Agricultural approaches to increase water use efficiency
By Dr. Louis Ehlers, Manager: Agricultural Services Development, Omnia Fertilizer
Many scenarios for global climate change indicate an increase in drought and associated water shortages. For South Africa it is predicted that land surface temperatures could rise by between 1.5 and 3.0°C over the next 50 years. This increase could in turn result in a reduction in annual average rainfall of between 20 and 50% in the West and between 5 and 30% in the East of the country (Rautenbach, 2014). Together with this, it is estimated that the world population will increase to 9.15 billion by 2050 and that grain production would have to increase by 43% (FAO, 2009) to meet the increased demand. Maximum yield per unit of water consumption (or water use efficiency), as well as a clear understanding of the mechanisms involved in plant drought resistance, would form the basis for achieving this objective.
Definition of water use efficiency (WUE)
In agricultural terms, WUE (kg/ha/mm) is defined as grain yield obtained per unit area (kg/ha) with a certain amount of water (mm) and it varies from crop to crop. The WUE of maize and sorghum (C4 plants) are two to three times higher than the WUE of wheat and oats (C3 plants) and there are even differences between cultivars.
So the question that needs to be asked is: what is the maximum yield that can be achieved with a specific (and limited) amount of water? Or put differently: how can a producer manage his farm to ensure that less water is lost from the production system and that the water taken up by the plant is converted more effectively into actual harvestable or usable product?
The answer is not so simple, but great progress is being made in research and development of technologies focusing on agronomic, physiological and plant nutritional approaches to increase the efficient use of water.
Agronomic approaches focus on management practices to increase transpiration and decrease water loss through evaporation, runoff and deep drainage. Examples include conservation tillage, early planting, cultivars with rapid early development and the combination of plant population and row spacing.
Conservation tillage (mulching) is an obvious practice that can reduce evaporation, but it can also reduce runoff, increase infiltration and reduce soil temperatures. With moisture conservation in sandy soils in semi-arid areas, the only advantage of mulch, however, is that it only decreases the rate of evaporation when the soil is wet, and not the total evaporation amount of water in the profile.
The principle of early planting, cultivars with rapid vegetative development and the combination of plant population and row spacing is that the soil surface is covered as quickly as possible by the leaf canopy to minimise evaporation losses. This practice is only applicable in certain areas. Usually, early rain is very unreliable and crops planted early will be easily exposed to drought, especially when planted on a dry soil profile. In the drier western areas the optimum plant population for most grain crops is directly related to the amount of water during the growing season. Plant population should therefore be adjusted very carefully to prevent the advantage of the limited evaporation from being exceeded by the increased water demand.
Physiological approaches include aspects such as artificial manipulation of the growth and development of plants by making use of natural or synthetic plant hormones, as well as irrigation techniques such as regulated deficit irrigation (RDI) and partial root zone drying (PRD). These techniques are mainly developed for permanent crops such as apples, pears, peaches and vine where the balance between vegetative growth and reproductive development is critical. The basic principle of RDI is to limit irrigation during the vegetative growth phase, followed by a normal irrigation programme during fruiting. With PRD, a part of the root system is kept moist while the other part is allowed to dry out. The part of the roots subject to drying, produces stress hormones which trigger the closing of the leaf stomata, while the wet roots supply the plant with adequate water.
Plant nutrition approaches
There is increasing evidence that the nutritional status of plants plays a critical role in increasing the plant's resistance to drought and that proper plant nutrition is a good strategy to improve WUE and productivity in plants. During a period of drought, nutrients such as nitrogen (N), potassium (K), magnesium (Mg), zinc (Zn), boron (B) and silicon (Si) may increase the production of antioxidants in plants to counter the toxic effects of free radicals produced during such periods of stress. Experimental results indicate that N applied as nitrates such as ammonium nitrate (ANO 21) and limestone ammonium nitrate (KAN 28) can increase the WUE of maize under dry soil conditions by as much as 94% until the age of eight weeks, compared to N in the form of Urea (46).
Other nutrients such as phosphate (P), K, Mg and Zn improve root growth which increases the amount of soil that can be exploited for water and other nutrients. Micro-nutrients such as B, iron (Fe), manganese (Mn) and molybdenum (Mo) improve WUE by reducing the adverse effects of drought by activating certain physiological, biochemical and metabolic processes in the plant.
Direct measurements and algorithms to specifically measure WUE in practice is a tool that can be used to determine whether crop production is restricted through water supply or other factors. A good understanding of the impact of different management practices on WUE enable growers to identify opportunities and to adapt practices in order to ensure optimal WUE, thereby reducing their risk. The key lies in the fact that the WUE of crops can be increased by a combination of optimal agronomic practices and a balanced fertilizer programme.
- Bacon, M.A., 2004. Water-use efficiency in plant biology. Blackwell Publishing Ltd, Oxford, UK.
- FAO, 2009. News Article: 2050: A third more mouths to feed.
- Rautenbach, H., 2014. Assessment / effect of climate change in South Africa. Water Institute, University of Pretoria.