Ground Cover North : Ground Cover 094 September-October 2011 - North
SEPTEMBER -- OCTOBER 2011 GROUND COVER Water use efficiency 31 Associate Professor Victor Sadras, from the South Australian Research and Development Institute (SARDI), explains the drivers of water use efficiency (WUE) and discusses implications for benchmarking and crop management Nitrogen and WUE: a question of balance Altogether, this means that nitrogen- deficient crops have low WUE. This applies to most crop species, soils and climates. It is clear then, that high water use efficiency requires high nitrogen rates. The red line in Figure 2 shows the positive effect of increasing nitrogen rate in WUE. But there is a catch: the crop uses each additional unit of nitrogen less efficiently to produce grain. The red line in Figure 2 shows how nitrogen use efficiency declines with increasing nitrogen supply. The green and red lines in Figure 2 describe how nitrogen fertiliser drives a trade-off between nitrogen use efficiency and WUE. There are two important consequences from this trade-off. First, many crops operating at, say, 60 to 80 per cent of maximum WUE may indicate a sensible solution to the nitrogen--water trade-off. It does not necessarily mean a grower is being inefficient, but has found the right balance between nitrogen and water. Second, we need benchmarks of water use efficiency that reflect the nitrogen input that growers are comfortable with. Part 2 outlines a method to benchmark WUE that accounts for nitrogen supply. * This work is part of the GRDC-funded project 'Improving crop and farm water use efficiency in Australia' (project DAS00089). mm and the nitrogen correction would return 16.6kg/ha/mm. We therefore select the lowest value, 14.7kg/ha/mm, as a benchmark for this combination of location and nitrogen supply. PART 3 Effects of location and agronomy on soil evaporation The default assumption in benchmarking water use efficiency is that soil evaporation is about 110mm. Soil evaporation is, however, extremely variable. It depends on the pattern of rainfall and crop ground cover. High frequency of small rainfall events, typical of the southern and western regions, increases the proportion of rain lost as soil evaporation. Reductions in leaf area caused by factors such as diseases, nutrient deficiency or soil compaction increase the proportion of rainfall lost through soil evaporation. Here, we use two boundary lines to represent the maximum and minimum soil evaporation as a function of latitude and general growing conditions (Figure 4). Under good conditions -- for example, agronomy favouring rapid ground cover and a large fraction of seasonal crop water use derived from stored soil water -- soil evaporation would range from 120mm in southern locations to 35mm in northern locations as rainfall shifts from winter to summer dominant. Under poor conditions -- for example, poor agronomy and nutrient deficiency and a large fraction of seasonal crop water use derived from in-season rainfall -- soil evaporation would range from 200mm in southern locations to 120mm in northern locations. Estimates of soil evaporation can be derived directly from Figure 4. In summary, the approach to benchmark wheat WUE originally derived by French and Schultz is both robust and simple enough to be used as a guide to the best yield that can be achieved with a certain amount of water use. The original benchmark of 20kg/ha/mm needs to be updated to 22kg/ha/mm to account for current varieties. Furthermore, here we developed simple-to- use corrections to tailor this benchmarking approach to location and agronomic conditions, particularly nitrogen supply. □ GRDC Research Code DAS00089 More information: Associate Professor Victor Sadras, 08 8303 9661, firstname.lastname@example.org; the GRDC Research Report Water use efficiency of grain crops in Australia by Associate Professor Victor Sadras and Dr Glenn McDonald will be available in late September from Ground Cover Direct (free phone 1800 11 00 44, email@example.com); www.grdc.com.au/DAS00089 PART 2 Benchmarking WUE: effects of location and nitrogen In benchmarking WUE, we use 20 kilograms of grain per hectare per millimetre as a convenient default. Here, we propose a three-step method to adjust this benchmark accounting for location and nitrogen. STEP 1 Use the curve in Figure 3A to account for the effect of nitrogen on WUE. For crops with abundant nitrogen supply (N supply > 200 kilograms N/ha) the benchmark approaches 22 to 24kg grain/ha/mm. For severely limited crops (N supply < 50kg N/ha), the benchmark would be as low as 5 to 6kg grain/ha/mm. For intermediate N supply, the benchmark can be estimated graphically using the curve in Figure 3A. STEP 2 Use the line in Figure 3B to correct for location. For a latitude of 41.5°S (Launceston, Tasmania, the southernmost location in this study) the benchmark would be around 24 to 25kg grain/ha/mm. This reflects the milder climate of this southern latitude. For a latitude of 23.5°S (Emerald, Queensland, the northernmost location) the benchmark would be about 12kg grain/ha/mm. For intermediate locations, the benchmark can be estimated graphically using the line in Figure 3B. STEP 3 Select the lowest value from steps 1 and 2. For example, if we want to estimate the benchmark for Dalby (latitude = 27.1°S) with intermediate N supply (100kg N/ha), the location correction would return 14.7kg/ha/ Figure 1 Well-fertilised crops had a greater capacity to withdraw water from the soil profile than nitrogen-deficient crops. Soil depth (cm) 0 --20 --40 --60 --80 --100 0 5 10 15 Soil water content (mm) low N high N maturity Measurements on GladiusA wheat sown 3 June 2010 at Roseworthy Figure 2 Increasing nitrogen rate improves water use efficiency (green line) but reduces nitrogen use efficiency (red line) 0 100 200 300 Water use efficiency (kg grain/ha per mm) Nitrogen use efficiency (kg grain/ha per kg N) 20 15 10 5 0 40 30 20 10 0 Nitrogen rate (kg N/ha) WUE NUE Figure 3 Water use efficiency benchmark in response to (A) nitrogen and (B) location. 0 100 150 50 200 250 WUE benchmark (kg/ha/mm) 24 22 20 18 16 14 12 10 8 6 4 2 0 --44 --30 --32 --34 --36 --38 --26 --28 --40 --42 --24--22 WUE benchmark (kg/ha/mm) 24 22 20 18 16 14 12 10 Nitrogen supply (kg N/ha) Latitude (B) (A) These curves were derived from simulations with the APSIM model using characteristic soils and long-term climate records for 43 locations in combination with a broad range of initial soil water content and nitrogen availability. Figure 4 Soil evaporation of wheat crops as a function of latitude and agronomic and environmental conditions Soil evaporation (mm) 'Good' conditions to achieve low soil evaporation include good nutrition and a large proportion of total crop water use accounted for by stored soil water. 'Poor' conditions leading to high soil evaporation include nitrogen deficiency and a large proportion of total crop water use accounted for by in-season rainfall, particularly small events. These lines were derived from simulations with the APSIM model using characteristic soils and long-term climate records for 43 locations in combination with a broad range of initial soil water content and nitrogen availability. --44 --30 --32 --34 --36 --38 --26 --28 --40 --42 --24 --22 200 150 100 50 0 Latitude good condition poor condition PART 1 The role of nitrogen Adequate nitrogen supply is essential to maximise water use efficiency. A nitrogen- deficient crop will have low water use efficiency (WUE) for the following reasons: n Nitrogen deficiency means slow canopy development and possibly incomplete canopy closure. This means a large proportion of rainfall is lost as unproductive soil evaporation. Less biomass means less grain per unit of rainfall. n Nitrogen deficiency means pale green crops and fast yellowing of bottom leaves. This leads to restricted canopy photosynthesis. Again, less biomass and less grain per unit rainfall. n Nitrogen deficiency means crops may be unable to withdraw soil water effectively, as illustrated in Figure 1. n Nitrogen deficiency means crops produce less grain per unit of above- ground biomass at anthesis. Our trials in the mid north of South Australia, for example, showed nitrogen-deficient crops produced 1.48 grains per kg of dry matter at flowering and crops with higher fertiliser rates achieved 1.73 grains per kg dry matter at flowering. In summary, nitrogen deficiency leads to wasteful use of water, lower photosynthesis, reduced water uptake from deep soil layers and less grain per unit growth.
Ground Cover 095 November-December 2011 - North
Ground Cover 093 July-August 2011 - North