One consequence of decreasing size of land holdings in the Central Kenya Highlands is a shift from extensive to more intensive mixed crop/livestock farming systems including acquisition of external inputs to feed livestock and replenish soil nutrients. Inorganic fertilisers are too expensive for most smallholders. The scope of this study was to evaluate manure management options that could best conserve nutrients and improve manure quality.
A survey documented current and potential manure management options, and evaluated manure physical characteristics and nutrient concentrations that could be associated with manure quality. Results suggest that modification of traditional livestock housing (boma) to the zero-grazing system may have beneficial effects on some aspects of manure quality.
When steers were fed with a basal diet of napier grass and dairy meal concentrate, high concentrate levels resulted in both faeces and urine of significantly higher N than the low concentrate levels. It was possible to conserve urinary N when wheat straw was applied at the relatively high amount of 1.8 kg liveweight/yr, about 720 kg/400 kg cow liveweight/yr.
The effects of simple strategies for combining cattle faeces, urine and rejected maize stover forage on the conservation of nutrients during storage/composting and on maize productivity over two seasons after field application of manure were investigated. The strategies were; S - faeces (F) + urine (U) + feed refusals (FR) mixed in a deep-litter system by steers; F+U+FR; F+FR; F+U and F, with all except S collected and mixed manually.
On average, 28 and 18% of the N input as feed was recovered in faeces and urine, respectively. Of the total N excreted, faecal N contributed between 47 and 76% (mean 61%) while urinary N ranged between 24 and 53% (mean 39%). Greatest loss of N during the accumulation phase was observed in heaps with high moisture contents from the addition of urine. During the composting phase, manures with maize stover refusals manually added had the greatest N losses. Overall, N losses ranged between 34 and 63% with F+FR resulting in the lowest loss. S and F+U+FR showed the highest overall N losses.
The use of bedding as a means to conserve N in manure is a traditional practice in mixed farming in many climates and agricultural systems. However, in this experiment, the addition of feed refusals was either insufficient or ineffective in conserving N added to the manure heap as urine. The combination of F+FR very effectively conserved the N in the faeces and feed refusals. In S and F+U+FR treatments, the additional N from urine was completely lost, and even lower total N in the final compost (5.5 and 5.6 g N kg-1 LWi, respectively) was obtained than with the urine-free F+FR (6.5 g N kg-1).
In the F+FR treatment, 4.7 g N kg-1 LWi from urine was also available for direct return to the soil, possibly as a liquid slurry feed to perennial fodder crops, resulting in a 11.2 g N kg-1 LWi return to the soil. It is not clear to what extent this urine N would contribute to crop production if applied as a liquid, since loss of N from urine applied to the soil can be both rapid and extensive. However, in the F+FR manure scenario, any additional gain from the direct use of urine, however small, would be a net benefit above the alternative of adding the urine to the manure heap.
Calculations based on waste derived from livestock at densities found in the Central Kenya Highlands, suggest that small farms (mean 0.45 ha) potentially produce composted manure equivalent to 112 kg N/ha of the whole farm area, if optimum collection and composting strategies are followed. Given the first season uptake efficiency of 43.9% of applied N from F+FR manure, a smallholder farmer potentially produces enough composted manure for a first season maize crop uptake of 47.6 kg N/ha over the whole farm area, an amount quite close to the recommended rate of 50 kg N/ha
Experimental manures were tested in maize (Zea mays) production field trials. Maasai manure (kraal or boma manure) obtained from the pastoralists of Kajiado in the Rift Valley, and a farmer's own manure were also tested. All of the experimental manures, applied at a rate to give 75 kg N ha-1, significantly improved grain yield in the first season compared with an unfertilised control. The greatest yield (F+FR, 4336 kg ha-1) was significantly higher than the lowest yield (F+U, 2916 kg ha-1). First season maize stover yield, which represents nutrients that can recycled through the livestock, was similarly affected by manure quality.
With the best manure collection strategy, F+FR, the manure from one steer (mean liveweight 212 kg) would be worth an extra 356 kg of maize grain and 295 kg of stover above the control level from 0.1 ha of land over two seasons.
Laboratory N-mineralisation and manure lignin, polyphenols, NDF-N and the C:N ratio were examined to correlate quality parameters with crop response. Manures that attained low C:N ratios and maintained high N concentrations at the end of composting promoted higher maize yields. The importance of the C:N ratio as an indicator of manure quality is suggested even in the second season where a significant negative correlation of total C:N ratio with grain yield was observed. However, Maasai manure gave the highest grain and stover yield, despite having the lowest N concentration and highest C:N ratio.
With Maasai manure included, significant positive correlations were found between lignin concentration and grain, stover and total yield in the first season, and grain and total yield in the second season. Significant correlations were also found between lignin + polyphenols and lignin:N ratio for some yield parameters, while negative correlations were found between polyphenol concentration and the second season grain and total yield.
Simple indices of manure quality are required that will enable farmers to combine manure more effectively with strategic quantities and placements of inorganic fertilisers and so more precisely meet the nutritional needs of crops. In this survey, links between manure nutrient concentration and C:N ratio, and the manure texture, colour, smell and biological activity observed visually were investigated. There appears to be scope for the development of decision tools for manure-compost quality.
In conclusion, the diet fed to the animals and the type of organic materials added to the manure had an effect on the manure quality as assessed by nutrient content and crop response. Livestock make an important contribution to the sustainability of intensive smallholder farming through their contribution to soil fertility. This research has shown that increases in crop yields on smallholder farms in the Central Kenya Highlands, gained from simple techniques for better care of manure during collection and storage, can be substantial and enduring. It may well be that the contribution this research makes to enhancing the competitiveness of the smallholder sector in Central Kenya is increased where improved manure management can be linked to cultivation of higher value horticultural crops.
As human populations continue to grow towards an anticipated figure of over 8 billion by the year 2020 there is considerable anxiety that the food inequalities prevalent in the world today will worsen over the next 20 years (Pretty et al, 1996). Optimists calculate that, in absolute terms, the planet should be able to sustain this huge population through increases in crop production. It is predicted that in developing countries, two-thirds of the increase in food output will in fact come from rising crop yields, the rest (20%) will be achieved through expansion of the arable area into marginal and degraded lands, and from increased cropping intensity (13 %) (Alexandratos, 1995). However, these advances will have a disappointing impact upon mitigation of the impending crisis if disparity of access to food is not resolved for the poorest households.
Amongst strategies for improving access to food is ensuring that local capacity for staple food production is retained or, better still, enhanced (Pretty et al, 1996). This may seem an obvious suggestion but is becoming increasingly difficult to attain. Rising population densities in rural areas render the average size of agricultural landholding too small even for subsistence crop production. The risk that rural families will lose access to viable land units providing a year-round food supply is very real. This has led to a popular paradigm that rising rural populations place increasing pressure on land through increased cropping intensity and that this threatens the fundamental bio-physical factor underpinning food security _ soil fertility (Donovan & Casey, 1998).
The effect of increasing population on landholdings can be seen in high agricultural potential areas of Embu District in Kenya's Eastern Province. Here, soils are dominated by the humic nitisol soil type and rainfall is in the range of 1200 and 1500 mm per annum. Intensive, small-scale agriculture is the main source of household income. Two major cash crops are grown: tea at high altitudes and coffee lower down the mountainsides. Macademia nuts and vegetables are fairly recent additions to the range of cash crops. Maize and beans are the major staples grown. Dairy cattle production is widespread amongst farms with 70% of households owning cattle (Kihanda et al, unpublished data). Although this intensive mixed farming currently maintains populations of up to 800 persons/km2 (Imbernon, 1997), there is concern that this is reaching an upper limit and there is now a steady flow of poor people out of the high potential areas to the low potential, semi-arid areas in search of land to sustain livelihoods.
In Embu District, land was demarcated in 1963. Families were allocated land holdings of between 5 and 15 acres (2-6 ha). In a recent survey, farmer groups were asked to describe and map changes on a typical farm in the immediate area. Box 1 describes the changes on a typical farm.
The intensive farming systems of Central Province, Kenya, where most of the research described in this publication was carried out, are similar and have been described in detail by Lekasi et al (1998). For example, farm sizes in Murang'a District ranged from 0.4 to 12.5 ha with a mean of 1.8 ha. Thirty-three percent of farms occupied less than 1 ha. In Kiambu District, farm size averaged 1.4 ha (range: 0.1-4.3 ha). Fifty two percent of farms were less than 1 ha. Mean household size for the sample was seven individuals. All farms grew a mixture of food crops. Higher altitude farms in the sample grew coffee as a cash crop. Vegetables such as potatoes (Solanum and Ipomea), kales, french beans, tomatoes, citrus fruit and bananas were grown partly for home consumption and partly for sale. Maize and beans (Phaseolus) were grown ostensibly for home consumption.
Dairy cows were owned by all households in the survey by Lekasi et al (1998) since this was one of the criteria for inclusion in the sample. All dairy cows on farms in the sample were kept in permanent confinement and fed by cut-and-carry. Livestock, particularly dairy cattle are, however, clearly an important enterprise in the Central Kenya Highlands generally. Staal et al (1997) and Harris et al (1997) estimate that 77 and 85%, respectively, of agricultural households in rural areas around Nairobi own dairy animals.
1.2 Declining soil fertility?
As stated above, there is a popular conception that the high population densities now present in the East African Highlands (Swift et al, 1994) pose a serious threat to the maintenance of soil fertility. In Kenya, losses of N and P were estimated at 42 and 3 kg/ha/yr, respectively, in the period 1982 to 1984 (Stoorvogel et al, 1993). The long-term decline in soil fertility is thought to be mainly due to increased cropping intensity and to the limited use of inorganic fertiliser. Smaling et al (1992) estimated that N and P fertiliser use in Kenya was only 6 and 3 kg/ha/yr in 1981.
As little or no land is available to be left fallow to sustain soil fertility in the smallholder farming systems of Central Kenya Highlands, farmyard manure (FYM) and crop residues have been the main organic resources available as soil amendments for soil fertility replenishment in these systems (Woomer & Muchena, 1996).
Although the evidence that soil fertility is in decline in sub-Saharan Africa is purportedly incontrovertible (Smaling et al, 1997), alternative views have been advanced. Scoones & Toulmin (1999), for example, point the scientific world to reasons why the evidence for decline needs to be carefully interpreted. Weak methodologies for scaling up plot/single season soil fertility and crop productivity data to supra-national levels lie at the heart of the problem.
However, there is little doubt that in some areas `nutrient mining' is prevalent and action to slow or reverse this process is required. Constructive approaches have sought solutions in indigenous knowledge and practices. To this end some authors have documented examples where centuries of population pressure have seen farmers engage in a range of strategies for sustainable intensification of food production. These examples pervade the `grey literature'. Experiences have been brought to the international arena through organisations such as the Information Centre for Low-External-Input and Sustainable Agriculture and through mainstream publications including Tiffen et al (1994), Reij et al, (1996) and Mortimore (1998).
1.3 Intensification and livestock
In order to adapt to increasing population and decreasing farm size, while attempting to maintain or increase productivity, farmers in Embu District of Kenya adopt a range of strategies (Box 2). It is clear that in the absence of widespread opportunities for off-farm employment the way to sustain the increasing population in the future was thought to be through greater farm diversity and productivity. Subsistence agriculture, i.e. the growing of food crops was not seen as a priority; quite the contrary, the consensus was that diversification into market-oriented products such as horticultural crops and dairy was regarded as the key to improving food security.
In the high potential farming systems of East Africa, the conservation and efficient use of nutrients is paramount to ensuring their productivity. There is tentative evidence to suggest that livestock are the major conduit for nutrient flow onto farms through feed collected and brought onto the farm (Shepherd & Soule, 1998). A major feature of the strategy for successful intensification proposed by the Embu farmers was the inclusion of an intensive dairy enterprise. This is considered to offer more than medium-term financial viability to small farms. Mixed farming appears to have had particular appeal to poor farmers in locations where external fertility inputs are not available (Winrock, 1992). The presence of cattle underpins strategies for the sustainable intensification of smallholdings. One important advantage of integrated farming is the opportunity to convert by-products and waste from one activity into inputs for another. The livestock provides inputs such as manure for crop production with crop products such as residues and fodder being used in livestock production.
Livestock production systems are currently being scrutinised with regard to their negative environmental impact (Steinfeld et al, 1997) but are also globally recognised for their major contribution to the income and welfare of the poorest people (Livestock In Development, 1999). One purpose of the research undertaken in this project was to contribute to evidence that livestock enterprises on intensively managed mixed-farms actually make a positive contribution to livelihoods of the poor and also help sustain the farming system.
In discussions, farmers in Embu District ranked manure as the most important output from cattle despite the proximity to a local milk market. Statements made by older members of the group concerning the use of manure included "my land was very infertile in 1965 so I bought cattle to improve my land through manure" and "without livestock many things will not move or grow". This latter reaction arose in response to the notion that as farm sizes reduced so would the opportunities for keeping cattle. There was general consensus that the communities would continue to keep cattle despite land pressure. Other surveys (Lekasi et al, 1998) in Kiambu and Murang'a Districts, Central Province Kenya ranked manure a close second behind milk in importance as a cattle product. However, a key question is whether, in these intensive small mixed farms, there is sufficient quantity and quality of soil fertility inputs from stall-fed livestock to sustain intensive cropping and facilitate the rate of turnover of nutrients.
In recent years, with increasing cost of inorganic fertilisers, scientific interest has turned towards the evaluation of organic fertilisers based on locally-available resources including crop residues, animal manures and green manures (Reijntjes et al, 1992). Research has focused on the quality, quantity and methods of application of biological materials (Myers et al, 1994). These studies now complement a wealth of research conducted over the last half century in East Africa demonstrating the positive responses of crops to livestock manure (e.g. Pereira & Jones, 1954).
From the 1960s, when the use of organic fertilisers, particularly livestock manure, might be considered to be at a nadir, manure is now used by over 95% of all smallholder farmers in the Kenya Highlands (Karanja et al, 1997; Harris et al, 1997). Utilisation of cattle manure as a soil amendment is an integral part of the smallholder crop-livestock farming systems of the Kenya Highlands and of East Africa in general. Manure produced in these systems is usually applied prior to planting of field crops such as maize, beans and potatoes as well as vegetables such as kale, cabbages and tomatoes, and cash crops such as coffee.
The beneficial role of manure in crop production has long been recognised. The capacity of manures to provide nutrients, especially N, P and K is one such benefit. Other benefits that have been demonstrated include an increase in cation exchange capacity (CEC), pH, water holding capacity, hydrolylic conductivity and infiltration rate, and decreased bulk density.
Studies on utilisation of composts and FYM for crop production in East Africa have been reported since the 1930s (Beckley, 1934; Beckley, 1937; Mehlich, 1965). Many crop response trials have looked at rates and methods of application, effects on soil chemical and physical properties and effects on soil moisture dynamics (Dagg et al, 1965) and more recently biological properties and soil organic matter dynamics have attracted some interests (Kapkiyai et al, 1999). What these trials have lacked is that they have not considered the different factors that affect the quality of the manures. The chemical composition of cattle manure is influenced by the diet of the animal and by the way the manure is collected, stored and handled before utilisation (Kirchmann, 1985; Kemppainnen, 1989; Mugwira & Murwira, 1997).
In order to optimise and maintain manure quality, proper knowledge is required for manure collection, storage and utilisation that would minimise nutrient loss and yet allow the nutrients to be readily available to the plants. For instance, by analysing manures that have been derived from different diets with different organic materials added and with storage in pits or heaps, covered or not covered.
