critical for zero tillage practices. However, it can take some time-roughly 5 years-before the benefits are evident. After that, zero tillage with residue retention resulted in higher and more stable yields than alternative management. Conventional tillage with or without residue incorporation resulted in intermediate yields. Zero tillage without residue drastically reduced yields, except in the case of continuous wheat which, although not high yielding, still performed better than the other treatments with zero tillage and residue removal. Zero tillage treatments with partial residue removal gave yields equivalent to treatments with full residue retention (average maize and wheat yield of 5868 and 5250 kg ha(-1)). There may be scope to remove part of the residues for fodder and still retain adequate amounts to provide the necessary ground cover. This could make the adoption of zero tillage more acceptable for the small-scale, subsistence farmer whose livelihood strategies include livestock as a key component. Raised-bed cultivation systems allow both dramatic reductions in tillage and opportunities to retain crop residues on the soil surface. Permanent bed treatments combined with rotation and residue retention yielded the same as the zero tillage treatments, with the advantage that more varied weeding and fertilizer application practices are possible. It is important small-scale farmers have access to, and are trained in the use of these technologies. (c) 2004 Elsevier B.V. All rights reserved.
