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Early Weed Control Strategies in Oilseed Rape

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dr david ellerton hutchinsons optimFor any crop, good weed control is vital in ensuring maximum profitability as well as contributing to an overall anti resistance strategy says Dr David Ellerton, Hutchinsons Technical Development Director.

Managing resistant weeds

With ever increasing resistance in a range of grassweeds to ALS inhibitors including sulphonylureas (such as mesosulfuron/iodosulfuron, flupyrsulfuron and pyroxsulam) and to ACCase inhibitors, such as fenoxaprop and clodinafop, weed control in cereals is becoming increasingly difficult.
However, the presence of break crops (e.g. oilseed rape and field beans) in the rotation does present the opportunity to use a wider range of products with alternative modes of action as part of a rotational anti-resistance strategy.

Autumn 2014 showed the benefit of creating stale seedbeds, encouraging germination of a range of volunteers and grassweeds prior to drilling winter cereals - which could then be controlled with applications of glyphosate based products and further cultivations.

“This approach not only reduced the grassweed and volunteer cereal populations, but also destroyed a significant number of resistant plants.”
However, due to early drilling of the crop, oilseed rape growers do not usually have the same opportunity he notes which often leads to weeds emerging within the crop, resulting in early competition and consequent problems with their control.

“Nevertheless, if we are in a position this autumn where there is adequate moisture in seedbeds, then this could lead to rapid grassweed germination and give growers the opportunity to burn off weeds prior to oilseed rape drilling.”

Every effort should be made this autumn to utilise this vital cultural control method if at all possible.

“While focused on the control of grass weeds and optimising performance from residual graminicides, a clear benefit from utilizing the ‘Micro-Wing’ establishment system is an overall reduction in weed populations, both grass and broad leaved. We have seen significant reductions in charlock, cranesbill and cleavers emerging within the crop at Brampton,” he adds.

Pre-emergence herbicide use

Dr Ellerton reminds growers that the first opportunity to apply selective herbicides in oilseed rape occurs within the 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 the crop.”

He advises that whilst there are a range of products are available, product choice should be guided by the expected weed spectrum.

“Metazachlor alone will control weeds such as chickweed, mayweed and speedwells and offers a limited effect on black grass. The addition of dimethanamid-P will give improved control of cranesbill, poppy, shepherds purse and charlock.”

“Another option is the combination of quinmerac and metazachlor which adds cleavers, poppy, speedwells and red dead-nettle to the metazachlor susceptible weeds,” he says.

”A further alternative is to tank mix metazachlor based products with clomazone which offers additional control of cleavers, fools parsley, hedge mustard, shepherds purse and chickweed.

“However it is important to remember that while metazachlor may be applied post emergence of the crop, clomazone should only 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 active ingredients include the ALS inhibitor imazamox, as found in Cleranda, as a means of controlling weeds such as charlock and oilseed rape volunteers in Clearfield® varieties of oilseed rape.

“With this latter product it is vital that growers ensure the crop they are treating is a Clearfield® variety before spraying or they will destroy the crop. It is also essential to plan a strategy to control Clearfield® volunteers in following crops as they will be unaffected by ALS products which are often the usual methods of control.”

Environmental impact management

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

Therefore, growers should be aware of the restrictions placed on the label of metazachor based products limiting applications to a maximum individual dose of 750 gai/ha and a total dose of 1000 gai/ha over a three year period in order to avoid metazachlor exceeding the DWD limit, he says.
“In addition to this, stewardship guidelines have recently been issued to try to further reduce the risk of peaks appearing in water. This covers a number of strategies including use of a 6m grass buffer strip or 5m no-spray zone next to water, employing minimum tillage techniques moving only the top 4-6 cm, not applying if drains are flowing or heavy rainfall imminent and avoiding the use of metazachlor on drained fields after the end of September.”
“A further approach is to replace some of the metazachlor with active ingredients such as dimethanamid or quinmerac, or utilize an alternative to metazachlor from the same group of chemistry, such as dimethachlor.”

He adds that many growers will be applying metazachlor at a similar time to slug pellets, where metaldehyde is causing similar problems in water.  “The use of these active ingredients combined with issues around autumn applied propyzamide and carbetamide means that oilseed rape growers, in particular, will have to pay considerable attention to avoiding contamination of water this autumn, 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 passage of herbicides and metaldehyde to field drainage systems. This improves the overall performance by retaining active ingredient where it can deliver best performance and reducing the incidence of drain and groundwater contamination.

Avoiding weed competition

Post emergence, volunteer cereals and susceptible grassweeds should be controlled as early as possible from the 1 to 2 leaf stage of the weed, before they begin to compete with the crop.

“Effective products include a number of established graminicides such as fluazifop, propaquizafop, tepraloxydim or cycloxydim. A relatively recent addition to the armoury of grass weed actives is clethodim which has performed well in Hutchinsons trials, particularly on black grass.”

“However it should be remembered that this product has major restrictions on tank mixing and sequence gaps, as well as a cut off for application at the end of October in order to avoid potential crop damage.”

“Where grassweeds in a particular field are known to suffer from a high degree of resistance to fops and dims, an alternative to graminicides is to apply a low rate of carbetamide in combination with a silicon based wetter from mid-September, at the 3 to 4 leaf stage of the crop, to keep the grassweeds in check, followed by an application of propyzamide when soil conditions are suitable, normally from the start of November.”

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

“Complete absence of emerging weeds is always the preferred outcome, but our work is highlighting the fact that black grass emergence will occur in peak germination periods and we must expect and plan for that. Minimising the impact of direct competition and seed return are sound objectives in a long term control strategy, whereas 100% control is not.”

Early weed control in oilseed rape is a vital part of the strategy to maintain the profitability of this key break crop and is also crucial in reducing the weed burden in following cereals. Your local agronomist will be able to guide you as to the most cost effective approach on your farm and keep you updated on product developments, as well as ensuring minimum impact on the environment.