By Dr Victor Sadras and Dr John Angus
The poor water-use efficiency of wheat crops in southern Australia is not unique to Australian farms, a study has found.
The study, to benchmark local crop performance against crops in three other low-rainfall regions of the world, North America, China and the Mediterranean basin, found that low water-use efficiency was a general problem.
The fact that crops rarely reach their potential efficiency is related to management and environmental constraints typical of dry environments.
Good management practices can overcome much of the inefficiency but they may not be enough on their own to achieve potential water-use efficiency in a dry environment.
The study found that management practices can improve the water-use efficiency of single crops, but larger gains are more likely to come from management at the farming system level.
At the crop level, practices to improve water-use efficiency include timely sowing, management of weeds and stubble and better matching of resources, particularly water and nutrients.
Fertiliser management is crucial in systems where high-risk farming due to uncertain rainfall is tackled with a low-input strategy.
At the farm level, more intensive cropping is leading to large gains in productivity and efficiency in the use of resources.
The process of intensification is not just happening in Australia but is under way in many farming systems worldwide.
Greater diversity and intensity of cropping at the expense of fallow and pastures is possible in the agro-ecosystems of southeastern Australia, where wheat-fallow and wheat-pasture rotations have been dominant.
These have been the main findings from the study, which sought to compare water-use efficiency of wheat in south-eastern Australia with the other dry environments.
Severe water deficit in the critical stages of flowering, grain set and filling is a common feature of all four environments studied.
The maximum ration of yield per unit water use was close to 22 kilograms of grain per hectare per millimetre, but few crops reached ratios close to this value (Figure 1).
Figure 1. Frequency distribution of water use efficiency of wheat crops in four regions of the world. Water use efficiency is calculated as the ratio of grain yield (kg/ha) to total water used (mm).
Average water-use efficiency was 9.9kg grain/ha/mm for south-eastern Australia, 9.8kg grain/ha/mm for the China Loess Plateau, 8.9kg grain/ha/mm for the northern Great Plains of North America, 7.6kg grain/ha/mm for the Mediterranean basin, and 5.3kg grain/ha/mm for the southern-central Great Plains.
These values represent 31 to 44 percent of the maximum potential value estimated for these environments.
Evaporative demand during flowering and grain filling accounted for most of the variation in mean water-use efficiency among environments (Figure 2).
Figure 2. Water use efficiency decreases with increasing evaporative demand around flowering. Evaporative demand, which is the drying power of the air, was calculated as longterm Penman-Monteith evapotranspiration.
Once the effect of evaporative demand is taken into account, it is clear the average efficiency is in line for all four regions. This again indicates that the gap between actual and potential water-use efficiency is not only a local Australian problem. Likely reasons for the low efficiency include:
Large soil evaporation losses are the main constraint for high water-use efficiency of rainfed crops in dry environments. A modelling study in southern Australia estimated soil evaporation can account for 30 to 80 percent of the growing season rainfall.
Under particular combinations of rainfall, soils and topography, runoff and deep drainage are other potentially important "wasteful" pathways for rainwater. Water that runs off or drains below the root zone is not available for crop growth. Nonetheless, from a production viewpoint, the main leak in the systems is upwards, in the form of evaporation from the soil.
Reduced row spacing, early vigour and good supply of nutrients can favour rapid ground cover and reduce soil evaporation.
The benefits of rapid use of water early in the season, however, have to be weighed against the depletion of soil water reserves for critical stages of grain set and filling.
Adequate fertilisation can dramatically increase water-use efficiency, largely through the reduction in soil evaporation.
For example, in a study of Mallee wheat crops, the gap between potential and actual yield declined at a rate of 109kg grain/ha per unit increase in topsoil phosphorus content between eight and 30 milligrams of phosphorus per kilogram of soil.
Other experiments with barley in Mediterranean locations showed a threefold increase in water-use efficiency in response to fertilisation, but the fact remained that (even with adequate nutrition) for each millimetre of water used productively in crop transpiration, one or two millimetres were wasted through soil evaporation.
In summary, unproductive terms of the water budget - chiefly soil evaporation - are major constraints for the achievement of high water-use efficiency in dry environments.
Management practices and breeding can help reduce these losses, but key features of these environments, such as soil type, timing, size and frequency of rainfall events may impose a limit to their effectiveness.
Timing of rainfall is a crucial determinant of grain yield and water-use efficiency, as it can impose agronomic and physiological constraints to the crop.
Agronomically, the onset of rainfall determines sowing opportunities, thus constraining season length and yield.
On average, the yield of wheat crops in the Mallee drops about one percent for each day"s delay in sowing from mid- April.
Options to deal with uncertain opening rains are good agronomy, such as direct drilling and good weed control, to allow sowing with the first rain, and possible future genetic increase in coleoptile length, combined with moisture-seeking seeding equipment to allow sowing into subsurface moisture before rain.
Physiologically, rainfall around flowering is critical because this is a crop stage particularly susceptible to environmental stresses.
Long-term simulations at Whyalla, SA, showed seasonal rainfall accounted for 42 percent of the variation in grain yield, whereas water availability in the 30-day period bracketing anthesis accounted for 42 percent of the remaining variation.
For more information:
Dr Victor Sadras, CSIRO Land and Water, 08 8303 8543, email@example.com
GRDC Research Code: CSO212, program 4