Measuring water use efficiency (WUE)
WUE is not easily calculated for grazing systems where the ‘growing season’ is hard to define. However, calculating WUE at the paddock level is a valuable objective measure of current pasture performance. The following is a guide to using the information to determine priorities for fertiliser applications and pasture species manipulation:
• Define the start date for your ‘year’. This should ideally include all of the normal reliable growth periods for a year. As an example, in areas where late autumn to spring rainfall predominates (or is the most reliable), choose a start date that occurs before the normal autumn break. In northern summer rainfall areas, a date in winter may be more appropriate.
• Pasture mass readings (kg DM/ha) are made at the start and end of the defined year.
Calculate the pasture consumed (kg DM/ha) for the defined year from grazing records that estimate total intake by grazing animals. This is calculated from paddock records that include the length of each graze in days (Gl); the number of animals grazing in the herd (No); and the average predicted daily intake per animal (Di). Pasture consumed at each graze is calculated as:
(Gl x No x Di)
Pasture consumed at each graze is then added up through the defined year to estimate total consumption for the year. If pasture growth rates are not known for the farm, refer to Tool 4.3, ‘Daily pasture growth estimates for localities and regions across southern Australia’ in Module 4: Pasture utilisation as a guide to mid-monthly estimates of pasture growth rates (kg DM/ha/day) in average seasonal conditions.
• Add the rainfall recorded for the period.
• Pasture growth = Pasture mass at end of grazing + Consumption + Decay – Pasture mass at start of grazing.
• WUE = Pasture growth (kg DM/ha) divided by rainfall (mm) = kg DM/ha/mm of rain.
• Pasture and allowance information are collected from grazing records.
Guidelines for the use of WUE
WUE should be used as an individual farm benchmark only. Recent research has shown that indices based on rainfall are poorly correlated to pasture growth and should not be used for comparison between regions or across a region. Recommended use in this module includes the following:
• As a guide, temperate improved pastures may have a maximum potential WU E of 18kg DM per mm of rain and native pastures a potential of 10kg DM per mm of rain. These maximums have been assessed under experimental conditions and it is likely that they overestimate what is achievable in commercial field practice.
• Latest research suggests that 15kg DM/ha/mm of rain is a more reasonable target for WUE with temperate improved pastures when all nutrient and species limiting factors have been corrected.
• The length of growing season, as measured by plant available soil moisture (PAW), is a major determinant of variation in pasture growth. As variability in PAW increases it is likely that potential WUE will decrease due to the inability to manipulate stock intake in line with fluctuations in pasture growth rates and the need to store ‘dry standing feed’.
Suggested methodology
1. Classify pasture zones into levels of potential pasture production once all limiting factors responsive to management have been corrected.
• High potential pasture zones are those where there are no physical limitations to growth, for example deep clay loams on moderate to level slopes, with no limiting factors such as soil chemistry (salinity or acidity), proximity to riparian zones and aspect.
• Low potential zones are those where significant physical and soil limitations exist, or have been identified for use for pasture production at significantly less than their natural potential because of management constraints, for example areas identified for native pasture conservation.
• Most properties will also have one or more intermediate zones between these extremes due to variability of physical and soil properties across the property.
2. Assess WUE in paddocks where current pasture performance is judged to be closest to the potential level of performance for each representative class. WUE in these paddocks become the current internal benchmarks for performance.
3. Assess WUE in all other paddocks and compare the assessment against the appropriate internal benchmark.
4. Use the assessments to determine priorities for fertiliser applications and pasture manipulation.
5. Raise internal benchmarks for WUE as paddocks are improved to a performance level above the current internal benchmark.
6. Initially (and then every three years) compare internal benchmarks against suggested targets developed in recent research. If the gap is large, there may be a case for accelerated investment to raise WUE and pasture performance across the property. Analysis should include assessment of the variability of PAW at this site and the relevance of stated targets to this property. Analysis can be complex and advice should be sought.
7. If WUE is outside the desired level, consider fertiliser application or pasture species manipulation to boost the amount of rainfall (water) that is converted into pasture growth.
