Slash-and-burn farmers in the hills of Laos urgently need techniques that can sustain rice yields and reduce weed pressure under short fallow periods. For this, the potential of pigeon pea (Cajanus cajan (L.) Huth) as a fallow crop was evaluated in variety, establishment, and cropping systems studies conducted at Houay Khot station (19 degrees N) from 1992 to 1995. Introduced perennial varieties were not superior to local material for the parameters evaluated. Some semi-perennial varieties produced grain yields up to 2.2 t ha(-1) but were inferior in weed suppression and survival 12 months after planting. All varieties lost much of their weed competitiveness after the first picking of pods. In studies without weeding, < 10% initially established plants were present 15 months after planting. Compared with pigeon pea, Gliricidia sepium and Leucaena leucocephala performed better in biomass production, weed suppression, persistence, and self-regeneration after cultivation of rice. In a rotation study, rice yield was 1.3, 1.5, 2.3, 1.6, 1.8, 1.8, and 2.2 t ha(-1) (LSD 5% = 0.5 t) for continuous rice, rice-pigeon pea intercropping, rice after pigeon pea, rice after cowpea, rice after Stylosanthes hamata, rice after maize, and rice after fallow, respectively. Compared with continuous rice, intercropping or rotation with pigeon pea reduced nematode (Meloidogyne graminicola) infestation of rice and weed biomass in the rice crop. In spite of the beneficial effects, rotation or intercropping systems with pigeon pea will only be viable if economic benefits from harvested grain or from using plant parts as livestock feed can be obtained. Because of high mortality and weed invasion pigeon pea is not suitable for fallow improvement if fallow duration is more than one year.
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