This is far less than the yield of about 7–8 t ha −1 observed in research fields ( Shehu et al., 2019). Despite the increase in production, maize yields in Nigeria have remained low over the last decades hovering around 1.8–2 t ha −1 ( FAOSTAT, 2018).
Significant increase has been witnessed in its production in the past three decades in the savannas of Nigeria, even in the semi-arid Sudan Savanna (SS) zone where rainfall is erratic ( Kamara et al., 2009 FAOSTAT, 2017). Maize ( Zea mays L.) is a principal food security and a major commercial crop in Nigeria providing food, animal feed, and industrial raw materials ( Badu-Apraku et al., 2009). This is probably due to the poor soil fertility in this location. Though Yelwa is in the SGS, lower yields and narrower sowing windows were simulated for both varieties than for those of the other locations. For the medium-maturing variety, IWDC2SynF2, planting should be done by the first week of July. In the Sudan savannah (SS) where the growing season is 90–120 days, planting of 2009EVDT can be delayed up to the third week of July. In the Northern Guinea savannah (NGS) where the length of growing season is 150–180 days, the optimum planting window is June 15–July 19 for both varieties at Zaria and June 8–July EVDT and June 8–August 2 for IWDC2SynF2 at Sabon Gari. The planting window that gives attainable yield at Yelwa, is June 15–July EVDT and June 8–28 for IWDC2SynF2. In the Southern Guinea savanna (SGS) where the length of growing season is 180–210 days, the best planting window was June 8–July EVDT and June 8–July 26 for IWDC2SynF2 in Abuja. Planting from June 15 to 28 simulated the highest mean grain yield for both varieties in all the agro-ecologies. The seasonal planting date analysis showed that optimum planting windows for 2009EVDT and IWDC2SynF2 depend on the variety, agro-ecozones and sites. The evaluation with the experimental data showed that the model performance was reasonable and accurately predict crop phenology, total dry matter (TDM) and grain yield for both maize varieties. The model was run for 11 planting windows starting from June 1 and repeated every 7 days until 16 August using long-term historical weather data from the 7 selected sites representing three agro-ecological zones (AEZs).
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The Agricultural Production Systems Simulator (APSIM) model was calibrated and validated and used to identify the optimum planting windows for two contrasting maize varieties for three agro-ecologies in the Nigeria savannas.