As featured in Arable Farming Magazine March 2021
Desmedipham loss puts focus on pre-ems
by Arable Farming
Greater use of residual sprays and the need for careful timing of post-emergence treatments are inevitable consequences of weed control without desmedipham.
After a season when sugar beet crops were ravaged by virus yellows, growers will be doing everything possible to help speed growth and propel crops towards adult plant resistance.
David Allison, Frontier Agriculture sugar beet crop protection specialist, says: “Recent conversations about sugar beet have been dominated by virus yellows.
This is understandable, but it shouldn’t distract from the need to consider weed control without desmedipham.
“For more than a generation, the likes of Betanal maxxPro and other desmedipham-containing products made weed control easy.
These products delivered a broad-spectrum of activity and high crop safety.
Without desmedipham weed control at the pre-emergence stage will need to be better and the first post-emergence application delayed until the crop is at the expanded cotyledon stage.
This may involve more regular inspections if the spray is to be well timed,” he adds.
Although strategies will need to evolve in response, growers should not despair, says Mr Allison.
“It is important to consider how programmes can be adapted to avoid placing undue pressure on phenmedipham and ethofumesate.
On land known to harbour a high weed burden, a pre-drilling application of Roundup [glyphosate] may be the best policy, while a robust residual will almost certainly be necessary to provide control through until the first post-em spray can be applied.”
For some growers there will be the temptation to return to straights, but the workload these exert on the sprayer operator mean this is unlikely to be the favoured approach of many.
For those who prefer the convenience of a formulated product, there is a choice of Betanal Tandem or Powertwin, both containing phenmedipham + ethofumesate, but in many situations, these will need a residual such as metamitron to bolster control, Mr Allison says.
Last year, the introduction of varieties tolerant to the acetolactate synthase (ALS) group of herbicides introduced a new weed control system under the Conviso Smart brand.
This promised to simplify weed control while expanding the range of weed species which could be reliably controlled.
“The loss of desmedipham is perhaps the catalyst needed for growers to take a closer look at what Conviso Smart has to offer.
“It certainly adds to control but should not be used in isolation.
In my experience the Conviso One herbicide fits best at the end of a programme comprising classic herbicides of both a residual and contact nature,” Mr Allison adds.
Benefits beyond weed control
With sugar beet growers facing their first full season without desmedipham, 2021 could be the year the sector witnesses a resurgence in the use of metamitron and quinmeracbased pre-emergence herbicides.
That is according to Bill Lankford, herbicides technical specialist at Adama UK, who says that as well as giving weed control an early season kick-start, the inclusion of a pre-emergence treatment as part of a sequenced herbicide programme can also boost crop yields.
He says: “With desmedipham no longer available, sugar beet growers aren’t entirely able to depend on post-emergence treatments alone to achieve the necessary level of weed control.
“Herbicide programmes will therefore need a little more thought this year.
Unfortunately, the choice of pre-emergence herbicides is limited due to the revocation of chloridazon and the removal of pre-emergence usage from the lenacil label.” Goltix (700g/litre metamitron) and Goltix Titan (525g/litre metamitron and 40g/litre quinmerac) can fill the gap, says Dr Lankford, with the inclusion of quinmerac in the latter protecting against several of the more difficult to control weeds, such as cleavers, orache and bindweed.
Because modern sugar beet varieties are so quick to germinate, pre-emergence herbicides should ideally be applied within 24-48 hours of drilling and certainly within an absolute maximum of five days post-drilling, Dr Lankford adds.
And while he recognises there might be some reticence in terms of adding a pre-emergence treatment to herbicide programmes – due to the additional input cost, extra workload and the need for adequate soil moisture to make pre-ems work effectively – he believes that the pros often outweigh the cons.
In addition to the potential yield boost – use of Goltix Titan delivered a yield benefit of 7.5-21 tonnes/hectare over untreated crops in Adama trials – pre-ems can create some latitude in the timing of post-em treatments and workload management advantages, says Dr Lankford.
Concerns over weed control in challenging conditions
The loss of desmedipham is a big blow for weed control in sugar beet, especially as alternative products generally lack the same efficacy in more challenging conditions, says Hutchinsons’ root crop technical manager Darryl Shailes.
Targeting weeds early at the cotyledon stage is key to effective control and minimises potential yield losses from crop competition.
“However, last spring showed how difficult this can be in practice when very dry conditions hinder the efficacy of residual chemistry and weeds are able to grow away in gappy crops,” says Mr Shailes.
Controlling larger weeds such as fat hen will be ‘virtually impossible’ in some conditions, so growers must focus on using robust residuals and pre-emergence treatments this spring and target smaller weeds post-emergence, he adds.
He suggests products based on ethofumesate + metamitron, or quinmerac + metamitron, with ethofumesate added where grass-weeds are a concern.
Post-emergence, where brassica weeds and polygonums are a problem, then lenacil + triflusulfuron is likely to be needed.
“Targeting smaller weeds is likely to take more agronomist involvement and increase the number of applications and cost for growers,” Mr Shailes adds.