WithTheGrain: Right conditions needed to maximise post-em effectiveness

Author: | Date: 01 Jul 2017

A number of different factors will influence the effectiveness of post-emergent herbicides, including frost and moisture stress.

The challenge for growers is knowing the right time to spray, or whether or not it is worth leaving the sprayer parked in the shed and saving some money on post-emergent herbicide applications.

University of Adelaide Associate Professor of weed management Dr Chris Preston says one of the biggest issues currently facing growers is the amount of resistance in grass weed populations to post-emergent herbicides.

“There are a lot of products which aren’t going to work in certain areas, so knowing the resistance status of your weeds is going to decide what chemistry can and can’t be used,” he says.

Dr Preston’s colleague, University of Adelaide postdoctoral fellow Dr Peter Boutsalis, has been studying the level of resistance among weed populations to pre- and post-emergent herbicides in the southern region as part of the GRDC’s random herbicide resistant weed survey.

This research has found varying resistance to herbicides across the southern region, as high as 96 per cent for some weed populations to chlorsulfuron in southern Victoria (table 1).

To try and maximise the control of grass weeds in cereals and break crops, growers are encouraged to revisit the STAR principle when considering post-emergent spraying: stress, timing, application and rate.

Table 1: Percentage of paddocks identified to contain herbicide resistance. The results in the table represent data from paddocks chosen at random in the last five years. Source: Boutsalis, 2017.
South Australia
South East2012789061628016-
Eyre Peninsula2014344780471817

Note: Paddocks were scored as resistant if the seeds collected exhibited more than 20 per cent survival in a pot test conducted the following winter. Therefore, samples that exhibited one per cent to less than 20 per cent survival were scored as non-resistant.

Bayer Crop Science Australia’s integrated weed management website provides regionalised herbicide resistance development in the form of maps.

Stress and timing

Dr Preston says dry and frosty weather will impact the efficacy of post-emergent herbicides, particularly Group A herbicides.

In 2013 and 2014, the University of Adelaide ran trials to examine the effect of frost on clethodim applied to annual ryegrass under controlled environment conditions using a frost chamber. Seedlings of annual ryegrass were sprayed at the three-leaf stage with increasing rates of clethodim before or after exposure to three consecutive nights of -2 degrees Celsius (°C) to simulate frost events.

This research found that simulated frost reduced clethodim activity on susceptible annual ryegrass populations, particularly events occurring before clethodim application (figure 1).

Line graph showing Response of clethodim resistant annual ryegrass populations identified in Victoria and Western Australia to clethodim treatment with three simulated nights of frost prior to or post application. Control plants were not sprayed with herbicide and plant survival was assessed 28 days after completion of frost treatment. Source: Saini et al, 2015.

Figure 1: Response of clethodim resistant annual ryegrass populations identified in Victoria and Western Australia to clethodim treatment with three simulated nights of frost prior to or post application. Control plants were not sprayed with herbicide and plant survival was assessed 28 days after completion of frost treatment. Source: Saini et al, 2015.

“We had a lot of dry weather across the country early in the growing season, as well as a lot of frosty weather,” Dr Preston says.

“Group A herbicides don’t take well to frosty conditions and therefore performance is poor. If growers are experiencing a lot of frost then they will have to delay spraying.

“The catch, however, is that the ideal timing for Group A herbicides on grass weeds is when they are at the two-to three-leaf stage, so if spraying is delayed in the hope that environmental conditions improve then the weeds could get too big.”

Dr Preston says some products, such as Targa® and Select®, do perform well on larger weeds. However, the performance of such products is always better on smaller weeds.

“Ideally, we want to get the product out before weeds get too big, otherwise it puts the herbicide under more stress and if the environment is against you then there is more likely to be a failure,” Dr Preston says.


Not only does frost impact the performance of herbicides on weeds, it also makes herbicides more damaging to the crop, Dr Preston says.

“Herbicides are less damaging to the crop in dry conditions, but performance on weeds is reduced,” he say.

“All the weeds are going to slow down. The issue we’re going to have is if there is good subsoil moisture the weeds will continue to grow and it is just frost that will slow them down.

“If there is minimal subsoil moisture, the weeds will just be hanging on and they aren’t going to take up any product.

“However, even though it’s dry it still might be worth getting out and controlling the weeds early. If it is dry and frosty, it might be best to just park up sprayer and wait for conditions to hopefully improve.”

Application and rate

The amount of stress on the weed will also influence the application technique, Dr Preston says.

“If there are environmental stresses and the products aren’t going to work as well on weeds then it is important to get everything else right,” he says.

“That includes water rates, adjuvants and tank mixes. Just doing those things to get the application right will help with the efficacy of herbicide on stressed weeds.

“The same goes for product rates. Under tough conditions, cutting the rate means you won’t get the control you want.”

Despite some cases of ryegrass becoming increasingly resistant to clethodim, Dr Preston says the chemical is still an important part of ryegrass management, particularly in break crops.

Even in resistant populations, higher label rates of clethodim still have some activity on ryegrass, Dr Preston says.

“As part of our trial work, we have been looking at a system of management for herbicide resistant ryegrass,” he says.

“We have sown into populations of ryegrass with high levels of clethodim resistance and found that clethodim still is useful at higher label rates.

“It stunts the weeds, making them less competitive and later in the season we can get good levels of control with tactics like crop topping.”

Dr Preston says it is critical to read the critical comments and general instructions on product labels when applying post-emergent herbicides in tough conditions.

“Just doing that and following those instructions will ensure growers are getting the most out of what they are applying, because those products are going to struggle,” he says.

GRDC research codes: UCS00020, UA00158, UA00144

More information

Chris Preston,
08 8313 7237,

Useful resources

University of Adelaide fact sheet - Maximise clethodim performance: Impact of frost

GRDC integrated weed management hub

GRDC Project code: UCS00020, UA00158, UA00144