Mixed outcomes from conservation practices on soils and Striga-affected yields of a low-input, rice–maize system in Madagascar

Abstract

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.

From rice-like plants to plants liking rice: A review of research on weeds and their management in African rice systems

“…Practising no-tillage however generally leads to higher weed biomass (Olofintoye, 1989;Saito et al, 2010a). In a rice-maize rotation a Conservation Agriculture (CA) approach with Stylosanthes guianensis as cover crop and source for mulch, controls erosion (Rodenburg et al, 2020) while the loss of weed control from refrained tillage can be compensated by its weed suppression (Randrianjafizanaka et al, 2018). Crop residue mulching may contribute to weed suppression, but earlier work showed that removal (Becker and Johnson, 1999b) or burning of residues (Akanvou et al, 2000) resulted in lower weed biomass levels in the following crop.…”

Adoption by adaptation: moving from Conservation Agriculture to conservation practices

“…However, there are also studies that show limited yield effects (e.g. Kitonyo et al, 2018;Rodenburg et al, 2020) or yield declines (Corbeels et al, 2014;Mupangwa et al, 2016;Rosenstock et al, 2014) following CA, or studies that only show yield increases from an integrated approach when it was combined with additional weed control and fertilizers application (e.g. Ngwira et al, 2014).…”

“…Stylosanthes spp. could be an alternative cover crop that produces high amounts of biomass (Rodenburg et al, 2020). Other solutions to reduce this trade-off are identifying alternative feed stocks (Jaleta et al, 2013;Valbuena et al, 2012), producing more maize biomass as feed, and introducing smallscale mechanization to further reduce the dependency on animals for traction (Baudron et al, 2014).…”

“…One important trade-off that potentially hampers adoption is the likelihood of a short-term yield penalty (Bruelle et al, 2015;Droppelmann et al, 2017;Masvaya et al, 2017;Thierfelder, Matemba-Mutasa, et al, 2015). While immediate benefits from CA practices, such as soil conservation, are evident (Rodenburg et al, 2020), consistent yield benefits are often only obtained after several years of implementation (Giller et al, 2009). Smallholders who need to decide whether a change of cropping practice would be a beneficial and wise decision to take may be discouraged by reduced yields in the first years following a change from conventional to conservation agriculture.…”

Évaluation multicritère et participative de variétés de riz pluvial dans une gamme contrastée d’environnements à Madagascar

“…Cependant, on constate une saturation progressive des bas-fonds (Breumier et al, 2018) et pour soutenir la demande croissante en riz, la recherche et ses partenaires de développement sont engagés depuis plus de trente années dans l'extension de sa culture sur le domaine strictement pluvial (Raboin et al, 2013 ;Ozaki & Sakurai, 2020). Avec comme but de développer des variétés adaptées aux contraintes spécifiques de ce domaine pluvial, le FOFIFA (acronyme malgache traduit par Centre National de Recherche Appliquée au Développement Rural) et le CIRAD (Centre de Coopération Internationale en Recherche Agronomique pour le Développement) ont lancé un programme de sélection variétale de riz pluvial spécifique pour la zone écologique d'altitude comprise entre 800 et 1 300 m. Les deux principales contraintes de cette zone sont la fertilité du sol, qui est bien moindre dans les plateaux que dans les bas-fonds (Ozaki & Sakurai, 2020) et la présence d'une plante parasite des céréales, Striga asiatica (L.) Kuntze, qui fait l'objet d'une attention particulière dans le cadre des recherches menées sur la résistance variétale du riz (Randrianjafizanaka et al, 2018 ;Rodenburg et al, 2020 ;Scott et al, 2020). Les principaux objectifs de ce programme, outre le contrôle de S. asiatica, est de veiller à une bonne efficience de l'utilisation de l'azote (Rakotoson et al, 2017), à la résistance aux maladies comme la pyriculariose (Dusserre et al, 2017) et à la qualité des grains (Raboin et al, 2014).…”