Crop-Livestock Interactions Affecting Soil Fertility in Sub-Saharan Africa
Manure to improve soil fertility is a major benefit of crop-livestock integration. It is thought to be an input whose quantity could be increased greatly, whose value could be improved by simple techniques, and whose use could improve soil fertility and structure. Research from different African sites has confirmed that manure raises yields and improves soil organic matter content.
In apparent conflict to these results, farm management studies have shown that farmers use manure on small areas or sometimes not at all. Where it is used, it is used in an agronomically inefficient way; it is rarely stored, mixed with litter, composted, or incorporated to reduce losses of dry matter and of nutrients to leaching, to evaporation, and to microbial action.
Perhaps chemical fertilizers and mulches have displaced manure, or the economics of intensifying manure production and use have been unfavorable. To look at these arguments, a brief overview of research results is presented and a general perspective given.
Manure and Mulches
Experimental work has concentrated on the effects of organic matter returned to the soil by animal manuring, mulching, green manuring, and composting. The work has focused on semiarid and subhumid areas, and on their major cereals: maize, sorghum, and pearl millet. Methods have consisted of plots continuously cultivated with tractors or animals, from which above ground residues have been removed. Large quantities of cattle dung or mulch, usually 5-10T fresh matter per hectare, are generally incorporated at plowing. Moderate quantities of chemical fertilizers are applied at 25-150kg of nutrients per hectare.
Most of the published work answers some basic questions clearly. However, if these experiments were to provide results for extension, then some of their methods were weak:
Treatments correlated with factors that increase yields.
Maximized yield does not fit farmers’ need to maximize profits or utility.
Control plots produced bias toward farmer practices.
Test quantities greater than normally used by farmers.
Manure. To compare manure with fertilizer a product/material ratio was calculated. The general indication is that animal manures produce weak crop responses. The low concentration of manure makes it much less efficient in terms of transport and application costs than chemical fertilizers. A ton of fresh manure, at a response rate of 6-7 kg grain/kg N and containing 0.75% N, probably produces about 50 kg of grain in the short term. A ton of diammonium phosphate (18-46-0), at a similar rate, would give 1,170 kg grain per ton of material.
Mulches. The results for crop residues and grasses are more variable. It appears that a ton of fresh incorporated crop residue produces about 20-40 kg grain.
Soil effects of manures and mulches. Soil studies looked at the extent to which manure replaces fallow and complements chemical fertilizer. General conclusions from this work, conducted over many years, are as follows:
Manure or chemical fertilizer alone cannot maintain fertility.
Manure is superior to cereal crop residues; manure might be inferior to groundnut shell mulch.
Composted crop residues and manure might replace chemical fertilizers in some situations.
The relative efficiency of manure and crop residues without fertilizer cannot be determined from these experiments.
Reviews of fertilizer response are summarized in Table 2. The data suffer several biases:
Estimates are from standard treatments applied for only one year at several sites.
Standard treatments are badly adapted to different environments.
Selection bias occurrs in reporting because unsuccessful trials are sometimes not reported.
Response estimates are made from plots of unknown fertility or history.
Bearing these biases in mind, the order of physical responses to N is reasonably clear. In rainfed conditions, among the four major cereals of Sub-Saharan Africa, the ranking is rice, maize, sorghum, and pearl millet.
A General Perspective
Studies have shown that large additional quantities of manure are necessary to stabilize crop production in several environments. Those quantities are not readily available and, even if they were, do not allow growth, but only maintain yields at base levels. Also, manure supply depends on biomass production in the system and, unlike fertilizers, is not an external input. Therefore, manuring cannot be a strategy for overall agricultural growth and is not comparable to fertilizer in that sense.
The insufficiency of manure cannot be remedied simply by integrating animal and crop production for two reasons:
Manure supply is a function of stocking rate. At some point, further increases in manure supply for crop production compromise land for crop production.
Technical efficiency in manure use is a function of land use intensity. In general, intensive techniques are not adopted in sites with low population density. Failure to use these techniques is economically rational because the value of manure nutrients is less than the labor necessary to use them.
Thus, the major constraint on manure use is supply, both of the material itself and of the resources, especially labor, necessary to use it.
Although high transport and application costs tend to favor the choice of fertilizers over manure, farmers and extension agents in many countries choose manure because fertilizer is not available or the cost is prohibitive.
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