Saved summer moisture sets up yield potential
When summer weeds are controlled, increases in plant-available water (PAW) at sowing significantly increase the percentage of years when above-average yields are obtained.
This is the recurring message of research being compiled in a new integrated weed management (IWM) manual, soon to be released by the GRDC.
Modelling research by Rural Solutions and CSIRO at Quorn in South Australia found that in years where PAW was above the median at seeding, yields were at least 2.6 tonnes per hectare. When PAW was below median, the median yield was 0.4t/ha (Table 1).
Other trial research outlined in the manual found that in the SA Mallee, managing weeds in summer fallow could increase soil water at sowing by between six and 21 millimetres. This small water saving resulted in yield increases of up to 0.68t/ha, depending on the amount and distribution of in-crop rainfall and soil nitrogen status.
The Independent Consultants Australia Network’s John Cameron – who runs a series of IWM workshops around Australia – is compiling the manual.
He says the research compiled in the manual shows that summer weed control is both a water and nitrogen story.
“It’s an absolute ‘no-brainer’ that if you have significant summer rainfall, controlling weeds is one of the best investments you can make on the farm,” Mr Cameron says.
“If you can store some of the water for the next crop, even small increases in stored water, particularly if that water is stored deep in the soil, can be extremely valuable in contributing to the next year’s yield.
“With in-crop season rainfall, if you average 20 kilograms of grain per millimetre PAW you’re doing very well. However, a soil type that is capable of storing water deeper in the profile could provide in-crop season rainfall equivalent to 60kg grain/mm PAW. Under this situation, an extra 10mm of water could be worth an extra 0.6t/ha of grain in the bin.”
The second part of the equation for summer weed control is plant-available nitrogen.
“The weeds you haven’t controlled in the summer fallow tie up and rob a lot of the plant available nitrogen that would otherwise have been available to the crop.”
When it comes to preferred methods of summer weed control, Mr Cameron says grazing options are less effective because they’re slower to reduce weed populations.
“There hasn’t been the history of glyphosate use on the summer weed species in the southern and western regions that there has been in the northern region. As a result, many southern and western region paddocks still have quite a few years of glyphosate use before problems arise.
“The use of double knock and strategic tillage both increase the effective life of glyphosate.”
That said, Mr Cameron still makes the point that every use of a herbicide is selecting for resistance and this needs to be kept in mind.
“The economics of controlling summer weeds are so obvious that we need to do it. Tillage is also an option, but one that costs more and has disadvantages in respect to soil water and ground cover.
“If you had to choose between tillage and letting weeds grow, letting weeds grow will lose more water every time.”
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The new manual, Summer fallow weed management in the southern and western grains regions of Australia – a reference for grain growers and advisers, will be released soon. It will be available from Ground Cover Direct, email email@example.com or free phone 1800 11 00 44.
End of Ground Cover Issue [#109] (Southern edition)
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GRDC Project Code ICN00012
Region South, National, North, West