Maize is grown in most cropping regions of the world, being the fourth largest crop produced worldwide. It is grown both as a human staple and for stock feed, under irrigated and dry/and conditions and by subsistence and commercial farmers. However, in the cropping region of northern Australia, grain sorghum is the preferred option for a summer cereal, with maize a relatively minor crop. Despite the higher yield potential and often with a price premium for maize compared with sorghum, farmers prefer sorghum because of the perception that maize is less reliable in the hot, dry environments of northern Australia. The contention that maize is adversely affected by high temperature and water deficit at flowering is often advanced as a reason for the preference for grain sorghum. This thesis attempts to disassociate the impacts of water deficit and high air temperature per se. To many farmers and agronomists, high air temperatures are analogous to developing crop water stress because evaporative demand is also driven by temperature. However, it may be that air temperature alone impact adversely on maize yield
This thesis seeks to quantify the impacts of high air temperatures and water stress on maize grain set and yield using field-based experimentation and simulation modelling. Grain sorghum is included in field experiments for comparative purposes; however, the thesis concentrates on maize production in environments that are likely to be affected by high temperature and water stress, either independently or in combination. Earlier research has shown that anthesis is a key growth stage affected by water and temperature stress, with reductions in grain set, grain number and grain yield being reported However, these stresses display similar symptoms under field conditions, and there have been no studies examining their independent and interactive effects under field conditions, particularly in the Australian environment.
The first stage of the research consisted of field experiments that provided data on the relative performance of maize and sorghum under water-limiting and high temperature environments. The results from the first of two field experiments, in which maize and sorghum were sown on three dates in various sowing patterns, showed evidence that temperatures > 38˚C around anthesis affected the grain setting process, which caused reduced yields.
However, the evidence was not conclusive because the possible impact of water deficiency that may also have influenced grain set and ultimately grain yield could not be separated from the apparent temperature effects. Consequently, a second field experiment involving seven planting dates at weekly intervals under water and nutrient non-limiting conditions was conducted to gain data on the effect of high temperature on pollen viability and grain set in the absence of water stress. This experiment produced evidence of adverse effects of high temperature independent of water stress, and indicated that the timing of high temperature exposure was critical for any impact to occur.
Data from the field experiments were then used to examine the use of different high temperature thresholds in APSIM-Maize and to justify changes to the critical temperature threshold for the detrimental impacts of high air temperatures. The model predicted phenology, temporal changes in LAI, grain number, grain yield and soil water extraction reasonably well when assessed by the Root Mean Square Deviation for conditions when high temperature did not occur. However, when high temperature occurred, the threshold temperature for the impact of high temperature needed to be lowered from 38˚C to 36˚C to achieve satisfactory predictions.
The experimental and modelling studies showed that high air temperature per se can reduce grain set and yield of maize. Given the accumulated evidence presented, the application of APSIM for exploring the risk of maize production in northern Australia was examined. The model was evaluated by asking several questions, like how important are plant available water at sowing and sowing date options in avoiding high temperature effects around anthesis. Such information may address farmer perceptions of maize production risks under dryland conditions.
Of the three factors considered in this research (sowing date, water supply at planting and risk of high temperature stress), limited water supply at planting has the greatest impact on maize yield, and planting an less than a full profile is difficult to justify in the study region. Economic assessments conducted support the findings from the yield predictions, and should be tailored for local conditions and for a wide range of economic parameters.
A significant potential exists to extend dryland maize production into areas of north – eastern Australia previously considered too risky, thereby improving the supply of this sought after feed grain and providing an additional option for producers in developing sustainable production system. APSIM-Maize facilitates investigation of a range of management options, and thus can provide guidance on appropriate agronomic practices. Clearly, further studies can be done with additional crop production inputs e.g. cultivars, plant population to supplement the guidance provided in the present study. It is this capacity of models that should be used to examine production systems and provide guidance to their management.
This thesis has made a significant contribution to better understanding the influence of environmental stimuli on maize growth, development and yield. There is now a greater confidence in quantifying the impacts of high temperatures at flowering, soil water supply at sowing and sowing date on maize grain set and yield. Consequently, changes have been made to the APSIM-Maize model that improves its simulation of maize performance in northern Australia and elsewhere. APSIM-Maize can now be used by researchers and with industry to assist in maize becoming a more viable option in dry land farming.