Summer weed control makes economic sense

Author: Alistair Lawson | Date: 07 Jan 2016

Colin McMaster in a crop

NSW DPI research and development agronomist Colin McMaster says by controlling summer weeds growers can double yields in subsequent crops.

Following widespread heavy rainfall in late 2015, growers in the southern cropping region are being urged to implement a summer weed management strategy.

Not only do summer weeds rob growers of valuable soil moisture and nitrogen, recent research suggests it is also robbing growers of substantial profits through large yield losses.

Return on investment

New South Wales Department of Primary Industries research and development agronomist Colin McMaster has led much of the research into the effects of summer weeds on plant available water and nitrogen in following crops and the yield benefit from controlling summer weeds.

His research has found that for every dollar invested in herbicides during the summer fallow period, the return on investment ranged from $3 per hectare to $8/ha.

He says the return on investment added up over a diverse range of seasons, irrespective of the amount of rainfall received.

“Even during the wet season of 2010 when moisture was unlimited, the return on investment came in the form of more nitrogen in the soil, but in drier years the return on investment came from a combination of stored moisture and nitrogen,” Mr McMaster says.

“We wanted the herbicide costs associated with each spray treatment to be conservative - on the higher side - so we allowed $17/ha for lower rates of herbicide and $24/ha for higher rates, including application costs. Either way, it resulted in a really good return on investment.”

‘Buying’ a spring

Mr McMaster says controlling summer weeds has the added advantage of reducing the pressure on low spring rainfall, which has been a scarce commodity for many growers in recent seasons.

In Mr McMaster’s trials, controlling summer weeds increased plant available water at sowing by up to 86 millimetres in central NSW.

“Spring rainfall is ideal to finish crops, but if growers have got stored moisture from summer rain then they are going to be sweating on it less,” he says. “There is nothing growers can do about controlling what happens in spring but if they try and save as much moisture as they can during summer by controlling weeds which rob moisture and nitrogen, it’s a cost-effective way of buying some crop security in spring.”

In some of Mr McMaster’s trials in central west NSW, yields of subsequent crops halved when summer weeds - broadleaf and grasses - were not controlled. Trials in 2010 following a very wet season saw wheat yield increase by up to 1.7 tonnes per ha, which Mr McMaster attributed to the additional nitrogen available to the crop, as evident by soil test results.

“The problem during the summer period is if there is a green plant growing, it will be taking moisture out of ground which will also remove nitrogen,” he says. “In our trials we found that for every mm of moisture lost during the summer fallow to weed growth, we also lost 0.65 kilograms per ha of nitrogen.

“We’re predominantly losing nitrogen through the weed’s skeleton, but also because the weed is drying the soil out and therefore reducing nitrogen mineralisation. Temperature, moisture and organic carbon percentage are key drivers for nitrogen mineralisation, therefore a great opportunity for growers to build free nitrogen during the summer months when seasonal conditions allow.

“Like many people, I always thought of the summer fallow period as a way to conserve moisture, but nitrogen is also a big aspect.”

Getting the right kill

The most effective way to kill most summer weeds is to spray them when they are young and actively growing. Waiting until the weeds are moisture or heat stressed reduces herbicide efficacy.

During trial work, Mr McMaster and his team waited 10 days after a significant summer rainfall event - greater than 25mm - before they inspected for weeds and then sprayed.

Trial plots showing where summer weeds were controlled (background) and where they were not (foreground).

A trial showing where summer weeds were controlled (background) and where they were not (foreground).

“We were leaving it for 10 days and then going out and looking to see what was germinating. If nothing has germinated then logic and common sense has to be used,” he says.

“Therefore, the longer there is a green plant growing, the more moisture and nitrogen will be lost.

“Obviously we need to give the weeds time to germinate and emerge, but a real practical risk is if the weeds are left for 20 days after a rainfall event and then weather conditions arrive that aren’t conducive to spraying, growers could lose weeks. That 10-day rule gives a good opportunity for growers to get over their whole program.”

Rain during harvest may present some logistical problems for growers as there may not be enough labour and time for both harvest duties and weed control. To that end, Mr McMaster’s advice for growers was to get on the front foot immediately after harvest.

“After harvest try to clean the weeds up because a fair bit of damage can be done,” he says. “Keep an eye on the weeds that come up and control them when they are smaller and easier to control. The bigger the weeds get, the more difficult they are going to be to control and the more damage that has already been caused.”

Know your enemy

Mr McMaster says it is important growers can identify the weeds they need to control and be aware of any herbicide resistant issues amongst the weed spectrum.

“Growers really need to be consulting with their adviser about weed types, density and the growth stage they are at in order to come up with a suitable control strategy,” Mr McMaster says. “There isn’t just one recipe to follow for all weeds. Each paddock is going to be different.

“Herbicide resistance and weed species will influence the chemistry growers use. In some situations glyphosate on its own will do the trick, but other scenarios will require an additional spike or the use of a double knock or cultivation.”

GRDC-funded research by the CSIRO has shown that summer weeds are now likely to be causing more crop yield loss than winter weeds. Adelaide-based CSIRO farming systems scientist and research group leader Dr Rick Llewellyn says there are three contributing factors to that finding:

  • Growers are continuing to keep winter weed densities in-crop quite low
  • Summer weeds are becoming increasingly difficult and costly to control
  • Stored moisture is of very high value to crop production

Dr Llewellyn says the summer weeds causing the biggest losses in southern cropping systems were heliotrope, melons, fleabane, panic grass and caltrop with weeds such as Lincoln weed, windmill grass and wireweed being particularly difficult and costly to control in some areas.

“Even though about two thirds of land for cropping in the southern region may receive a herbicide treatment for summer weeds and about a third of growers use at least some cultivation for summer weed control, we are still seeing high total costs to yields from summer weeds.

“It’s a part of the farming system that is increasingly costly and is in need of more management options, particularly as new herbicide-resistant or herbicide-tolerant summer weeds become more important.”

More information

Colin McMaster, NSW DPI, 0427 940 847,; Rick Llewellyn, CSIRO, 08 8303 8502,

Useful resources

GRDC summer fallow weed management manual 

GRDC Project Code CWF00013, CSA00043

Region South