Crop residue management in relation to sustainable land use : a case study in Burkina Faso

IntroductionTraditional agricultural production systems in Sub-Saharan Africa were based on transfer of nutrients by grazing animals from rangeland to cropland, combined with fallowing. These systems are under increasing pressure as a result of rapid population increase. In most of the semi-arid and sub-humid zones, the area of rangeland, required to maintain cropland productivity, is not available anymore. The associated continuous decline in soil productivity induces risks of food shortage and irreversible soil degradation, and every effort should be made to avoid further deterioration of the resource base. Integration of crops and livestock and the associated intensified nutrient cycling are often advocated as desirable developments towards sustainable land use. Performance of these integrated systems hinges on the management of crop residues and manure, which represent the main source of organic matter and nutrients for animal and soil.The objective of the present study was to analyse the consequences of alternative management techniques of crop residues, from a sustainable crop-livestock farming perspective in the North Soudanian zone of Burkina Faso. The specific objectives were: (1) To establish response curves describing the effect of (varying degrees of) selective consumption of crop residues (as single feeds and in various combinations) on animal production; (2) to evaluate the effect of alternative systems of feeding crop residues on labour requirements and household income; (3) to evaluate the effect of alternative systems of feeding residues on crop production and on organic matter, nitrogen and phosphorus balances at farm level; (4) to determine the trade-offs among various objectives associated with alternative crop residue management techniques.The approachThe potential contribution of crop residues to ruminant feeding in the four agro-ecological zones distinguished in Burkina Faso was estimated, applying the Java Program developed at the Animal Production Systems Group of Wageningen University (Chapter 2). The number of animals that can be fed and their production were estimated on the basis of availability and quality (digestibility and N content) of crop residues (cereal stovers and legume haulms).Feeding trials were carried out with sheep to determine the relationships between the quantities of crop residues offered on the one hand, and intake and digestibility of ingested material on the other hand. In Chapter 3, the effect of selective consumption on intake and digestibility of sorghum, cowpea and groundnut residues is described. Two additional feeding trials were conducted as a basis for optimising the use of cowpea and groundnut haulms as supplement to sorghum stover (Chapter 4). The models used for analysis of the data allow description of the combined effects of animal selectivity for the better fractions of cereal stover, and of supplementation, on intake and digestibility of ingested material. Results of the feeding trials complemented by literature data served as the basis to derive iso-production curves. Based on the isoquant concept, least-cost rations were derived for a range of combinations of cereal stovers and supplements.The possibilities for optimal utilisation of crop residue at farm level was analysed using a linear programming technique. To quantify the relevant household resources, constraints and objectives, farm surveys were conducted (Chapter 5). The above-mentioned iso-production curves served as the basis for quantification of the technical coefficients for animal feeding activities, while for crop activities various models from the literature were applied to the region under consideration. A Multiple Goal Linear Programming (MGLP) model (referred to as HOREB, Household level Optimal crop REsidue allocation in Burkina Faso) was developed to determine the effect of various crop residue management techniques on farm productivity, economics and sustainability, in terms of balanced organic matter, nitrogen and phosphorus budgets (Chapters 6 and 7). The main results are discussed in Chapter 8, with special attention for the contribution of crop residues to sustainable land use in Sub-Saharan Africa.Contribution of crop residues to ruminant feeding at regional levelOn the basis of current crop production, 54 and 98 % of the current ruminant population can potentially be maintained during the dry season in the Sub-Sahelian and North-Soudanian zones, respectively. There might even be scope for increasing the contribution of crop residues to animal production in these areas, by increasing production of legume haulms and allowing selective use of stover leaves.It may also be profitable to take advantage of the capacity of animals to use feeds selectively by excess feeding. This requires, however, collection techniques and conservation structures that minimise the reduction in nutritive value of the scarce highest quality residues (legume haulms). The objective of rearing animals, either maximisation of animal production or maintaining the maximum number of animals, determines the most suitable feeding techniques. Maximising animal production is associated with inclusion of smaller quantities of cereal stovers in the ration and with a smaller number of animals kept. Maintaining the maximum number of animals allows the use of more cereal stovers and results in higher manure availability for arable fields, and so may contribute to maintaining cropland productivity.Optimising crop residue use at animal levelResults of the feeding trials show that unsupplemented sorghum stover can hardly be used for animal production, due to its low intake and digestibility, and its low protein content. Even maintenance can only be reached if selective consumption of leaves is allowed; In our trials, sheep reached the maintenance level only when 87 g OM kg -0.75d -1was offered, of which only 53 % was eaten. Offering cowpea and groundnut haulms, maintenance can be achieved with 36 and 39 g OM kg -0.75d -1, respectively. These results illustrate, that when coarse, low quality forages are fed alone, selection is not a waste, but a prerequisite for animal production and even for maintaining animals. Excess feeding of sorghum stover (allowing selective consumption of leaves), significantly reduced the quantity of supplement (cowpea and groundnut haulms) needed to reach a desired level of intake of digestible organic matter (IDOM). Up to 54 g OM kg -0.75d -1of groundnut haulms was needed for 1.4 times maintenance (33.6 g DOM kg -0.75d -1) when animals ingested 90 % of the sorghum stover, but only 29 g OM kg -0.75d -1was needed, when animals were allowed to eat only 50 % of the sorghum stover offered.Optimising crop residue use at farm levelThe results of farm surveys indicate that availability of a donkey cart for transport allowed storage of significantly more high quality crop residues (cowpea haulms). The stored residues served as animal feed (100 % for legume haulms and 70 % for cereal stovers), the remainder being used as building material, source of fuel for cooking, etc. Crop residues remaining in the fields are commonly grazed and only the non-edible parts serve as soil amendment. Factors such as relationships between farmers and herdsmen, location of fields, herd composition and production system, and available means of transportation determine the quantities of crop residues collected.On a farm (6.2 ha available land) with 210 md available labour per month, where crop residues are not collected for stall feeding in the dry season and no capital is available to buy fertiliser, a maximum of 3.2 t staple food can be produced in normal (average rainfall) years. When the household does collect crop residues for sheep stall feeding and aims at maximum gross margin, attainable staple production hardly changes, when no external inputs are used. The household could produce 4.5 t staple, when maximising total gross margin under balanced OM and nutrient budgets, provided sufficient working capital to buy external inputs is available. Model results suggest that reduction of the current crop area by 0.5 to 1.2 ha (depending on the restrictions imposed on nutrients budgets) is necessary to allow integration of sheep stall feeding, because of the required labour for residues collection. Integration of sheep stall feeding could then contribute to restriction of the expansion of the crop area without compromising households food security.Availability of labour for crop residue collection and storage is the major limiting factor for the integration of crops and stall feeding of sheep. Labour availability dictates, in combination with the objectives of animal keeping, the selection among the different feeding techniques. Chopping, requiring substantial investment in labour, is selected when the farmer aims at maximising availability of manure, security or savings, for which a maximum number of animals is fed at maintenance. Treatment with urea requires, in addition to chopping, working capital to buy urea. This option is hardly ever selected because of the combined limitations of labour, capital and low remuneration. At the current urea price and labour availability conditions, excess feeding is the best method of feeding in all scenarios.Maximising crop gross margin is always associated with soil mining; 20-27 % of calculated farm gross margin originates from soil mining, if soil nutrients and organic matter are valued at inorganic fertiliser and stover prices, respectively. From 200- 344 kg OM, 15-21 kg N and 1.5-1.8 kg P per ha are lost annually from the system when animals are not stall fed (extensive feeding system). When crop residues are collected for stall feeding of sheep and the farmer aims at maximum total gross margin, annual losses are 175-192 kg OM, 12-15 kg N and 1.1-1.2 kg P. The results of these analyses clearly illustrate that intensive management of crop residues in integrated crop-livestock farming systems cannot maintain soil nutrient status. Nutrients from external sources are necessary, either in the form of concentrate or fertiliser. Moreover, maximum gross margin per manday of labour invested in sheep feeding activities is 0.20 kF, which is much lower than the actual labour wages in the area. Hence, if off-farm employment is available, stall-feeding of sheep is not an economically attractive alternative. Economic profitability can be improved if capital availability allows utilisation of concentrate, i.e. 0.28 kF extra gross margin can be attained per kF invested in cotton seed cake. Potentially, attainable gross margin from sheep feeding is 182 kF per household, when crop residues would be optimally collected (80 % of the cereal stovers and 86 % of the legume haulms produced).ConclusionsIntegration of crops and livestock is often considered as a step towards sustainable agricultural production, because of the associated intensified organic matter and nutrients cycling through intensive crop residue management. The main advantage of the integration of livestock and crops is the added value derived from crop residues (especially those of legumes) in terms of animal products and income. Intensive management of crop residues in integrated crop-livestock farming appears a means of guaranteeing subsistence in an environment characterised by growing competition for organic matter and nutrients between animals and soil. Sustainable production, at regional level, can only be triggered by external inputs. However, the current price ratios of fertiliser and grains are hardly conducive to fertiliser utilisation. The prospects for sustainable crop-livestock production are intimately linked to the price of external inputs and to the low availability of working capital. Investment in concentrate is remunerative, but lack of cash is a serious constraint for their acquisition. Improved credit facilities for farmers may stimulate intensification of livestock production and thereby increase nutrient availability for cropland, and hence sustainable land use practices.