Saved summer moisture sets up yield potential

Stored soil moisture terms

  • Plant-available water capacity (PAWC) The maximum amount of water available to the crop if the soil profile is full. It varies with soil and crop type. Measurements are expressed as millimetres of plant-available water per metre of soil profile, for example, 150mm/m, or as a total for the soil that takes into consideration the depth of the soil profile. If the 150mm/m example has a depth of 1.5m with no subsoil constraints, PAWC would be 225mm.
  • Plant-available water (PAW) In many seasons, the maximum water storage capacity (PAWC) is not reached for reasons including insufficient rainfall, fallow weeds, run-off and evaporation. In these cases, the actual water present is described in terms of the PAW, that is, ‘how full is the bucket?’. It is expressed in millimetres of PAW.
  • Fallow efficiency The percentage of fallow rainfall still present at sowing. To calculate it, use the following formula: Fallow efficiency (%) = (PAW (mm) at fallow commencement – PAW (mm) at fallow end) / Fallow period rainfall (mm) x 100.

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.

“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.” 

– John Cameron

“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.”

TABLE 1 Modelled effects of PAW on clay loam soils at seeding against simulated yields at Quorn, South Australia, from 1900 to 2009.
All years Years with above-median PAW at seeding Years with below-median
PAW at seeding
Number of observations 110 55 55
Median yield (t/ha) 1.3 2.6 0.4
Number of years < 0.7 t/ha 39 (35%) 6 (11%) 34 (62%)
Number of Years > 2.0 t/ha 49 (45%) 37 (67%) 12 (22%)
 SOURCE: Mudge and Whitbread, 2010

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.”

More information:

John Cameron
02 9482 4930

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 or free phone 1800 11 00 44.

End of Ground Cover Issue [#109] (Southern edition)
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Ground Cover Supplement issue #109 – Frost


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