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Why post-emergence herbicides still have a place

  • Control levels vary by field situation
  • Still effective against weeds with metabolic resistance, particularly when plants are small
  • Any reduction in black-grass head numbers reduces seed return  

Spring is the final stage of the wheat herbicide programme. Contact-acting herbicides like Monolith (mesosulfuron + propoxycarbazone) and its predecessor Atlantis WG (mesosulfuron + iodosulfuron) are the last chance to control black-grass in wheat crops, other than hand-pulling or resorting to spraying off the entire crop.

Recent seasons have seen growers focus their chemical weed control efforts at the pre-emergence timing while, using a post-em herbicide has fallen out of favour with some growers and agronomists. We examine the reason for this change and what value a post-em can still bring to the herbicide programme.

Resistance appears

Since its launch 15 years ago, there’s no denying that numerous farms have seen a decline in performance from Atlantis WG (mesosulfuron + iodosulfuron) and other mesosulfuron based products. When it first appeared on the market, control above 95% was commonplace – it appeared to be a miracle product getting near total control of grass weeds in a cereal crop.

These exceptionally high levels of control couldn’t be maintained but why has there been a drop in performance and why has it not affected all farms in the same way?

“There’s clearly been a build-up of resistance to mesosulfuron in black-grass,” says Bayer Development Manager Dr Gordon Anderson-Taylor. “Interestingly, the main form of resistance is enhanced metabolism rather than target-site – although we do see some target site resistance as well.”

Target site resistance and enhanced metabolism resistance explained:

Target Site: Within the cells of a weed, the active substance has a specific ‘target site’. The herbicide binds to the target site causing a chain of events that results in plant death. With target site resistance a mutation in the form of the target site means the herbicide can no longer interact with it in the same way. A simple analogy is a lock and a key – if you change the lock then the old key no longer works.

Enhanced metabolism resistance (EMR) is about the ability of the plant to identify and metabolise (digest into harmless substances) the herbicide before it has the chance to take full effect. EMR exists on a spectrum from reduced sensitivity to the herbicide all the way to very strong resistance.

When Atlantis was first launched there was some discussion among weed scientists about which form of resistance was likely to be a bigger problem. As it turned out, enhanced metabolism resistance became the most troublesome.

“I think the reason enhanced metabolism resistance is more of a problem is because it is not specific to mesosulfuron,” says Dr Anderson-Taylor. “It can provide resistance to other herbicides too, so it can slowly build throughout the rotation. On the other hand, target site resistance is highly specific, so even though the range of crops and herbicides used in the UK is fairly narrow, there has been enough diversity to keep target site resistance in check on many farms.”

Added to that, it is important to remember that the type and degree of resistance you see on farm depends on the genetic make-up of the original population. Herbicides select from what is already there, they don’t ‘create’ resistance, but they do allow it to become a very common trait unless other modes of action or cultural control methods are used.

What level of control is still achievable?

Variation in the resistance status of populations mean there is no easy answer to this. In a series of trials running from 1999–2016 the average control from Atlantis WG started at over 90% but step-by-step reduced to around 50%. However, the average figures masked the fact that from 2012 onwards there were two distinct groups; one with control below 50% and, another that still got at least 70% and above. 

In either case, the post-em can still provide useful levels of control within the herbicide programme. Newer mesosulfuron products such as Monolith for spring and Atlantis OD (mesosulfuron + iodosulfuron) for autumn offer a further 5–10% improvement.

“It’s important to look at how the post-em fits within the herbicide programme,” says Dr Anderson-Taylor. “The recent trend has been to focus spend and effort at the pre-em timing with fairly substantial stacks and sequences being used. But at pre-em it is a case of diminishing returns. The first product like Liberator (flufenacet + diflufenican) does the heavy lifting, the next product might give an additional 10%, and then the next 5% and so on.

“At some point, using a post-em will control more than simply stacking more products at pre-em and if you look closely at the numbers that might come earlier than some people think.”



Focus on reducing seed return

Controlling black-grass in wheat provides an immediate benefit by improving yield but long-term the aim is to degrade the seedbank by minimising seed return. One reason black-grass has become so prevalent is that it is still possible to get a decent crop of wheat despite black-grass headcounts of more than 100/m2. However, such high counts are returning huge amounts of seed to the soil and eventually will make the crop uneconomic.

Cutting seed return means there is less risk from weeds in future crops so that farmers can focus on margin rather than weed control. Post-em herbicides are a useful tool to reduce seed return even; even on tough populations using a post-em can reduce the headcounts by 30–50% which equates to a huge number of seeds across a field. Each head can produce between 80–150 viable seeds, so reducing a headcount of 50/mto 35/mequates to over 1000 fewer seeds per metre or 10 million per hectare!

When looked at through the lens of seed return, the small improvement in control offered by Monolith can translate into a significant reduction in the number of seeds returned. In trials, the average control of black-grass from Monolith was 75.3% whereas, Atlantis averaged 66%. Using the example of a starting headcount 50/magain applying Monolith instead of Atlantis would result in 4 fewer heads or 300–600 seeds per square metre. This demonstrates that small improvements in control across the herbicide programme can add up to big differences in the weed burden in the soil.

“Clearly in a situation where the pre-emergence residual herbicide application has left a high population of survivors it may be appropriate to spray off dense patches when these are detected,” says Dr Anderson-Taylor. “In this case as a selective post-emergence treatment is unlikely to prevent significant seed return because the plant numbers are simply too high.

“When a selective post-em treatment is used, it is important to continue to maximise control with aggressive management of stubborn patches. Spraying off remaining patches later in the season and/or hand-rogueing should be the final stage of the weed control programme when heads appear above the canopy in May to cut down viable seed return even more.”

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