The Yanesha Indians of the Central Amazonian Lowlands of Peru practice a bush-fallow system in which the fallow is cleared and burned after three to five years. They relay crop maize, cassava and plantains along with many minor crops. In the first eighteen months, maize, cassava and a first plantain crop are harvested. Farmers weed their fields four to six times during this period. In the following year plantain yields and weeding frequency decrease, and by the last weeding soil fertility is low and herbaceous weeds dominate. Trees and shrubs gradually take over, close the canopy and the land remains fallow until it is cleared for the next crop cycle.
Charles Staver of Cornell University conducted research (with the support of USAID’s Central Selva Natural Resource Management Project and the Palcazu Rural Development Program of the Government of Peru) to shorten fallows in the Yanesha system according to the following rationale: “The period when plantain production is minimal and trees and shrubs do not yet predominate is time lost for cropping and for the productivity-regeneration process which depends on tree and shrub canopy closure. Could the action of naturally regenerating trees and shrubs be enhanced by relay-cropping additional trees and shrubs with maize-cassava-plantain? Could these trees and shrubs begin during the cropping phase to suppress herbaceous weeds and accumulate woody biomass, without depressing crop yields, without setting back natural regeneration, and without increasing labor costs substantially? Shortening the fallow in this way would permit accelerated rotations without a change in crop sequence, without sacrificing crop yield, and without a major alteration in the woody nature of the bush fallow.”
Researchers chose Inga edulis and Desmodium ovalifolium to test this idea. Inga edulis is a fast-growing easy-to-establish tree that is already common in fallows in the area. It has a somewhat open canopy and by itself does not totally suppress herbaceous weeds. Desmodium ovalifolium is a shade-tolerant, slow growing forage legume. It is aggressive once established and has woody stems that form a dense thicket.
Four experiments were conducted using this plant combination (Des/Inga) in local cropping systems to determine whether 1) it suppresses the build-up of herbaceous weeds during and after the crop cycle; 2) it accumulates woody biomass (leaves, twigs and trunks) without setting back natural regeneration of trees and shrubs; 3) it does not reduce crop yields or 4) greatly increase labor costs for weeding; and 5) Desmodium does not become a weed in the following crop cycle.
Experiment II compared herbaceous weed build-up under different weeding schedules with and without Des/Inga in the same plantain/cassava intercrop. One year into the crop cycle, plots with Des/Inga had significantly fewer weeds than plots without under weeding regimes similar to those practiced by farmers. A weeding schedule that is more frequent than farmers' practices suppressed more weeds than Des/Inga after one year. Two years into the cropping cycle and one year after the last weeding in all treatments, weed biomass in Des/Inga plots was much lower than that in other treatments, including the one that had been weeded more frequently.
In experiment III, papaya and annual crops were planted in all treatments after clearing. A year later when only papaya remained, one treatment was not cropped or weeded, the second was again cropped in annuals, the third with cassava/plantain polyculture and the fourth as the third with the addition of Des/Inga. The last three treatments were weeded for the second year. Woody biomass was measured one year after each treatment’s last weeding. The Des/Inga treatment had less naturally regenerated woody biomass than any other treatment. Total biomass including Desmodium and Inga was statistically the same as in the treatment that was left untouched after one year and greater than in all other treatments. Thus the rate of woody biomass recovery on two-year Des/Inga plots is superior to that on two-year plots without Des/Inga and comparable to that on plots cropped for only one year.
These experiments indicate that adapting the current bush fallow system to include fallow-regenerating plants in the cropping cycle could accelerate the process of recovery without adversely affecting crop yields and therefore could increase the proportion of time land could be sustainably used for cropping. The effect of the Des/Inga combination on herbaceous weed suppression and on woody biomass accumulation is especially encouraging. Potential difficulties in the system are the increased labor required for seed collection, planting and weeding, the suppression of naturally regenerating trees and shrubs and displacement of secondary crops. This research was conducted where annual rainfall is 6247 mm. This specific polyculture may not work in conditions of lower rainfall because of competition for water. However, the principle of accelerating fallow by relay-cropping locally adapted species into the crop mix could be used in other agroclimatic conditions.