Pre emergent herbicides: part of the package for FTR management?

Take home messages

1. Feathertop Rhodes grass (FTR) is generally a difficult and costly weed to control with knock-down herbicide strategies
2. A number of residual herbicides provide useful FTR activity but highly unlikely to be standalone options
3. Need to incorporate a range of integrated weed management (IWM) approaches to achieve effective long term management
4. Monitoring for new incursions and effective patch management appear to be key tools for this species
5. Individual paddock rotations may need to change to enable use of effective residual chemistry in preceding fallows or in-crop         

The issue

Feathertop Rhodes grass - Chloris virgata has become an important weed management issue in many parts of the northern grains region, having previously been more common as a weed of non-crop areas. Some of the likely reasons for this change are:

• FTR is a species with higher levels of natural tolerance to glyphosate and as such has been selected by glyphosate dominated fallow/roadside management strategies
• FTR is generally poorly controlled by either paraquat alone or even a double-knock of glyphosate followed by paraquat
• DAF Qld research has shown that FTR is a one of the first weed species to colonise bare areas (faster than weeds such as awnless barnyard grass, fleabane and sowthistle under the same conditions) and can germinate on rainfall events as small as 10mm
• FTR prefers to germinate from the soil surface with negligible emergence occurring when seed is at depths of 2cm or deeper in the soil
• Minimum/zero tillage practices are likely to have increased the threat posed by this weed as cultivation or seed burial can be effective FTR management tools

It was always expected that as our reliance on knockdown herbicides increased – both in fallow and in-crop, there would come a time when the industry would need to successfully apply more integrated weed management (IWM) approaches for a range of weed targets. For many growers, that time has been with us a few years now! Residual herbicides of course are far from the solution but merely another tool that may assist in the control of a range of weed species.

Residual herbicides for FTR management pre-crop or in fallow

Previous NGA, DAF Qld and NSW DPI trial activity/reports have focussed on the potential of residual herbicides, particularly in fallow, to aid the management of problem fallow weeds such as fleabane or FTR. Using a residual herbicide in fallow appears a logical first step as it should maximise the amount of herbicide present at the time the weed is most likely to be active. However an obvious downside is that in a fallow situation there is no crop competition to assist weed suppression/ control and often application happens under adverse conditions.

Although good levels of control can be achieved, variability in control and the issues that have often dogged residual products still need to be better understood and managed e.g. consistency of efficacy across varied soil types, incorporation requirements and stubble loadings, plantback restrictions and how to best maximise crop safety without reducing weed efficacy.

Research has shown that a range of herbicides can provide useful residual FTR activity, however at this stage there is still only one registered fallow herbicide option:

• 100g/ha isoxaflutole 750g ai/kg (eg Balance®)

Additional product registrations for fallow use are being sought. Other herbicides used to provide residual control of summer grass weeds in pre-plant or fallow situations (eg Dual® Gold, Flame® or Treflan®) have been noted to reduce emergence of FTR.

Residual herbicides for FTR in non-crop situations

FTR frequently dominates in non-crop areas with a potential for re-infestation of cropping.Imazapyr based herbicides (e.g. Arsenal®, Arsenal® Express) are registered for control in non-crop areas.

Residual herbicide in-crop

Utilising residual herbicides in-crop will allow the use of additional IWM approaches. In-crop use benefits from:

• Crop competition
• Change in crop being grown
• Residual herbicide often applied under better conditions than in fallow or where a level of mechanical incorporation occurs
• ‘Increased disturbance’ planting may provide benefits for FTR management via weed seed burial or removal of early weed emergence

Currently there are no registrations for residual control of FTR in-crop although there are a range of Group A herbicides with knock-down registrations for FTR control in summer crops such as cotton, mung beans, soybeans, sunflower and peanuts.

Residual herbicide strategies for awnless barnyard grass control (e.g.  Dual® Gold, Flame®, Treflan® and Stomp®) applied in a range of summer crops have been noted to reduce the emergence of FTR.

FTR is predominantly a summer weed but often the first cohort of emergence occurs during the winter crop phase. Screening of registered winter crop residual herbicides for activity against spring emergences of FTR (in addition to other summer grass weeds) has been conducted for the past three seasons with encouraging activity from a range of herbicides used in either cereal or chickpea production. Residual herbicide strategies for the control of a range of both grass and broadleaf weeds (e.g.  Balance®, Treflan®, Stomp®, Sakura® and Terbyne®) applied in a range of winter crops have been noted to reduce emergence of FTR.

Other tactics certainly required

Effort is continuing into determining how to ensure more consistent levels of efficacy from at-planting residual herbicides. However in nearly all cases follow up weed management will be required. In fallow situations this often results in the need for follow up fallow applications but may allow the use of optical sprayers to limit both the area treated and cost.

With in-crop application we have the added benefit of crop competition but also a number of rotation crops in which Group A herbicides are registered to allow knock-down control of FTR escapes. Sorghum is still seen as the potential ‘blow out’ crop for FTR management. Although there are a range of residual herbicides used in sorghum for summer grass management, their length of activity is certainly not season long and there are currently no in-crop knock-down options to control survivors. When combined with the wide row spacings used in sorghum, this creates an environment where FTR often flourishes.

Other options that should be considered for FTR management:

• Salvage cultivation of mature plants and inter-row cultivation for crops such as sorghum when grown on wide rows
• Patch cultivation of new incursions (including manual weeding and chipping)
• Residual herbicides applied to new patches after other tactics employed
• Strategic cultivation for seed burial (providing further tillage is not employed as this may simply return seeds to the soil surface)
• Burning may be a useful tool where blow outs have occurred in patches or even in larger areas

Conclusions – the role for residual herbicides

Our management issues with FTR (together with weeds such as fleabane and awnless barnyard grass) have in part stemmed from an over reliance on glyphosate in the summer fallow. To successfully manage these weeds the industry must continue to adopt a range of integrated management practices using both non herbicide measures eg:

• crop rotation
• strategic and salvage tillage
• intensive patch management for new incursions
• salvage burning
• a wider range of herbicide options (eg residual herbicides) as well as double-knock strategies

This is required to effectively manage these weeds but also to ensure we don’t lose glyphosate completely. FTR is a species that appears well suited to intensive patch management as it doesn’t appear to be as easily dispersed by wind compared to some of our other fallow weeds eg sowthistle.

Profitability is of course still paramount. The suggestion with these problem weeds is to focus on individual paddocks and adjust rotations to crops that most suit your environmental conditions but also enable the use of effective residual herbicides in the previous fallow or even in crop. Particularly for FTR, the seed bank is only short lived and two years of effective management can ensure that paddocks return to full flexibility of rotational choice.

Acknowledgments

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

Richard Daniel
Northern Grower Alliance
Ph: 07 4639 5344
Email: richard.daniel@nga.org.au

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