Pre-emergent herbicides part of the solution but much still to learn
Author: Richard Daniel and Anthony Mitchell, Northern Grower Alliance | Date: 24 Jul 2015
Take home messages
- The use of a disc planter for incorporation by sowing (IBS) of residual herbicides resulted in significantly reduced wheat emergence for all four herbicides evaluated
- The disc planter ‘set-up’ actually increased the risk of crop damage
- These results reinforce the need to only use narrow point tynes when using residual herbicides with IBS recommendations
- Residual herbicides are important tools for the effective and economic management of key summer fallow weeds
- Use of residual herbicides in fallow may suit operations with access to optical sprayers
- Individual paddock rotations may need to change to enable use of residual chemistry in preceding fallows or in-crop
The widespread adoption of minimum tillage has provided many agronomic and sustainability benefits to our farming system. However it is a system that has led to an overreliance on knockdown herbicides to achieve effective weed management. As a consequence we are faced with management issues in two main ‘herbicide-driven’ scenarios; control of herbicide resistant weeds e.g. annual ryegrass (ARG) - Lolium rigidum and a selection of weed species with higher levels of natural herbicide tolerance e.g. feathertop Rhodes grass - Chloris virgata (FTR) and flaxleaf fleabane - Conyza bonariensis.
Residual herbicides are an important tool to assist in the control of weeds in both these scenarios but the issues that have always dogged residual products will be important to better understand and manage 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.
This paper is split into two sections. The first section deals with results from two trials conducted in 2013 evaluating the crop safety and efficacy of registered residual herbicides for the control of ARG in wheat. The second section is a more general discussion of the fit for residual herbicides in the farming system.
1. Winter cereal ‘at planting’ residual herbicides and crop safety
In these trials the majority of treatments were managed by the incorporation by sowing (IBS) approach. IBS specifies the use of narrow point tynes on the planting equipment. This approach helps to ensure sufficient soil is thrown across the inter-row space to effectively ‘incorporate’ the herbicide plus it removes most of the herbicide treated soil from the planting furrow to improve crop safety. The negative consequence is that IBS generally provides poor weed control in the zone immediately around the planting row. In many cases, post sowing pre-emergent application (PSPE) is also being evaluated as it provides more uniform weed efficacy but requires herbicides or rates with improved crop safety together with reduced incorporation characteristics.
What was done?
At a site near Mullaley NSW, the grower had adjoining paddocks which he intended to plant with the same wheat variety (Crusader) but with two different planters; a tyned planter in one paddock and a single disc in the second. The trials were located within ~300m of each other.
Both trials were sprayed and planted on the 20/6/13, with a single tank of each treatment used to spray both trials. Planting occurred immediately after herbicide application.
Although the two trials were located within 300m of each other, there were distinct differences in the soil type. The soil type where the disc planter was used was a black vertosol with the tyne planter used in a lighter red soil.
The site had good planting moisture with ~48mm of rain received in the three weeks prior to planting. Table 1 shows the rainfall received between planting and the assessment of crop establishment, three weeks after planting. There was a total of 31 mm received in the first 9 days after planting. The only rainfall in the first week after planting was 4mm on day 4. It was not considered that this level of rainfall would have created a high risk scenario.
Table 1. Rainfall in first 21 days after planting
Date (days after planting)
Crop safety results
Figure 1 shows the wheat emergence data relative to the untreated. The actual plant population of the untreated in the disc sown trial was ~108 plants/m2 and ~66 plants/m2 when planted by tynes. NB the two planters were not set up to plant an equivalent rate of seed.
Figure 1. Wheat emergence as a % of untreated (11/7/13, 21 days after planting)
UTC = untreated control. All treatments applied in 70 L/ha total volume using AIXR110015 nozzles at 300 kPa * = significantly reduced wheat emergence compared to untreated within same trial
In the tyne planted trial, the mixture of 2.5 L/ha Boxer® Gold (800g ai/L prosulfocarb and 120g ai/L s-metolachlor) with either 1.5L/ha trifluralin 480g ai/L (eg TriflurX®) or 1.4 L/ha pendimethalin 440g ai/L (eg Stomp® 440) resulted in significantly reduced wheat emergence compared to the untreated. There was no significant difference between the untreated and any other treatment.
In the disc planted trial, all treatments significantly reduced wheat emergence compared to the untreated. TriflurX alone, Boxer Gold alone and all Boxer Gold mixtures also significantly reduced wheat emergence compared to 118g/ha Sakura® (850g/kg pyroxasulfone) or Stomp alone.
Depth of sowing
Depth of sowing can impact on crop safety with disc planted seed generally shallower than tyne plantings. Seedlings were dug up to measure the effective planting depth with the results shown in Table 2. Samples were also evaluated from the ’guess rows’ in the disc planted trial as it appeared that wheat emergence was less affected in those rows.
Table 2. Mean depth of sowing under different planting configurations
Disc ‘guess rows’
Mean depth (cm)
Depths shown are a mean from ~40 seedlings in each comparison
A PSPE application of Boxer Gold was also evaluated in both trials. This treatment was expected to cause the greatest level of crop damage as there was no removal of herbicide above the planting furrow.
This treatment actually resulted in significantly improved emergence counts compared to the IBS treatment in the disc sown trial. This indicated that the single disc setup was actually causing greater levels of crop damage by concentrating treated soil over the planting furrow rather than ‘removing’ treated soil.
Crop safety summary
- Wheat emergence was significantly reduced by all herbicide treatments when disc planting was used for IBS
- Sakura or Stomp alone were significantly safer than TriflurX or Boxer Gold alone or Boxer Gold in mixture with either TriflurX or Stomp when planted with discs
- Crop safety was dramatically improved when the tyned planter was used for IBS with only Boxer Gold plus Stomp or TriflurX 1.5L/ha significantly reducing plant stand
- Depth of sowing may have contributed to crop safety with the guess rows in the disc planted area appearing less affected although only marginally deeper (~2mm)
- Soil type may also have contributed to the varied level of crop affect between the two sites
In the tyne planted trial there was a population of ~13 ARG plants/m2 in the untreated. Similar levels of ARG control were achieved by all IBS treatments (~93-97% control) with the only exception being Stomp. Stomp provided significantly lower levels of control than the other IBS treatments at 94 days after planting (Figure 2). Boxer Gold applied PSPE in this trial also resulted in significantly poorer control than the IBS treatment.
In the disc planted trial there was a more variable population of ~7 ARG plants/m2. There was no significant difference in level of ARG control between any herbicide applied as IBS (~87-99% control). Boxer Gold applied PSPE provided similar levels of control to the IBS treatment (Figure 2).
Figure 2. % annual ryegrass control based on counts (22/9/13, 94 days after planting)
UTC = untreated control. All treatments applied in 70 L/ha total volume using AIXR110015 nozzles at 300 kPa
* = significant ARG control compared to untreated within same trial.
- High levels of ARG control were achieved by most IBS treatments
- The most consistent product were Boxer Gold or Sakura
- Weed control from Boxer Gold was significantly reduced in one of the two trials when applied by PSPE
Conclusions - winter cereal ‘at planting’ residual herbicides and crop safety
This project was conducted due to commercial crop safety concerns arising from the use of residual herbicides at planting for ARG control. These two trials highlighted some key points:
- Crop safety was significantly reduced when a disc planter was used for incorporation
- The disc setup appears to have exaggerated crop safety issues by planting seed in an area with increased herbicide concentration
- Observation suggested that small differences in planting depth may have impacted on crop safety in this scenario
- Additional trial activity in 2015 is showing similar crop safety results and reinforces the need to only use these herbicides with tyned planters
This work reinforces some of the difficulties growers and agronomists face with the use of residual herbicides. Crop safety and efficacy are influenced by a range of factors including planting equipment, planting depth, soil type, stubble load together with rainfall quantity and timing. As an industry we need to have a more thorough understanding of the impacts from these
(and perhaps other factors) to ensure we get the best from these important weed management tools.
Residual herbicides for fallow weed management 2.
NGA has been involved in a large number of projects to screen registered herbicides for residual control of ‘difficult fallow weeds’. Important target species have included fleabane, FTR and awnless barnyard grass - Echinochloa colona.
For both fleabane and FTR, the primary reason for evaluating residual herbicides was the high cost and difficulty of knockdown control of these weeds in the summer fallow. For awnless barnyard grass, it is broadly distributed, previously effectively controlled with glyphosate but we now have concerning levels of glyphosate resistant populations in the northern region.
Residual herbicide registrations in fallow
- 100g/ha isoxaflutole 750g ai/kg (eg Balance®)
- 0.86-1.2kg/ha of terbuthylazine 875 g ai/kg (eg Terbyne® Xtreme)
- Prior to sorghum only for knockdown and residual control: 700mL/ha FallowBoss® Tordon® (300g ai/L 2,4 D + 75g ai/L picloram + 7.5g ai/L aminopyralid) + 3-5L/ha atrazine 600g ai/L
Additional product registrations for in-crop knockdown and residual herbicide use, particularly in winter cereals, are still being sought. There are a range of commonly used herbicides, registered in winter cereal and broadleaf crops that can provide useful residual fleabane activity. Trial work to date has indicated that increasing water volumes from 50-100L/ha may help the consistency of residual control with application timing to ensure good herbicide/ soil contact also important.
Feathertop Rhodes grass
- 100g/ha isoxaflutole 750g ai/kg (eg Balance)
Additional product registrations for in-crop and fallow use are still being sought. Similar to fleabane, there are a range of commonly used herbicides that provide useful residual FTR activity. Other herbicides used to provide residual control of summer grass weeds have been noted to reduce emergence of FTR.
Awnless barnyard grass
- 150-200mL/ha imazapic 240g ai/L (eg Flame®)
- 100g/ha isoxaflutole 750g ai/kg (eg Balance®) – suppression only
There are a number of registered active ingredients that provide residual ‘in-crop’ control of awnless barnyard grass from an ‘at planting’ application. These include
- 2kg/ha atrazine 900g ai/kg in sorghum and 2.5-3.3kg/ha in maize
- 1-2L/ha Dual® Gold (960g ai/L s-metolachlor) in Concep II sorghum seed safener treated sorghum, maize, soybeans and sunflowers and 1L/ha in cotton
- 1.8-2.2L/ha Stomp® Xtra (pendimethalin 455g ai/L) in mung beans, soybeans and sunflowers and 2.2L/ha in cotton when pre plant incorporated
- 1.2-1.7L/ha trifluralin 480g ai/L (eg TriflurX®) in mung beans and sunflowers and 1.2-2.3L/ha in soybeans and cotton
Optical weed sprayers
One of the reasons knockdown herbicides are generally preferred over residuals is the consistency and robustness of control. Effective management of any weed species is heavily focussed on reducing the seed bank ie stopping any weed from setting seed.
Even under ideal conditions, it is highly likely there will be survivors from residual herbicide use in fallow partly due to the absence of crop competition. Use of a residual herbicide in fallow frequently suits a follow up with an optical spray operation. An alternative use for this technology is to apply the residual herbicide through an optical sprayer when a fallow knockdown is being applied. This approach can limit the area of application of the residual (and reduce potential crop effect issues) but also focus the residual herbicide in the patches with highest potential for subsequent weed emergence.
Conclusions - residual herbicides for fallow weed management
Our management issues with fleabane, FTR and awnless barnyard grass have all in part stemmed from an over reliance on glyphosate in the summer fallow. To successfully manage these weeds the industry must start 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 and even salvage burning together with a wider range of herbicide options eg residual herbicides as well as double-knock strategies to effectively manage these weeds but also ensure we don’t lose glyphosate completely.
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.
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.
Our sincere thanks to Josh Bell for his patience and co-operation with the annual ryegrass trials and Aaron Goddard (Landmark) for trial site assistance.
Northern Grower Alliance
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GRDC Project code: NGA00003
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