Maize yield prediction using seasonal weather forecasts and a crop growth simulation model.
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Maize production in marginal tropical regions is at great risk from rainfall variability. Farmers would benefit from the ability to forecast production likelihood. In this study we sought to develop a simple maize production decision support tool for Masvingo by using seasonal weather forecasts and a crop production model to forecast maize yields prior to the season. Downscaled ENSO-based statistical seasonal forecasts from RAINMAN were tested against those downscaled from a Global Circulation Model (GCM) using Climate Predictability Tool (CPT). RAINMAN was found to perform better at forecasting total seasonal rainfall than CPT. RAINMAN predictions were 69 % correct in all rainfall categories for the 1991/92 - 2006/07 seasons as opposed to 44 % for CPT (p< 0.05). RAINMAN had a higher hit rate than CPT and was not biased to any rainfall category while CPT was biased towards the normal and dry/below normal rainfall categories. Monthly rainfall predictions by RAINMAN were validated. The tool explained 65 % to 81 % (p<0.05) of the rainfall variability of the agricultural season (October to April), except for December and March where it explained 37 % and 48 % of the variability, respectively. We generated monthly weather series for the five phases of the Southern Oscillation Index (SOI). These formed the climatic scenarios used to run the crop production model (AquaCrop). Simulated agrometeorological scenarios included three planting dates, optimal and poor fertility levels, and three maize cultivars. Simulated maize yields ranged from 1.2 t/ha to 5.9 t/ha. Average yields were low for poor fertility levels. 100-day (early maturing) maize cultivars produced better yields under poor fertility levels. 140-day (late maturing) maize cultivars attained highest yields (5.9 t/ha) for good rain conditions (neutral, rising, and positive SOI and (20 %) probability of rainfall occurrence) and minimum yields (1.2 t/ha) under poor fertility. 100-day and 140-day maize cultivars produced higher yields when planted late (7 December). 125-day cultivars produced better yields when planted early (29 October) or on the medium planting date (16 November). The variance in yields under the given agrometeorological scenarios point towards the importance of considering maize cultivar and planting date selection. It was clear that maize production at Masvingo should preferably be done under good fertility.