Sustaining our herbicide options into the future

Take home messages

• Resistance to pre-emergent herbicides is increasing across southern Australia.
• New pre-emergent herbicides are becoming available; however, it is vital that these are used appropriately to get the best results.
• Rotating pre-emergent herbicide modes of action and using other weed management practices will be essential to managing resistance to these new herbicides.

Resistance to pre-emergent herbicides in south-eastern Australia

Pre-emergent herbicides have become more important for the control of grass weeds, particularly annual ryegrass, in the past decade as resistance to post-emergent herbicides has increased. However, resistance to trifluralin is now common across many cropping regions of South Australia (SA) and Victoria (Vic) (Table 1). Worryingly, resistance to the Group J and K pre-emergent herbicides has also been detected in random weed surveys. In some parts of SA, resistance to triallate is also becoming common. This means that it will become more difficult to control annual ryegrass with the current suite of herbicides available.

Table 1. Resistance to pre-emergent herbicides in annual ryegrass populations from random surveys in South Australia and Victoria. Samples were considered resistant to a herbicide if more than 20% of individuals survived the herbicide application.

New pre-emergent herbicides

There are several new pre-emergent herbicides coming to market in the next few years. As with previous recent introductions of pre-emergent herbicides, it is important to understand their best use in different environments and farming systems. Some of these products will be new modes of action, which will provide an opportunity to manage weeds with resistance to existing herbicides. However, it will be important to rotate these new herbicide modes of action to delay resistance.

Grass herbicides

Luximax

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

Cinmethylin has high water solubility and moderate binding to organic matter in soils. Cinmethylin will move readily into the soil with rainfall events but will be held up in soils with high organic matter. Less rainfall will be required to activate the herbicide similar to Boxer Gold^® (prosulfocarb + S-metolachlor). Persistence of Luximax is generally good, but it degrades sufficiently quickly so that plant backs in subsequent years are not likely to be a problem.

Wheat is not inherently tolerant of cinmethylin, so positional selectivity (keeping the herbicide and the crop seed separate) is important. Knife-points and press-wheels are the only safe seeding system and the crop seed needs to be sown 3cm or deeper. Obtaining crop safety with Luximax will be challenging on light soils with low organic matter. Heavy rainfall after application can also see the herbicide move into the crop row and cause crop damage. Due to its behaviour, Luximax is not generally suitable for dry seeding conditions. 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^TM

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

Wheat is most tolerant to bixlozone, followed by barley and then canola. The safest use pattern will be incorporated 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.

The behaviour of Overwatch in the soil appears to be similar to Sakura. It needs moisture to activate and has low to moderate water solubility. 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 are all tolerant of 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.

Persistence in the soil is medium; however, extended use of carbetamide in the pasture seed industry in the 1990s led to enhanced soil breakdown. This is unlikely to be a problem in grain production, as pulse crops are not grown every year. However, these soils also developed enhanced breakdown of propyzamide.

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.

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 its use much safer under adverse conditions. Devrinol-C offers an alternative pre-emergent herbicide to propyzamide or trifluralin for canola.

BAY167

BAY167 is an experimental product from Bayer. 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 2023.

The behaviour of this herbicide in the soil will be more similar to Sakura, compared to Boxer Gold. It will require more rainfall to activate and will have similar persistence to Sakura. It will most likely work best 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. It will require more rainfall after application than Boxer Gold does, so the post-emergent application will be more suited to higher rainfall regions.

Broadleaf herbicides

Callisto^®

Callisto, active ingredient, mesotrione is a pre-emergent Group H herbicide from Syngenta with expected registration in 2020. It will be registered as for IBS, knife-point press wheel use in wheat and barley. It will control a range of broadleaf herbicides including brassicas, legumes, capeweed and thistles.

Wheat is more tolerant than barley, and in both cases, positional selectivity is important for crop safety. Mesotrione has high water solubility and medium mobility in soils. High rainfall resulting in furrow wall collapse could result in crop damage. Callisto has moderate persistence with plant backs of only nine months, provided 250mm of rainfall has occurred. Callisto offers an alternative to post-emergent Group H herbicide mixtures, where early weed control is important.

Reflex^®

Reflex, active ingredient fomesafen, is a Group G herbicide from Syngenta with expected registration in 2021. It will be registered pre-emergent and post-sowing pre-emergent (PSPE) in pulse crops for control of broadleaf weeds; IBS only in lentils. It will have similar weed spectrum to Terrain®, but will likely provide better control of brassicas, sowthistle and prickly lettuce.

Fomesafen has more water solubility than flumioxazin (Terrain), so will be more mobile in the soil. It does not bind tightly to organic matter. Pulse crop safety is good, except for lentils, which are most sensitive. Care will be needed in lentils on light soils with low organic matter. Fomesafen persistence is good; however, plant backs are expected to be nine months provided 250mm rainfall has occurred.

Voraxor

Voraxor, from BASF, contains the active ingredients trifludimoxazin and saflufenacil, which are both Group G herbicides. Voraxor will provide broadleaf weed control and some annual ryegrass control as a pre-emergent herbicide in cereals. It is expected to be registered in 2021.

Voraxor is a little more mobile in the soil compared to Reflex^® and considerably more than Terrain. Voraxor will offer a broader spectrum of broadleaf weed control compared to Terrain and more annual ryegrass control. However, annual ryegrass control will not be as good as with current annual ryegrass pre-emergent standards. This means that it will be best used where broadleaf weeds are the main problem and annual ryegrass populations are very low. Grass pre-emergent herbicides cannot be tank mixed with Voraxor and will have to go out as a separate application.

Managing resistance to the new pre-emergent herbicides

The availability of new modes of action, particularly for annual ryegrass control, is a valuable aid to maintaining no-till in grain production. However, overreliance on any herbicide mode of action can lead to resistance. Some of the annual ryegrass populations with widespread resistance to other herbicides already have low level resistance to napropamide and bixlozone. In addition, there are an increasing number of Group H and Group G herbicides becoming available. Care needs to be taken to rotate herbicide modes of action through the cropping rotation to delay the onset of resistance. Other weed management practices such as crop competition, crop topping and harvest weed seed control should be employed where appropriate.

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