Optimising maize yield under flood-recession cropping in the Zambezi Valley floodplains, Northern Zimbabwe

Abstract

Flood-recession cropping (FRC) can alleviate effects of droughts, dry spells, and poor soil fertility, which are the major constraints to improving food security for smallholder farmers in semi-arid areas. The study aim was to evaluate sustainable options for optimising flood-recession maize (Zea mays L.) yield in the mid-Zambezi Valley. This study determined: farmers’ perceptions on FRC’s socio-economic importance and production challenges; maize agronomic practices; floodplain soil fertility; crop establishment method (tillage); cultivar and fertilizer effect on maize yield; and seasonal rootzone water content and salinity in maize fields. Questionnaires were administered to 123 FRC households for baseline survey. Soil samples were collected at 0.20-m depth increments to 1.00 m, from 24 sampling points and analysed using core method for bulk density (BD), hydrometer method for texture, loss on ignition for organic carbon (OC), Kjeldahl procedure for total N, 0.01M CaCl2 method for pH and ICP for Mehlich-3 extractable elements. Field experiments were conducted from 2016-2018 at Zhoubvunda 1, Zhoubvunda 2 and Mukumbura, replicated thrice per site. Experiment-1 was a 4*4 factorial CRD comprising NPK basal fertilizer [7:6:6 (6-8% S)] and N top-dressing fertilizer (NH4NO3), each at 0, 75, 150 and 225 kg/ha using an early-maturing maize cultivar (SC513); Experiment-2 was a 3*4*4 factorial split-split plot design with two cultivars, medium (SC627) and late maturing (SC727) added to Experiment-1; and Experiment-3 was a CRD that compared two crop establishment methods namely furrow only (F) and furrow+holing-out (F+H). Percent emergence was determined 3 weeks after crop emergence (WACE) in 6.0 m × 2.7 m net plots. Days to 20%, 50% and 80% tasselling and silking were determined from 7-7.5 WACE. Maize yield was measured from 1.8 m × 1.8 m net plots. Soil moisture content was measured using the TDR method and salinity samples were collected up to 1.0 m-depth at 7-14-day intervals. Salinity was measured using an Electrical conductivity and Temperature Meter. Survey data were analysed using descriptive statistics and Binary Logistic Regression Model in SPSS Version 20.0. Soil and crop data were analysed using ANOVA, Pearson's correlation and t-Test in GenStat 18th Edition. Flood-recession cropping was ranked first among livelihood sources, contributed >50% household income, and produced food and high value crops. Major production challenges were pest damage, inadequate labour and equipment. Maize plant spacing was 1.00 m × 0.60 m, 69% of farmers practised F+H, 97.6% did not apply fertilizers, 68.3% planted retained seed, and 87.0% planted >4 seeds per station. Better-resourced farmers were more likely to practise F+H. Floodplain soils were medium textured and fertile: 1.20-1.40 g/cm3 BD, 0.36% N, 2.04% OC, 7.70-8.60 pH, and had high base concentrations. Across sites, the mean exchangeable sodium percentage values ranged from 4.14 to 7.32, were therefore below 10 which is the critical level for soil structure damage. Maize yield and percent emergence were >20% higher under F+H than F. High SC513 grain yield, 6.20-8.45 t/ha, ≥2 times yield from farmers’ fields, was recorded without fertilizer. Averaged over NPK and N top-dressing fertilizer rates, the late-maturing cultivar, SC727, out-yielded earlier maturing cultivars, but had a low harvest index (<0.40). NPK fertilizer response was higher at 75 and 150 kg/ha for early and later-maturing cultivars respectively. Benefit from N top-dressing fertilizer, up to 75 kg/ha was evident at 0 kg/ha NPK fertilizer, in an increasing trend with increasing days to maturity of cultivar. Soil moisture depletion was higher in the effective maize rootzone (0.0-0.4 m) but mostly remained above wilting point. Soil salinity reached levels that can reduce maize yield by 10-50%, with highest potential effect on cultivars that require >100 days to mature. It was concluded that productivity of FRC can be improved through selecting appropriate crop establishment methods and maize cultivars; micro-dosing with NPK fertilizer and; nutrient stewardship to reduce rootzone salt accumulation. Further studies should focus on long-term experiments for (i) developing FRC fertilizer recommendations, (ii) screening maize cultivars for tolerance to salinity and determining: (iii) effect of crop establishment method on (a) maize root development and yield; and (b) erodibility and erosion of floodplain soils. Key words: Crop emergence; Fertilizer; Flood-based farming; Production challenges; Soil fertility; Soil moisture content; Soil salinity.