Water extraction, water-use and subsequent fallow water accumulation in summer crops

Take home messages

• Consider both soil water extraction as well as subsequent fallow accumulation when considering different summer crop options.
• Cotton and maize had higher water use than sorghum, but less efficient fallows.
• Mungbean water use and soil water extraction is often lower than summer cereal crops, but differences are often diminished after the subsequent fallow.
• Differences in soil water extraction under different sorghum configurations are small and seasonal, but impacts on subsequent fallow efficiency could be significant.

Introduction

The efficiency that soil water accumulates during fallows and crops ability to extract that water and convert into yield is a key driver of farming system productivity and profitability. While a large amount of work has been done on winter crops in farming systems, significantly less information is available on the relative water extraction of different summer crops and their impact on subsequent fallow water accumulation. Some previous work was conducted in the western farming systems projects and in Central Queensland that examined the impact of different sorghum crop configurations on water extraction and fallow accumulation.

The current GRDC-funded farming systems projects have also gathered useful information on the soil water dynamics during and after different summer crop options in the farming system. This paper aims to provide an update on some of this information and improve understanding of how crop choice and management might influence residual soil water at the end of the crop and accumulation during a subsequent fallow.

Differences in crop water extraction between summer crops

Amongst the various summer crops grown across farming systems research sites, there are 3 cases where there are opportunities to draw comparisons of soil water dynamics during and after summer crops of different types.

1. Core farming systems site 2016/17

At the farming systems experiment at Pampas in 2016/17 a range of summer crops (maize, sorghum and cotton) were sown in the same season, with similar crop history and starting soil water (220 mm plant available water) (see Table 1). This allows a useful comparison of the extent of soil water extraction between these crops under the same conditions and the soil water accumulation during the subsequent fallow. Mungbean was also sown, but later in the same summer season.

In this season 136 mm of rain fell after soil sampling at the end of August and before sowing maize, sorghum and cotton on 5 October. Hence, the soil profile was full at sowing for each of these crops. Similarly, 265 mm of rain, replenished the soil profile to > 200 mm before Mungbean sowing on 10 Jan 17. Hence, all crops began with a full soil profile.

Soil sampling post-harvest in all these crops revealed only small differences in soil water – ranging from 130 mm in mungbean to 175 mm in sorghum. Effective crop water use over this period, after estimated soil evaporation was subtracted, was similar in maize and cotton, about 30 mm lower in sorghum and about 80 mm lower in mungbean. Despite relatively high crop water use, mungbean and maize yields (1.4 and 3.4 t/ha respectively) resulted in much lower returns per mm of crop water use ($ 0.70-0.80/mm) compared to cotton (4.1 bales/ha) and sorghum (6.8 t/ha) which produced higher returns per mm of crop water use ($ 1.80-2.00/mm).

A dry winter followed these crops with little soil water accumulating. Soil water status after sorghum and maize was maintained, while this declined after cotton and mungbean – presumably due to lower ground cover following these crops. Hence, at the start of the next summer cropping season, soil water was 20 mm lower after cotton than it was after maize, and this was significantly lower than after sorghum (35 mm). Over the whole annual sequence (from 30 Aug 16 to 20 Sept 17), including the crops water extraction and subsequent fallow accumulation, the relative change in soil water was 20 mm less for maize than cotton, 50 mm higher for sorghum than cotton and 65mm higher for mungbean than cotton.

Table 1. Comparison of soil water extraction, crop water use efficiencies and subsequent fallow soil water status between summer crop options sown at the core farming systems experiment, Pampas during the summer of 2016/17

^{† -} Calculated from soil samples taken earlier; ^A – Total rainfall + soil water extraction – APSIM predicted soil evaporation (290 mm)

Price assumptions used in calculations were 10 year median port prices less $40/t cartage costs. These were $221/t for sorghum, $667/t for mungbean, $281/t for maize and $1090/t for cotton ($537/bale plus seed).

2. Core farming systems site 2017/18

In the subsequent summer cropping season, data from the core experimental site at Pampas allowed for comparisons of soil water extraction and crop water use between sorghum (solid plant 1 m row spacing, 60,000 plants/ha), high density sorghum (solid plant 0.5 m row spacing, 90, 000 plants/ha) and mungbean (see Table 2). The crops compared here had a common crop history (following maize in 2016), and soil nutrient status was also similar. This showed little difference in soil water extraction between these different crops, but effective crop water use was estimated to be 20 mm less in mungbean than sorghum in the same summer. Interestingly there was no difference in sorghum crop water extraction, or crop water use efficiency between the standard or high density configurations.

Table 2. Comparison of soil water extraction, crop water use efficiencies and subsequent fallow soil water status between summer crop options sown at the Core Farming Systems experiment, Pampas during the summer of 2017/18.

^A – Total rainfall + soil water extraction – APSIM predicted soil evaporation (200-220 mm)

Price assumptions used in calculations were 10 year median port prices less $40/t cartage costs. These were $221/t for sorghum and $667/t for mungbean.

3. Billa Billa farming systems site 2016/17

Two separate comparisons of summer crops are possible at the Billa Billa Farming systems site in summer 2016/17 (Table 3). The first, between spring sown sorghum crops with different starting soil waters (after a long-fallow after wheat, and after a short-fallow following mungbean). Both sorghum crops finished with similar post-harvest soil water status, despite nearly 100 mm difference in starting soil water. The sorghum crop with the higher availability of water yielded significantly more and ended with a much higher crop grain WUE than the crop starting with more marginal soil water. This translated into double the gross margin return per mm.

The second comparison can be made between sorghum and mungbean crops sown in January following a pulse crop the previous winter. Both crops started with a similar soil water status, but soil water was about 100 mm lower following the sorghum crop compared to the mungbean crop in July after harvest. This was largely driven by the difference in maturity timing between the crops, with the sorghum crop having access to an additional 115 mm of in-crop rain that fell after the mungbean crop was mature. So in the case of the mungbean crop, this 115mm would have added to finishing soil PAW levels. Despite these differences in post-harvest soil water at July 2017, differences in soil water were negated at the end of the subsequent fallow (in March 2018); with soil water similar at this time. Over the whole annual cycle, there was only a marginal difference in the change in soil water, with sorghum extracting more soil water than mungbean but higher subsequent fallow efficiency after sorghum made up for this difference.

Table 3. Comparison of soil water extraction, crop water use efficiencies and subsequent fallow soil water status between summer crop options sown at the Billa Billa farming systems experiment during the summer of 2016/17

^{† -} calculated from soil samples taken earlier; ^A – Total rainfall + soil water extraction – APSIM predicted soil evaporation
Price assumptions used in calculations were 10 year median port prices less $40/t cartage costs. These were $221/t for sorghum and $667/t for mungbean.

Impact of sorghum configuration on crop water extraction and water accumulation

Little contemporary work has examined the effects that different sorghum crop configurations, such as solid planting, single skip or double skip, have on crop water use, crop water-use-efficiency and subsequent fallow accumulation. The information presented here is from research conducted previously by others.

In the study of Routley et al 2003 (Table 4), few differences in soil water change and crop water use were statistically significant due to high site variability. However, at 3 of the 4 experimental locations double-skip sorghum crops extracted 20-40 mm less soil water than solid plant. Single skip sorghum was intermediate in soil water extraction and crop water use, with around 10 mm less soil water extraction but these small differences are hard to assess experimentally.

Interestingly in these data sets, the high rainfall year at Croppa Creek (> 400 mm in-crop rain) showed a significant yield penalty and lower crop WUE under the double skip configuration compared to single skip or solid plant. In contrast in the low rainfall season at Bungunya (165 mm in-crop rain), the single-skip and double-skip crops yielded similarly to solid plant crops but due to lower soil water extraction had higher crop WUE. Analysis over a wider range of environments and seasons has shown that double-skip or single-skip row sorghum crops only outperform solid-plant sorghum in dry growing seasons or when soil water at sowing is marginal (e.g. <60% full profile) (Whish et al. 2005).

Other locations have also shown marginal differences in total crop water use and soil water extraction between different sorghum row configurations. Results in 10 experiments Nebraska in the USA show no significant difference in total crop water use or extraction between solid, single-skip or double skip configurations (Abunyewa et al. 2011)

Table 4. Effect of sorghum configuration on soil water change, crop water use, yield and crop water-use efficiency over 4 seasons and locations

Source: Routley, R., Broad, I., McLean, G., Whish, J., and Hammer, G. (2003). The effect of row configuration on yield reliability in grain sorghum: I. Yield, water use efficiency and soil water extraction. Proceedings of the Eleventh Australian Agronomy Conference, Geelong, Jan 2003.

The impact of the different row configurations on subsequent fallow water accumulation is also a critical factor to consider. It is expected that narrower rows with more even ground cover should improve soil water infiltration during a fallow after sorghum, while wide-row crops would be less efficient at accumulating water. However, there is little information on this currently. An experiment in Emerald conducted in 2006 (Table 5), showed that sorghum sown on narrow rows (0.5 m) had higher average ground cover at the end of the subsequent long fallow and had accumulated about 20 mm more soil water compared to sorghum on wide rows of 2.0 m. Other differences were not significant but intermediate row spacings accumulated soil water between these two extremes.

Further examination of the impact of narrow row (0.5 m) and higher density sorghum crops on subsequent fallow water accumulation is expected in the coming 12 months from farming systems experiments.

Table 5. Effects of sorghum row spacing on ground cover at the end of the subsequent fallow and fallow water accumulation.

Source: Routley R, Lynch B, Conway M (2006) the effect of sorghum row spacing on fallow cover distribution and soil water accumulation in Central Queensland. In Proceedings of the 13^th ASA Conference, 10-14 September 2006, Perth, Western Australia.

Acknowledgements

The research undertaken as part of this project 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 like to thank the project teams and collaborators contributing to the management and implementation of the farming systems experiments across the northern region. Particular thanks and recognition must go to Richard Routley for reminding me of the past work on sorghum configuration effects on water extraction and fallow efficiency presented here.

References

Abunyewa A, Ferguson RB, Wortmann CS, Lyon DJ, Mason SC, Irmak S, Klein RN (2011) Grain sorghum water use with skip-row configuration in the Central Great Plains of the USA. African Journal of Agricultural Research 6 (23), 5328-5338.

Routley R, Lynch B, Conway M (2006) the effect of sorghum row spacing on fallow cover distribution and soil water accumulation in Central Queensland. In Proceedings of the 13^th ASA Conference, 10-14 September 2006, Perth, Western Australia.

Routley, R., Broad, I., McLean, G., Whish, J., and Hammer, G. (2003). The effect of row configuration on yield reliability in grain sorghum: I. Yield, water use efficiency and soil water extraction. Proceedings of the Eleventh Australian Agronomy Conference, Geelong, Jan 2003.

Whish J, Butler G, Castor M, Cawthray S, Broad I, Carberry P, Hammer G, McLean G, Routley R, Yeates S (2005) Modelling effects of row configuration on sorghum yield reliability in north-eastern Australia. Australian Journal of Agricultural Research 56, 11-23.

Contact details

Lindsay Bell
CSIRO Agriculture and Food
203 Tor St, Toowoomba Qld, 4350
Ph: 0409 881 988
Email: Lindsay.Bell@csiro.au