Author: Rebecca Barr | Date: 27 Mar 2015

Wild radish in a paddock

Wild radish is one of the most widespread broadleaf weeds in Australian grain cropping and has developed resistance to many herbicide modes of action. 

SOURCE: Peter Newman

It’s one of the buzz phrases of grain growing in recent years – integrated weed management. But how can growers use IWM to take control of weeds and prevent resistance without losing financial returns?

GRDC-supported research has shown that while IWM principles may seem to be expensive and more complex than chemical control alone, using a range of control measures will pay for itself – both financially and in giving growers more control options for the future. There are simple and easy tools available for growers to identify the best IWM tactics for their cropping program.

The key principles of IWM are stopping weed seed set, rotating crop types and chemical modes of action, testing for resistance, adhering to label rates, double-knock techniques and, perhaps most importantly, aiming for 100 percent control.

IWM economics

While many growers may see the short-term costs of investing in extra weed management diversity as an impediment, modelling has shown that using IWM results in significantly higher returns in the longer term.

CSIRO agricultural economist Marta Monjardino was part of the University of Western Australia team which developed RIM (Ryegrass Integrated Management) – a GRDC-supported bioeconomic model which calculates the economic return from different weed management strategies, based on growers’ specific circumstances.

“There is no ‘right way’ to use integrated weed management. The options that will provide the best results will depend on the weed density, resistance status, preferred rotation, and other factors that are different for every grower,” Dr Monjardino said. “RIM was originally developed so that growers and advisors can feed their own situation into the model to find out the best economic balance between the tactical decisions for this year and the strategic outcomes they want over a 10-year period.”

RIM was originally a ryegrass model but has also been extended to wild radish. It accounts for the dynamics of weed behaviour and crop biology, potential herbicide resistance development and the effect of a range of weed control options over 10 years. For example, if a grower models the use of green manuring, the program will take into account the increased return from crop and weed seedbank benefits in following years. RIM has also proven particularly useful in demonstrating to growers when harvest weed seed control practices will be profitable.

“What we’ve found from RIM is that, over the long term, there’s no choice but to use a diverse range of practices. However it’s unlikely the whole suite of tools will be required for any grower. Some measures are only profitable for particularly high weed densities. For example, we showed that a three-year pasture phase can provide excellent control of the weed seed bank, and will still pay off over 10 years for a severe infestation,” Dr Monjardino said.

Dr Monjardino is currently working with the Australian Herbicide Resistance Initiative (AHRI) to update the model to include brome grass, which is becoming an increasingly important weed in south-eastern Australia.

RIM is a computer-based tool designed for growers and advisors to use, with tutorials and training workshops available, and a new user-friendly version can be downloaded for free from the Australian Herbicide Resistance Initiative website (see Useful Resources).

Testing for resistance

University of Adelaide researcher Dr Chris Preston says once growers have experienced resistance to a certain herbicide, there is huge value in testing weeds to find out what herbicides will work.

“While there are some rules about what types of cross-resistances there might be between herbicides, there are lots of variations; cross-resistance is not absolute,” he said. “These variations can sometimes help growers make a decision about what product they might use.”

For example, in a GRDC-funded Fast Track trial in the Griffith, NSW, region, a large number of seed samples were collected and sent in for testing. While all the samples tested positive for Logran® resistance – which had been expected – surprisingly, two thirds of the growers’ samples showed no resistance to Hussar®.

“So they suddenly had a product they could use, even though the technical rules would say you’re likely to get cross resistance. You’d never know that unless you tested,” Dr Preston said.

Chemical control

Tim Condon kneeling in a paddock

Agronomist Tim Condon (pictured) advises growers to mix and rotate modes of action when using chemical weed control.

The key to effective chemical weed control is to understand what the herbicide can achieve and optimise application to improve efficacy.

Delta Agribusiness senior agronomist Tim Condon says that while the majority of growers adhere to label rates, the technique of application still needs to be considered.

“Growers are aware of the need to use the appropriate label rates to achieve the required levels of weed control, however there’s still more focus required on application. A critical factor is nozzle selection, for example when applying a pre-emergent, the herbicide must be making it onto the soil, not the standing stubble, so the nozzle selection and water rate need to be adjusted to ensure correct application,” he said.

Mr Condon gives three tips for optimised chemical control – apply a double-knock, manage expectations, and mix the modes of action.

“A double-knock is traditionally the application of two knockdown herbicides around a week apart, for instance a full rate of glyphosate followed by a full rate of gramoxone. However a double-knock does not have to be chemical, and it doesn’t have to be limited to two steps. The principle is to use a subsequent technique to control the survivors of the first. One effective method is a triple-knock, which is a full rate of glyphosate, followed by a heavy sheep grazing, then a full rate of gramoxone,” Mr Condon said.

Following the knockdown, the next control option is the effective use of pre-emergent herbicides. Mr Condon advises growers should not have too high expectations of the control pre-emergents can achieve.

“Achieving anything over 80 percent control is very respectable. The degree of control may range from 70 up to 95 percent when applied well, but to expect 100 percent control is unrealistic. IWM is all about depleting the seedbank – reducing the numbers. It’s a long-term process and growers need to take into account that even in a good system, some weeds will generally survive. It’s all about stopping any weed seeds from entering the seedbank using as many techniques as practical,” he said.

Mr Condon’s final tip is to mix and rotate modes of action strategically.

“As per the double-knock principle, mixing modes of action means that if herbicide A doesn’t control a certain weed, then herbicide B will. Continuous use of a single mode of action will select for resistance, so the importance of mixing and rotating modes cannot be understated. The newer modes of action are often more expensive than the older ones, but sticking to the cheaper chemicals will select for resistance, and will eventually mean the new modes of action will be the only ones left. By mixing and rotating, growers can ensure the cheaper herbicides remain effective,” he said.

Rotation control

Tractor amongst trial plots

Forage harvesting to simulate grazing in Southern Farming Systems IWM trial at Inverleigh. The plots contain (left to right) serradella heavily infected with radish, persian clover and sub-clover. 

SOURCE: Corinne Celestina

Another important principle within IWM is crop rotation. Southern Farming Systems (SFS) is managing a GRDC-funded trial into the effect on weed populations of pasture ‘breaks’ in a farming system.

The four-year trial at Inverleigh and Lake Bolac in the Victorian high rainfall zone has involved sowing nine different pasture crops for between one and three years, starting in 2012,  followed by a return to a traditional grain rotation (canola, wheat, then barley). The crops selected for the trial were sub-clover, lucerne, Arrowleaf clover, Persian clover, forage oats, ryegrass, Balansa clover, forage peas and serradella. Each of these pasture and fodder species is subjected to different weed control measures over the course of the trial, including in-crop herbicides, silage, hay, grazing and manuring.

SFS projects coordinator Annieka Paridaen said the trial was initiated after noting that while many growers in the high rainfall zone are mixed farming, the animals and crops were rarely integrated.

“Growers would have ‘crop’ paddocks, and ‘pasture’ paddocks, but weren’t rotating between the two. We saw great potential in integrating the two practices in terms of weed control,” she said.

The trial has run for three seasons, with all pasture rotations complete.

“We have proven that with a crop that does not go to harvest, such as hay or silage, we can achieve almost 100 percent prevention of weed seed set. By counting seedbank numbers, and using the combination of cutting hay, then grazing, then spraying, we saw a weed reduction in the following year of about 90 percent. This can deliver excellent returns, with the sale of hay, feed for livestock and a huge reduction in weeds,” Ms Paridaen said.

While chemical control is part of the weed equation, Ms Paridaen says the SFS trial has shown there is a problem with relying on chemical control.

“In the trial, it’s not necessarily the weeds in autumn that are the problem, a knockdown and pre-emergent chemistry can control those weeds. It’s the weeds germinating in spring that create the biggest issue for the following year. Up-front herbicides are a great tool, but if you’re missing the spring germinating weeds every year, it’s difficult to get on top of the problem. By stopping those weeds setting seed by other means, it’s reducing the seed-bank for the future,” she said.

A group of people looking at a trial plot

Local growers and agronomists inspecting an Southern Farming Systems IWM trial paddock at Lake Bolac during 2013.

SOURCE: Corinne Celestina

SFS tested different crops but found no clear winners at this stage. Oats provide high bulk early, and strong competition with autumn weeds, while clovers may be slow early on but increase their bulk in spring, improving weed competition. While the perennial crops such as sub-clover and lucerne are slow in the first year, they provide better crop competition in the second and third years.

So far, Ms Paridaen has found that methods which physically remove the seed, such as hay or silage, are having the best effect on weed seeds, but it is critical to time it right based on weed maturity.

“Only one week late will make a big difference, and the timing has to be based on weed maturity, not crop maturity. The weed has to be stopped from setting seed,” she said.

A demonstration trial has indicated there may be benefit in hay-topping which prevents weed regrowth, however further research is required to confirm this.

So what does Ms Paridaen recommend?

“It depends on the severity of the problem. We’ve shown already that two years of pasture can reduce the numbers significantly, so if the weed problem is costing yield, a two-year break, cutting hay and grazing can pay itself back as well as reduce weed numbers by around 90 percent,” she said.

“Most importantly, stick to the plan. We’ve seen many serious weed problems where the grower set out a rotation plan, but then changed at the last minute, maybe they sowed wheat when they were going to sow a hay-blend, or else instead of cutting for hay, they took oats through to harvest. If you have a weed problem, without action it will only get worse, and one year of poor control can undo 10 years of good work. Integrated management is a long-term commitment.”

More Information

Marta Monjardino, 08 8303 8413, Marta.Monjardino@csiro.au

Tim Condon, 02 63 862 118, tcondon@deltaag.com.au

Annieka Paridaen, 03 5265 1666, aparidaen@sfs.org.au

Useful Resources

Integrated Weed Management Hub

RIM: Ryegrass Integrated Management

GRDC Project Code SFS00022, UWA0014

Region South, North