Conservation tillage of rainfed maize in semi-arid Zimbabwe: A review

• We reviewed maize conservation tillage experiments in semi-arid Zimbabwe.
• Tied ridging and mulch ripping outperformed conventional tillage in dry years.
• A conservation tillage system that performs well under wet conditions is required.
• Smallholder farmers face resource constraints to adopt conservation tillage.
• Conservation basins are an option only for poor farmers and as fall-back.

Food security in Sub-Saharan Africa, particularly in semi-arid tropics (41% of the region; <800 mm annual rainfall often coming in storms and >6 months of dry season) is threatened by droughts, dry spells and infertile soils. In Zimbabwe, 74% of smallholder farming areas are located in semi-arid areas mostly in areas with soils of low fertility and water holding capacity. The dominant crop in these areas, maize (Zea mays L.), is susceptible to drought. Under smallholder farming in Zimbabwe, conventional tillage entails cutting and turning the soil with a mouldboard plough thereby burying weeds and crop residues. Seed is planted by hand into a furrow made by the plough, ensuring that crops germinate in relatively weed free seedbeds. Inter-row weed control is performed using the plough or ox-drawn cultivators and hand hoes. Conventional tillage has been criticised for failure to alleviate negative effects of long dry spells on crops and to combat soil loss caused by water erosion estimated at 50 to 80 t ha−1 yr−1. Therefore, conservation tillage has been explored for improving soil and water conservation and crop yields. Our objective was to determine the maize yield advantage of the introduced technology (conservation tillage) over conventional tillage (farmers’ practice) based on a review of experiments in semi-arid Zimbabwe. We use a broad definition of conservation tillage instead of the common definition of ≥30% cover after planting. Eight tillage experiments conducted between 1984 and 2008 were evaluated. Conventional tillage included ploughing using the mouldboard plough and digging using a hand hoe. Conservation tillage included tied ridging (furrow diking), mulch ripping, clean ripping and planting pits. Field-edge methods included bench terraces (fanya juus) and infiltration pits. Results showed small yield advantages of conservation tillage methods below 500 mm rainfall. For grain yields ≤2.5 t ha−1 and rainfall ≤500 mm, 1.0 m tied ridging produced 144 kg ha−1 and mulch ripping 344 kg ha−1 more than conventional tillage. Above 2.5 t ha−1 and for rainfall >500 mm, conventional tillage had ≥640 kg ha−1 yield advantage. Planting pits had similar performance to ripping and conventional tillage but faced digging labour constraints. Experiments and modelling are required to test conservation tillage seasonal rainfall thresholds. Constraints to adoption of conservation tillage by smallholder farmers necessitate best agronomic practices under conventional tillage while work on adoption of alternative tillage methods continues.