Arable News

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Early weed control strategies in oilseed rape

For any crop,good weed control is vital in ensuring maximum profitability as well ascontributing to an overall anti resistance strategy says Dr David Ellerton,Hutchinsons Technical Development Director.

Managingresistant weeds

With everincreasing resistance in a range of grass weeds to ALS inhibitors includingsulphonylureas (such as mesosulfuron/iodosulfuron, flupyrsulfuron andpyroxsulam) and to ACCase inhibitors, such as fenoxaprop and clodinafop, weedcontrol in cereals is becoming increasingly difficult.

However, the presence of break crops(e.g. oilseed rape and field beans) in the rotation does present theopportunity to use a wider range of products with alternative modes of actionas part of a rotational anti-resistance strategy.

Autumn 2014 showed the benefit ofcreating stale seedbeds, encouraging germination of a range of volunteers andgrassweeds prior to drilling winter cereals – which could then be controlledwith applications of glyphosate based products and further cultivations.

This approach not only reduced thegrassweed and volunteer cereal populations, but also destroyed a significantnumber of resistant plants.

However, due to early drilling of thecrop, oilseed rape growers do not usually have the same opportunity he noteswhich often leads to weeds emerging within the crop, resulting in earlycompetition and consequent problems with their control.

Nevertheless, if we are in a positionthis autumn where there is adequate moisture in seedbeds, then this could leadto rapid grass weed germination and give growers the opportunity to burn offweeds prior to oilseed rape drilling.

Every effortshould be made this autumn to utilise this vital cultural control method if atall possible.

While focused on the control of grassweeds and optimising performance from residual graminicides, a clear benefitfrom utilising the Micro-Wing establishment system is an overall reduction inweed populations, both grass and broad leaved. We have seen significantreductions in charlock, cranesbill and cleavers emerging within the crop atBrampton, he adds.

Pre-emergenceherbicide use

Dr Ellerton reminds growers that thefirst opportunity to apply selective herbicides in oilseed rape occurs withinthe first 48 hours after drilling (before the seed has the opportunity to chit)with the use of certain metazachlor based products.

If this opportunity is missed,application will have to wait until the fully expanded cotyledon stage of thecrop.

He advises that whilst there are a rangeof products are available, product choice should be guided by the expected weedspectrum.

Metazachlor alone will control weedssuch as chickweed, mayweed and speedwells and offers a limited effect on blackgrass. The addition of dimethanamid-P will give improved control of cranesbill,poppy, shepherds purse and charlock.

Another option is the combination ofquinmerac and metazachlor which adds cleavers, poppy, speedwells and reddead-nettle to the metazachlor susceptible weeds, he says.

A further alternative is to tank mixmetazachlor based products with clomazone which offers additional control ofcleavers, fools parsley, hedge mustard, shepherds purse and chickweed.

However it is important to remember thatwhile metazachlor may be applied post emergence of the crop, clomazone shouldonly be used as soon as possible after sowing, pre-emergence of crop and weed,or serious crop damage can result.

He adds that other potential activeingredients include the ALS inhibitor imazamox, as found in Cleranda, as ameans of controlling weeds such as charlock and oilseed rape volunteers inClearfield varieties of oilseed rape.

With this latter product it is vitalthat growers ensure the crop they are treating is a Clearfield variety beforespraying or they will destroy the crop. It is also essential to plan a strategyto control Clearfield volunteers in following crops as they will be unaffectedby ALS products which are often the usual methods of control.

Environmental impact management

One consideration to bear in mind whenapplying some of these products is that metazachlor is regularly being reportedat levels above that permitted under the Drinking Water Directive (DWD).

Therefore, growers should be aware of therestrictions placed on the label of metazachor based products limitingapplications to a maximum individual dose of 750 gai/ha and a total dose of1000 gai/ha over a three year period in order to avoid metazachlor exceedingthe DWD limit, he says.

In addition to this, stewardshipguidelines have recently been issued to try to further reduce the risk of peaksappearing in water. This covers a number of strategies including use of a 6mgrass buffer strip or 5m no-spray zone next to water, employing minimum tillagetechniques moving only the top 4-6 cm, not applying if drains are flowing orheavy rainfall imminent and avoiding the use of metazachlor on drained fieldsafter the end of September.

A further approach is to replace some ofthe metazachlor with active ingredients such as dimethanamid or quinmerac, orutilize an alternative to metazachlor from the same group of chemistry, such asdimethachlor.

He adds that many growers will beapplying metazachlor at a similar time to slug pellets, where metaldehyde iscausing similar problems in water.  Theuse of these active ingredients combined with issues around autumn appliedpropyzamide and carbetamide means that oilseed rape growers, in particular,will have to pay considerable attention to avoiding contamination of water thisautumn, if we are to avoid further restrictions on the use of these products.

The Micro-Wing establishment technique,developed at Brampton, has a major and positive impact in reducing the passageof herbicides and metaldehyde to field drainage systems. This improves theoverall performance by retaining active ingredient where it can deliver bestperformance and reducing the incidence of drain and groundwater contamination.

Avoidingweed competition

Post emergence, volunteer cereals andsusceptible grassweeds should be controlled as early as possible from the 1 to2 leaf stage of the weed, before they begin to compete with the crop.

Effective products include a number ofestablished graminicides such as fluazifop, propaquizafop, tepraloxydim orcycloxydim. A relatively recent addition to the armoury of grass weed activesis clethodim which has performed well in Hutchinsons trials, particularly onblack grass.

However it should be remembered thatthis product has major restrictions on tank mixing and sequence gaps, as wellas a cut off for application at the end of October in order to avoid potentialcrop damage.

Where grass weeds in a particular fieldare known to suffer from a high degree of resistance to fops and dims, analternative to graminicides is to apply a low rate of carbetamide incombination with a silicon based wetter from mid-September, at the 3 to 4 leafstage of the crop, to keep the grassweeds in check, followed by an applicationof propyzamide when soil conditions are suitable, normally from the start ofNovember.

Dr Ellerton believes that the strength ofHutchinsons work at Brampton is that field based issues are demonstrated and,as many growers are now aware, late February germinating black grass is becominga very real issue in WOSR crops. Sequencing planned applications of carbetamideinto February is having a noticeable impact on the development of these latewinter emerging plants.

Complete absence of emerging weeds isalways the preferred outcome, but our work is highlighting the fact that blackgrass emergence will occur in peak germination periods and we must expect andplan for that. Minimising the impact of direct competition and seed return aresound objectives in a long term control strategy, whereas 100% control is not.


  • Written by: Farmers Guide
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