Bush Fallow Enhancement Research in Peru

Many tropical farming systems depend on fallow periods to restore soil fertility. During these periods, soil structure, chemistry and organic content improve and nutrients accumulate in woody biomass. At the same time, herbaceous weeds are eliminated, weed seed banks reduced, and pest cycles broken. Pressure on limited land resources to produce more in less time has pushed farmers to shorten fallow periods, resulting in degradation and yield losses. Research in Peru seeks to accelerate soil regeneration in a bush fallow cropping system by relay planting local species Inga edulis and Desmodium ovalifolium into the crop sequence.

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.

Herbaceous weed suppression

Experiments were set up to test the effect of the Des/Inga combination on herbaceous weeds under different planting densities, weeding schedules, and crop combinations. In experiment I, Desmodium alone, Desmodium in combination with two different densities of Inga, and neither were planted between alternating rows of plantain and cassava. Results showed that weed biomass decreases as Desmodium cover density increases. Plots with Inga had significantly less weed biomass than those without Inga, and there were significantly fewer weeds in the second year under Inga than in the first.

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.

Woody biomass build-up

Data collected from the same experiments indicate that plots with Des/Inga accumulate woody biomass at rates equal to or greater than that of control plots. However, these plants suppress the biomass accumulation in naturally regenerating trees and shrubs. In Experiment I, no significant difference in number of tree and shrub stems was found between plots with and without Inga whether or not Inga stems were included. Biomass of naturally regenerating trees and shrubs, measured in stem basal area, was greater in plots without Inga than with, but if Inga was included, total biomass was greater in the Des/Inga plots. Experiment II had similar biomass results, although the Des/Inga plots had fewer naturally regenerating tree and shrub stems than the other treatments.

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.

Effect on yields

Experiment II is the only one in which crop yield data was taken. Results did not show Des/Inga to have a negative effect on cassava and plantain yields, and it may actually have a slightly positive effect (compared to natural weeds) on the plantains. Differences in yields seem to be due more to weeding regimes than to presence or absence of Des/Inga.

Weeding time

In Experiment II, data werecollected on weeding rate and on total weeding time per treatment. The treatment that was weeded most frequently required the most total weeding time although individual weedings were accomplished more quickly than in other treatments. Des/Inga treatments took more total time than remaining treatments because they were weeded more frequently and because the rate of weeding was slower at the beginning when care had to be taken not to weed out the germinating Desmodium and Inga seedlings. Late planting of Des/Inga reduced labor costs of establishment since planting was done after crop cover had begun to suppress weed growth. This experiment was conducted after a relatively short fallow resulting in vigorous weed growth at the beginning of the cropping period. Overall, Staver estimates a 30% to 35% increase in weeding time with Des/Inga present.

Desmodium as a weed

Experiment IV examined Desmodium’s recovery ability during and after the crop cycle and its potential to act as a weed. A two-year old thick mass stand of Desmodium was slashed and burned and planted in rice. Weed regrowth by species and crop yield approximation data were taken under different weeding regimes. More frequent weedings significantly increased rice productivity, showing that Desmodium was acting as a weed. Samples taken three months after rice harvest showed that frequent weedings do not adversely affect Desmodium’s ability to reestablish itself after crop harvests. Desmodium reestablishment came from seed germination and not from sprouting of pre-burn roots and stems.

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.

Staver, Charles, 1989. Shortened bush fallow rotations with relay-cropped Inga edulis and Desmodium ovalifolium in wet central Amazonian Peru, in Agroforestry Systems 8: 173-196. Kluwer Academic Publishers, Netherlands.