Striga hermonthica seed bank dynamics: process quantification and modelling

Key words: Weed control, integrated management, parasitic weed, population, sorghum, millet. This thesis presents a study on the quantification of seed bank dynamics of the parasitic weed Striga hermonthica. The main objectives were to quantify transition rates between different stages of the life cycle, determine these under different conditions and control strategies and to develop and use a population model to project long-term seed bank dynamics. To this end, field experiments were performed in Mali, with sorghum in 2002, 2003 and 2004, and Niger, with millet in 2004. Three demographic processes behind Striga hermonthica seed bank replenishment were determined and quantified, namely (1) recruitment of Striga plants, (2) survival of emerged Striga plants to maturity and (3) fecundity (number of seeds produced per mature plant). Striga seed production was highly variable between years and sites, because of high variability in recruitment. Different control strategies reduced Striga at different stages in the life cycle. Intercropping the host cereal with non-host trap crops mainly reduced recruitment and survival while late weeding acted almost solely on survival to maturity. A critical re-assessment of the seed bag method used to determine Striga seed mortality in the soil, led to the conclusion that the method overestimates seed mortality. It was suggested to adapt the seed bag method in order to measure seed mortality more accurately. An adapted seed bag burial method and a soil sampling method were used simultaneously to determine processes and rates of seed bank depletion under bare soil, fallow and different crop covers. Results suggested that the main cause of seed bank depletion was germination of seeds. Both methods yielded similar seed bank depletion percentages and most germination was found in soil under host crops, sorghum and millet, followed by intercrops of a host and a non-host trap crop, non-host crops, fallow and bare soil. The information and insights obtained were used to develop a spatially explicit, stochastic Striga population model, with which long-term effects of crop systems and control was modelled. The spatial patterns of emerged Striga plants after point inoculation with stochasticity in the attachment stage of Striga resembled spatial distribution of Striga that is typically observed in farmers’ fields. Sensitivity analysis showed that only two slope parameters for the dispersal curve of seeds and seed death other than germination in response to millet roots were of minor importance for population growth. The model indicated that intercrops of host cereals and non-host crops showed higher potential to reduce the Striga seed bank than did rotations of these. The implications of the findings are discussed in the context of integrated Striga management and participatory research.