This report is about water use efficiency of grain crops in Australia: principles, benchmarks, and management
It looks at the water availability is a major constrain for production of grain in Australia, and improving water use efficiency is a primary target of growers, breeders, and agronomists.
The goal of this study is to provide decision makers with tools to understand and improve water use efficiency in rainfed systems where water deficit is a perennial problem.
The key sections are:
Chapter 1 Crop growth and yield: physiological principles
Capture and efficiency in the use of resources driving crop growth
- Crop growth and yield depend on the ability of crops to capture above-ground and soil resources, and on the capacity of crops to transform these resources into biomass.
- As a rule of thumb, shortage of resources (drought, nutrient deficit) and soil constraints (compaction, salinity, alkalinity) reduce crop growth by reducing the capture of resources, rather than efficiency in the use of resources
- Capture of water by the crop, measured as transpiration, was similarly reduced from 110 to 60 millimetres, whereas biomass per unit transpiration was largely unaffected, at about 58 kilograms per hectare per millimetre.
Annual crops have typical ‘windows’ when yield is more sensitive to stresses
- Grain number is reduced when stress occurs in critical developmental windows. Compared to unstressed controls, which produce 100 percent the (potential) number of grains (horizontal line), stressed crops have severe depressions in grain set when stressed at critical windows
- The rate of flower mortality is higher under poor environmental conditions.
- The process of floret mortality is under genetic control and responds to the environment: the plant will kill more florets in a poor environment.
Chapter 2 Water use efficiency: climate and crop drivers
Crop biomass and grain yield depend on photosynthesis. Photosynthesis involves the uptake of carbon dioxide (CO2) through stomata, which are pore-like, specialized cells in the surface of leaves.
Water use efficiency and vapor pressure deficit
Liquid water moves from soil to root, and from root to shoot.
- Vapour pressure deficit has a large impact on water loss and little direct impact on CO2 uptake.
- In Australia, vapor pressure deficit at the critical window around flowering of typical wheat crops increases northwards and inland
- The French and Schulz parameter – 20kg/ha per millimeter – was originally derived under South Australian conditions
Water use efficiency and rainfall patterns
- Seasonality and size of rainfall events influence crop water use efficiency.
- For a given soil type, run-off and deep drainage are more likely where large rainfall events dominate.
- Collectively, vapor pressure deficit and rainfall patterns are the main climate determinants of location-specific water use efficiency.
Water use efficiency and nitrogen availability
- Nitrogen-deficient soils reduce water use efficiency. First, a nitrogen-deficient crop will have impaired photosynthesis; hence, above-ground dry matter per unit transpiration will drop.
- Second, nitrogen deficiency reduces the ability of the crop to capture soil water and increases soil evaporation in association with a smaller canopy and root system.
- The gain in water use efficiency is achieved at the expense of reduced yield per unit of nitrogen fertilizer.
- The nitrogen-driven trade-off between water and nitrogen use efficiency is universal; it has been documented for wheat, rice, maize, canola and forage grasses, among other crops.
Water use efficiency and seed composition
- The trade-off between leaf photosynthesis and water loss is rather robust: there is not much difference between crop species, except for maize and sorghum, which have a higher photosynthesis per unit water loss than small grain crops.
- Cereals have a much greater water use efficiency than oilseed crops. This reflects the low energy cost of starch relative to fat as main products stored in grain
Chapter 3 Benchmarking wheat water use efficiency: accounting for climate and nitrogen
In this chapter, location-specific parameters for benchmarking wheat water use efficiency for crops grown with a wide range of nitrogen supply are presented.
French and Schultz parameters: expected effects of climate and nitrogen
• The model of French and Schultz has two parameters. One is the slope of the line representing the best yield for a given water use.
• The approach of French and Schultz has known limitations; for example, it does not account for timing of rainfall
• The notion of a single parameter representing maximum yield per unit water use and a single parameter representing soil evaporation is a simplification.
• First, the slope of the line decreases with increasing vapor pressure deficit
• Second, soil evaporation is greater in locations and seasons with a dominance of small rainfall events and where a greater proportion of total water use is derived from in-season rainfall
• Third, nitrogen deficit reduces the slope and increases soil evaporation.
Estimating maximum yield per unit water use by location and nitrogen A three-step procedure to derive the ‘slope’ parameter representing maximum yield per unit water use accounting for nitrogen and location is proposed.
• For intermediate nitrogen supply, maximum yield per unit water supply can be estimated graphically using this curve.
• For a latitude of –23.5˚ (Emerald, the northernmost location), maximum yield per unit water use would be about 12kg grain/ha/mm.
• Select the lowest value, 14.7kg/ha/ mm, as a benchmark for this combination of location and nitrogen supply
Estimating soil evaporation as a function of location and agronomy
• Assuming a single soil evaporation parameter to benchmark water use efficiency is therefore a very coarse simplification and possibly the main source of error in estimating water use efficiency using the French and Schultz approach
Chapter 4 Crop management, water use, and water use efficiency
The amount and distribution of rainfall are major factors influencing crop water use. While farmers have no control over rainfall, by using different management practices they can affect how much of the rainfall is used by the crop and how efficiently it is used.
• Following captures out-of-season rainfall and can increase the amount of water available for crop growth.
• Following is very important for winter crop production in the northern cereal zone where rainfall shows a strong summer incidence.
• While fallowing efficiency is often low, leading to small increases in available soil moisture and crop water use, the benefits of this moisture can still be high
• The value of subsoil moisture over a wider range of soils and environments needs to be evaluated.
• Water use efficiency is most commonly considered for individual crops and often the focus is on management of the crop during the current growing season to improve water use efficiency
• Measurements of water use efficiency in rotation experiments show consistent increases in water use efficiency when wheat is grown after a legume
• In Queensland, sowing wheat after chickpeas increased water use efficiency by a similar amount (27 per cent), compared to continuous wheat (11.7kg/ha/mm compared to 9.2kg/ha/mm).
Time of sowing
• Arguably, time of sowing is the most important management practice that will affect water use efficiency and yield.
• Early sowing increases the effective length of the growing season and using varieties with patterns of development that take advantage of this improves the overall efficiency of water use.
• While the optimum sowing time for a particular location can be variable depending on the maturity type of a variety, the flowering ‘window’ that provides the highest yields is generally more stable
Water use and water use efficiency
• Time of sowing generally has a small effect on total crop water use but can have a marked effect on water use efficiency.
• The highest water use efficiencies are consistently achieved when the crop is sown at the optimum time
• Sowing a variety at a time that is either too early or too late for its maturity type will reduce yield and water use efficiency, but may not greatly affect total water use.
• Plant density and row spacing determine the spatial arrangement of plants in crops.
• Reducing soil evaporative losses can improve water use efficiency by channeling more moisture through transpiration, which directly contributes to growth and yield
• By affecting the early growth of the crop, plant arrangement also affects the evapotranspiration during the pre-anthesis period and the balance between pre- and post-anthesis water use.
• Sowing rate can alter water use efficiency by influencing early crop vigour, and the pattern of water use before and after flowering.
• Most crops have considerable plasticity in their growth and can compensate for variation in plant density and row spacing
• The optimum plant density is affected by the amount of available moisture as well as the ability of the crop to compensate for changes in plant density
• Higher sowing rates increased biomass production and reduced the available soil moisture at anthesis, especially in the two years with low rainfall
• Studies in a range of environments with wheat crops tend to show that increasing the row width will reduce yield and lower water use efficiency
• The effect may be greater when row width increases and high plant sowing rates are maintained, leading to a high degree of crowding in the row
• While increased row spacing can lead to increased bare soil evaporation and reduce water use efficiency, the effect may not be important in crops with small canopies and the yield reductions, in this case, may be most strongly related to increased competition within the row
• Grain yield responses to row width in pulses can vary depending on crop and seasonal conditions, but in most cases, there are small and non-significant effects of row spacing or a reduction in yield with wide row spacing
• The present evidence suggests that using wide rows may have limited benefit to the efficient use of seasonal rainfall or may cause significant reductions in efficiency.
• Crop nutrition can affect a number of aspects of crop growth related to water use and water use efficiencies, such as root growth, the rate of canopy development, biomass production and harvest index
• Nitrogen is the nutrient required in largest amounts by crops and its supply can greatly affect growth, yield and water use efficiency
• Improving the supply of nitrogen from following or from growing wheat after a legume increases the water use and water use efficiency of the following wheat crop
• The supply of nitrogen can be used to manipulate canopy development and biomass production and water use.
• The choice of variety can be an important aspect of managing crops for maximum water use efficiency.
• The response to the time of sowing is affected by the maturity type of the variety and to improve yield and water use efficiency, variety needs to be matched to sowing date.
Tolerance to subsoil limitations
• Soil properties that restrict plant growth, such as alkalinity, acidity, salinity or high concentrations of boron or aluminum lead to the incomplete use of available soil water and limit yield per unit rainfall.
Tolerance to disease
• Both root and foliar diseases can reduce water use and water use efficiency.
• Variation in soil properties and landform can lead to high spatial variation in growth and grain yield and this is reflected.
In conclusion, the guide states that the principles outlined in Chapters 1 and 2 are used to interpret crop responses to practices including fallowing, crop rotation, planting arrangement (sowing rate and row spacing), crop nutrition, variety selection and precision agriculture.