Getting the best out of the new products for ryegrass control

Take home messages

  • Resistance to herbicides is common in annual ryegrass in southern Eyre Peninsula including to the pre-emergent herbicides Trifluralin® and Avadex® Xtra.
  • New pre-emergent herbicides are becoming available; however, it is vital that these are used appropriately to get the best results.
  • Matching pre-emergent herbicides to soil types and seeding systems can help reduce crop damage.

Herbicide resistant weed survey Eyre Peninsula 2019

A weed resistance survey of 170 annual ryegrass samples randomly collected from the Eyre Peninsula at harvest in 2019 showed high frequencies of resistance to the Group A and Group B herbicides (Table 1). Resistance to the Group A herbicides is higher in the Lower Eyre Peninsula (EP) compared to the Upper EP. In contrast, resistance to the Group B herbicides is high across the region. Resistance to Intervix® in annual ryegrass on the EP is as common as resistance to the sulfonylurea herbicides.

Table 1. Extent of resistance in annual ryegrass populations from Eyre Peninsula in 2019. Resistant populations are those with >20% survival to the field rate of the herbicide.

Herbicide

Group

Lower EP

Upper EP

Total

  

Resistance (% of populations)

Diclofop-methyl

A

84

19

61

Axial®

A

71

8

49

Clethodim

A

14

0

9

Oust®

B

97

79

90

Intervix®

B

97

88

94

Glyphosate

M

1

8

3

Paraquat

L

0

0

0

Trifluralin®

D

66

17

49

Propyzamide

D

0

0

0

Avadex® Xtra

J

33

3

23

Arcade®

J

10

0

7

Boxer Gold®

J + K

4

0

3

Sakura®

K

1

0

1

For the pre-emergent herbicides, resistance is highest to trifluralin and then the Group J herbicides, Avadex® Xtra and Arcade®. Resistance to the pre-emergent herbicides is higher in the Lower EP. There is less resistance to Boxer Gold® and Sakura® and no resistance found to propyzamide (Table 1). The extent of resistance to herbicides in annual ryegrass, particularly in the lower EP means herbicide rotations and other weed management strategies need to be used to delay the resistance to existing and new herbicide modes of action.

New pre-emergent grass herbicides for annual ryegrass

There are several new pre-emergent herbicides that have been released or will be released in the next few years. It is important to understand their behaviour to get the best use out of these herbicides.

Devrinol-C®

Devrinol-C®, active ingredient napropamide, is a Group K herbicide from UPL registered in 2019. Devrinol-C® is registered for annual grass weed control in canola.

Its active ingredient, napropamide, is not as water soluble as metazachlor (Butisan®) and has less movement through the soil. Canola has much greater tolerance to napropamide compared to metazachlor making it much safer in the high rainfall zone. Devrinol-C® offers an alternative pre-emergent herbicide to propyzamide or trifluralin for canola.

Luximax®

Luximax®, active ingredient cinmethylin, is a new mode of action herbicide (Group Z) from BASF registered in 2020. Luximax® is a pre-emergent herbicide for annual ryegrass control in wheat, but not durum. It will also provide some suppression of brome grass and wild oats. In our trials, control of ryegrass is as good as with Sakura®.

Its active ingredient, cinmethylin, has higher water solubility than many other wheat pre-emergent herbicides. This means cinmethylin will move readily into the soil with rainfall events. Less rainfall will be required to activate the herbicide similar to Boxer Gold® (prosulfocarb + S-metolachlor). Persistence of Luximax® is generally good.

Cinmethylin has quite high binding capacity to soil organic matter and this is important in achieving crop safety. Wheat is not inherently tolerant of cinmethylin, so positional selectivity (keeping the herbicide and the crop seed separate) is crucial. Knife-points with press-wheels is the only safe seeding system and the crop seed needs to be sown 3cm or deeper. Heavy rainfall in the first few days after application can also result in the herbicide causing crop damage. Mixtures with trifluralin, triallate and prosulfocarb are good and can provide some additional ryegrass control; however, mixtures with Sakura®, Boxer Gold® or Dual Gold® are likely to cause crop damage and need to be avoided.

Overwatch®

Overwatch®, active ingredient bixlozone, from FMC is a Group Q herbicide that will be available for sowing in 2021. Overwatch® controls annual ryegrass and some broadleaf weeds and will be registered in wheat, barley and canola. Some suppression of barley grass, brome grass and wild oats may occur.

Wheat is most tolerant to bixlozone, followed by barley and then canola. The safest use pattern will be incorporation by sowing (IBS) with knife-points and press wheels to maximise positional selectivity, particularly with canola. Some bleaching of the emerging crop occurs often, but in our trials, this has never resulted in yield loss. In situations where the crop grows poorly, for example, water logging, high root disease, etc., the crop may have more difficulty growing away from the initial bleaching effect.

Overwatch® has a little more water solubility than Sakura®. The level of ryegrass control in our trials has been just behind Sakura®. Mixtures with other herbicides can increase control levels and in our trials in the high rainfall zones, the mixture of Overwatch® plus Sakura® has been very good.

Ultro®

Ultro®, active ingredient carbetamide, from Adama is a Group E herbicide that will be available from 2021. Ultro® will be registered for the control of annual ryegrass, barley grass and brome grass in all pulse crops.

Pulses have reasonable tolerance to Ultro®, so crop damage should be rare. Ultro® provides the best control of annual ryegrass when used pre-emergent. Ultro® has relatively high water solubility, so is more effective on weeds like brome grass that tend to bury themselves in the soil. Persistence of Ultro® is shorter than Sakura®.

Mateno® Complete (BAY167)

Mateno® Complete is a new product from Bayer containing the active ingredient aclonifen. It will be a new mode of action pre-emergent and early post-emergent herbicide for the control of grass and some broadleaf weeds in wheat and barley. Registration is expected in 2022.

The behaviour of this herbicide in the soil will be more similar to Sakura® than Boxer Gold®. It will require more rainfall to activate and will have similar persistence to Sakura®. It will most likely work best for annual ryegrass as a pre-emergent IBS herbicide. The timing of the early post-emergent application will be similar to Boxer Gold®, at the 1 to 2-leaf stage of annual ryegrass. The post-emergent timing will require more rainfall after application than Boxer Gold® does, so will suit higher rainfall regions.

Managing crop safety risk with pre-emergent herbicides

Due to their nature of being soil active herbicides, pre-emergent herbicides can be prone to causing crop damage. This is one reason why knife-point and press wheel seeding systems are recommended for many pre-emergent herbicides. These seeding systems move herbicide treated soil out of the crop row and reduce the potential for herbicide to move into the crop root zone.

The main features of pre-emergent herbicides that need to be considered are water solubility, binding to organic matter and the inherent tolerance of the crop to the herbicide. Herbicides with higher water solubility, lower binding to organic matter and lower inherent crop tolerance are more likely to cause crop damage.

Soil type and rainfall patterns are also important in understanding crop damage from pre-emergent herbicides. Soil types low in organic matter and with coarser particles will allow herbicides to move further in the soil. Herbicides applied when the soil is dry will move further with the opening rainfall event, than if the soil is already moist. Finally, many pre-emergent herbicides lead to root pruning if they come into contact with the crop roots. Any soil constraints that lead to reduced crop growth can magnify the effect of the herbicide on the crop. Even herbicides that we traditionally consider to have high safety on the crop, such as Sakura® in wheat, can result in crop damage under the wrong conditions. In certain soil types with low organic matter, Sakura® can move into the crop root zone and cause damage.

Managing for crop safety has several aspects. The first is ensuring the right pre-emergent herbicide is being used for the crop, soil type and expected rainfall. Secondly, is ensuring that the herbicide treated soil is not dragged or thrown into the crop row by the seeding or subsequent operations. The third component is choosing the appropriate rate. Where a rate range is present on the label, choose the lowest rate for problem soil types. This may require a mixing partner to be added to achieve the desired level of weed control. The final component is good agronomy. Make sure the crop is sown at the correct depth, has the right nutrition and protection from pathogens and insects.

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.

Useful resources

GRDC Publications: Soil behaviour of pre-emergent herbicides

Contact details

Dr Chris Preston
School of Agriculture, Food & Wine
University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au

GRDC Project code: UCS00024, UA00158