Using a post-emergence herbicide is, for many, the final step in an integrated weed management plan. It’s the last opportunity, bar hand-rogueing or spraying off the crop with glyphosate, to control black-grass and minimise seed return for the following season.
While levels of control from post-emergence herbicides have dropped in recent seasons as resistance has increased, they are still a really important final part of programmes – every black-grass plant removed means you are getting that one step closer to the 97% control you need to make sure the problem is not getting worse in the following crop.
There are also a number of steps you can take to get the best from any post-emergence application. Here are four tips to get the best out of any post-emergence herbicide spray for black-grass control.
Resistance to post-emergence herbicides is becoming a widespread problem, so the mode of action chosen to target your black-grass should be chosen carefully. If you are concerned about resistance, consider getting a resistance test to better understand your population. Fops and Dims were widely used in the 80s and 90s but target site resistance to these actives is common.
ALS-inhibitors such as Atlantis OD (mesosulfuron + iodosulfuron) and Monolith (mesosulfuron + propoxycarbazone) are also affected by resistance. The effects of the most usual type of resistance, enhanced metabolism, can be reduced by applications at early growth stages when plants can’t metabolise the herbicide as effectively.
The newer formulations of mesosulfuron-based products such as Atlantis OD, Monolith or Hamlet (mesosulfuron + iodosulfuron + diflufenican) offer improved control of black-grass over Atlantis WG (mesosulfuron + iodosulfuron). For example, typically Monolith outperforms Atlantis WG applied at the same time by about 10%. While these differences are relatively small, every percent matters when it comes to black-grass control and seed return, so it’s an improvement worth having.
Contact post-emergence herbicides should be applied when weeds are actively growing and, ideally, at growth stage 11-13 for Atlantis and 21-32 for Monolith. Contact herbicides are absorbed through the leaves of the plant, so application should be made to a dry leaf, with no wet weather forecast for at least two hours – ideally more – for drying.
Temperature is also important as it determines active growth. Generally, applying before a cold spell works better than applying immediately after. When coming out of a cold period it is best to wait until growth has visibly started rather than spraying on the first warm day.
Post-em treatments should only be delayed when the weed is not actively growing such as during an extended period of cold weather, or when the crop is stressed. Delaying applications allows weeds to get bigger and more likely to survive herbicide applications.
The condition of your sprayer is important because post-em applications work by targeting the leaf surface which needs to be a precise application. At growth stage 11-13 black-grass weeds can be hard targets due to their small size, vertical stature and hairless, waxy surface.
During application, forward speeds should not exceed 12kph to ensure an even coverage and spray droplets should be fine to medium sized, ideally from a flat fan nozzle to increase the likelihood of leaves retaining the spray. Correct sprayer set up is even more crucial where water volumes are reduced below 200 L/ha.
Post-ems should always be applied at the full rate to help delay the development of enhanced metabolism resistance. To get the best from your post-em, always use the adjuvant recommended. This will optimise efficacy and give a more consistent performance.
Tank mixtures should always be supported by the manufacturer and should not contain a high number of different components. The more components used, the less effective they are each likely to be.
At post-emergence a half rate of Liberator (flufenacet + diflufenican) or other residual herbicide mixed with Atlantis OD can be applied if conditions are dry or where black-grass germination is delayed in autumn to control plants emerging in the crop and also extend residual protection across the winter.
Combining other chemical actives including fungicides and growth inhibitors at post-emergence can have a number of risks because the wetters and stickers found in these can reduce the efficacy of the herbicide and falter leaf uptake. They can also increase the risk of crop damage. In particular, chlorothalonil should not be mixed with Atlantis OD or Monolith.
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