Impacts of crops and crop sequences on soil water accumulation and use

Take home messages

• Efficiencies of fallows over the crop sequence were 22% ± 4% - this can be used to estimate average fallow water accumulation but large variation in fallow efficiency (FE) exists for individual fallows
• Lower fallow efficiencies can be expected in low intensity crop sequences (i.e. waiting for full profile before sowing) and in systems with high frequency of legumes
• Higher intensity systems where crops are grown on lower soil water thresholds have higher fallow efficiencies
• While grain legumes (chickpea, fababean, fieldpea, mungbean) often leave more residual soil water at harvest than cereals, this difference is diminished due to lower subsequent fallow efficiencies and hence soil water is often similar at the sowing of the next crop
• Despite the inefficiencies of fallows and similar efficiencies of rainfall use, accumulating more water prior to sowing crops typically increases Crop water use efficiency (WUE), and crop gross margins and achieved higher returns per mm an individual crop.

Introduction

The efficiency that soil water accumulates during fallows and availability of that soil water for use by crops are key drivers of farming system productivity and profitability. Using fallows to accumulate soil water to buffer subsequent crops against the highly variable climate is critical in northern grain production systems. A range of factors can influence the efficiency of fallows (i.e. the proportion of rain that accumulates in the soil profile) including ground cover, seasonality or timing of rainfall events, the length of the fallow and residual water left at the end of the proceeding crop. Further, while accumulating more soil water prior to sowing a crop is always preferable, this often requires longer fallow periods, meaning there are additional costs for maintaining that fallow and the number of crops grown declines. Here we analyse the data from farming systems experiments across seven locations (Emerald, Pampas, Billa Billa, Mungundi, Narrabri, Spring Ridge and Trangie) over the past four years to explore the question; ‘how much does the farming system (i.e. mix of crops and their frequency) and different crops influence the accumulation and utilisation of water?’

We explore several factors influencing the accumulation of water during fallows and the availability of water for subsequent crops.

1. How does crop intensity (i.e. the proportion of time in crop or fallow) influence the accumulation and use of water in the farming system?
2. How much does crop choice (e.g. legume vs cereal or other) impact on water extraction and subsequent fallow water accumulation?
3. What is the value of additional soil water for subsequent crop productivity and water use efficiency, and is this sufficient to compensate for longer-fallows required to build this soil water?

Across these projects a common set of farming system strategies have been employed to examine how changes in the farming system impact on multiple aspects of the farming system (outlined below).

Cropping system strategies impacts on rainfall utilisation and soil water dynamics

Here we compare the differences between different farming system strategies over the whole 4 experimental years to see how they differ in terms of fallow efficiency and water use. That is, how efficiently is water used in the system if it is modified  to increase/decrease crop intensity, change the mix of crops to grow grain legumes or other break crops more frequently, or increase the nutrient supply to the farming system. Across 10 different contexts we compare the following modifications to the farming system strategies:

• Baseline – an approximation of common farming system practice in each district: dominant crops only used; sowing crops on a moderate soil water threshold to approximate common crop intensities (often 0.75-1.0 crops per year); and fertilising to median crop yield potential
• Higher crop intensity – increasing the proportion of time that crops are growing by reducing the soil water threshold required to trigger a planting opportunity (e.g. 30% full profile)
• Lower crop intensity systems – only growing crops when plant available soil water approaches full (i.e. > 80% full) before a crop is sown and higher value crops are used when possible
• Higher legume frequency – crop choice aims to have every second crop as a legume across the crop sequence and uses high biomass legumes (e.g. fababean) when possible
• Higher crop diversity – a greater set of crops are used with the aim of managing soil-borne pathogens and weeds. This implemented by growing 50% of crops resistant to root lesion nematodes (preferably 2 in a row) and 2 alternative crops are required before the same crop is grown
• Higher nutrient supply - increasing the fertiliser budget for each crop based on a 90% of yield potential rather than the baseline of 50% of yield potential

Efficiency of fallows under different farming systems

Here we have analysed the efficiencies of all fallows within different farming systems across sites in order to examine how different strategies may impact on the efficiency of water accumulation during fallow periods (Table 1). That is, we calculated the ratio of all rain falling during fallow periods to the total accumulated soil water over these fallows across the whole crop sequence (not just individual crops).

Firstly, this shows that there are significant environmental influences on the efficiency of fallows, associated with the timing of rainfall events. Over our four experimental years, environments with more winter-dominated rainfall had lower fallow efficiencies – this is likely due to smaller and less frequent rainfall events occurring during summer fallows meaning that soil water accumulates less efficiently. Overall, though most baseline systems tended to achieve fallow efficiencies of 22% ± 4% over the whole cropping sequence. This is consistent with long-term simulations which show fallow efficiencies of 21-24% for cropping systems with crop intensities of 0.75-1.0 crops per year (i.e. 66-75% time in fallow). Robinson & Freebairn (2017) show fallow efficiencies of 25-30% under no-till in historical research but our data suggests that using a generic 30% FE may over-estimate fallow water accumulation in most cases.  Earlier research mostly examined systems where winter cereals were a larger component of the farming system, and cropping systems used now with higher proportion of legumes and summer crops are likely to achieve lower fallow efficiencies (see further results below).

Significant differences in the efficiency of fallows are also found between different farming systems treatments tested across the sites.  Key findings are:

• Higher crop intensity increased fallow efficiencies at most sites. This is due to less time in fallows and fallows having lower soil water content meaning higher infiltration rates. The higher crop intensity system at Narrabri so far has similar crop intensities and hence fallow efficiency is similar to the baseline system
• Conversely, systems with lower crop intensity systems had lower fallow efficiencies owing to longer fallows and a greater proportion of rain and time in fallows. The main exception here was at Mungundi where the low intensity system has achieved a similar fallow efficiency to the baseline at this point in time
• Systems with higher legume frequencies had lower fallow efficiencies (5% lower), particularly where they were reliant on summer rain accumulation. At several locations this effect was large, particularly where legumes were followed by a long-fallow period. This is due to the lower and less resilient cover provided by grain legume crops than cereals
• On average, systems aimed at increasing crop diversity have achieved similar fallow efficiencies to the baseline systems. However, there was large site-by-site variability, half the sites had an increase and half lower FE.  There was significant differences in how increasing crop diversity is achieved across the various locations (e.g. some involve alternative winter break crops, some involve long fallows to sorghum or cotton), which is likely to bring about these variable results.

Table 1. Comparison of efficiencies of fallow water accumulation (i.e. change in soil water/fallow rainfall) amongst different cropping system strategies at 7 locations across the northern grains region.

*Crop system does not yet vary from the baseline in this regard

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Figure 1. Relationship between the proportion of rain falling during fallow periods (i.e. time in fallow) and efficiency of fallows in the crop sequence (i.e. proportion of fallow rain accumulated in soil) across all farming system locations. Only baseline, low and high intensity systems are plotted, excluding altered crop diversity or legume frequency as this changes fallow efficiencies (see Table 1).

The effect of crop intensity on fallow efficiency is further illustrated in Figure 1. This shows a negative relationship between % of rain falling during fallows (i.e. time in fallow) and the fallow efficiency across all sites. That is, fallow efficiency declines dramatically as the time in fallow (or rain during the fallow) increases. This shows that at a point where 70% of the rain at a location is falling during the fallow that fallow efficiency is declining to 0.16, while in a system where 50% of rain falls during fallows the fallow efficiency is 0.25. What this means is for an environment receiving an average of 600 mm of rainfall per year, a farming system that captures 50% of the rain in fallows (1.3 crops per year), would accumulate 77 mm of water/yr during the fallow period (i.e. 0.5 x 600 mm = 300 mm in fallow @ 0.25 fallow efficiency = 77 mm) and 300 mm/yr would occur in-crop – Total crop water use = 377 mm (63% of rainfall). In contrast a farming system receiving 70% of rain in the fallow period (e.g. 0.6-0.7 crops per year), would accumulate 67 mm in fallow/yr and in-crop rain would be 180 mm per year – Total crop water use = 247 mm per year (41% of rainfall). These results are consistent with the differences in rainfall utilisation between cropping systems of different intensities across all the farming systems research sites (see Table 2).

What this means is that a crop grown after a longer fallow in a lower intensity system must generate 1.5-times the gross margin per mm of water used to be equally profitable. This is achievable in most cases (see results later in this paper) but it does mean that these crops must be managed to maximise their WUE in order to make up for the lower utilisation of water across the system.

Table 2. Differences in the percentage of total rainfall that was used by crops (i.e. in crop rain + change in soil water from sowing to harvest) between cropping systems treatments varying in crop intensity across farming systems experiments.

*Crop system does not yet vary from the baseline in this regard

Crop-by-crop effects on fallow efficiency

Across the farming systems sites we have monitored fallow water accumulation following a range of different crops – over the 4 research years, we have collected data on residual soil water and final soil water for over 306 different crops. Here we have collated this data in order to compare how different crop types impact on subsequent fallow efficiencies (Figure 2). This data shows the high variability in fallow efficiency that occurs from year to year but is also demonstrates some clear crop effects on subsequent fallow efficiencies.

Figure 2. Summary of observed fallow efficiencies following different crops and different fallow lengths (SF – short fallows 4-8 months, LF – long fallows 9-18 months) across all farming systems sites and treatments between 2015 and 2018; winter cereals include wheat, durum and barley; other pulses include fababean and fieldpea. Boxes indicate 50% of all observations with the line the median, and the bars indicate the 10^th and 90^th percentile of all observations. Italicised numbers indicate the number of fallows included for each crop.

This data clearly shows the higher fallow efficiencies that can be achieved from a winter cereal crop than winter grain legumes and to a lesser degree, canola. The median fallow efficiency following winter cereals was 0.27, while following chickpea and other grain legumes this was 0.14, with canola intermediate (0.19). Median fallow efficiencies following sorghum were also similar to wheat (0.26), but efficiencies of short-fallows during winter after sorghum were more efficient than long fallows. This difference between short and long fallows was less obvious following winter cereals. This is likely due to winter fallows being more efficient than summer fallows, due to lower evaporation losses (and possibly lower soil water content at the start of the fallow). Hence, short fallows after sorghum occurring in winter are more efficient, while long-fallows spanning into summer are less efficient. This also explains the similar fallow efficiency of short (summer) and long fallows (summer + winter) after winter cereals.

What this means is that, the impacts of a particular crop on the accumulation of soil water in the following fallow should be considered in the cropping sequence. For example, a fallow receiving 400 mm of rain after a winter cereal would accumulate 108 mm on average, while the same fallow after a grain legume would have only accumulated 56 mm. This difference could have a significant impact on the opportunity to sow a crop and/or the gross margin of the following crop in the cropping sequence.

Residual water and fallow efficiency effects on soil water after legumes vs cereals

While we have observed lower fallow efficiency following grain legumes in the farming system, a frequently mentioned benefit of legumes is the residual soil water left at harvest that can be used in subsequent crops. In Table 3 we have compiled cases where chickpeas and wheat have been grown in the same season to compare the residual water at harvest and the accumulation of water until the sowing of the following crop. On numerous occasions we observed higher residual soil water at harvest after pulse crops (chickpeas, fababeans or field peas) compared to after wheat. This was often associated with rainfall later in the crops development where the winter cereals were able to extract this water while the pulses were finishing and did not utilise this additional water. On average across these 7 comparisons chickpea had 41 mm more soil water post-harvest compared to wheat, however, at the end of the subsequent fallow this difference was greatly reduced so that on average only 10 mm more water remained in the soil profile after chickpea compared to wheat or barley.  What this means, is that you shouldn’t bank on the additional moisture after a grain legume translating into additional soil water available for subsequent crops.

Table 3. Residual soil water at harvest and subsequent fallow water accumulation between chickpea and wheat compared across 7 sites/years

Fallow length effects on crop water use efficiency and gross margin

While we have shown above that there are a range of factors that affect fallow efficiency, it is important to factor in how effectively the subsequent crop turns the water available into grain and gross margin. From the seven farming systems sites, 42 crops had eight common crops at the end of fallows of varying length (Table 4). These comparisons showed that in 41 of the 42 crops, longer fallow periods (under the same seasonal conditions) have resulted in more plant available water (PAW) at planting of the common crop. The only crop that didn’t increase was an 18 month fallow, which didn’t increase from 2/3 full despite 700mm rainfall over a 12 month period.

In every comparison, higher PAW at planting resulted in increased grain yield, which in seven of the eight comparisons improved crop water use efficiency (WUE) i.e. grain yield/(in-crop rain + change in soil water) (WUE). The comparison where higher grain yield didn’t translate to higher water use efficiency was the highest yielding crop, with the highest WUE in these comparisons (ie. sorghum at Pampas in 2016/17). However, it is important to also factor-in the fallow rain required to achieve the higher plant available water at sowing. Here we have calculated this as the rainfall use efficiency (RUE) of these crops, i.e. grain yield/ (prior fallow rain + in-crop rain). This shows that once the efficiency of fallow water accumulation is considered then in most cases there was little difference in productivity of the systems in terms of kilograms grain produced per mm of rain (exclusions were a chickpea crop following a 18-month fallow at Pampas in 2017 and a sorghum double-crop at Pampas in 17/18).

While this shows that across fallow lengths leading into crops there is little difference in system productivity, this does not necessarily translate to system profitability. The crops with a longer fallow lead in had higher crop gross margins due to their higher yields. In 6 of the 8 comparisons between crops, higher gross margin returns per mm were achieved for crops with a higher PAW at sowing due to longer fallows prior. The two cases where the shorter fallow crops (wheat at Emerald in 2016, and sorghum at Pampas in 2016/17) had higher $ GM/mm, both had higher crop margins and high starting PAW (> 100 mm) at sowing. Across these comparisons the marginal gain in profit per mm of additional water at sowing ranged from $0.5-14.9, but was mainly between $1.1/mm and $2.2/mm.

Table 4. Comparison of yield and water use of crops with varying lengths of preceding fallow, for a range of crops and locations. Double crop is 0-4 month fallow; Short fallow is 4-8 month; long fallow is 9-18 months.

Conclusions

Overall these farming systems experiments have shown that systems with less time in fallow increases system water use and WUE through higher fallow efficiencies. However, significantly higher returns for crops sown on higher plant available water more than compensates for the low efficiencies of fallow water accumulation. This trade-off will be further influenced by the cost structure and risk appetite of the farming enterprise and the availability of labour, since higher intensity systems will increase inputs of labour and machinery and increase risk of crop failures. This is explored further in other papers. Though, this does mean that it is more critical to optimise management and inputs for crops following long-fallows in order to convert the extra water efficiently into yield outcomes.

Further reading

Water use and accumulation

Lindsay Bell, Andrew Erbacher (2018) Water extraction, water-use and subsequent fallow water accumulation in summer crops.

Freebairn, David (2016) Improving fallow efficiency.

Kirsten Verberg, Jeremy Whish (2016) Drivers of fallow efficiency: effect of soil properties and rainfall patterns on evaporation and the effectiveness cf stubble cover.

Local farming systems experiments

Andrew Erbacher, David Lawrence (2018) Can systems performance be improved by modifying farming systems? Farming systems research – Billa Billa, Queensland

Darren Aisthorpe (2018) Farming Systems: GM and $ return/mm water for farming systems in CQ.

Jon Baird, Gerard Lonergan (2018) Farming systems site report – Narrabri, north west NSW

Andrew Verrell, Lindsay Bell, David Lawrence (2018) Farming systems – Spring Ridge, northern NSW.

Lindsay Bell, Kaara Klepper, Jack Mairs, John Lawrence (2018) Farming system impact on nitrogen and water use efficiency, soil-borne disease and profit.

Lindsay Bell, Kaara Klepper, Jack Mairs, John Lawrence (2017) Improving productivity and sustainability of northern farming systems: What have we learnt so far from the Pampas systems experiment?

Lindsay Bell, David Lawrence, Kaara Klepper, Jayne Gentry, Andrew Verrell, and Guy McMullen (2015) Improving northern farming systems performance.

Acknowledgements

The research undertaken as part of this project (CSA00050, DAQ00192) is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. We would also specifically like to thank all the farm and field staff contributing to the implementation and management of these experiments, the trial co-operators and host farmers

References

Robinson JB, Freebairn DM (2017) Estimating changes in Plant Available Soil Water in broadacre cropping in Australia. In ‘Proceedings of the 2017 Agronomy Australia Conference’, 24 – 28 September 2016, Ballarat, Australia. www.agronomyconference.com

Contact details

Lindsay Bell
CSIRO
PO Box 102, Toowoomba Qld, 4350.
Mb: 0409 881 988
Email: Lindsay.Bell@csiro.au