New actives and herbicide use patterns relevant to broadacre cropping in Central NSW

Author: Christopher Preston, Peter Boutsalis and Gurjeet Gill, University of Adelaide | Date: 28 Feb 2017

Take home message

  • New pre-emergent grass control options from Group J and Group K are available for 2017.
  • Understanding the characteristics of these products is essential to get the best out of these herbicides.
  • Resistance has already occurred to many pre-emergent herbicides and has recently appeared to Group J herbicides.
  • Rotation of pre-emergent herbicides and the use of tactics that stop weed seed returning to the seed bank are essential for stewardship of these products.

New pre-emergent herbicides and use patterns

The increasing resistance to post-emergent herbicides in grass weeds has meant greater reliance on pre-emergent herbicides for weed control. There are new use patterns and new actives available for the control of several weeds pre-emergent. It will be important to use these herbicides under the correct circumstances to get the best out of them.

Butisan®, active ingredient metazachlor, is a new pre-emergent grass herbicide for canola.  It is currently under review for registration with the APVMA. It is a Group K herbicide and the active ingredient is similar to S-metolachlor in behaviour (Table 1). Compared to other pre-emergent herbicides available for use in canola, metazachlor is more water soluble and has lower binding to organic matter. This means metazachlor is likely to be more mobile within the soil than other pre-emergent herbicides. This will be an advantage where there is limited moisture at or shortly after application. Less rainfall will be required to activate the herbicide and it is more likely to move into and control weeds in the crop row. However, on light soils with low organic matter, the herbicide can be easily moved by heavy rainfall too deep to control weeds. Butisan has activity on wild oats and wireweed as well as annual ryegrass, so will be useful where these weeds occur. Metazachlor also has a relatively short half-life. In our trials, it struggled with very large annual ryegrass populations as a result of its short persistence. With larger grass weed populations and in higher rainfall regions, a mixing partner will be essential to obtain high levels of control.

Table 1. Solubility and binding to organic matter (KOC) characteristics of pre-emergent herbicides for grass weed control in canola compared with S-metolachlor.

Pre-emergent herbicide

Trade name (example)

Solubility
(mg L-1)

KOC
(mL g-1)

S-metolachlor

Dual Gold®*

480

226

Metazachlor

Butisan

450

45

Propyzamide

Rustler®

9

840

Trifluralin

Triflur-X®

0.2

15,800

Simazine

Gesatop®

5

130

* In canola, Dual Gold is only registered for control of toad rush.

Arcade®, active ingredient prosulfocarb, has been registered for the control of annual ryegrass in wheat and barley. Prosulfocarb is a Group J herbicide that is already widely used as the main component of Boxer Gold®. Arcade, due to the lack of the S-metolachlor component, will have different behaviour to Boxer Gold. It will not be as water soluble as the water solubility of prosulfocarb is more similar to pyroxasulfone, the active ingredient in Sakura®. The lack of water solubility means that Arcade is not suited to post-emergent use. Arcade does not control wild oats and an additional herbicide, such as Avadex® Xtra will be required where wild oats occur.

Arcade has a rate range of 2.5 to 3 L ha-1, however, our trials indicate that every bit of 3 L ha-1 will be required to control annual ryegrass.  Arcade will provide the opportunity for a range of mixtures with other pre-emergent herbicides. Good mixing partners will include trifluralin and triallate (Avadex Xtra). There is also the opportunity for mixing different ratios of Arcade and metolachlor or S-metolachlor. This will be particularly useful in light soils where damage to wheat from Boxer Gold can occur.

Resistance risks and stewardship for pre-emergent herbicides

Resistance to trifluralin (Group D) is extensive across South Australia and Victoria and is increasing in New South Wales. Annual ryegrass is the main weed with resistance to Group D herbicides, although there is also resistance in fumitory. It takes about 12 to 15 years of use of trifluralin for resistance to evolve to this herbicide. Propyzamide (e.g. Rustler) is also a Group D herbicide. While there is some cross-resistance between trifluralin and propyzamide, field experience suggests most trifluralin resistant populations are well controlled by field rates of propyzamide.

Resistance to triazine herbicides, such as atrazine, is present in several weed species in Australia: annual ryegrass, barnyard grass liverseed grass, silvergrass, wild radish, Indian hedge mustard and stinging nettle. Many of these have occurred from post-emergent use of triazine herbicides; however, pre-emergent triazine use also selects for resistance. Resistance to triazine hebricides typically occurs after 10 to 15 years of use, but in some cases has taken a lot longer.

Resistance to Avadex Xtra (Group J) has appeared in annual ryegrass in South Australia, New South Wales and Victoria in recent years. These populations are also resistant to Arcade and some to Boxer Gold (Table 2). Our research suggests resistance to Group J herbicides is likely after 7 to 8 years of use, so will occur more rapidly than resistance to Group D or Group C.

Table 2. Survival (%) of 4 annual ryegrass biotypes to field rates of pre-emergent herbicides in a winter pot trial.

Herbicide

Population*

EP162

198-15

375-14

SLR31

Roseworthy-R

S

Sakura

0

0

0

0

0

0

Boxer Gold

32

29

0

0

0

0

Avadex Xtra

54

43

100

0

60

0

Trifluralin

7

20

-

79

0

0

*EP162: was identified from the 2014 random weed survey of southern Eyre Peninsula; 198-15: identified from central NSW after poor control with Boxer Gold; 375-14: identified from the Yorke Peninsula SA after poor efficacy with Avadex Xtra.; SLR31: is a trifluralin resistant and multi-resistant population; Roseworthy R: was selected with high rates of Avadex Xtra in a field trial from a susceptible population and resistance enriched after recurrent selection; S: is a susceptible field population.

There is no field-based resistance to Group K herbicides in weeds in Australia, but this may be because this mode of action has not been as widely used as some others. However, there is only a limited amount of resistance to Group K herbicides throughout the world, indicating this mode of action is generally robust from a resistance perspective. Much of the resistance that does occur appears to be cross-resistance from use of other herbicides; as typically these populations have resistance to multiple modes of action. This cross-resistance represents a risk.

Given these risks for resistance to pre-emergent herbicides and the increasing number of opportunities to use Group K herbicides in the rotation, it is important that strategies to minimise the effect of resistance to pre-emergent herbicides are put in place. Rotating pre-emergent herbicide options across the crops grown is one useful tactic, rather than using the same mode of action every year. Ensuring pre-emergent herbicides are used in such a way that high levels of weed control are achieved is also useful. Crop competition in cereals and canola are good partners with pre-emergent herbicides, as competition reduces the amount of seed set by survivors and later emerging weeds. Failure is more likely where pre-emergent herbicides are used in very high weed density situations and these should be avoided, by reducing weed numbers with other tactics. Reducing the amount of weed seed going into the seed bank should be included as often as practical.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

Contact details

Dr Christopher Preston
University of Adelaide
Ph: 0488 404 120
Email: christopher.preston@adelaide.edu.au