Maize yields can vary between different production areas in South Africa, even under optimal moisture and soil nutrient status conditions. The reasons for these differences between production areas are difficult to determine due to the direct differences between temperature, rainfall, sunlight, photoperiod and hybrid differences.
This past summer in the western production areas was characterised by unprecedented prolonged high temperatures with little to no rainfall.
At the beginning of the season, soil moisture was fair to good, and planting was done with the hope of regular rain showers throughout the season. In most cases there was no rain during January and February, and measured by planting done in late December, this meant that panicle stage, R1 and R2 of the plant development, occurred directly in the most critical phase of head development, between 65 and 75 days after planting, in other words no rain and extremely high temperatures.
High temperatures and water shortages increase stress
Higher temperatures increase the transpiration rate of crops, the demand for groundwater supply increases, and the result is the start of drought stress.
The increased water demand under high temperatures is enormous – it doubles between temperatures of 27°C and 35°C.
Maize reacts to water stress by closing the stomata. This helps to reduce water consumption, but unfortunately it reduces the absorption of CO₂, which is necessary for photosynthesis.
Vapour pressure deficits
High temperatures directly reduce yields by reducing pollination and photosynthesis, but the greatest losses occur with the interaction between high temperatures and water stress. Increased temperatures increase the evaporative demand deficit between the leaves and ambient air temperatures. This increases the need for ground water with consequent dry stress that occurs if there is too little moisture absorption from the soil.
Vapour (moisture) moves from areas of high concentration to areas of low concentration. It can be assumed that plant parts are 100% saturated with water, and as long as the surrounding air has less than 100% relative humidity, this means that moisture is always extracted from the leaves, that is transpiration. The greater the difference between moisture inside the plant and air moisture, the faster transpiration from the leaf surface takes place. Temperature is extremely important in this equation, because vapour pressure deficit increases exponentially with increased temperature, even if the relative humidity remains constant (Figure 1).
Hirasawa and Hisao (1999) demonstrated in experiments that photosynthesis and growth of maize plants decrease from temperatures in the mid 30s. Temperatures of 35 ⁰C were the order of the day during January and February 2024. Decrease in photosynthesis is also observable under irrigated conditions, although the decrease is greatest under dryland conditions (Figure 2).
The difference in photosynthesis rate for maize plants over daily time between dryland and irrigated maize (Hirasawa and Hisao, 1999).
Effect of high night temperatures on maize yields
Research shows that above average night temperatures during the reproductive phase have a detrimental effect on growth and reduce yield with fewer kernels and lower kernel weight. Two thoughts exist as to why high night temperatures result in lower yields:
- Rate of respiration (the process where sugars formed by photosynthesis in the plant are converted into energy used for growth, reproduction and other growth processes) increases within the plant, which places a higher need for sugars for energy for the respiration process, which then results in too little sugars being available for starch formation; or
- Higher temperature increases the phenological rate of plant development. This in turn causes the plants to reach maturity earlier, with little or no sugars available for starch and for ear formation.
The second thought is the process most supported by research, and is primarily considered the major reason for yield reduction.
Development is driven by average heat units or growing degree units (GDU)
Development or rate of development is determined by the average daily heat units.
GDU = ((max temp ⁰C + min temp ⁰C)/2) – 10 where the maximum ⁰C per day does not exceed 30 ⁰C, nor can it be less than 10⁰C; for example if the maximum temperature is 35 ⁰C, 30 is used as the maximum; but if the night temperatures are 26 ⁰C or 11 ⁰C respectively, this means the difference is 16 GDUs versus 10,5 GDUs.
In other words, if temperatures increase, the phenological processes (rate) of the plant must also increase. The problem under these conditions are that the effectiveness of the various processes under which the development stages must take place decreases. The effectiveness of the various processes decreases drastically or does not take place at all.
The result is reduced yields.
If both thoughts are in play, along with reduced moisture and high temperatures, the result may be maize plants without any ear development. First, all available sugars are used for growth, then the various phenological processes occur so quickly that the plant reaches maturity faster than planned, in other words, before the plant can go through its normal reproductive phases in time, the plant has already reached maturity and began to die.
Curling maize leaves in the mornings while sufficient moisture is present indicate heat stress.
High temperatures and moisture stress early in the vegetative growth stage
The current harvesting season started earlier than usual in some areas, due to the early death of the plants, which developed faster than usual, as explained above. Many fields are already harvested at 180 days compared to the normal 210 days. Although some crops are very susceptible to moisture stress during critical developed stages, VT (plume) in maize and R3 (flower) in soya beans, both crops are very sensitive to very high temperatures and moisture deficits in their early vegetative growth stages.
Above normal heat decreases photosynthesis and transpiration, and at the same time leads to poor root development. The combined effect of all these factors leads to a reduction in yield. The combined effect of heat stress and moisture stress is much greater than each individual effect, for both soya beans and maize.
Heat stress is a function of intensity, duration and rate of air temperature increase. If air temperature increases, soil temperature increases and in the absence of soil moisture, the soil temperature is even higher.
During periods of high temperatures, sufficient moisture must be available for normal plant uptake and to counteract high soil temperatures. In most cases, plants overcome the stress by absorbing moisture from deeper soil layers.
If soil temperatures rise above optimum conditions, water and nutrient uptake decreases, with consequent damage to one or more plant parts, depending on phenological plant development. Very high heat coupled with warm winds cause stomata to close more quickly, resulting in a reduction in transpiration for both maize and soya beans.
Heat stress and associated moisture stress cause the maize root volume to become small and concentrated just below the soil surface, that is no large and deeply distributed root volume.
If heat and moisture stress continues, the root volume will become lower and parts will become corky in the very dry topsoil. This helps the plants lose moisture through roots in droughts, but the opposite is that it negatively affects growth and development.
Conclusion
Moisture stress alone is usually given as the cause for low yields during droughts, but it is more complex than only moisture stress. Moisture stress with associated high long-term heat stress and the effects of hot winds, are enormous. The high air temperatures raised soil temperatures, which limited root development and spilled over into lower yields. Roots develop underground, so it is difficult to study the different effects, and this is a field that still needs to be studied more deeply and extensively.
It is clear that the limited root development of the past season has given rise to lodging in various different fields, regardless of genetics. Early demise of roots to protect plants can cause root pathogens such as Fusarium spp to occur saprophytically or secondarily on the roots.
The weather conditions therefore gave rise to poor root development which led to poor uptake of nutrients and moisture from the soil. The plants developed at an accelerated rate, and during the reproductive phase the plants cannibalised nutrients from the stems to feed ears. In the case of soya beans, nitrogen-fixing organisms also did not develop sufficiently to bind nitrogen from the atmosphere, with associated low yields.
References
Corteva Agriscience – Pioneer Agronomy. Feel free to contact the Pioneer Agronomist in your area for expert advice and recommendations.
