2. SUMMARY REPORT ON WORKSHOP

2.3 Technical sessions

2.3.4 Soil fertility and erosion

The objective of research under this theme is to improve productivity of soil through fertility maintenance/enrichment and erosion control brought about by the incorporation of trees into cropping systems. The principal technology that has received major research attention in this regard is hedgerow intercropping. The improved fallow system has also shown great potential for soil productivity enhancement and sustainability, and is being given increasing research attention. Five presentations on this theme were given during the meeting.

The first paper presented in this session was on "performance of hedgerow species and their effect on crop yields grown in their alleys in the highlands of Burundi". This paper reported work from two hedgerow intercropping trials with a range of species.

The first experiment involved 3 provenances each of Calliandra calothyrsus and Cassia (senna) spectabilis, and one provenance of Leucaena diversifolia. The second experiment, which had similar objectives, involved three provenances each of Leucaena diversifolia and Leucaena leucocephala, and one provenance each of Calliandra calothyrsus, Cassia spectabilis and Leucaena collinsii. The control in each experiment was a plot without hedgerows.

Generally, the Calliandra and Leucaena diversifolia showed much better growth and productivity of prunings. Across the two experiments, pruning biomass in year two ranged from 0.1 tons/ha dry matter (Leucaena leucocephala) to 2.8 tons/ha dry matter (Calliandra calothyrsus Guatemala).

Initial crop yield responses from the various treatments (including the control plot) were very low due to the poor status of the soil. A short fallow was introduced into the system after 1990 (the first season was left uncropped and trees unpruned). This significantly influenced the growth of the trees and their effect on crop yields.

The effect of the various species on crop yields was generally positive for most of the Calliandra and Leucaena diversifolia plots. Yield gains in excess of 30% were obtained from hedgerow intercropping plots over yields obtained in the control plots.

These trials have indicated the wisdom in exploring incorporation of fallows and other soil management options into hedgerow intercropping. This could result in an overall beneficial and more flexible production system.

Sesbania sesban is one MPT species that has also received a lot of research attention in both hedgerow intercropping and improved fallow systems. A presentation from Maseno (Kenya) discussed the residual effects of the species in hedgerow species screening trials.

The trial involved 7 hedgerow species and a control plot without trees. All the species established well and gave good biomass production in one year of growth. Calliandra calothyrsus was reported as having given the highest fresh weight pruning yield of 45.6 tons/ha, followed by Sesbania sesban with 31 tons/ha in year one. Hedgerow growth and productivity was maintained in all the species, except Sesbania sesban, which died out in the second year, and therefore, produced no prunings for incorporation in the soil.

Generally, crop yields were reported to have been higher in the hedgerow intercropping plots compared to the control plot, throughout the experiment (1989 -1992). In 1992, the control plot yield in the major season was 0.8 tons/ha, compared to a yield range of 1.74 - 2.73 tons/ha for the hedgerow intercropping plots.

Sesbania sesban plot was reported to have maintained a high level of production throughout the trial period and actually gave the highest yield (2.73 tons/ha) in 1992 first season. Since no mulch was produced within the Sesbania plots since 1990, the implication was that there is a positive and long-lasting residual effect from the Sesbania, which continued to influence crop productivity up to 3 years after the Sesbania died out.

In discussions that followed this presentation, questions were raised on the validity of the results, given a number of design and implementation problems that were associated with the trial. It was advised, however, that the issue of residual effect of Sesbania should, if necessary, be studied in a new well-designed trial. The current trial needs to be examined critically and decisions made as to its future.

A number of papers were also presented on farmer assessment of hedgerow and improved fallow systems, as well as on the economics of such systems. One such presentation was titled "biological, economic and farmers assessment of hedgerow intercropping trials on farms in Western Kenya". The objectives were to confirm crop yield responses observed on-station and also to test profitability of the system.

The research involved 50 farmers, and two tree species Leucaena leucocephala and Calliandra calothyrsus, were used in the establishment of the hedgerow intercropping plots.

Ail the plots were farmer-managed. Evaluation was on labour and other input data, tree productivity data and farmer assessments. Farmer assessments involved formal interviews with trial farmers at the end of each season, to obtain their views and opinions with regard to the technology. Views and impressions of field technicians were also recorded.

Results were generally positive from the farmers' perspective, with over 50% of farmers reporting yield gains in their hedgerow intercropping plots. Technicians, however, reported observing increases in only about 20% of cases. Only 10% of farmers were however, reported to have dropped out of the trial.

It was suggested in follow-on discussions, that the methodology for such trials should be reviewed and strengthened to allow a more rigorous monitoring and recording process. Information from continuous monitoring in such trials could hold the ace in explaining the differences in responses among various farms. The meeting was informed that a lot more data is being collected than was reported on, but that a full analysis and use of the data remains to be done.

A second trial aimed at understanding farmers' impressions and practices with regard to fallowing, and their impressions on Sesbania sesban as a fallow tree in Western Kenya, was presented. This study was done through a targeted survey.

Results showed that in spite of the high population density and very small farm sizes, a significant number of farmers in the area (about 50%) still practice fallowing for soil fertility purposes. About 75% of the farmers were familiar with Sesbania sesban and had it growing in their farm lands and fallow lands. In most cases, the Sesbania established naturally, but were protected by the farmers. It was not obvious that a significant number of farmers do plant Sesbania to intensify their fallows.

Some further research is required to confirm benefits from dense Sesbania fallows in farmer-managed systems.

A related trial to assess the economics of improved fallows with Sesbania in Western Kenya was presented in another paper. The objective was to observe how farmers will manage Sesbania fallows, and to assess the profitability implications of such systems.

Generally, Sesbania fallows resulted in 60% yield gain (after fallow) over the continuous cropping plots. Economic analysis revealed that, in situations where a farmer provides all labour from within the family, and does not involve hired labour, a maize yield increase of 65%, following the improved fallow, is required to break even. Where the farmer hires 50% of the labour, then only a 12% maize yield increase is required to break even. This is because labour costs saved by not cultivating during the fallow period are much higher in the latter scenario. It was also noted that profitability from fallows increases when base yields are low and when farmers use hired labour during cultivation.

In a general technical paper presented at the end of this session, Dr. M.R. Rao discussed the potential role of agroforestry for soil improvement. He listed agroforestry potentials for soil improvement as including:

• Hedgerow intercropping

• Rotational tree fallows

• Biomass transfer

• Soil conservation techniques - contour hedge (tree, tree/grass)

• Scattered trees on crop land

Focusing on hedgerow intercropping, he reported that an analysis of research results across sites have shown mixed results. Positive crop yield responses have been obtained in some sites such as Maseno (Kenya), Chalimbana (Zambia) and Yaounde (Cameroon), while negative results have been obtained from other sites, e.g. Msekera (Zambia), Machakos (Kenya) and Mashitsi (Burundi).

He explained that competition between the hedgerows and the crop has a major influence on the eventual crop productivity within the system. This was illustrated with the following equation:

Dr. Rao also stressed that in trials on hedgerow intercropping, it is very crucial that appropriate control treatments are defined and used. The trial should also be designed in such a way that the control plots are not encroached upon in any way by the hedgerow trees, above or below grown.