Researchers gain ground on soil water challenge

Author: | Date: 15 Apr 2019

image of David Lawrence
DAF extension officer David Lawrence says early results suggest that cover crops can increase net water storage across fallows with limited ground cover and deliver dramatic yield increases in subsequent cotton and wheat crops. Photo DAF

The effective capture and storage of water is a holy grail for northern grain growers who have long strived to harness the productive benefits of every millimetre of rainfall.

It’s underpinned enormous change in farming techniques over the years with the adoption of practices like zero tillage, but researchers believe that significant additional productivity gains are possible with a well-managed cover cropping program.

Across the region, typically 20-40 per cent of rainfall is transpired by dryland crops, up to 60 per cent of rainfall is lost to evaporation, and a further 5-20 per cent lost in runoff and deep drainage.

Every 10 mm of extra stored soil water available to crops could increase dryland grain yields for growers by up to 150 kg/ha, with corresponding benefits to dryland cotton growers as well.

The Grains Research and Development Corporation (GRDC) and the Cotton Research and Development Corporation (CRDC) are currently investing in a joint cover crop research project to quantify the impact of different stubble loads on the accumulation of rainfall, the amount of water required to grow cover crops with sufficient stubble loads, the net water gains/losses for the following crops and the impacts on their growth and yield.

Conducted by the Department of Agriculture and Fisheries (DAF), the research spans two trial sites at Bungunya and Yelarbon and in southern Queensland, respectively comprising a double skip sorghum, long fallowed to dryland wheat rotation and, by comparison, a pivot irrigated cotton, short fallowed to pivot irrigated cotton rotation.

According to DAF extension officer David Lawrence, early results suggest that cover crops can increase net water storage across fallows with limited ground cover and deliver dramatic yield increases in subsequent cotton and wheat crops.

“Initial work has certainly shown that cover crops can increase fallow water storage and improve crop performance and returns in northern farming systems,” Dr Lawrence said.

“In each experiment, a cover crop treatment provided the highest plant available soil water by the end of the fallow however the best cover crop treatment depended on the length of the fallow.

“A later spray-out, with more resilient cover, was best in the longer fallow but delaying spray-out too long had a dramatic effect on water storage.”

At the same time, a cover crop saved two to three fallow herbicide sprays equating to ~$40/ha and dramatically improved establishment at one of the sites.

Most notable though were the early yield results for the subsequent cotton and wheat crops at each site.

While yields and returns were increased by the best cover crop treatment at each trial, yield effects appeared to be in excess of those expected from the increased soil water storage according to Dr Lawrence.

image of coorangy trial site
At the Bungunya site for example, the biggest yield increases were from the cereal cover crops, especially the late-terminated millet and the sorghum. Photo DAF

“At the Bungunya site for example, the biggest yield increases were from the cereal cover crops, especially the late-terminated millet and the sorghum,” Dr Lawrence said.

“The water differences at planting (end of the fallow) may explain some of the yield difference at this site. However, the establishment of the wheat crop was dramatically better where cover crops were used, more so where cereals were used but also for lab lab.

“The expected yield increases from the higher fallow water storage alone would typically be ~200 kg grain in wheat (WUE 15 kg grain/mm water) for the mid-terminated millet (worth ~$50/hectare), ~280 kg grain for the late millet (worth $75/ha) and ~540 kg grain for the late +rolled millet (worth $150/ha). These gains would represent net returns of $20/ha, $45/ha and $120/ha respectively, and are not inconsistent’ with growers experiences.

“However, the measured yield gains for these treatments were 950 kg/ha, 1461 kg/ha and 1129 kg/ha respectively, representing increased returns of between $250 and $380 /ha.

“These are quite remarkable results and while unlikely to occur often, may demonstrate the value of retained surface moisture for good establishment.”

While the industry is becoming increasingly more adept at utilising available soil water and improving individual crop performance in dryland systems, the effective capture and storage of rainfall across the whole farming system remain major challenges for northern grain and cotton growers.

For a number of years GRDC farming systems research investments have been assessing ways to improve this system water use, and to achieve 80 per cent of the water and nitrogen limited yield potential in northern cropping systems.

This work adds to past research by GRDC’s Eastern Farming Systems project and Northern Growers Alliance (NGA) trials that suggested cover crops and increased stubble loads can reduce evaporation, increase infiltration and provide net gains in plant available water over traditional fallow periods.

“Consequently, cover crops may be a key part of improved farming systems providing increased productivity, enhanced profitability and better sustainability,” Dr Lawrence said.

Cover crops are typically used in southern Queensland and northern NSW to overcome a lack of stubble and protect the soil following low residue crops such as chickpea and cotton or following skip-row sorghum with uneven stubble and exposed soil in the ‘skips’.

Growers typically plant white French millet and sorghum and spray them out within ~60 days to allow recharge in what are normally long fallows across the summer to the next winter crop.

Research to date has shown that allowing these cover crops to grow through to maturity led to significant soil water deficits and yield losses in the subsequent winter crops.

“However, the Eastern Farming Systems project showed only small deficits, and even water gains, accrued to the subsequent crops when millets were sprayed out after six weeks, with average grain yield increases of 0.36 tonnes/ha,” Dr Lawrence said.

“Furthermore, NGA work supported by GRDC is indicating that the addition of extra stubble (from 5-40 t/ha) after winter crop harvest appears to reduce evaporation, with initial studies showing between 19 mm and 87 mm increases in plant available water.

“These gains will be valuable if validated in further research and captured in commercial practice.”

For more information on the cover cropping research project, Dr Lawrence’s 2019 Goondiwindi GRDC Grains Research Update paper is available.

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Toni Somes, GRDC
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GRDC Project code: DAQ00211