Research assesses soil water strategies

image of Andrew Erbacher
Department of Agriculture and Fisheries (DAF) research agronomist Andrew Erbacher (pictured here with MCA Goondiwindi-based agronomist Stuart Thorn).

The desperately dry conditions affecting large tracts of Queensland and New South Wales highlight all-too-well the plight of cropping in a highly variable climate.

Although having stored soil moisture this year has proven akin to winning the lottery, northern growers have long recognised the value of efficient storage and use of soil moisture in driving farming system productivity and profitability.

Fallow efficiency is the proportion of rain that falls during the fallow that becomes available to plants and it’s a key decision-making tool when weighing up crop options and planting opportunities.

It’s well known that fallow efficiency can be influenced by a range of factors including ground cover, timing and intensity of rainfall events, the length of the fallow and residual water left at the end of the preceding crop.

However questions have lingered over how much the type, mix and intensity of crops grown affects this accumulation and utilisation of water.

Scientists have begun to address these issues as part of a flagship five-year research investment by the Grains Research and Development Corporation (GRDC) examining the key limitations, consequences and economic drivers of farming systems in the northern region; assessing the potential of different farming systems and crop sequences to meet emerging challenges; and testing the impacts of modifications of the farming system on attributes such as nutrients, water, pathogens, soil health, yield and economics.

The trial work has been conducted at a core site at Pampas near Toowoomba and six regional centres across central and southern Queensland (Emerald, Billa Billa, Mungindi) and northern NSW (Spring Ridge, Narrabri and Trangie).

Department of Agriculture and Fisheries (DAF) research agronomist Andrew Erbacher said the research had unearthed some key messages regarding the likely efficiencies of higher and lower intensity cropping systems, the retention and availability of soil water following legume crops, and the importance of starting soil moisture reserves.

“Overall the farming systems experiments have shown that systems with less time in fallow increase system water use and rainfall use efficiency (RUE) through higher fallow efficiencies,” Mr Erbacher said.

“That said, one of the more important observations of the research was that despite the inefficiencies of long fallows, accumulating more water prior to sowing crops is a significant driver of profitability.

“Stored water acts as a buffer against variable rainfall patterns, so sowing with more stored water typically increases crop water use efficiency (WUE) and can achieve higher returns per mm on an individual crop basis.

“The trade-off between opting for higher or lower intensity systems 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.

“What this means from a practical perspective is that it’s more critical to optimise management and inputs for crops following long fallows in order to convert the extra water efficiently into yield outcomes.”

Over four experimental years, data has been collected on residual soil water and final soil water for over 306 fallows following different crops, to compare how different crop types impact on subsequent fallow efficiencies.

This data clearly showed that higher fallow efficiencies can be achieved from a winter cereal crop compared to winter grain legumes and canola.

Mr Erbacher said this signalled the importance of considering the impacts of a particular crop on the accumulation of soil water in the following fallow, when making management decisions pertaining to the cropping sequence.

“For example, a fallow receiving 400mm of rain after a winter cereal would accumulate 108mm on average, while the same fallow after a grain legume would have only accumulated 56mm prior to a planting opportunity,” Mr Erbacher said.

“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.”

While lower fallow efficiency was observed following grain legumes in the farming system, it’s a commonly held belief that one of the major benefits of legume crops is the residual soil water at harvest and the accumulation of water until the sowing of the following crop.

In a number of instances, researchers 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 crop’s 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 chickpea had 41mm more soil water post-harvest compared to wheat, however chickpea stubble achieved lower fallow efficiency. At the end of the subsequent fallow the difference in soil water was greatly reduced so that on average only 10mm more water remained in the soil profile after chickpea compared to wheat or barley,” Mr Erbacher said.

“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.”

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