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a b s t r a c tConservation agriculture (CA) is based on minimum soil disturbance, permanent soil cover, and crop rotation; it is promoted as a sustainable alternative to systems involving conventional tillage. Adoption of CA changes weed dynamics and communities and therefore necessitates adjusting weed control methods. The objectives of this review are to summarize literature concerning CA principles and their interactive effects on weed life cycles and community composition, briefly review CA-appropriate cultural practices for additional weed control, and identify areas where further research is needed. No-till systems accumulate seeds near the soil surface where they are more likely to germinate but are also exposed to greater mortality risks through weather variability and predation. Assuming no seed input into the system, germinable seedbanks under no-till decrease more rapidly than under conventional tillage. Reducing tillage may shift weed communities from annual dicots to grassy annuals and perennials. Surface residues lower average soil temperatures and may delay emergence of both crops and weeds. Germination and growth of small-seeded annuals will suffer from restricted light availability, physical growth barriers and potential allelopathic effects from surface residue. Crop rotation affects weeds via allelopathy and altered timing of both crop management and resource demands. Rotations should incorporate crops sown in varied seasons (e.g., autumn and spring), annuals and perennials, different herbicides, and/or various crop families. Literature indicates implementing no-till without crop rotation can result in severe weed problems; greater rotational crop diversity results in easier weed management. Additional cultural practices for CA include: (i) selecting highly competitive varieties; (ii) altering planting dates; (iii) preventing weed seed recruitment; (iv) adjusting planting arrangement, densities, and fertilizer placement; and (v) microbial bio-controls. Further research is needed concerning: (i) the interactive effects of tillage and surface residue on weeds; (ii) the use of models and/or meta-analyses to predict weed responses, and to identify intervention points in CA; and (iii) the weed-suppressive potential of longer (4+ years) rotations.

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Densely populated, intensively cropped highland areas in the tropics and subtropics are prone to erosion and declining soil fertility, making agriculture unsustainable. Conservation agriculture in its version of permanent raised bed planting with crop residue retention has been proposed as an alternative wheat production system for this agro-ecological zone. A five years field experiment comparing permanent and tilled raised beds with different residue management under rainfed conditions was started at El Batá n (Mexico) (2,240 m asl; 19.31°N, 98.50°W; Cumulic Phaeozem) in 1999. The objective of this study was to determine the soil quality status after five years of different management practices. The K concentration was 1.65 times and 1.43 times larger in the 0-5 cm and 5-20 cm profiles, respectively, for permanent raised beds compared to conventionally tilled raised beds. The Na concentration showed the opposite trend. Sodicity was highest for conventionally tilled raised beds and for permanent raised beds it increased with decreasing amounts of residue retained on the surface. Permanent raised beds with full residue retention increased soil organic matter content 1.4 times in the 0-5 cm layer compared to conventionally tilled raised beds with straw incorporated and it increased significantly with increasing amounts of residue retained on the soil surface for permanent raised beds. Soil from permanent raised beds with full residue retention had significantly higher mean weight diameter for wet and dry sieving compared to conventionally tilled raised beds. Permanent raised beds with full residue retention had significantly higher aggregate stability compared to those with residue removal. A lower aggregation resulted in a reduction of infiltration. Principal component analysis (PCA) was performed using these soil physicochemical variables that were significantly influenced by tillage or residue management. The PC1 and PC2 separated the conventionally tilled raised beds from the permanent raised beds and PC3 separated permanent raised beds with at least partial residue retention from permanent raised beds with no residue retention. These clear separations suggest that tillage and residue management have an effect on