Two field experiments were undertaken with three sowing dates (28 of December, 28 of January and 24 of March) under irrigated conditions and three sowing dates (June 19, July 19 and August 19) under rain-fed conditions. Two levels of NPK (no fertiliser and fertiliser: 120 kg N ha-1, 90 kg P2O5 ha-1 and 60 kg K2O ha-1) and two maize varieties were used in a split plot design with four replications on a Gleyic Lixisol (Silitic) in the Northern Region of Ghana. Highest yield was obtained for SD3 and the lowest SD2 compared to SD1.

Biomass and harvest index was highest for SD1 followed by SD3 and SD2 respectively. The results from this study suggest that air temperatures (>35° C) at anthesis decrease grain number per cob and grain yield under irrigated and rain-fed conditions. Analysis of fertilised and unfertilised plots revealed that the negative heat stress effect around anthesis on the number of grains per cob and grain yield was only significant in unfertilised plots. Sowing date had no significant interaction with maize variety. However, fertiliser application dampened the negative effect of heat stress and increased water use efficiency by 2.3 times. Comparing the SIMPLACE model output to the observed field data, the duration of development phases were predicted with acceptable accuracy among the three sowing dates.

The heat stress component of SIMPLACE gave a good prediction for yield under heat stress when no other stress (water, nutrients) occurred. The estimations of the final yield showed an over estimation when the model was run with no heat stress condition in the rainy season experiment in particular under nutrient stress.

Under changing climate, the impact of heat stress on maize yield can be minimised by sowing late in March for the dry season or in June for the rainy season to avoid the combined effect of heat stress and terminal drought. However, the use of NPK fertiliser can help to reduce the effect of heat stress.