Pre-emergence weeding is often unavoidable, especially in the management of grasses against which post-emergence is not always enough. But many questions arise. How can this intervention be more successful and what is the technical and economic optimum if we wish to limit the investment to sowing?
Blackgrass and ryegrass infestations are still progressing and are becoming a priority target in cropping systems. (©Terres Inovia)
Heavy infestations of vulpins and of ray-grass can, at the start of the cycle, limit the biomass of the crop, which is nevertheless necessary for the good behavior of the plant in the face of flea beetle larvae.
In addition to all agronomic levers known at the scale of the rotation and unavoidable (rotation with spring crops, stale seedbeds, occasional ploughing), it is imperative, in rapeseedto properly assess the technical, economic and environmental issues related to the grass management.
Avoid early post-emergence against ryegrass
The use of the specialty Mozzar/Belkar in post emergence has generally solved the problems of dicotsespecially heavy infestations of geraniums and of gaillets. This choice also makes it possible to disinvest in pre-emergence, a decision that is all the more motivated by the risk of drought which can lead to crop reversal.
Dry soil conditions have steered growers towards early post-emergence with pre-emergence products such as Novall or Alabama applied at the first-leaf cotyledon stage. In both cases, the fight against broadleaf weeds becomes more effective.
But just like a pre-emergence in dry conditions at the beginning of August, the management of grasses is lacking, despite the applications of propyzamide at the onset of winter. Indeed, a failing pre-emergence means basing the graminicidal action even more on propyzamide (Kerb Flo, etc.) with potential dissatisfaction and impacts on water quality that we want to be as limited as possible. .
If early post-emergence maintains a good level of efficacy against vulpinprovided that the herbicide is based on metazachlor and that the application is carried out at the beginning of September, it is quite different against the ray-grass. Indeed, this grass rises very early, at the same time as rapeseed. The root and anti-germination action of the herbicide then stalls on weeds during emergence.
Foliar graminicides are mainly used against volunteer cereals (in early application around 2-4 rapeseed leaves) or wild oats. Use on ryegrass and foxtail is a solution, when it still works. It must be exceptional (recovering from a failing pre-harvest) in order to prevent the résistance settles down. In this case and to reduce this risk, then apply a propyzamide-based herbicide (such as Kerb, Ielo, etc.). Herbicides based on clethodim (Centurion, Ogive VXT, Foly R, etc.) are somewhat less impacted by resistance.
How to manage pre-emergence in dry conditions?
It should be remembered that napropamide (Colzamid, etc.) in pre-plant incorporated is by far the most effective solution against foxgrass and ryegrass.
In the herbicides of collected alone, chloroacetamides offer an action against grasses: metazachlor, dimetachlor, dmta-P and pethoxamide. In pre-emergence and against foxtail, a product based on metazachlor is essential (Novall, Alabama, Rapsan TDI, Sultan, Bandoneon, etc.).
Summary of efficacy on foxgrass and ryegrass of sowing applications based on Terres Inovia trials (©Terres Inovia)
When the soil moisture conditions are favorable to the effectiveness of root herbicides (rainfall during the summer before application, possibly rain after application), the Terres Inovia trials show that infestations of foxtail and ryegrass are reduced by 50 to 80% .
On the other hand, when the conditions are dry (before as well as after application), the efficiencies drop sharply and are between 10 and 40%. So how to optimize the effectiveness of these solutions?
To fight against foxtail, metazachlor-based preemergence can be applied to rapeseed early post-emergence (rapeseed at the cotyledon stage which begins to mark the row) and during preemergence of foxtail, that is to say at the beginning of September. In the trials conducted by Terres Inovia in 2021 and 2022, delaying the application saves 20 to 30 points of effectiveness on blackgrass.
To fight against ryegrass which emerges at the same time as rapeseed, the situation is more delicate. the deposition from pre-emergence to a wetter period is therefore more perilous. At best, even if sowing is done at the beginning of August in dry conditions, we will wait for the return of the rains to apply the pre-emergence herbicide (quickly before the germination of rapeseed and ryegrass). The herbicide will then be more effective and more persistent.
Since April 2021, the conditions of use of the metazachlor limit this active substance to a maximum application of 750 g/ha every 4 years or a maximum application of 500 g/ha every 3 years. The calculation is based on rapeseed sowing in 2021.
The detection of active substances such as metazachlor, dimetachlor, propyzamide or their metabolites in water is detrimental to the entire rapeseed sector. From the prescriber to the user, everyone is responsible for guaranteeing the durability of these highly strategic solutions in the fight against grasses in particular. To benefit from it tomorrow, it is essential to respect good usage practices adapted to the local context.
Limit the investment to sowing to avoid too much loss in the event of a reversal
Faced with uncertainties about the establishment of the crop (late emergence and early attacks of adult flea beetles), one may be tempted to skip pre-emergence and only invest in herbicides once the diagnosis has been made. successful implantation (regular emergence before September 5-10). This is very easy against broadleaf weeds, especially with Mozzar post-emergence.
However, deadlock is not advised in medium or high grass pressure. It is therefore necessary to maintain a pre-emergence application but it is possible to optimize its application. As mentioned earlier, early post-emergence is only possible in the fight against blackgrass.
Limiting the investment to sowing means choosing a very basic pre-emergent herbicide (or Colzamid incorporated pre-sowing) which remains optimal in the fight against grasses. Solutions based on metazachlor (Sultan, Rapsan, Springbok) or dimetachlor (Terox, Colzor Uno) are preferred on ryegrass. On foxtail, it is better to stay on metazachlor-based solutions. The cost will then be between 30 and 45 €/ha. The rest of the investment will be made once the establishment is successful, against volunteer cereals, broadleaf weeds or to “finish” the control of grasses with propyzamide (Kerb Flo, etc.)
The different strategies are summarized in the table below :
(© Terres Inovia)
Maintain good conditions for propyzamide efficacy
This year, many dissatisfactions on the effectiveness of propyzamide have been raised. The first question posed is that of résistance. But in fact, no populations of foxgrass or ryegrass resistant to propyzamide have been discovered in field crops. The monitoring carried out by the firms also goes in this direction. Efficacy defects are most often explained by more numerous and more developed populations (dense root hair, deep rooting) at the time of application of propyzamide at the start of winter.
Pour optimize efficiency of the active substance, the application must be done in moist and cool soil conditions (soil temperature < 10°C) in the months of November (preferably) or even December at the most, and on poorly developed weeds. It is best to avoid applications before heavy rainfall to limit the impact on water quality. Finally, you have to know how to be patient because the effectiveness of propyzamide is a long-distance race. The effectiveness is judged 3 months after application. In a regulatory concern, as in a concern for water quality, the single application is the basic rule
Another reminder of the regulations:
In its technical instruction of May 27, 2021 “controller’s vademecum”, the DGAL writes the following instruction: “The dose and the maximum number of applications apply to all commercial specialties whose composition is strictly identical. Except in special cases (), this cumulative use of PPPs containing the same active substance leads to the maximum authorized application dose being exceeded and is therefore not permitted. »
This means that it is forbidden to successively apply two propyzamide-based products even if these products have a different MA number (eg: Kerb Flo then Barclay Propiz).
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