Mixed outcomes from conservation practices on soils and Striga-affected yields of a low-input, rice\u2013maize system in Madagascar

Jonne Rodenburg,Alain Paul Andrianaivo,Patrice Autfray,Lala Harivelo Raveloson Ravaomanarivo,Lucie Büchi,Ibnou Dieng,Meva Tahiry Randrianjafizanaka

doi:10.1007/s13593-020-0612-0

Abstract

On upland soils in tropical Africa, common production constraints of rice and maize on smallholder farms are poor soil fertility—resulting 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.

Highlights

Rice (Oryza sativa L. and O. glaberrima Steud.) and maize (Zea mays L.) are among the main staple cereal crops in Africa, but yields on smallholder farms are often low (Shiferaw et al 2011; Seck et al 2012)
There, farmers rotate upland rice with maize at mid and high altitudes, on poor and erosive soils (Dusserre et al 2012) that are heavily infested by Striga asiatica (L.) Kuntze
The field was selected as representative cropping environment for central Madagascar, with typical soil and landscape characteristics and Striga asiatica infestation

Summary

Introduction

Rice (Oryza sativa L. and O. glaberrima Steud.) and maize (Zea mays L.) are among the main staple cereal crops in Africa, but yields on smallholder farms are often low (Shiferaw et al 2011; Seck et al 2012). Average yields of rain-fed upland rice in Africa range from 1.0 to 2.5 t/ha (Tanaka et al 2017), and attainable yield gains of up to 2.9 t/ha have been estimated (Nhamo et al 2014). There, farmers rotate upland rice with maize at mid and high altitudes, on poor and erosive soils (Dusserre et al 2012) that are heavily infested by Striga asiatica (L.) Kuntze. If uncontrolled, this parasitic weed causes average yield losses of 73% in rice (Rodenburg et al 2016) and 80% in maize (Rusinamhodzi et al 2012)

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