Paddock Practices: Feathertop Rhodes grass and sorghum – does this work?
Date: 16 Aug 2021
Managing FTR in sorghum
- Select low pressure paddocks
- Apply first residual herbicide in late winter
- Follow with another residual application at planting or early post-emergent. Ensuring this application is applied before weed germination
- Maximise crop competition
- Employ inter-row cultivation where necessary
- Harvest early, and have a management plan to remove any patches after harvest.
Managing feathertop Rhodes (FTR) grass in sorghum can be extremely difficult, however a well-planned and integrated strategy can be successful.
Complexity in FTR management is related to the wide-row spacing commonly used in sorghum, where the lack of crop competition in the inter-row may allow FTR to establish and compete. Once established there are very few effective in-crop control options for FTR, apart from inter-row cultivation. Also challenging in wet years is the likelihood that residual herbicides may ‘run out’ before harvest and late germinations of FTR may occur.
If incorrectly managed, a sorghum crop can quickly become the weak link in a FTR management plan, resulting in a ‘blow out’ of FTR that can take several years to get back under control.
However, by carefully choosing paddocks, integrating good fallow management, residual herbicides and non-herbicide options like crop competition and inter-row cultivation, sorghum can remain a viable summer crop option.
Planning a FTR strategy for sorghum
Know your paddock history – The most important tactic underpinning effective management is to know the paddock history, and only grow sorghum in a paddock where the FTR seedbank is low. Identify paddocks the summer before planting sorghum and put effective management strategies in place to stop weed seed set and then maintain a clean fallow in the months prior to planting.
Growers should avoid planting sorghum in paddocks where the FTR seedbank is known to be high, or young FTR plants have emerged before the sorghum planting window. Where FTR seedbank is high, growers should either maintain a clean fallow over the coming summer or grow a broadleaf crop such as mungbean where there are additional options to manage FTR.
Managing FTR in mungbean crops is often much easier than in sorghum. Mungbean will be planted on narrower rows, thus increasing crop competition. Additionally, the time to row closure and the total growing period is much shorter in mungbean than sorghum.
These factors mean residual herbicides applied at planting are more likely to provide full season control in mungbean crops.
Additionally, there is the option of applying a registered post-emergent Group 1 (A) herbicide to control in-crop escapes.
If seedling FTR are present and sorghum is still the preferred crop choice, full-cut cultivation is likely to be required to remove existing plants prior to planting, as there are no effective herbicide options for this situation.
Full-cut cultivation may also bury a significant portion of the FTR seedbank. Germination of FTR seed from below 2cm is substantially reduced.
Start with a clean fallow – Using herbicide tactics to control established FTR is difficult. The most consistent knockdown herbicide options for control of FTR seedlings in fallow typically involves a Group 1 (A) herbicide followed by a paraquat double knock. However, due to plantback constraints to sorghum with Group 1 (A) herbicides, this strategy cannot be implemented close to sorghum planting. Glyphosate, or a glyphosate followed by paraquat double knock, which is commonly used pre-plant for a range of other weeds is typically ineffective on FTR, even if the plants are small.
To ensure the fallow is clean of FTR leading into sorghum planting, a residual herbicide will normally be required in the winter fallow to control early spring germinations that are likely to occur following the first spring rainfall events. Residual herbicide options registered for application to fallows prior to planting sorghum include Valor® (flumioxazin) or Dual Gold® (s-metolachlor).
Both herbicides can provide effective residual control of FTR in spring and typically provide similar length of control at their registered application rates. Length of persistence for both depends on the amount and frequency of rain following application.
Valor® controls a much wider range of broadleaf weeds than Dual Gold®, however Valor® has a one-month plantback period to sorghum. Whereas Dual Gold® can be applied at any time up to the six-leaf sorghum growth stage, provided sorghum seed is treated with either Concep® II or Epivio® C seed safener.
The use of imidazolinone (IMI) herbicides in the fallow, or the previous crop, can often be an effective management tool to provide residual control of a range of grass and broadleaf weeds. However, plantback constraints with these herbicides limit their use to well in advance of conventional sorghum planting. For example, there is a 10-18 month plantback period, including a requirement for a minimum of 500mm rainfall, before conventional sorghum can be planted following an imazapic application.
With the release of imidazolinone-tolerant sorghum (e.g. Sentinel® IG), growers now have an additional management tool that allows for IMI-tolerant sorghum varieties to be grown in paddocks where soil residues of IMI herbicides may still be present.
Intervix® (imazamox + imazapyr) herbicide is registered for post-emergent application in IMI-tolerant sorghum varieties to control a range of grass and broadleaf weeds. However, Intervix® is poor at controlling emerged FTR and therefore post-emergent use of Intervix® in IMI-tolerant sorghum should not be relied on for FTR management within the crop.
Crop competition – Sorghum is typically grown on one metre or wider row spacing. Under this planting configuration the lack of crop competition allows FTR to establish, especially in the inter-row.
Wide row spacing, in combination with a relatively long growing season and lack of in-crop herbicide options for control are primary reasons why sorghum is often a ‘blow-out’ crop for FTR seed production in northern farming systems.
Research from the Queensland Department of Agriculture and Fisheries (DAF) and the Queensland Alliance for Agriculture and Food Innovation (QAAFI) has shown that increased crop competition can be a useful management tool for control of FTR, even in a crop such as sorghum. Reducing row spacing from 1m to 50cm can reduce FTR seed production by more than 50 per cent (Figure 1); while increasing crop density can also contribute to reduced FTR seed production (Figure 2).
Figure 1: Narrowing sorghum row spacing greatly reduced seed production on FTR plants.
Figure 2: Increasing sorghum crop density (plants/m2) can reduce seed production on FTR plants.
Additionally, if FTR establishment can be delayed by as little as 4-6 weeks after the establishment of the sorghum crop, then FTR seed production may be drastically reduced (Figure 3).
Figure 3: FTR seed production (seeds/plant) and retention (% of seeds/plant) in sorghum in response to different transplanting time (weeks) after sorghum emergence. (No herbicides were applied in this trial).
The use of residual herbicides at planting is often an effective way to achieve this requirement of four to six weeks of weed free conditions post sorghum emergence.
Decisions to manipulate crop row spacing and population for improved weed management should always be made with the knowledge of stored soil moisture, seasonal forecasts and yield potential for the target environment.
Residuals at planting – An effective residual herbicide program is essential for paddocks with known FTR pressure. Typically, this will involve applications of Dual Gold®, or Dual Gold® + atrazine where additional broadleaf weed control is required. Several application strategies have proven to be effective.
If Dual Gold® is to be applied in the winter fallow to control first spring germinations, then a split application strategy can be implemented, with the second ‘top-up’ application being applied at, or immediately after, planting.
Alternatively, Valor® may be selected for the winter application (providing there is at least one month before sorghum planting), with the Dual Gold® then applied at planting. In addition to increasing the length of residual protection, this strategy utilises two different modes of action which is advantageous for resistance management.
To extend the length of residual control even further, sorghum growers may elect to apply Valor® in the winter fallow, followed by a split application of Dual Gold® with the first application at planting and the remainder applied early post-emergent in the sorghum crop.
Splitting Dual Gold® application generally increases the total length of residual control, as opposed to a single application of the same total application rate. However, when splitting applications, ensure that the total volume of product does not exceed two litres per hectare (2L/ha) in any one crop and that applications made post-emergent to the crop are applied before the six-leaf stage of the sorghum crop.
When using Dual Gold® herbicide in sorghum, always ensure that the seed has been treated with a seed safener (Concep® II or Epivio® C).
Inter-row cultivation where needed – Where residual herbicides have not been used, or have been ineffective, FTR may establish early in the sorghum crop before canopy closure. There are no registered and effective post-emergent herbicide strategies to control these escapes in sorghum.
Crops grown on 1m or wider rows will allow the possibility of inter-row cultivation. FTR seedlings are shallow rooted and will normally be removed by a shallow inter-row cultivation.
Where inter-row cultivation is employed, this may disturb previous applications of residual herbicide applied before planting, especially where the cultivation is aggressive e.g. where the cultivation event is deep or also being used for furrow forming for surface irrigation.
Where inter-row cultivation is planned as part of a FTR management strategy, consider a post-emergent application of Dual Gold®, to provide ongoing residual weed control after cultivation. If a shallow cultivation is to be implemented, then applying the Dual Gold® immediately prior to cultivation can utilise the shallow cultivation to incorporate the herbicide. If an aggressive cultivation is to be used (i.e. furrow forming for flood irrigation) then it is probably more effective to apply the Dual Gold® after the cultivation, providing there is rain forecast to assist with incorporation. Ensure that any post-emergent Dual Gold® is applied before the 6-leaf stage of the sorghum
Have a plan for post-harvest FTR management – In some situations, residual herbicides may ‘run out’ before sorghum harvest and FTR may establish late in the crop, particularly if there are gaps in the plant stand.
Residual herbicides are most likely to ‘run out’ in years of heavy and consistent rainfall. These seasons are also likely to favour FTR germinations and possibly may also restrict machinery access to the field.
In situations where FTR has established late in the sorghum crop, it is important to have a strategy ready to implement immediately post-harvest.
Glyphosate is typically the herbicide of choice for sorghum desiccation, but it is unlikely to be effective in reducing any FTR present prior to harvest. However, a well-timed glyphosate application may be able to bring sorghum harvest forward, which may assist in being able to implement follow-up FTR management before plants can set seed.
In low weed pressure situations, FTR is likely to be present as single plants or in small patches. Established FTR plants are often noticeable during harvest. Ensure that the harvester operator records these patches (via GPS tagging as they are identified) and commit to manage these areas as soon as possible after harvest.
For larger patches or heavier pressure situations, removal of established FTR plants post-harvest is likely to require cultivation. If plants have already shed seed, plan on the inclusion of a residual herbicide following the cultivation event to control the next flush of emergence.
This information has been developed by ICAN in partnership with GRDC to focus attention on strategies to improve the management of FTR in the Northern Region. To follow the progress of demonstration sites established to showcase best management approaches for FTR control, or to register for upcoming FTR workshops and field walks head to the Independent Consultants Australia Network website.
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