HERBICIDE RESISTANCE UPDATE - WEST OF THE WARRUMBUNGLES

GRDC code:

DAQ00136 Risk assessment and preventative strategies for herbicide resistance in the northern grain region (Phase III).

Background

The cropping area west of the Warrumbungle ranges has many challenges, in the form of herbicide resistance. Currently the main issue is wild oat resistance to various herbicide groups. This could exist as populations resistant to one herbicide group only or as many as three groups. Another serious issue is the presence of glyphosate resistant annual ryegrass (ARG), in small patches. With a dominance of summer fallows in this district and the confirmation of glyphosate resistant awnless barnyard grass (BYG) and liverseed grass in the north-west plains, it may only be a matter of time before these additional threats extend southwards.

Many areas have limited summer crop options and grow predominantly winter crops in a zero tillage system. Such a system is ideal to develop glyphosate resistant summer fallow weeds such as BYG, liverseed grass, sowthistle and fleabane. Such a system may also enable multiple resistant wild oat and ARG populations to develop.

Wild oats: Controlling populations with resistance to 1 herbicide group

Essentially there are three herbicide groups available for post-emergence wild oats control in wheat. They are represented by Group A (eg. Topik®, Wildcat®, Axial®), B (eg. Hussar®, Atlantis® and Crusader®) and Z (eg. Mataven®) herbicides. A common occurrence is wild oats with resistance to Group A herbicides only, leaving at least 2 herbicide groups for effective post-emergence activity.

For people with Group A resistant wild oats, the data in Table 1, would be relevant to their needs. It shows that a combination of pre- and post-emergence herbicides (not Group A) will generally prevent wild oat seed production and thus minimise the density of resistant seed returning to the seed bank.

For farmers that have multiple resistance to two herbicides groups (either A and B or A and Z), could use a pre-emergence herbicide followed by the remaining useful post-emergence option and get excellent levels of control.

Table 1: Controlling Group A resistant wild oats, North Star 2007.

SST = Selective Spray Topping – late post-emergence to prevent seed production.

However, there are some cases of multiple resistance to all three post-emergence herbicide groups. In this case, data in Table 1 would be irrelevant as no post-emergence option would be effective (refer to Table 2 instead). Reliance solely on pre-emergence herbicides would result in populations of wild oats increasing. Surviving plants from trifluralin and Avadex® Xtra treatments tend to be large and produce more seed than what is lost from the germination process.


Table 2: Controlling multiple resistant (Groups A, B and Z) wild oats, Collie, 2006.

Note: This population had confirmed complex resistance to groups A, B and Z, however it was shown in previous trials that it was partly susceptible to Atlantis® and Sertin® This is the reason for their inclusion in 2006

Changes to crop type / rotation may open the door to the use of other herbicides (Table 2). Although this wild oat population can be kept in checkI in wheat with pre-emergence herbicides + Atlantis®, alternative crops can be grown with better weed control outcomes.

Chickpeas grown on conventional row spacing or wide rows resulted in excellent control and enabled the inclusion of simazine, trifluralin and Avadex® Xtra as useful pre-emergent options. Trial results showed that inter-row spraying of Gramoxone® in wide row chickpeas could be useful, if it were to be registered for this use. .

The same principles apply to canola which enables the inclusion of atrazine trifluralin, Avadex® Xtra and Dual® Gold.

Long fallowing paddocks is another alternative. It is important to note that the Flame® treatment did not control wild oats well and a follow-up application of glyphosate was required to prevent seed set.

Poor wild oat control was reported in Clearfield® canola after using OnDuty®. This may be due this populations’ strong level of resistance to Hussar®, another Group B herbicide. This is a likely reason why Flame® did not work well in the fallow. Despite the failure of Clearfield® canola, Roundup Ready® Canola should work since the population seems susceptible to glyphosate.

Double knocking – to control glyphosate resistant annual ryegrass (ARG) and barnyard grass (BYG)

Double knock tactics are defined as “the sequential application of two different control options on the same flush of weeds, where the second option is designed to control the survivors of the first.”

The benefits of double knock tactics are
- reduction in weed pressure
- reduced reliance on one weed control strategy
- reduces the risk of herbicide resistance
- can reduce herbicide resistant weeds

Results from double knocking annual ryegrass

Three experiments investigated the effects of double knocking annual ryegrass. Many interesting findings were obtained, as this technique is affected by several variables. The major conclusions are;
• Growth stage of ARG is critical when the second knock is a contact herbicide. These herbicides are best suited to small weeds.
• As the growth stage increases, the second knock (contact herbicide) dose rate can increase to allow for bigger plants.
• Application of Group L herbicide should be limited to ARG no greater than the early tillering stage.
• Paraquat was consistently more effective than Spray.Seed® on a L/L basis (Figure 1).
• Glyphosate can be used as the first knock treatment but should never be used as the second knock treatment (Figure 1).
• Glyphosate can suppress the growth of resistant plants for periods of up to 2 weeks, allowing a greater period of time between knocks. Being a winter weed, the cooler conditions will also reduce growth rates and extend the permissible time between knocks.
• The incorporation of a residual herbicide improved ARG kill and extended the period of control.

Figure 1. Double knocking annual ryegrass: Interaction between glyphosate and Group L herbicides (Spray.seed® and paraquat), Spring Ridge 2004.

Note: T1 = First time of application (2 to 3 leaf stage), T2 = treatment to be applied 8 days after T1 (3 leaf stage), T3 = treatment to be applied 14 days after T1 (early tillering). Control score of 3 = 80% control, score of 4 = 95% control and score of 5 = 100% control.

The strategic use of pre-emergence herbicides to control ARG is critical. Many people decide not to use pre-emergence herbicides because of restrictions on opportunity cropping options. From observation, most paddocks have set rotations planned and usually a few residual herbicides could be used without serious plant-back issues for subsequent crops. Depending on the crop to be sown, a residual herbicide can often be selected that will not interfere with cropping sequence.
From the extensive range of herbicides tested, the following pre-emergence herbicides were highly effective
• Glean and Logran (no plant back issue with cereals)
• Atrazine (suitable prior to TT Canola, sorghum)
• Simazine (suitable prior to chickpeas)
• Dual® Gold (Canola, sorghum and chickpeas tolerant)
• Flame® at 200mL/ha (can be used in fallows prior to winter cereals)

Results from double knocking barnyard grass experiments

The principles of double knocking BYG are very similar to ARG. However, there were a few differences. Firstly, growth rates of BYG are much faster and the time between knocks is much less than for ARG. There are also differences in the pre-emergence herbicides that work. For BYG the best herbicides are Dual® Gold, Flame® at 200mL/ha and rates of atrazine at least 3L/ha.

Double knock on glyphosate resistant plants: In March 2007, the first experiment (at Bellata) conducted on glyphosate resistant barnyard grass investigated basic combinations of glyphosate followed by either paraquat or Spray.Seed®. In addition, single applications of glyphosate, paraquat and Spray.Seed® were made as a comparison to the double knock treatments.

Although resistant to glyphosate, barnyard grass was controlled well with 1.5 and 2.0L of glyphosate 450 per hectare (Table 3). This indicates that glyphosate can provide at least partial control of glyphosate resistant barnyard grass, provided the plants are less than the 3 leaf stage and actively growing. Evidence from a recent glasshouse experiment indicates that resistant biotypes exhibit greater levels of resistance as plants size increases.

Similar levels of control were seen with paraquat and Spray.Seed® in this experiment, however paraquat is preferred due to its lower cost. The levels of control were excellent but did not control all weeds. All the double knock treatments resulted in 100% control in this trial.

Table 3.Percent control of glyphosate resistant barnyard grass following some double knock and single treatments,

T1 = 1st knock, barnyard grass at 1 to 2 leaf stage
T2 = 2nd knock, mostly 1 to 2 leaf due to 33 mm of rain on night of 1st knock, however some were at early tillering stage. Sprayed 5 days after the 1st knock

A follow-up experiment was conducted at Bellata in November 2007 to validate the effects seen in the previous experiment. Some differences were observed, namely the barnyard grass plants were at the 1 to 4 leaf stage for the 1st knock, much larger than the 1 to 2 leaf stage in the previous experiment. In addition, at the 2nd knock, barnyard grass plants were also larger (between 2 leaf and 3 to 4 tillers stage).

Results from this experiment (Table 4), indicate the robustness of the double knock technique. Despite, treating slightly larger weeds, the technique constantly resulted in between 99 and 100% control.

Table 4. Percent control of glyphosate resistant barnyard grass following some double knock and single treatments, assessed 24 days after 2nd knock.

T1 = 1st knock, barnyard grass at 1 to 4 leaf stage
T2 = 2nd knock, 2 leaf up to 3 to 4 tillers. Sprayed 7 days after the 1st knock

Repeated applications of glyphosate at 1 and 2L/ha resulted in less than commercially acceptable control, although more than half the plants were killed. There seems to be a slight advantage using paraquat rather than Spray.Seed® (Tables 2 and 3).

The addition of a residual herbicide in the second knock improved control slightly over the standard non-residual double knock treatments. The addition of Flame®, Dual Gold® and atrazine are good examples of residual herbicides that are active against barnyard grass. Having a residual herbicide in the second knock should provide longer-term excellent control, minimising the reliance on the double knocking technique if several flushes of weeds occur in a wet fallow season.

What does herbicide resistance mean for farmers?

Management
• Improved management of weeds. This would include getting samples of weeds tested for their resistance status and planning control strategies accordingly. Monitoring herbicide performance should be a higher priority.
• Making radical changes to cropping systems if severe multiple resistant wild oats limits control options. This usually occurs in rotations that are dominated by wheat with few winter fallows. An understanding that wild oat seed production is the factor that makes infestation persist, so efforts to minimise seed production will result in excellent outcomes.
• Better resources are needed in a timely manner. Double knock requires sequential applications with limited days in between sprays. There must be adequate staff or resources in place if large areas of weeds are to be treated to prevent weeds growing too large. Since timing is critical with double knocking, growers need to be aware of weed growth stages.
• Be flexible. Excessive rain may mean that it is hard to spray early enough. Herbicide rates may need to be adjusted to suit the size of weeds. Glyphosate, Spray.Seed® and paraquat have a range of rates on the label to adjust for increased size of weeds.
• Understand plant back periods if using residual herbicides. The use of residual herbicides will have implications for subsequent crops.

Costs
• Changing cropping systems to combat complex multiple resistance in wild oats has some level of risk. However, continuing to abuse post-emergence wild oat herbicides will ultimately result in serious resistance issues and poor weed control, leading to poor yields and forced changes to operations.
• Inclusion of residual herbicides early in the fallow phase (ARG and BYG) should take pressure off post-emergence applications and should reduce costs (follow-up sprays).
• Selection of paraquat (Gramoxone®) rather than Spray.Seed® is cheaper and also provides slightly better control of grass weeds.

To dos
• Wild oats- If the season allows, grow a summer crop. This allows two winter fallow periods and the used of glyphosate on Group A, B and Z resistant weeds.
• Wild oats – A combination of a pre- and post-emergence treatment is recommended if a post-emergence herbicide still works on wild oats.
• BYG and ARG in fallow - Apply double knock tactic to small weeds, preferably 2 to 3 leaf.
• BYG and ARG - Use residual herbicides (with a desiccant herbicide) for the second knock if possible.
• All weeds - Monitor weed growth stages frequently and get weeds tested for resistance status when suspected.
• Target the double knock technique for the big flushes of weeds, usually early in the season.

Contact details

Tony Cook
NSW DPI
Ph: 02 67631250
Fx: 02 67631222
Email: tony.cook@dpi.nsw.gov.au

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