A farming systems scientist says the efficiency with which soil water accumulates during the fallow period and the availability of that moisture for crop use are key drivers of profitability.

Dr Lindsay Bell, a principal research scientist with CSIRO Agriculture and Food, says this is particularly true for summer-dominant rainfall areas where accumulating water in fallows is critical to buffer against variable in-crop rain.

Dr Bell says several factors influence the amount of rain accumulating in the soil profile.

“These include ground cover, rainfall timing, fallow length, and the residual water left at the end of the preceding crop,” he says.

“While accumulating soil water before sowing a crop is always preferable, this often requires longer fallows, resulting in additional costs for maintaining that fallow and the number of crops grown declines.”

Nonetheless, he says, crops with access to stored soil moisture are often more efficient and less reliant on in-crop rainfall to drive their final yield.

Accordingly, optimising the water use efficiency of the farming system is a balancing act between maximising fallow water accumulation and the capacity of crops to convert available moisture into grain (crop water use efficiency).

Crop water use efficiency

Dr Bell defines crop water use efficiency as the grain produced per millimetre of water available to the crop. This includes growing season rainfall plus the soil water at sowing minus the residual water left at harvest.

He says data collected through the GRDC Northern Farming Systems Project (a collaboration between GRDC, the Queensland Department of Agriculture and Fisheries, New South Wales Department of Primary Industries and CSIRO) have enabled researchers to examine these relationships for key crops in the region.

“Figure 1 shows the average water use efficiency for wheat was 17.3 kilograms/hectare/millimetres, for chickpeas was 8.2kg/ha/mm and for sorghum was 20.8kg/ha/mm,” he says.

Figure 1: Relationship between crop water use and grain yield across northern farming systems crops.

Note: The slope of the line indicates the water use efficiency of each crop, and the X-intercept is the estimate of the minimum water available to produce grain for each crop.

Source: Dr Lindsay Bell, CSIRO

“However, there is always significant variability in crop water use efficiency due to growing season differences, the timing of rainfall and/or other factors that might reduce crop performance, such as nutrient deficiencies and disease.”

Dr Bell says the data showed that the best 20 per cent of crops achieved a water use efficiency of 23.2kg/ha/mm for wheat, 11.8kg/ha/mm for chickpeas, and 25.1kg/ha/mm for sorghum.

“Figure 1 also shows a minimum amount of water is required before a crop will produce yield, where the line of best fit crosses the x-axis. This is the water required to grow sufficient biomass to produce grain,” he says. “It’s 60mm for chickpeas, 100mm for wheat and 200mm for sorghum. Sorghum is higher because it grows during summer with a higher evaporative demand.”

He says across all crops, those with lower starting soil water achieved lower crop water use efficiencies. These crops likely encountered water stress, meaning they could not convert biomass into grain yield.

Equally, he says, the data demonstrated that crop water use efficiency often declines at higher water availability when the crop does not use surplus rainfall, which is often lost as runoff or via higher evaporation.

“For each crop, there are critical soil moisture levels where crops are more likely to maximise their water use efficiency: 110 to 180mm plant-available water for wheat, 80 to 160mm for chickpea and more than 140mm for sorghum,” he says.

“While the outcome for each crop will result from subsequent seasonal conditions, these critical thresholds indicate a trigger for when to sow these crops, so they use water most effectively to produce grain.”

Fallow efficiency

Dr Bell says fallow efficiency is the percentage of rainfall accumulating in the soil during a fallow. Hence, achieving a higher fallow efficiency can significantly increase the available water for subsequent crops.

For example, he says, a fallow receiving 400mm of rain with an efficiency of 25 per cent will have accumulated 100mm of soil water by sowing, while a fallow with an efficiency of 20 per cent would have only accumulated 80mm. This difference could significantly affect the opportunity to sow a crop and/or the gross margin.

“Environmental conditions such as the timing of rainfall greatly influence fallow efficiency, which can vary dramatically from season to season,” he says.

“Over the past seven years, our measurements show that most current cropping systems achieved fallow efficiencies of 22 per cent ± four per cent. This is consistent with long-term simulations showing 21 to 24 per cent fallow efficiencies.”

The data showed that environments with winter-dominant rainfall had lower fallow efficiencies over summer, likely due to smaller and less-frequent rainfall events occurring during summer fallows.

Other findings included:

• higher crop intensity increased fallow efficiency at most sites due to less time in fallows and lower soil water contents, meaning less water is evaporated;
• lower crop intensity systems had lower fallow efficiencies due to longer fallows and more rain over that time, meaning evaporative losses were higher; and
• higher legume frequencies had lower fallow efficiencies (five per cent lower), particularly where they relied on summer rain accumulation. This effect was large at several locations, chiefly where legumes were followed by a long fallow period. This was due to the lower residue cover, which breaks down faster following grain legume crops than cereals.

On average, Dr Bell says systems aimed at increasing crop diversity achieved similar fallow efficiencies (Table 1) to the baseline systems (regionally accepted practice).

“There were significant differences in how increasing crop diversity was achieved across the various locations. For example, some involved alternative winter break crops, some involved long fallows to sorghum or cotton, which will likely bring these variable results.”

Table 1: Comparison of fallow efficiency (per cent of fallow rain accumulated in soil water at sowing) and percentage of rain available to crops for different cropping systems averaged across the northern grains region experiments. The table shows the percentage of sites where that system either increased or decreased relative to the baseline (otherwise, they were the same).

1. In the low intensity cropping system, there were no sites where fallow efficiency increased. 2. In the high intensity cropping system, there were no sites where fallow efficiency decreased. 3. In the high nutrient cropping system, there were no sites where the percentage of rain available increased. 4. In the high nutrient and high intensity cropping systems, there were no sites where the percentage of available rain decreased.

Source: Dr Lindsay Bell, CSIRO

Crop legacies

Dr Bell says the previous crop greatly influences how much water will be available in the soil when the next crop is sown. The previous crop influences water accumulating over the fallow period and any residual moisture left from the previous crop.

“Across the sites monitored, fallow water accumulation was measured after more than 350 separate crops, including situations where there were variations in residual soil water and final soil water,” he says.

“The data highlights the large variability in fallow efficiency that occurs and demonstrates some clear crop type effects on subsequent fallow efficiencies.”

It shows:

• after winter cereal crops, fallow efficiencies are higher than after winter grain legumes and, to a lesser degree, canola. The median fallow efficiency following all winter cereals was 27 per cent (including wheat, barley and durum), while after chickpeas and other grain legumes, the fallow efficiency was lower at 19 per cent, with canola at 23 per cent;
• after sorghum, fallow efficiencies are typically lower than after winter cereals (17 per cent) due to a combination of longer fallows after sorghum. Short fallows after sorghum are generally higher-efficiency than long fallows (21 per cent compared with 16 per cent);
• fallows after cotton are the lowest efficiency (12 per cent) because they are often longer with less residue; and
• fallow length also affects fallow efficiencies. Across the data, longer fallows were generally less efficient – long fallows of more than nine months have a 16 per cent efficiency, short fallows (4 to 9 months) have a 23 per cent efficiency, while fallows involving a double crop (less than four months) have a 33 per cent efficiency.

Residual water

Dr Bell says drier soils result in typically more-efficient fallows than those with more residual moisture. Hence, lower fallow efficiencies do not always translate into less soil water at sowing.

For example, he says, legume crops often (but not always) leave more soil water at harvest, and despite lower fallow efficiency following grain legumes, they may leave a similar amount of water for the next crop (Table 2).

Table 2: Residual soil water at harvest and subsequent fallow water accumulation after chickpeas and wheat compared across sites and years.

Source: Dr Lindsay Bell, CSIRO

“On numerous occasions, we observed higher residual soil water at harvest after chickpeas, faba beans or field peas compared to after wheat,” he says.

“On average, this was 41mm more soil water post-harvest than wheat, often associated with rainfall later in the crop development, where the winter cereals could extract this water while the pulses were maturing and did not use the additional water.”

However, at the end of the subsequent fallow, he says this difference was greatly reduced so that, on average, only 10mm more water remained in the soil after chickpeas compared to wheat or barley.

Accordingly, he says, do not bank on the additional moisture after a grain legume translating into additional soil water available for subsequent crops. However, fallow efficiency is not the only contributor to soil water in the following crop.

Water capture and profit

Dr Bell says fallow and time in-crop drive differences in water use and its conversion into profit over the farming system.

“The trick is to find the right balance between the time in fallow and time in crop to accumulate sufficient water to maximise crop water use efficiency while at the same time not dramatically reducing the overall system water capture,” he says.

“Crop choice, like introducing more legumes or more diversity, has small positive or negative effects on total system water use, but big differences are driven by crop intensity.”

Higher-intensity systems almost always increased the proportion of total water use compared to the baseline, while lower-intensity systems reduced total water use.

For example, he says to consider an environment that averages 600mm of rainfall annually.

“A lower-intensity farming system where a crop is receiving 70 per cent of rain in the fallow period (0.6 to 0.7 crops per year) with fallow efficiencies of 0.16, would accumulate 67mm in fallow per year and in-crop rain would be 180mm per year – resulting in total crop water use of 247mm per year (41 per cent of rainfall),” he says.

“In contrast, a farming system that captures 50 per cent of the rain in fallows (1.2 to 1.4 crops per year) with fallow efficiencies of 0.30, would accumulate 90mm of water per year and 300mm per year would fall in-crop – resulting in a total crop water use of 390mm (65 per cent of rainfall).”

For a crop grown after a longer fallow in a lower-intensity system to be equally profitable, he says, it must generate 1.6 times the grain or gross margin per millimetre of water use.

“From the farming systems data, we have eight examples of where a common crop was sown at the end of fallows of varying length and different starting water,” he says.

“In every comparison, higher levels of plant-available water at sowing resulted in increased grain yield, which in seven of the eight comparisons improved crop water use efficiency.”

In any calculation, he says, it is important to factor in the fallow rain required to achieve the higher plant-available water at sowing. This was calculated as the rainfall use efficiency of these crops, which is grain yield divided by prior fallow rain plus in-crop rain.

“When the efficiency of fallow water accumulation is considered, in most cases there was little difference in the productivity of the systems in terms of kilograms of grain produced per millimetre of rain,” he says.

“However, there were clearer differences in the system gross margin per millimetre of rain if crops were sown outside the optimal range of soil water (either too high or too low), and so converted rainfall ineffectively into profit compared to crops grown in the same season with optimal soil water at sowing.”

For example, in wheat, he says all the crops sown with pre-planting plant-available water of less than 100mm achieved lower $/mm returns. For sorghum, two crops sown with less than 140mm plant available water achieved lower $/mm returns.

Across these comparisons, he says the marginal gain in profit per millimetre of additional water at sowing ranged from $0.50 to $14.90 but was mainly between $1.10/mm and $2.20/mm.

Dr Bell says these experiments have shown that farms with less time in fallow increase system water use and water use efficiency through higher fallow efficiency.

“However, crops sown on suboptimal plant-available water did not achieve a higher conversion of water into profit and hence applying appropriate thresholds to sow your crops enables system water use efficiency to be optimised.”

More information: Lindsay Bell, lindsay.bell@csiro.au