WithTheGrain: Summer weed control imperative to preserve moisture for upcoming season

Author: | Date: 01 Feb 2018

Following poor performing crops in some areas of the low- and medium rainfall zones, farmers are reminded summer weed control could assist with profits this season, presenting what could be a $5 return for every $1 investment in weed control methods.

La Trobe University senior lecturer Dr James Hunt says the main priority in the coming months for growers operating a continuous cropping system with no livestock is summer weed control.

Dr Hunt says a rough rule of thumb to come out of the GRDC’s National Water Use Efficiency Initiative was for every dollar invested in summer weed control there can be a $5 return in the form of additional yield due improved stored soil water and nitrogen.

Failure to control summer weeds can also lead to a ‘green bridge’ which can increase pre-sowing populations of a range of pests and diseases such as cereal rusts, mites and viruses.

In the case of paddocks which may not have been harvested due to dry conditions in 2017, summer weed control will be critical in order to preserve as much moisture as possible for the coming season’s crop.

“Weeds are the big one. Tidying up volunteers and other background summer weeds is very important,” Dr Hunt says.

“Growers in the low-rainfall (LRZ) and medium-rainfall zones (MRZ) should aim to keep fallows as clean as possible to give the new season crop a good start. This involves assessing fallows following the first round of spraying to check weed control and following up with another round of spraying as a double knock if it is required.”


According to La Trobe University senior lecturer Dr James Hunt paddocks not harvested in 2017 or droughted should be watched closely for weed growth to protect nutrients and existing soil moisture this summer.

Generally, herbicide efficacy is highest when summer weeds are treated early (three- to five-leaf stage) at registered label rates.

It is important to use methods which reduce spray drift when controlling summer weeds. The herbicide product label will give guidance about spray quality, buffer (no-spray) zones and wind speed requirements.

Weather conditions should be continually monitored at the site of application. This includes always measuring and recording wind speed, wind direction, temperature and relative humidity – along with herbicide and water rates at the start of spraying and at the end of every tank (according to the label requirements).

Dr Hunt says stubble management options are limited where a sparse legume crop may have been harvested on lighter soils and there is the possibility of wind erosion due to a lack of crop residue, however cultivation can help in paddocks that are completely bare.

“Cultivating creates some roughness in the paddock and break up the wind fetch, but as those furrows slump they need to be cultivated again to remain effective,” he says.

“It’s obviously not an ideal situation to have paddocks without any cover at all. Even a droughted legume crop should still have enough cover to stop growers from having to cultivate.”

Methods for managing heavy stubble loads over summer comes down to personal preference for many, with mulching, slashing or burning considered viable options. Erosion concerns and maintaining soil fertility are common reasons given for avoiding burning, but in areas with large amounts of stubble remaining come seeding time, it can present some benefits in reducing the stubble load.

In terms of stubble load, for every tonne per hectare of grain harvested, about 1.5 tonnes per hectare of stubble will be left as residue. Prevention of wind and water erosion requires 70 per cent soil cover with anchored stubble, which equates to about 2-3 t/ha of cereal stubble.

This is a general rule across the southern region, although it may need to be lifted on lighter Mallee soils, where more straw will need to be retained to avoid erosion.

More information

Dr James Hunt
03 9032 7425

Useful resources

GRDC Update Paper: Insights from the Grain and Graze Program

GRDC GrowNotes: Spray Application Manual

GRDC Stubble Management Fact Sheet