This paper provides a retrospective look at a systems-oriented research program, on the increasing occurrence of parasitic weeds in rainfed rice in sub-Saharan Africa, to qualitatively assess merits and identify challenges of such approach. We gained a broad contextual overview of the problem and different stakeholders’ roles, which enabled identiﬁcation of entry points for innovations in parasitic weed management. At the crop level parasitic weed infestation is associated with poor soil fertility and water management. Farmers’ infrequent use of inputs to control them was caused by various factors, ranging from fears of undesired side effects (agronomic) to a lack of quality control of products (institutional). Furthermore, there may be enough extension agents, but they lack the required training on (parasitic) weed management to provide farmers with advice, while their organizations do not provide them with the necessary means for farm visits. At even higher organizational levels we observed a lack of coherent policies on parasitic weed control and implementation of them. Merits and challenges of an integrated multi-stakeholder and multi-level research project are discussed. Keywords: multi-disciplinary; trans-disciplinary; agricultural innovation systems (AIS); farmer participation; multi-stake-holder; crop protectionDownload document
Schut, M., Rodenburg, J., Klerkx, L., Hinnou, L.C., Kayeke, J., Bastiaans, L., 2015. Participatory appraisal of institutional and political constraints and opportunities for innovation to address parasitic weeds in rice. Crop Protection 74, 158-170
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The parasitic weeds Striga asiatica and Striga hermonthica cause high yield losses in rain-fed upland rice in Africa. Two resistance classes (pre- and post-attachment) and several resistant genotypes have been identified among NERICA (New Rice for Africa) cultivars under laboratory conditions (in vitro) previously. However, little is known about expression of this resistance under field conditions. Here we investigated (1) whether resistance exhibited under controlled conditions would express under representative Striga-infested field conditions, and (2) whether NERICA cultivars would achieve relatively good grain yields under Striga-infested conditions. Twenty-five rice cultivars, including all 18 upland NERICA cultivars, were screened in S. asiatica-infested (in Tanzania) and S. hermonthica-infested (in Kenya) fields during two seasons. Additionally, a selection of cultivars was tested in vitro, in mini-rhizotron systems. For the first time, resistance observed under controlled conditions was confirmed in the field for NERICA-2, -5, -10 and -17 (against S. asiatica) and NERICA-1 to -5, -10, -12, -13 and -17 (against S. hermonthica). Despite highStriga-infestation levels, yields of around 1.8 t ha−1 were obtained with NERICA-1, -9 and -10 (in the S. asiatica-infested field) and around 1.4 t ha−1 with NERICA-3, -4, -8, -12 and -13 (in the S. hermonthica-infested field). In addition, potential levels of tolerance were identified in vitro, in NERICA-1, -17 and -9 (S. asiatica) and in NERICA-1, -17 and -10 (S. hermonthica). These findings are highly relevant to rice agronomists and breeders and molecular geneticists working on Striga resistance. In addition, cultivars combining broad-spectrum resistance with good grain yields in Striga-infested fields can be recommended to rice farmers in Striga-prone areas.
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Over the last decades rice (Oryza spp) became more important as a staple food crop for the African continent. In two decades, the locally produced rice in Africa doubled to almost 30 Mt in 2013. Still, the local production is insufficient to meet local demand and this particularly holds for Sub-Saharan Africa (SSA). Rice yields in SSA pertain to the lowest in the world. One of the major constraints accountable for these low yields are weeds. The parasitic weeds Striga asiatica and Rhamphicarpa fistulosa can cause devastating yield loses in SSA rice production. Recent field observations suggest that the presence of parasitic weeds influences the competitive relations between rice and non-parasitic (or ordinary) weeds. In the presence of S. asiatica the competitiveness of ordinary weeds was increased, whereas in the presence of R. fistulosa the ordinary weeds were further suppressed. Two pot experiments, carried out under greenhouse conditions, were used to study how the competitive relations between rice and the ordinary weed Mitracarpus villosus was affected by the presence of either S. asiatica or R. fistulosa. Plant dry biomass was used as a measure for the competitive effects. S. asiatica caused significant reductions in total pot biomass compared to pots with rice alone. Such a reduction was however not observed if next to rice also the M. villosus was present. This absence of a reduction in total pot biomass was not because the ordinary weed filled the gap that was created through the negative effect of the parasite on its rice host, rather rice biomass did not decrease in the rice-ordinary weed mixture. Emergence of S. asiatica in this mixture was lower, and this might be accountable for the minor effect of the parasite on the host. It is suggested that the root system of the ordinary weed might have disturbed the establishment of a connection between the host and the parasite, resulting in a reduced emergence of S. asiatica. R. fistulosa reduced rice biomass much more strongly than S. asiatica. In the rice-weed mixture, the ordinary weed was not able to profit from the reduced growth of the rice plant, as R. fistulosa grew fiercely and developed into a strong competitor. Consequently, the competitive ability of the ordinary weed, just as that of the rice plant, was strongly decreased. The experiments clearly show that the presence of a parasitic weed affects the growth and competitive relation between rice and ordinary weeds both directly and indirectly. The outcome of this complex multi-species interaction depends a lot on the parasitic weed species. R. fistulosa showed to be a damaging pest and utilized its ability to parasitize rice to gain a stronger competitiveness against the developed into the species dominating not only the rice, but also the ordinary weed. In case of S. asiatica, the ordinary weed reduced the infestation level of the parasitic weed and consequently the competitive relation between rice and the ordinary weed remained relatively undisturbed.Download document