What are we dealing with in the summer fallow?

The majority of growers have now been managing their summer fallow using zero till principles for at least 15 years. This has seen two significant changes in recent years.

• Species shift. Weeds suited to surface germinating (i.e.do not need to be buried) now dominate the spectrum e.g. the Chloris grasses (windmill and feathertop Rhodes), fleabane and sowthistle
• Herbicide resistance. The reliance on herbicides in the fallow is seeing herbicide resistance rapidly evolve. Many growers have been dealing with Group B resistance for many years however, in more recent years, the discovery of glyphosate (Group M) resistant weeds is becoming common.

Many growers have turned to the use of Group A (fops) and Group B (Flame) to manage grass weeds in fallow however this is likely to be a very short term strategy, as these modes of action develop resistance much faster than glyphosate.

How did we get here?

With any discussion on managing resistance we need to start with the basics and understand why we have the problem in the first place. When herbicides are applied to a weed population there is the potential that some individuals in the population will be resistant to that mode of action, and hence survive the application. If these individuals are allowed to survive and set seed then the population can eventually become dominated by these resistant weeds.

The speed of development of resistance depends upon many factors, however one of the most important is the natural frequency of resistant individuals in the population.

Some herbicide modes of action are much more susceptible to resistance development than others.

Table 1. Years of herbicide application before resistance evolves (Preston et al 1999).

^ All tradenames are registered trademarks of their respective owners.

Tradenames are provided as examples only to provide readers with an understanding of common herbicides from that herbicide mode of action group.

While it is important to understand the potential for herbicide resistance development, it is also important to understand that these intervals can be substantially extended if growers do not let any survivors return seed to the soil. This will mean that multiple different tactics will need to be employed, including non-herbicide strategies.

Our main targets

While there are a large number of weeds found in the summer fallow, some have become more prevalent than others, and these will be the ones that will be the focus of this paper – as they fit one, or both, of the characteristics above.

In addition to the resistance status it is also important to understand the ecology of the weed to help design a control strategy.

Table 2. Predominant summer fallow weeds in the northern region

Developing a control strategy for the summer fallow

By having a good understanding the effectiveness of control tactics available and looking for weaknesses in the ecology of the weed that can be exploited, integrated control strategies designed to aggressively drive down the weed seed bank in the soil can be developed.

In fallow situations, the lack of crop competition places additional importance on the effectiveness of other strategies.

Control in the winter crop is one of the major strategies to managing weeds in summer fallows. Weeds such as fleabane and sowthistle can germinate in early in the winter crop or in spring prior to harvest. Some of the summer grasses can also germinate on spring rainfall well before harvest, especially where gaps in the crop canopy allow light onto the soil surface. If these weeds establish before harvest, they can be extremely difficult to control after harvest.

Fleabane is a prime example. Germinations can occur in autumn and early winter. If not controlled in the crop the fleabane will continue to develop a large tap root, even if the above ground size is limited by competition from the crop. Registrations are now in place for the use of Amicide® Advance 700 (knockdown) or FallowBoss® Tordon® (knockdown and residual) in winter cereals.

For crops such as chickpeas there are very limited post emergent options so the key to preventing weeds being present at harvest is to ensure a robust pre-emergent strategy is implemented. A tank mix of Gesatop® and Balance applied at planting should provide control of the majority of key weeds until harvest.

Farm hygiene can be a low cost, but highly effective tactic in preventing new weed incursions into the paddock. Livestock can introduce weed seeds on their coat or in their stomachs; weeds can come in through hay and animal feed; weed seeds can be in farmer saved planting seed that has not been properly cleaned; dirty farm machinery and farm utes are an additional source of weed seeds; as is water flow across the paddock from flooding.

Strategies to reduce new weed introductions include: make sure fencelines, roadsides, tracks and around machinery sheds are kept clean of weeds; pay particular attention to weeds on perimeters of paddocks; always feed hay and other stock feed in a yard or small paddock close to facilities and monitor and remove weed germinations coming from the feed source; if livestock are fed hay and other fodder that may contain weed seeds, keep them in a small paddock for a week to allow the majority of weed seed to pass through before turning them out onto cultivation paddocks; always have planting seed graded to remove weed seeds; harvest from the cleanest paddocks to the dirtiest, and then clean down; always clean down contractors machinery before entering the paddock; if machinery breaks down in a paddock and has to be dismantled for repairs (and weed seeds are likely to be dislodged) then GPS mark the site and come back over the next 1-2 years and control weed escapes.

Knockdown herbicides have been the most common tactic employed in zero till fallows and are likely to continue to be the main strategy; however the simplicity of using glyphosate for ‘just about everything’ has to change.

The use of double knock program is increasing, either to control resistant weeds or those that require more than a single application to achieve the high levels of control desired. The first application is likely to be a systemic herbicide (typically Group A, Group I, Group M) followed by a contact herbicide (typically Group L).

Tips for effective double knock herbicide applications:

Initial application:

An effective double knock needs the first application to do the bulk of the work. If the herbicide selected is not likely to give at least 90% control as a single application then the doubleknock is probably unlikely to be effective.

• Target small weeds, especially with glyphosate resistance.
• Coarse droplet spectrum (required for drift reduction) dictates that water rates will need to be high (50L to 100L/ha) to have enough droplets for effective coverage. If using extremely coarse spectrum then water rates need to increase further.

Source: GRDC Nozzle Selection - Back Pocket Guide

Recommended spray quality x herbicide

	F	M	C	VC	XC	recommended recommended at 90-100L/ha recommended in stubble
Fop’s & Dims (A)
Imi’s (B)
Triazines (C)
DNA’s (D)
Phenoxy’s (I)
Pyridines (I)
Glyphosate (M)

Source: Nufarm Spraywise®

• Systemic products require good application conditions i.e. small weeds and not moisture stressed.
• Shallow rooted grass weeds (e.g. feathertop Rhodes, windmill, barnyard grass) will quickly become moisture stressed in summer. Weeds that ‘freshen up’ following morning dew and cooler temperatures are still moisture stressed.
• Don’t delay application, waiting for further germinations – the second knock will take care of these.
• Tank mixes
  • Don’t apply mixtures that will compromise the efficacy of the initial application. Use the second knock to clean up the rest of the spectrum.
  • Known mixtures to avoid include:
    • Group A herbicides and Group I herbicides (e.g. Verdict^® + Uptake^® with 2,4-D)
    • Products requiring oil based adjuvants with glyphosate when targeting grass weeds
    • 2,4-D with glyphosate when targeting sowthistle.

Second application:

The second application is designed to ‘finish off’ any weeds not completely controlled by the first application and to pick up any escapes from the first application, especially if they are resistant to the first mode of action.

• Spray in the opposite direction. This may pick up weeds that were physically shaded in the first application.
• When applying contact herbicides, coverage of the target is critical. If using a medium/coarse droplet spectrum high carrier rates are recommended. Use of an extremely coarse droplet spectrum is not recommended as the water rates required to maintain coverage become impractical.
• Herbicide and water rates for Group L herbicides are dependent upon weed size.
• The Spray.Seed® label recommends the following water rates for application in summer rainfall areas

Extra wetter is required when water volumes exceed 100L/ha.

• Apply the correct rate for the situation.

Syngenta SPRAY.SEED Stewardship Module

• Where a residual herbicide is to be added, generally it is more practical to include it in the second application as compatibility is usually better. The main exception is likely to be where there is rainfall forecast between applications where the rainfall can be utilised for residual herbicide incorporation and/or there is a risk of delay to the second timing.

Specific double knock situations

Glyphosate followed by paraquat or Spray.Seed may still be effective when initial low levels of glyphosate resistance are observed, however only on very small weeds and full label rates of both applications should be used. For the initial glyphosate application, do not tank mix 2,4-D when targeting sowthistle or oil based adjuvants when targeting grass weeds.

For grass weed targets, time applications approximately 5 to 7 days apart. For broadleaf weeds best results are seen from applications approximately 7 to 10 days apart.

Group I followed by paraquat or Spray.Seed . For broadleaf weeds such as fleabane or sowthistle use a Group I (2,4-D, FallowBoss Tordon, Tordon 75-D – check labels for rates and use situations) followed by paraquat or Spray.Seed. The addition of glyphosate in the first pass generally increases control of fleabane but reduces control of sowthistle.

Target small weeds (less than 1 month old). Reliability of control decreases on older plants, particularly fleabane. Time applications approximately 7 to 10 days apart.

Group A followed by paraquat . Two permits are currently are in place for fallow applications:

• PER13460 for quizalofop against windmill grass. Application rate is equivalent to 250 to 500mL/ha of Targa®bolt (200g/L) + Hasten® at 1L/ha. This use is restricted to NSW only.
• PER12941 for haloxyfop against feathertop Rhodes grass. This use is restricted to Queensland only and only preceding a mungbean crop.

In both cases, the use must be followed by a double knock of 250g/L paraquat of at least 1.6L/ha for resistance management and only one application is permitted per season. Activity is currently underway to expand registrations. However the constraint of only one double knock per fallow is expect to remain, so growers need to ensure this application is timed for maximum impact.

Paraquat followed by paraquat . While this is not a double knock for resistance management, under certain situations the use of two sequential paraquat applications can be beneficial. In particular, when dealing with grass weeds when conditions are not suitable for a systemic herbicide (e.g. glyphosate or Verdict) as the weeds are moisture stressed. Waiting for rainfall to freshen up the grass weeds is likely to see weeds develop to the point where they will not be adequately controlled by standard double knock options. In this situation, a better strategy is to apply paraquat as the first application followed by a second paraquat 14-21 days later when weeds start to show signs of any regrowth.

Optical (camera) sprayers e.g. WeedSeeker®, WeedIT® can be highly effective in providing economical control of low density populations in the fallow. Additionally, registration of high application rates by this technique may allow the control of resistant weeds that have survived a preceding application of a different mode of action.

Nufarm have recently received the following product registrations for use with the optical spot spraying technique:

In addition to the above registrations, APVMA Permit 11163 is in place to allow users to apply a range of other herbicides via this technique, including glyphosate; paraquat; paraquat + diquat; amitrole + paraquat; glufosinate; amitrole T; 2,4-D; triclopyr; fluroxypyr; quizalofop; haloxyfop, sethoxydim; clethodim; butroxydim; and fluazifop. See permit for application rates and use situations. However note that this permit is currently due to expire on 28^th February 2015.

With glyphosate becoming increasingly less effective, paraquat having weed size limitations and Group A herbicides limited to one application per fallow, growers are going to need additional strategies that do not solely rely on knockdown herbicides. Other useful strategies to consider include:

Monitoring is a critical step in weed management that is often overlooked. Following any treatment, always come back and check the paddock to ensure that the treatment is working the way it is expected.

Residual herbicides can be an excellent option to introduce a different mode of action and take some of the pressure off knockdown herbicides. Consider treating at least some of the fallow paddocks with a residual herbicide to take the time demands off double knock spray applications, especially in paddocks where the next crop rotation is known.

Some of the main pre-emergent herbicides available for use in the northern region include:

Cultivation has increased in frequency on a number of farms in recent years, especially where growers are dealing with ‘blow outs’ due to herbicide failure from resistance; or the lack of ability to apply knockdown herbicides in a timely manner, often due to climatic constraints or lack of spraying capacity.

While cultivation is generally seen as an ‘option of last resort’ in zero till systems it can be an effective tactic against shallow germinating weeds and may also be incorporated into other farming system requirements such as pupae busting , deep placement of nutrients or when paddock renovation is required.

If cultivation is to be contemplated, then consider the following:

• Where there is large amounts of vegetative material on the paddock herbicide coverage may be compromised, so burial of the above ground material may result in better coverage of subsequent applications
• Cultivate on a drying profile. This will minimise surface zone compaction and the ‘transplanting’ of weeds
• Weeds such as fleabane, sowthistle and feathertop Rhodes, windmill and barnyard grasses prefer to germinate on, or near, the soil surface. Burying seeds greater than 5cm (10cm for barnyard grass) is likely to prevent these seeds from emerging. However
  • Burying seeds generally increases their persistence and reduces the rate of natural mortality. The exception is the Chloris species (windmill grass and feathertop Rhodes grass) where seed mortality is ~12 months, regardless of whether it is left on the soil surface or buried
  • Unless using a mouldboard plough set up with soil skimmers, weed seed is likely to be distributed at a range of depths, which may still allow a proportion to emerge
  • If multiple cultivation passes are undertaken then mixing of the seed will occur throughout the profile
  • Any future tillage undertaken before final seed mortality will bring some seed back up towards the surface and enable germination and weed emergence.
  • Cultivation method will affect the proportion of seed buried to different depths
• Regardless of the cultivation technique employed some seed will always remain within the germination zone so these weeds need to be controlled to prevent the population regenerating. One of the best strategies to manage this is application of a residual herbicide with incorporation undertaken by the last cultivation pass in a multiple cultivation scenario; or applied after the cultivation but just before the next rainfall event, where rainfall is being used for incorporation.

Patch management techniques can be implemented when small patches of weeds have been identified. Mark these patches with GPS coordinates and take an active strategy to remove existing weeds before flowering and continue to monitor for subsequent germinations until the seed bank has been exhausted. Patch management tactics could include chipping or hand rogueing, spot spraying with an effective herbicide, or spot cultivation.

Growing a summer crop , rather than maintaining a fallow, could be a consideration in some situations. For example, growing a summer legume such as mungbean or soybean may allow growers to tackle a paddock with a summer grass weed problem. The option of a pre-emergent herbicide, crop competition and selective post-emergent herbicide from a different mode of action is likely to drive down weed numbers more than multiple fallow sprays of a single mode of action knockdown herbicide.

Likewise, if broadleaf weeds such as fleabane are a problem in the paddock, then a summer crop of sorghum allows the use of robust pre and post-emergent herbicide options.

If growing a summer crop for weed management, keep row spacing narrow to maximise competition from the crop and prevent light from reaching the soil surface, always use the maximum registered rate of pre-emergent herbicide to get the maximum length of control and avoid situations where there are limited reliable post-emergent options to control any subsequent emergence. For example, don’t grow sorghum in a paddock with grass weed problems as it is likely that the pre-emergent herbicides will run out of residual before harvest and no effective post-emergent options are available for over the top application.

Careful and thorough attention to preventing weed set seed in the fallow can quickly drive down the seed bank of most of our problematic weeds in the fallow. However, if weeds are allowed to return seed to the soil during the fallow phase then weed numbers are likely to continue to increase over time. Growers that have the weed seed bank under control will save herbicide costs; maintain crop rotational flexibility; be able to use diverse tactics that would otherwise be cost prohibitive; and by reducing the frequency of herbicide applications, will delay the onset of herbicide resistance.

Contact details

Name Mark Congreve
Organisation Independent Consultants Australia Network
Phone 0427 209234
Fax 02 9482 4931
Email mark@icanrural.com.au

® Registered Trademark.

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Reviewed by John Cameron