Chapter 4
Agroforestry

4.2 Improved fallows

Imperata often dominates abandoned fields as a fallow vegetation. If fire is prevented, the field may grow into shrubs and trees (Chapter 5). If fire is present, the Imperata fallow is a fire hazard for nearby fields and does not restore soil fertility. An improved fallow using a species other than Imperata should:

• Improve soil fertility by providing organic material and making more nitrogen and phosphorus available to the next crop. Often, this would be a nitrogen-fixing species or a species that produces a lot of foliage that decomposes rapidly.

• Protect the soil from erosion.

• Grow fast enough to outcompete weeds, including Imperata; cast enough shade that weeds die out before the next crop.

• Be easy to remove for the next crop.

• Possibly provide wood, forage or other products.

Suggest if:

• Fallow periods are too short or not effective

4.2.1 Establishing an improved fallow

It requires extra work and management for farmers to establish an improved fallow to avoid infestation of their fields by Imperata. This is especially true if they must work hard to convert a grassland to a fallow that will not benefit a crop until one or more years into the future. Some options are rotational alleycropping (Section 4.1.5), leguminous cover crops (Section 4.3.1), or fuelwood or timber plantations (Section 4.7).

Farmers can most easily improve the fallows that follow the crops they are already raising. They may choose combinations of the following approaches.

Maintain desirable trees and shrubs in the field. While you cultivate, plant, weed and harvest the crop, also plant trees and take care not to damage existing trees. See Section 4.8, Multistory Agroforestry, and Chapter 5, Assisted Natural Regeneration.

Rotational alleycropping (contour hedgerows). Allow existing contour hedgerows to grow and shade the alleyways (Section 4.1.5). If the hedgerows are far apart, or the hedgerow species is a small shrub, plant additional trees or shrubs within the row to close the canopy between hedgerows. When the fallow ends the trees are cut down and a contour hedgerow system will still be in place for soil erosion control. Crop productivity may increase compared to cropping an Imperata fallow.

This system is most likely to be used where farmers practice slash-and-burn cultivation in Imperata grasslands in situations where fires can be controlled. The tree hedgerows face risk of fire damage during the fallow period. Farmers that cultivate the soil by hand and have no access to fertilizers may find this system needs less labor in opening the land from Imperata. But there is considerable work required to cut the tree biomass prior to cropping. The hedgerow fallow system is less attractive to farmers with animal draft power. They are less likely to invest labor in hedgerow pruning because they normally get higher returns to their labor in land preparation.

Favor weed species other than Imperata. Farmers in many countries in Southeast Asia prefer fallows of Chromolaena odorata to Imperata because it enriches the soil and is easier to control after the fallow. Treatment of Imperata with glyphosate herbicide before the cropping period may stimulate Chromolaena establishment as a fallow species. Chromolaena tends to be favored over Imperata when the fallow period begins after cropping is discontinued. Both Chromolaena and its relative, Austroeupatorium inufolium, can be problem weeds. They are not palatable fodder, nor are they useful for thatch. Neither species should be introduced to an area, but they can be managed as effective fallows if they are already present.

Austroeupatorium inulifolium (syn. Eupatorium pallescens)

Austroeupatorium inulifolium is similar to Chromolaena but grows at higher elevations. The Dutch introduced it to Western Sumatra to smother Imperata in rubber plantations. In 1935, local farmers began to plant A. inufolium cuttings in Imperata on their farms for 3-4-year fallows. Compared to Imperata, A. inufolium provides much more organic material and readily available N and P to the next crop. Farmers found that with this species they were able to reduce their fallow period by half and obtain the same benefits. Farmers today sometimes scatter A. inufolium flowers (with seed) in their moist fields. More often, they clear their fields after the last harvest to receive windblown A. inufolium seed. After the fallow, they clear the A. inufolium and pile it between rows of vegetables. As it decomposes, they use it as mulch.

Plant tree or shrub seeds or seedlings to grow as the fallow following the crop. Commonly recommended species include Sesbania grandiflora, Crotolariajuncea, Flemingia macrophylla, Gliricidia sepium, Vitex pubescens, Trema orientalis and Leucaena leucocephala. Consider local native species that are known for providing mulch or that grow into secondary forest.

Economic fallows in Vietnam

These are many options to plant perennials that earn income during the fallow period. Vietnamese shifting cultivators from Song Be province sow cashew seeds at a 4m x 4m density when they plant upland rice. Seedlings are weeded but not fertilized. The cashew canopy closes after 3 or 4 years and nut production lasts until the 12th year of the fallow.

Time to plant fallow species

4.2.2 Ending the fallow

Timing. The elimination of weeds, crop insects, and diseases are critical factors in determining an appropriate fallow length. If the fallow period is likely to be short, the fallow needs to be managed carefully. Watch for dead trees and gaps where Imperata can grow. Replant fallow vegetation where necessary.

Fallow length

Clearing and nutrients. Farmers often burn when the fallow vegetation is bulky, but burning reduces the organic material and nitrogen returned to the soil. If wood is harvested or burned, take only the wood and leave the nutrient-rich leaves and small twigs as mulch. If wood is burned, spread the ash around. Make sure the fire does not escape.

Controlling soil erosion. Pile branches along the contour of the new field.

Successful fallow systems in the Philippines

Giant varieties of Leucaena leucocephala are used as an improved fallow by farmers in Occidental Mindoro, Philippines (elevation 50-600 m; four-month dry season).

Year 1: Farmers plant Leucaena at the same time as they plant upland rice or maize.

Years 2-4: Leucaena grows as a fallow crop, until weeds are suppressed.

Year 5: Farmers harvest Leucaena, sometimes making and selling charcoal. Leucaena regenerates from stump sprouts and seed. Farmers trim stump sprouts when they plant rice or maize, and trim them twice again during crop growth. Farmers uproot and mulch seedlings the first time they weed the crop.

Year 6: Leucaena grows into a fallow again (from stump sprouts and seed).

The Leucaena fallow is shorter than the local traditional bush fallow (6-8 years), with no decreases in crop yields reported by the farmers. Farmers sometimes burn the fallow to clear the Leucaena and remaining weeds. Soil organic matter is lower where fallows are burned.

Farmers in Naalad, Cebu, Philippines, have had a similar Leucaena fallow system for more than 100 years. They use the cut Leucaena to build low fences along the contour to catch soil and build up terraces.

Although these two examples are not in Imperata grasslands, Leucaena has been used elsewhere to suppress Imperata. These examples show that Leucaena fallows can be adopted by farmers and used for many years.