Exploration of agro-ecological options for improving maize-based farming systems in Costa Chica, Guerrero, Mexico

Abstract

In the Costa Chica, a region of Southwest Mexico, farming systems are organized in smallholder units. The dominant cropping systems are based on maize (Zea mays L.), either as monocrop or intercropped with roselle (Hibiscus sabdariffa L.). Continuous cropping, and unbalanced fertilizer management systems with an inadequate replenishment of organic matter stocks have caused depletion of soil fertility and low crop yields.

This thesis aimed to evaluate alternative cropping systems in terms of their contribution to on-farm productivity and to regeneration of the soil resource base. A set of approaches including farm surveys, on-farm experiments and model-based calculations was applied to characterize farming systems, identify main livelihood constraints and evaluate alternative cropping and farming systems. Main constraints identified were low yields of the major crops maize and roselle, low levels of nitrogen, potassium and soil organic matter, low resource use efficiencies, high production costs, limited marketing opportunities and low prices of products.

To address prevailing production constraints, farmer-managed experiments were established in two communities within the region. In on-farm experiments the legumes Canavalia (Canavalia brasiliensis Mart. Ex Benth) and Mucuna (Mucuna pruriens L.var. utilis (Wall ex Wight) Burk) were intercropped in (added to) maize monocrops and maize-roselle mixtures. Intercropping did not decrease maize and roselle yields, and resulted in major reductions of the weed biomass, as well as an increased N uptake by both the food crops and the cropping system as a whole. In nutrient management trials different sources of macro-nutrients were evaluated in maize monocrops and maize-roselle intercrops. The results showed that improvements at field scale are feasible in the short term. Partial replacement of mineral NPK by organic NPK in the form of vermicompost, leading to 10-20% lower total N and K inputs, did not result in lower maize yields or a reduced uptake of N and K. This suggests that the N and K from the vermicompost were utilized better by the maize crop than from the inorganic fertilizers due to lower leaching losses. An experiment on decomposition of and N release from aboveground biomass residues, crop root residues and vermicompost demonstrated that, although the pattern of decomposition varied depending on the type of organic material, most of the N was released within the cropping season. Particularly for vermicompost, only one third of its initial dry mass was decomposed, thus leaving significant amounts of residues for soil organic matter build-up. Model-based explorations were developed to assess the consequences of the experimental results at the field level for whole-farm performance. Results for eight case study farms demonstrated that changes in crop nutrition and animal husbandry can increase farm family income and improve organic matter balances. However, strategies to achieve these goals most effectively were distinct. To maximize family income required fertilizer-based cropping strategies, while rebuilding soil organic matter required investment in retaining, obtaining and applying sources of organic matter. Farms responded differently to the explored options, highlighting the need for crop nutrition strategies that are adjusted to the soil fertility status of individual fields to be most efficient. The explorations also showed that for six out of the eight farms the minimum family income standard could not be attained. The results imply that the current emphasis in policies to support smallholders by fertilizer subsidies requires adjustment to include promotion of technology development aimed at regeneration of the degraded resource base and to offer off-farm economic options.

Keywords: farm diagnosis, farming systems, soil degradation, intercropping, maize, roselle, legumes, nutrient management, vermicompost, crop residues, decomposition, explorations.