Joost Brouwer

Spatial dimensions of precision agriculture: a spatial econometric analysis of Millet Yield on Sahelian Coversands

The identification of local soil variability caused by within-field differences of macronutrients and ecological features is of paramount importance for the effectiveness of precision agriculture. We present several spatial statistical and econometric techniques to capture local differences in soil variation, ecological characteristics, and yield more effectively than the analytical techniques traditionally used in agronomy. The application of these techniques is illustrated in a case study dealing with precision agriculture in the West African Sahel. The production of millet on acid sandy soils constitutes a typical example of low soil fertility areas exhibiting small absolute but large relative differences in crop production conditions over short distances.

Soil Water Balance

In the more than 80 references related to banded vegetation patterns, five continents and 12 countries are represented. The early research concerned only the vegetation pattern itself (Clos-Arceduc 1956). Hypotheses regarding ecological functioning were then proposed between 1956 and 1970 and clearly synthesized by (1971) for seven countries. He defined the minimum common characteristics necessary for the existence of banded vegetation spatial structure. These characteristics are now well known and include a semiarid climate, high-intensity rainfall, and a gentle slope. These characteristics are cited in almost all the publications on banded vegetation. We will not further review them here but focus instead on what they imply: the importance the spatial redistribution of water in the dynamics and the functioning of tiger bush bands. The areas of sealed, substantially bare soils yield a high proportion of runoff. On encountering the thicket bands immediately downslope, the flow rate diminishes and infiltration occurs, thus providing an additional water supply to the thicket. This process is more active in the upslope part of the thicket and progressively decreases to nil on the downslope edge, where many dead trees are observed. The zone located immediately upslope of the thicket is often called the “pioneer front” (Table 5.1) and is a zone of active plant colonization.

An empirical analysis of the simultaneous effects of nitrogen, phosphorus and potassium in millet production on spatially variable fields in SW Niger

Low soil fertility is a major constraint for increasing millet production on the acid sandy soils of the West African Sahel. On these soils, all three macronutrients: nitrogen (N), phosphate (P) and potassium (K), may be expected to limit crop yields. The important question is therefore: which of them is the most critical and would, if applied in small amounts, increase yields significantly? This paper addresses this question with an empirical approach, thus avoiding the commonly observed difficulty in the interpretation of agronomic research, caused by the extreme local soil variability which characterizes Sahelian coversands. We actually exploit soil variability by using novel non-parametric techniques for data exploration in combination with spatial methods of parametric model estimation. Apart from N, P and K, the effects of surface crusting, local topography, manure levels, farmer behaviour and spatial dependence are taken into account, since these may confound the true effects of N, P and K. A quadratic formulation conforms best to the data and explains 81 percent of the yield variation. The equation highlights the importance of interactions among variables and thus confirms the possible impact of native soil conditions on the outcome of fertilizer treatments in experimental research. The results of much earlier, multi-year, research are confirmed remarkably well by this single year study. In addition, a spatially explicit assessment on the crop response to increasing nutrient levels highlights that blanket fertilizer applications are inefficient, because yield increases in some places will be accompanied by yield decreases at other sites. Cash-constrained farmers therefore have to resort to precision farming techniques to maximize returns from minimal external input packages. However, a large part of the good explanation of millet yield variability over space derives from spatial autocorrelation, and not directly from topsoil N, P and K. This calls for further research on the factors that affect millet yield and on the characterization and classification of sites, followed by experimental work to design site-specific fertilizer technologies.

Relevance of key resource areas for large-scale movements of livestock

Semi-arid rangelands show much spatial heterogeneity, with some parts producing more and better quality food for herbivores. The concepts of ‘Key Resource’ and ‘Key Resource Area’ have been developed to describe a resource that ‘provides good-quality forage’ and that ‘reduces (inter-)annual variation in forage supply’. Illius and O’Connor (1999) formalised these concepts, arguing that in key resource areas herbivores experience a density-dependency relation with food resources, generally during the dry season. In other areas, generally during the wet season, non-equilibrium conditions govern the relation between herbivores and their food resources. They further argued that it is implicit that key resources show lower inter-annual variability than occurs on the (alternative) dry-season range, buffering livestock densities from climatic conditions. Key resource and outlying areas must further operate in a source–sink manner. In this chapter, we discuss the various assumptions and conclusions regarding key resources and key resource areas, using the floodplains of the Sahel, especially those of Waza-Logone in Cameroon, as examples. Sahelian floodplain grasslands are intensively exploited during the dry season, with cattle densities on a year-round basis about five times as high as in surrounding drylands. We come to the conclusion that the inter-annual variability in the quantity of the forage production of the Sahelian floodplains is not less, but often greater than that of surrounding areas. Forage quality, however, may be more constant. The model of Illius and O’Connor would be more realistic if it included intra-annual variability in forage availability, variability in accessibility of that forage, and associated differences therein between the dry-season range and the wet-season range. The importance of a resource varies from year to year, depending among other things on inter-annual variability in rainfall in the wet-season grazing range and in (the catchment upstream of) the dry-season grazing range. When it is of great importance, it may be considered a ‘key resource’, but in another context the same resource is not necessarily a key resource. Because of this spatial and temporal variability in rainfall and forage availability, there is no unequivocal source–sink relationship between the Sahelian floodplains and the associated wet-season grazing ranges. Forage in a key resource area does not necessarily provide the only key resource in the grazing system. Water, for instance, can be important as well. We end by discussing what our findings mean for the key resource area concept of Illius and O’Connor, and by presenting a new definition of key resource area which is also relevant to other trophic systems