Pre-emergence weeding is often unavoidable, especially when caring for grasses against which post-emergence weeding is not always sufficient. But many questions arise. How can this intervention be more successful and what is the techno-economic optimum if we want to limit investment in sowing?
Blackgrass and ryegrass infestations are still progressing and becoming a priority target in cropping systems. (©Terres Inovia)
Strong infestations black gray and with rye grass can limit crop biomass at the beginning of the cycle, which is nevertheless necessary for good plant behavior against bug larvae.
In addition to everything agronomic levers known in the scale of crop rotation and inevitable (rotation with live crops, outdated beds, periodic plowing), it is necessary, in rapeseedto correctly assess the technical, economic and environmental problems associated with grass management.
Avoid early postemergence against ryegrass
Application of special mortar/belkar in after stairs generally solved the problems dicotyledonousespecially strong infestations geraniums and with underlings. This choice also allows forgoing investment in the pre-harvest period, a decision that is further motivated by the risk of drought that could lead to crop changes.
Dry soil conditions direct growers to early post-emergence with pre-emergence products such as Novall or Alabama at the first cotyledon stage. In both cases, the fight against broad-leaved weeds becomes more effective.
But just as before emergence in dry conditions in early August, weed management is lacking despite application propizamide at the beginning of winter. Indeed, a failed preemergence action means that the graminicidal action will be even more propizamide based (Kerb Flo, etc.) with potential dissatisfaction and water quality impacts that we want to keep as low as possible.
If a good level of effectiveness against is maintained early after emergence foxtailprovided that the herbicide based on metazachlor and application is carried out in early September, it is significantly different from rye grass. Indeed, this grass comes up very early, at the same time as rape. The root and anti-sprouting action of the herbicide then stops on the weeds at the time of emergence.
Foliar graminicides are mainly used against cereal grain crops (with early application of about 2-4 leaves of rapeseed) or wild oats. Using ryegrass and foxtail is a solution when it still works. It must be exceptional (recovery from a failed previous harvest) to prevent resistance settles In this case, and to reduce this risk, use a propizamide-based herbicide (eg Kerb, Ielo, etc.). Herbicides on the basis kletodyme (Centurion, Ogive VXT, Foly R, etc.) are somewhat less affected by resistance.
How to manage the pre-emergence period in dry conditions?
It should be remembered that napropamide (colzamid, etc.) in included before installation is currently the most effective remedy against foxglove and ryegrass.
As part of herbicides collected alone, chloracetamides act against grasses: metazachlor, dimetachlor, dmta-P, and pentaxamide. In the pre-emergence period and against foxtail, a drug based on metazachlor (Novall, Alabama, Rapsan TDI, Sultan, Bandoneon, etc.) is necessary.
Summary of forexgrass and ryegrass seed application performance based on Terres Inovia trials (©Terres Inovia)
When soil moisture conditions favorable to the effectiveness of root herbicides (precipitation during the summer before application, possibly rain after application), Terres Inovia trials show a 50-80% reduction in foxtail and ryegrass infestations.
On the other hand, when conditions is dry (before and after application), the efficiency drops sharply and ranges from 10 to 40%. Ago how to optimize the effectiveness of these solutions?
To control foxtail, metazachlor-based metazachlor can be applied to canola seedlings early after emergence (canola at the cotyledon stage beginning to mark the row) and during foxtail pre-emergence, i.e. in early September. In tests conducted by Terres Inovia in 2021 and 2022, delaying application saved 20 to 30 points in the effectiveness of black grass.
The situation is more delicate with the fight against ryegrass, which emerges at the same time as rape. in sedimentation therefore, from the pre-emergence period to the wetter period is more dangerous. At best, even if seeding is done in early August in dry conditions, we will wait for the rains to return to apply a pre-emergent herbicide (quickly before canola and ryegrass emerge). Then the herbicide will be more effective and more stable.
Detection of active substances such as metazachlor, dimetachlor, propyzamide or their metabolites in water is detrimental to the entire canola sector. From the prescriber to the user, everyone is responsible for the longevity of these highly strategic decisions in weed control. To benefit from it tomorrow, it is necessary to respect good usage practices adapted to the local context.
Limit the investment to the seed to avoid too much loss in case of a reversal
Faced with the uncertainty of crop establishment (late emergence and early attacks of adult bugs), one may be tempted to skip pre-emergent treatments and invest in herbicides only after successful implantation is diagnosed (regular emergence by 5-Sep-10). It is very easy against broadleaf weeds, especially with Mozzar after emergence.
however, cul-de-sac is not recommended with medium or high grass pressure. Therefore, it is necessary to support the pre-emergence application, but it is possible to optimize its application. As mentioned earlier, early post-emergence is possible only in the fight against blueberries.
Limiting seed investment means choosing the simplest pre-emergence herbicide (or incorporated Colzamid before seed) that remains optimal in weed control. On ryegrass, solutions based on metachlor (Sultan, Rapsan, Springbok) or dimetachlor (Therox, Kolzor Uno) are preferred. For foxtail, it is better to stop at solutions based on metachlorine. Then the cost will be from 30 to 45 euros/ha. The rest of the investment will be made when the facility is successful, against cereals, broadleaf weeds, or to “finish” grass control with propizamide (Kerb Flo, etc.)
The different strategies are summarized in the table below :
Maintain good conditions for the effectiveness of propizamide
This year there has been much displeasure with the effectiveness of propizamide. The first question raised is that resistance. However, in fact, propysamide-resistant populations of foxglove or ryegrass were not found in field crops. The monitoring carried out by the companies also points in this direction. Efficacy defects are most often attributed to larger and more developed populations (thick root hair, deep rooting) during early winter applications of propizamide.
for optimize efficiency of the active substance, application should be carried out in moist and cool soil (soil temperature < 10°C) in November (preferably) or even in December, as well as on poorly developed weeds. It is best to avoid application before heavy rainfall to limit the impact on water quality. Finally, you have to be patient because the effectiveness of propizamide is a long-distance race. The effectiveness is judged 3 months after application. In regulatory matters, as in water quality matters, single application is the rule of thumb
Another reminder about the rules:
In its technical instruction dated May 27, 2021, the “controller’s manual”, DGAL writes the following instruction: “The dose and maximum number of applications apply to all commercial specialties, the composition of which is strictly identical. With the exception of special cases (), this cumulative use of PPE containing the same active substance leads to exceeding the maximum permissible dose of use, and therefore is not allowed. »
This means that two propizamide-based products cannot be used consecutively, even if the products have different MA numbers (eg: Curb Flo followed by Barclay Propiz).
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