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Abstract:

(Sub)tropical highlands of the world are densely populated and intensively cropped. Agricultural sustainability problems resulting from soil erosion and fertility decline have arisen throughout this agro-ecological zone. Major changes are needed in land, livestock and water management in line with traditional lifestyles and customs to remedy the agricultural system. Can conservation agriculture, based on three basic principles (1) minimal soil movement, (2) retention of rational amounts of crop residues and (3) viable crop rotations, be an option towards these goals? It is essential to have an indication of the sustainability of a proposed agricultural system in addition to the potential, immediate benefits from introducing new technologies well before the catastrophic consequences of non-sustainability become apparent. Long-term experiments are best suited to test the sustainability of a given system. The International Maize and Wheat Improvement Centre (CIMMYT, Int.) has initiated various long-term agronomical trials (started 1991) at its semi-arid highland experiment station in Mexico (2240 masl; 19.31° N, 98.50° W; Cumulic Phaeozem) to investigate the long-term effects of tillage/seeding practices, crop rotations, and crop residue management on maize and wheat grown under rain fed conditions. The objective of this dissertation is to come to an integrated evaluation of the different management systems. Toward improved more stable yields? Small-scale maize and wheat farmers can obtain yield improvements through zero tillage, appropriate rotations and retention of sufficient residues (average maize and wheat yield of 5091 and 5407 kg ha−1), compared to the common practices of heavy tillage before seeding, monocropping and crop residue removal (average maize and wheat yield of 3411 and 4267 kg ha−1). Leaving residue on the field is critical for zero tillage practices. However, it may take some time—roughly 5 years—before the benefits become evident. After that, zero tillage with residue retention results in higher and more stable yields than alternative management. Toward increased physical and chemical soil quality? Zero tillage combined with crop residue retention improved chemical and physical conditions of the soil. Soil quality under conventional tillage was intermediate (irrespective of residue management), especially in its physical status. In contrast, zero tillage with removal of residues, led to low aggregate stability, high penetration resistance, surface slaking resulting in low time-to-pond values and high runoff. Throughout the growing season, the highest soil moisture content was found in zero tillage with residue retention while conventional tillage had intermediate soil moisture values. Zero tillage without residue retention had more days of soil moisture values under permanent wilting point, while zero tillage with residue retention had the least. Comparable results on the interaction between zero tillage and residue retention were obtained with permanent raised beds. Permanent raised bed planting with retention of crop residue results in more stable macroaggregates and as such a reduced erosion potential, as well as an increased protection of C and N in the microaggregates within the macroaggregates, as compared to conventionally tilled raised beds. However, the positive effect of permanent raised beds is lost when all residues are removed. Permanent raised beds with partial residue retention for both crops have provided acceptable levels of aggregate stability and C and N accumulation. Toward increased biological soil quality and soil health? Crop residue retention resulted in increased microbial biomass, micro-flora catabolic activity, increased populations of soil micro-flora that promote plant growth and suppress diseases (total bacteria, fluorescent Pseudomonas, Actinomycetes, total fungi, and Fusarium spp.) and reduced numbers of the nematode Pratylenchus thornei. The continuous, uniform supply of C from crop residues serves as an energy source for microorganisms. When residue was retained, zero tillage showed much higher microbial biomass, micro-flora catabolic activity and beneficial soil micro-flora populations than zero tillage without residue, especially for maize. Zero tillage on its own does not induce better soil health, but the combination of zero tillage with residue retention does. Zero tillage with residue removal is clearly an unsustainable practice. This highlights the importance of crop residue for dry-land rain fed production systems. The increased microbial diversity under zero tillage and residue retention may be useful for biological control and integrated pest management. In our agro-ecological environment, detrimental effects of soil-borne diseases were not observed for zero tillage with residue retention. Incidence of root rot and parasitic nematode populations were not correlated with yield. Although root diseases may have affected crop performance, they affected yield less than the positive effects of other critical plant growth factors such as infiltration and water availability. Toward a time efficient use of resources? The NDVI-handheld sensor was evaluated as a tool for crop monitoring and to provide within season prediction of yield. The handheld sensor is a non-destructive system that samples at a very high rate and, compared to other methods, can easily and time-efficiently measure a representative area for a plot. Final maize yield and biomass could be predicted 2.5 months before harvest by combining NDVI from the beginning of the season and NDVI measured at the beginning of the reproductive phase (coefficient of correlation in calibration ≥ 0.921 and 0.935 respectively). For wheat the period immediately after flowering is critical for yield prediction. The NDVI based crop growth curves clearly reflected crop performance and can explain in-season stress factors. Zero tillage induced different crop growth dynamics over time compared to conventional tillage. Zero tillage with residue retention is characterized by a slower, early crop growth, totally compensated for by an increased growth in the later stages, positively influencing final grain yield. Zero tillage with partial residue retention gave similar crop growth compared with full residue retention, indicating a promising potential to remove part of the straw for other uses (fodder, fuel etc.). Zero tillage with residue removal had low NDVI values throughout the growing season. Zero tillage with (partial) retention of crop residues results in time efficient use of resources, as opposed to conventional tillage, regardless of residue management, and zero tillage with residue removal. Toward a spatial efficient use of resources? The coefficient of variation (CV) for the NDVI measurement sequence in each plot was determined. Zero tillage systems without surface residue retention produced high CVs of the NDVI sequence and high spatial crop variability throughout the season, even after the vegetative period, as opposed to zero tillage with residue retention. Increased spatial variability throughout the season might reflect agronomical mismanagement leading to inefficient use of critical plant growth resources and increased fertility gradients. When one or more critical elements in the soil are limited, plant-to-plant competition will increase plant-to-plant performance variability and agronomical mismanagement will induce degradation processes resulting in increased spatial variability of crop performance. Comparison of the patterns of NDVI and soil moisture, made clear that soil moisture and water availability are limiting factors for our target zone, and any management practice that results in an increased time and spatial availability of this critical resource will increase production sustainability. Toward a more profitable agricultural system? Zero tillage and permanent raised bed planting with at least partial residue retention generate higher yields, lower costs, and higher incomes than the incumbent traditional agriculture management system, based on extensive tillage and residue removal. The zero tillage system with at least partial residue retention also demonstrates less income variability over years, decreasing risk. Moreover, the traditional farmer practices result in negative returns after variable costs. It is clear that an environmental friendly and profitable agriculture for the (sub)tropical highlands should be based on conservation agriculture practices. In the (sub)tropical highlands, a cropping system that includes zero tillage/permanent raised bed planting, crop rotation, and crop residue retention can have positive effects compared with commonly used farming practices of heavy tillage before seeding, monocropping and crop residue removal. Zero tillage with residue retention can clearly be a part of an integrated management scheme towards sustainable agriculture. It is clear that to develop new management practices to improve water use, reduce erosion and enhance human labour/(animal) power, the focus must be on the use of conservation agriculture both for rain fed as well as irrigated production systems which will need to be fine tuned for each system. Our results provide a sound basis to support the development of zero-till seeding equipment in order to extend the technology to the small-scale farmers of the target area.