Control summer weeds to reap yield benefits

Controlling summer weeds results in an average 60 per cent increase in seasonal WUE (harvest-to-harvest rain).

Controlling summer fallow weeds has emerged as one of the largest single contributors to improved water use efficiency across southern Australia

With CSIRO modelling indicating fallow rain contributes about a third of average wheat yields across southern Australia, it is no surprise that regional grower groups involved in the GRDC Water Use Efficiency Initiative put improved ways to capture, store and use summer rain at the top of their list when asked about the best way to lift water use efficiency (WUE) in their cropping systems.

Results from 21 trials carried out as part of the initiative overwhelmingly agree with the modelled prediction, with summer weed control delivering an impressive average increase in seasonal WUE of 60 per cent – well in excess of the 10 per cent target set at the start of the five-year research program.

Controlling summer weeds delivers an average $5.57 for every dollar invested in weed control costs (Table 1).

The measured increase in WUE and wheat yield was consistent across a range of seasons and soil types and resulted from conserved soil moisture and mineralised nitrogen that would otherwise have been used by the weeds.

Complete control of fallow weeds can deliver water use efficiency increases of up to 60 per cent and more than $5 for every dollar spent on weed control costs.

PHOTO: Dr James Hunt, CSIRO

Fallow management

A four-year fallow management trial set up on sandy and clay soils at Hopetoun in the Victorian southern Mallee by BCG and CSIRO is the first published study to quantify the impact of summer weeds on subsequent crops.

The combined impact of extra soil moisture and nitrogen at Hopetoun resulted in an average increase in wheat yields of 0.7 tonnes per hectare on sand and 0.9t/ha on clay. Crop yields increased in response to summer weed control across wet and dry growing seasons, but for different reasons. 

A simulation study based on the Hopetoun site calculated that in drier seasons the yield response was due to extra soil moisture, while in wetter years the yield increase resulted from extra soil nitrogen made available to the subsequent crop. This was supported by research in the central-west of NSW by Central West Farming Systems and the NSW Department of Primary Industries (DPI), which also found controlling summer weeds delivered reliable and summer-rainfall-driven responses in available soil water and nitrogen and subsequent crop yield (Table 1).

Further support for the yield benefits of fallow weed control was delivered by South Australian Research and Development Institute (SARDI) trials and the Hart Field Site Group, which found that in many seasons yield was limited by both water and nitrogen, and controlling summer weeds generated very reliable yield increases due to improvements in plant-available water and mineral nitrogen (Table 1).

Complete control of summer weeds should therefore increase productivity across the southern grain region, depending on current levels of control.

Stubble and soil moisture

Also examined as part of the Water Use Efficiency Initiative was the impact of stubble retention on stored soil moisture and subsequent crop yields. Unlike summer weed control, retaining stubble had no effect on stored soil moisture or wheat yield with the conclusion being that only very high stubble loads are able to reduce soil water evaporation.

Research established at Hopetoun examined a series of stubble treatments that reflected current farming practices, with residues either left standing or completely removed and summer fallow weeds either left uncontrolled or removed chemically. In other treatments stubble was slashed and summer weeds chemically controlled or standing stubble was cultivated to control summer weeds.

The results indicated that retaining stubble loads of 2.4 to 5.8t/ha did not result in more stored soil moisture or higher crop yields – with a small negative effect on grain yield measured on the clay soil in 2011 and a small positive effect in 2012.

Summer weeds and livestock

Growers often cite livestock as a reason for not controlling summer weeds because the fallow feed is deemed too valuable to remove.

However, a CSIRO whole-farm modelling study found the yield losses incurred by keeping summer weeds far outweighed their benefit as stock feed. Controlling summer weeds rather than leaving them for feed increased average farm income by about $74/ha at Hopetoun and $78/ha at Temora.

Many growers also worry that grazing stubbles will reduce subsequent crop yields. However, a four-year experiment at Temora run by FarmLink Research and CSIRO and at Condobolin by Central West Farming Systems and the NSW DPI, as part of the GRDC initiative, demonstrated that, provided summer weeds are controlled and 70 per cent ground cover maintained, allowing sheep to graze stubbles does not reduce subsequent grain yield. In some seasons there was a small yield benefit from grazing due to increased nitrogen made available via the grazed stubble.

A whole-farm modelling study predicted that grazing stubbles was more profitable than leaving them ungrazed, with a $4/ha increase in farm income from grazing stubbles compared with leaving them ungrazed at Hopetoun and a $21/ha increase at Temora.

While retaining stubble in no-till farming systems does not appear to improve crop yields (at least in the short term), the operational and soil structure benefits of the practice remain undisputed.

More information:

Dr James Hunt, research scientist, CSIRO Plant Industry,
02 6246 5066,
james.hunt@csiro.au

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GRDC Project Code CSP00111, UNF00001, CWF00013, DAS00089, BWD00012, FLR00005

Region South, North, West