In low-input rice-maize rotation systems in the hills of central Madagascar, farmers deal with erratic rainfall, poor soils, high soil erosion risks and infestation by the parasitic weed Striga asiatica (L.) Kuntze. Practices combining zero-tillage with permanent soil cover by intercropped legumes and crop residue mulches-known as Conservation Agriculture (CA)-are proposed as remedy against soil and climatic constraints. Implications of these practices for S. asiatica are unknown. A 4-season factorial experiment compared the current farmer practice of rice-maize rotation, involving seasonal tillage and crop residue removal (CONV), with three rice-maize rotation systems following CA with different cover crops, i.e. Vigna unguiculata (cowpea) and Mucuna pruriens (CACM), Vigna umbellata (ricebean) (CARB), and Stylosanthes guianensis (CAST). Performance of two rice varieties, NERICA-4 and-9, with partial S. asiatica resistance, were compared with the locally popular B22. Parasite emergence time, numbers, and seed bank sizes were recorded. In all CA practices S. asiatica infection was significantly reduced. Best results were obtained with Stylosanthes guianensis (CAST). This species also suppressed ordinary weeds much better than other cover crops. With CAST, average parasite emergence was delayed by 7.5 days (in rice) and 6.3 days (in maize) and infection levels were reduced by 79% (in rice) and 92% (in maize) compared to the conventional farmer practice (CONV). NERICA varieties delayed S. asiatica emergence by 5.7 days (NERICA-9) and 9.7 days (NERICA-4) and reduced infection levels by 57% (NERICA-9) and 91% (NERICA-4) compared to B22. In maize the residual effect of resistance of NERICA-4 resulted in a delay of 7.5 days in S. asiatica emergence and a reduction of 60% in parasite numbers. The best combinations delay S. asiatica emergence by 17.8 days (CAST + NERICA-9) and 19.1 days (CARB + NERICA-4) and reduce the parasite infection levels by 96% (CAST + NERICA-9 or-4) to 98% (CARB + NERICA-4) in rice, compared to CONV + B22. After two full rice-maize rotation cycles S. asiatica seed numbers in the soil (0-10 cm) were 76% (CACM), 78% (CAST) and 86% (CARB) lower

The parasitic weed genus Striga causes huge losses to crop production in sub‐Saharan Africa, estimated to be in excess of $7 billion per year. There is a paucity of reliable distribution data for Striga; however, such data are urgently needed to understand current drivers, better target control efforts, as well as to predict future risks. To address this, we developed a methodology to enable rapid, large‐scale monitoring of Striga populations. We used this approach to uncover the factors that currently drive the abundance and distribution of Striga asiatica in Madagascar. Two long‐distance transects were established across the middle‐west region of Madagascar in which S. asiatica abundance in fields adjacent to the road was estimated. Management, crop structure and soil data were also collected. Analysis of the data suggests that crop variety, companion crop and previous crop were correlated with Striga density. A positive relationship between within‐field Striga density and the density of the nearest neighbouring fields indicates that spatial configuration and connectivity of suitable habitats is also important in determining Striga spread. Our results demonstrate that we are able to capture distribution and management data for Striga density at a landscape scale and use this to understand the ecological and agronomic drivers of abundance. The importance of crop varieties and cropping patterns is significant, as these are key socio‐economic elements of Malagasy cropping practices. Therefore, they have the potential to be promoted as readily available control options, rather than novel technologies requiring introduction.

On upland soils in tropical Africa, common production constraints of rice and maize on smallholder farms are poor soil fertilityresulting from soil erosion and nutrient depletion-and infestation by witchweeds (Striga spp.). In Madagascar where these crops are often grown in rotation, combining legume cover crops with no-till and crop residue mulching-labelled conservation agriculture (CA)-may address these problems. Previously, it was shown that CA practices contribute to steep reductions in Striga asiatica infection. In the current study, a 4-year field experiment was conducted to test, for the first time, the hypothesis that CA practices also contribute to crop yield and soil improvements under Striga-infested conditions. The conventional mono-crop rice-maize rotation practice, involving seasonal tillage and crop residue removal, was compared to three rice-maize rotation systems following CA practices, each with a different legume cover crop option: (1) two short-cycle annual species, cowpea (Vigna unguiculata) and mucuna (Mucuna pruriens); (2) a long-cycle annual, ricebean (Vigna umbellata); and (3) a perennial, stylosanthes (S. guianensis). Rice yields, as well as yield variability, generally increased by changing from the conventional to a CA practice, and maize yields were variable and low in particular under the CA practices. CA practices significantly reduced soil displacement by rainwater runoff and increased soil nitrogen and pH levels (0-20 cm depth), in particular with stylosanthes as cover crop, but did not result in a significant change in soil organic carbon concentration. Rice yields correlated negatively with Striga asiatica plant numbers in years with moderate infection levels. This is the first study that shows mixed outcomes from CA practices in tropical cereal rotation systems on degraded, Striga-infested soils, and subsequent entry points for system improvements. Suggested improvements include judicious cover crop management, complementary fertilizer applications and selection of competitive, resistant and adapted crop varieties.

Species of the genus Striga, which belongs to the parasitic plant family Orobanchaceae (Joel et al., 2007), are among the most economically significant weeds affecting food security within sub-Saharan Africa (SSA) and cause severe losses in many staple crops (Scholes & Press, 2008). Striga has resulted in reported yield losses of between

The parasitic weed genus Striga causes huge losses to crop production in sub-Saharan Africa, estimated to be in excess of $7 billion per year, affecting subsistence farmers who frequently lack access to novel technologies proposed for control. Effective Striga management therefore requires the development of strategies utilising existing cultural and management practices. We report a multi-year, landscape-scale monitoring project for Striga asiatica in the mid-west of Madagascar, undertaken over 2019-2020 with the aims of examining cultural, climatic and edaphic factors currently driving abundance and distribution. Long-distance transects were established across the middle-west region of Madagascar, over which Striga asiatica abundance in fields was estimated. Analysis of the data highlights the importance of crop variety and legumes in driving Striga density. Moreover, the dataset revealed significant effect of precipitation seasonality, mean temperature and altitude in determining abundance. A composite management index indicated the effect of a range of cultural practices on changes in Striga abundance. The findings support the assertion that single measures are not sufficient for the effective, long-term management of Striga. Furthermore, the composite score has potential as a significant guide of ISM control beyond the geographic range of this study.