Table 13 shows the production of fresh composted manure per animal on an annual basis, calculated from the fresh weights of manure produced after 60 days collection and 84 days composting in the experiment in Section 3.2. This, together with the additional grain and stover per ton of manure applied, calculated from manure application rates and crop responses in Section 4.2, allows the calculation of the theoretical additional crop production per animal (mean liveweight 212 kg) from the use of its composted manure, and the area of land required to achieve this. With the best manure collection strategy tested in this experiment, F+FR, the manure from one animal is worth an extra 356 kg of maize grain and 295 kg of stover above the no animal/no manure level from 0.1 ha of land over the two seasons. There is also a considerable difference, between the best and worst experimental manure collection strategies, with the best strategies giving up to twice as much additional grain as the worst strategies, if only the composted manure is considered and not the direct application to the soil of waste materials not added to the compost heap.
The economic value of an organic resource, such as manure, to the farmer is the value of the increase in crop yield and/or quality that is derived from its use (Parr et al, 1986). Farmers can be expected to utilise organic materials to the point where the revenue from increment is equal to its price, assuming production and application costs are included, basically following the law of diminishing returns. However, the trend of this hypothesis is not always straightforward. In the kind of system in which this study was conducted, the need to produce sufficient crop yields for both subsistence and income generation normally dictates the proportion of available organic resources, that is allocated to the different crop enterprises without necessarily taking into account the individual crop needs. If this aspect is taken care of, then optimum benefit could be attained by judiciously allocating a particular crop enough manure to provide the more expensive of the most limiting nutrients.
A previous survey by Lekasi et al (1998) showed that the stocking densities of cattle and also of total ruminants was higher on small farms. Average numbers are shown in Table 14. This suggested that livestock numbers, especially cattle holdings, are not apparently constrained by farm size and indicated greater manuring potential for the smaller farms. Farmers were asked to estimate yearly production of manure from their ruminant stock using local units of measure and there was good agreement with the generally accepted figure that a ruminant produces 0.8% of its liveweight as faecal dry matter (DM) in a day (Fernandez-Rivera et al, 1995).
The average ruminant herd in each farm category was been divided into large cattle (bulls, cows and heifers) and small cattle (immatures and calves). Liveweights for large cattle were arbitrarily taken as 350 kg M, small cattle 100 kg M and sheep/goats as 25 kg (B. Lukuyu, KARI Muguga pers. comm.). This allowed the estimation of manure production per year. It was assumed that manure N content was 14 g N/kg. It was also estimated that steers produce 25 g urine/kg liveweight/day, a figure which agreed with that given for ruminants by Sundstøl & Owen (1993). Urine was assumed to contain 10 g N /l (Sundstøl & Owen, 1993). When it was assumed that all faeces and urine were captured and that no N was lost in the course of a year, then if all excreta were conserved, relatively large application rates of nutrients to farmland could be maintained. These earlier estimates for N are shown in Table 15. The results for the experimental manures described in Section 3.2 now provide data on actual production and losses during collection and composting, allowing the earlier estimates to be revised.
Clearly, previous estimates of urinary N production were much too high. Nevertheless, the small farms with livestock densities
found in an earlier survey (Lekasi et al, 1998) potentially produce composted manure equivalent to 112 kg N/ha of the whole farm
area. Data in Section 4 indicate that the quality of the manure produced as well as the N content may be important in determining
crop response.
Net N uptake from the manures, that is the N uptake above that achieved by an unfertilised crop, in the first season field trial of this experiment was determined and is shown in Table 16. These can be used to calculate apparent N recovery from the manure by the maize crop, giving an indication of the quality of the manure or its value as a fertiliser. All the manure types that had some organic materials added during storage also resulted in relatively higher N recovery compared with those without, except for the Maasai manure, based on the application used in the trial (75 kg N/ha).
Potential maize crop N uptake in the first season after application of manure, calculated from the available application rates in Table 15 and the uptake efficiency in Table 16 is, therefore, F+FR 47.6, S 47.4, F+U+FR 42.1, F 27.0 and F+U 18.2 kg N/ha based on the whole farm area of the small farm. Thus, a smallholder farmer using the F+FR strategy for manure collection, potentially produces enough composted manure for a maize crop uptake of 47.6 kg N/ha, an amount quite close to the recommended rate of 50 kg N/ha (FURP, 1994). At the same time, there is still 81 kg N/ha/yr from urine, which could be applied directly to the soil to make up for the shortfall. In contrast, similar calculations for when F+U are collected and composted without inclusion of feed refusals, suggest a mere 18.2 kg N/ha uptake leaving a shortfall which is unlikely to be obtained from the 41 kg N/ha/yr available in feed refusals if these are applied directly to the soil. The manner in which waste products are combined and composted clearly has a major impact on the efficiency of nutrient recycling into subsequent crops.
6.3 Value of livestock
Technical interventions aimed at improving soil fertility may not always be directly targeted at the soil. For example, using
green manure-legumes is a well-proven technique for improving soil fertility in the semi-arid tropics. However, green manure-legumes
that serve the single purpose of improving soil fertility have not often been adopted by poor farmers because they must be grown at
the expense of food crops on limited land holdings. Dual-, or better still, triple-purpose legumes (yielding food, feed and fertility)
may better serve the more complex livelihood objectives of households farming small land areas. The presence of productive
livestock enterprises on farms is one strategy that can offer a more profitable route for nutrient return to the soil. Jama
et al (1997) reported that Calliandra
calothyrsus foliage was much more economically attractive as a protein supplement for dairy cows than as a fertiliser
on smallholdings in high potential areas.
Similarly, crop residues are utilised for various purposes depending on the type available and the farming system. When left in the field after crop harvest they conserve soil moisture and recycle nutrients. However, in an intensive crop-livestock farming system, crop residues are frequently used as livestock feed while the manure and urine produced are used to produce crops and fodder (Tanner et al, 1995; Powell & Unger, 1997). This was confirmed in the surveys for this study. No households used organic materials, e.g. plant foliage, other than manure, directly as a fertiliser. Plant material was more likely to be fed to livestock or used as animal bedding than to be applied directly to soil. Some farmers considered adding unpalatable biomass (such as Grevillia and Eucalyptus foliage) directly to the manure heap. Most farmers considered it important, however, that the biomass be channelled through the animal (as feed) or through the animal unit (as bedding) whenever possible. Thus, livestock represent an integral step in the nutrient cycle within farms. Additionally, livestock (mainly dairy cattle) are the main reason for importation of exogenous nutrients onto highland farms (Shepherd & Soule, 1998) and farmers buy concentrates and forage on a regular basis to complement forage grown on farm (Lekasi et al, 1998).
