The potential of planted shrub fallows to combat Striga infestation on maize

Abstract

Two experiments were conducted on farms infested by the witch weed Striga hermonthica in western Kenya from 1997 to 1999 with the aim of testing whether short-duration planted shrub fallows that have potential to replenish soil fertility can reduce Striga on maize in rotation. In experiment 1, 1-year fallows of nine different species were compared with natural weed fallow and continuous maize for their effects on Striga on subsequent maize. In experiment 2, we tested whether Striga reduction by shrub fallows is associated with their in situ growing (i.e. root – soil interaction) or foliar biomass or combination of these two, using three fallow species. On a moderately Striga-infected field, fallows of Desmodium distortum, Sesbania sesban, Sesbania cinerascens, Crotalaria grahamiana and Tephrosia vogelii reduced Striga by 40 – 72% and increased maize yields by 224 – 316% compared with continuous maize. These species reduced Striga probably due to the combined effects of suicidal germination of Striga seed and increasing inorganic nitrogen (N) availability in the soil, which has a negative effect on Striga. Although Senna didymobotrya reduced Striga, it did not increase maize yield because of low quantity of N added to the soil through its foliar biomass. Senna occidentalis, Cajanus cajan and natural fallow did not affect Striga and maize yields. Although Tithonia diversifolia did not reduce Striga, it increased maize yield because of efficient nutrient recycling. On a highly Striga-infested field, planted fallows of C. grahamiana, S. spectabilis and S. sesban did not reduce Striga infection similar to natural fallow. Only in situ growing of shrub species, combined with incorporation of foliar biomass, significantly increased maize yields compared with continuous maize without inorganic fertilizer or green manuring of maize with only foliage of the fallow species. On moderately Striga-infected and N-depleted fields, planted fallows using species that germinate Striga seeds and produce high amounts of biomass such as S. sesban, C. grahamiana and T. vogelii can reduce Striga, but on severely Striga-infected fields perhaps longer-term testing may be needed to realize their positive effects.