Key points
- Cover cropping can improve fallow efficiency, provide a net water benefit and lead to better establishment and yields
- A three-year research project found improved crop establishment to be a significant secondary benefit from the practice
- ‘When’ and ‘what’ the next cash crop will be guides cover cropping options.
By providing increased ground cover at planting, cover cropping can improve establishment and yield of the subsequent cash crop.
Experience shows cover crops not only improve fallow efficiency and provide a net water benefit when grown as low cover fallows, but their potential to improve establishment and yields is equally important.
These are among the findings from a three-year GRDC research project into cover cropping completed last year. The project, ‘Quantifying the effectiveness of cover crops to increase water infiltration and reduce evaporation in the northern region’, was led by the Department of Agriculture and Fisheries’ (DAF) Dr David Lawrence, with DAF research agronomist Andrew Erbacher undertaking the trials in Queensland.
Mr Erbacher says cover crops help maintain a moist seedbed for longer, which leads to other benefits: “We focused on the soil water aspects of cover crops and found improved crop establishment to be a significant secondary benefit.”
He says there are a range of reasons growers are interested in cover crops, including biology and organic carbon. However, poor ground cover and not enough water to plant a cash crop are the main catalysts.
For growers interested in cover cropping, he says, the best place to start is to chat with an agronomist or neighbour. “Have a think – do you need to cover crop? Do you have poor cover?”
If yes, then it pays to think about the next cash crop following the cover crop. “What will it be and when will it be planted? The next cash crop will dictate the probability of getting a benefit, and also how best to manage the cover crop.”
These questions help determine the best timing for termination.
“A short fallow will favour an early termination, as the cover crop uses less water, and residue doesn’t need to last very long. In contrast, a long fallow needs stubble to last longer, so you need to wait for the cover crop to produce a resilient stem. This uses more water but will be more likely to receive enough rainfall to recover from the larger plant-available water (PAW) deficit.”
Research findings
The three-year project’s other findings were that fallows with high ground cover will generally have higher fallow efficiency from reduced evaporation and run-off. And a higher fallow efficiency provides more stored water at planting, improving the crop’s yield potential.
Dry, cracking clay soils can achieve fallow efficiencies greater than 70 per cent over a short period. When this is combined with higher fallow efficiency from increased ground cover, a carefully managed cover crop can recover the soil water used.
Reducing evaporation provides an additional benefit because it maintains moisture near the surface for longer. This can extend the planting window to better match the optimum date for maximum yield potential. It can also lead to larger areas of crop being planted without the need for time-critical rainfall.
Mr Erbacher says soil loss in run-off was one of the major drivers for the shift to zero or minimal-till farming systems in the late 1990s. “While not measured in our field trials, a simulation demonstrated the potential to reduce erosion with cover crops.”
The erosion modelling was undertaken by CSIRO research officer Brook Anderson. It showed a substantial long-term reduction in erosion from growing cover crops compared to bare fallows.
The model found that cover crops provide erosion control and facilitate increased water infiltration. The average net water storage benefit is 17 millimetres for short fallows and 15mm for long fallows.
“During drier years, cover crops offered little benefit, as erosion risk is inherently low, and the water lost through producing a cover crop is not replenished by rainfall. However, during wetter years, cover crops, while offering little benefit to water storage, substantially reduce erosion risk by reducing runoff volumes and sediment concentration in run-off water,” Ms Anderson says.
“Even small stubble levels of one tonne per hectare are predicted to eliminate sediment losses in up to 50 per cent of years.”
Ms Anderson says the modelling suggests that seasonal climate forecasts and PAW estimates should be considered when assessing cover cropping options.
Mr Erbacher says reducing erosion risk is important and leads to other benefits, including earthworks savings, while reducing the nutrient and pesticide losses that occur with sediment losses. “This delivers both economic, agronomic and environmental benefits.”
More information: Andrew Erbacher, 0475 814 432, andrew.erbacher@daf.qld.gov.au; Brook Anderson, brook.anderson@csiro.au; Neil Huth, neil.huth@csiro.au