Recent Conditions Favour Weed Germination

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impact of residual stacking on black grass controlTo date, relatively wet weather across much of the UK during July, August and September has resulted in moist soil conditions, ideal for weed germination says Dr David Ellerton, Hutchinsons technical development director.

“Growers should ensure that they create seedbed conditions that maintain this moisture, promote weed germination and enable them to utilise glyphosate to reduce overall weed levels, not just black grass, before drilling. On the worst black grass fields, delaying drilling will optimise this opportunity.”

He adds that to achieve successful weed control a suitable herbicide programme is crucial.

“In most cases, programmes should begin with a pre emergence application using a combination of active ingredients. The mix should include flufenacet as the base ingredient, plus additional pendimethalin, diflufenican, picolinafen, flurtamone, prosulfocarb, tri-allate, flumioxazine or flupyrsulfuron, depending upon the field’s grass weed population.”
“Pre emergence grass weed control is particularly important in winter barley, where there are few options available for post emergence application. Even fewer options are available for pre and post emergence weed control in winter oats and growers are likely to have to take advantage of EAMUs (formerly SOLAs), albeit at the grower’s risk.”

Dr Ellerton believes that the choice of herbicide combinations should take in to account field-by-field knowledge of previous performance of products on grass weeds or, ideally, on the results of resistance tests.

“Broad leaved weed spectrum should also be carefully considered in matching products to individual fields. Strategies for control of black grass, ryegrass and bromes are broadly similar and all of the above active ingredients offer good control of annual meadow grass, albeit at reduced dose rates.”

“On a positive note for annual meadow grass, chlorotoluron has returned to the market – at a significantly reduced dose - in combination with diflufenican and pendimethalin. Varietal restrictions still apply, so it is essential to check tolerance before making any application. Until recently this product was only cleared post emergence of the crop from the 1 to 3 leaf stage up until the end of October. However the product has now received clearance in winter cereals (not oats) for use pre emergence up to before GS 30.”

 He reminds growers that whichever combination of active ingredients are chosen, it is vital that the first application is made pre emergence rather than delaying to post emergence of the crop. Trials have shown reductions in grass weed control of over 20% in some cases from a delay of only a few days.

“When pre emergence applications are planned, growers should remember the importance of sowing at the correct depth to avoid crop damage. Drill-depth restrictions will vary between products but seed which is covered by 33mm of settled soil should be suitably protected for most products.”

As a general principle, the more serious the grass weed infestation the higher the herbicide dose required and the greater the number of active ingredients that should be applied - a technique known as ‘stacking‘ he says.

“This is particularly important when there is known resistance to post-emergence ALS inhibiting active ingredients, such as iodosulfuron, mesosulfuron or pyroxsulam. Where post emergence applications of ALS products with grass weed activity are necessary, they should be made once the majority of grass weeds are at 1 to 3 leaves and weather conditions are suitable for active growth.
 It is also important to include a residual component to extend the length of control.”

“Applications made in the autumn are usually more consistent than those made in the spring, when target weeds are also larger and enhanced metabolism resistance is more of an issue. In addition, delaying until the spring usually results in lower yields, due to an extended period of weed competition prior to application.”

“Nevertheless, timing trials with contact herbicides show very clearly that seedbed moisture, as well as temperature and weed size, is critical to achieve optimum control, so applications should only be made where conditions are appropriate.”

Accurate spray application

Delivering the product to the target via the sprayer is as important as choosing the right products to use advises Dr Ellerton.

He adds that when the target is the soil, then spray quality is less critical and provided good and even coverage is achieved, a satisfactory result should follow. When the target is an emerged weed (grass or broadleaved), then in addition to ensuring active weed growth and dry leaves at application, attention should also be paid to suitable forward speed for good boom stability, spray quality which should be fine-medium and that the boom height is suitable to the type of nozzle being used e.g. 110°FF should run at 50cm above the target.

“All of the above details can play a major part in determining optimum efficacy of autumn contact herbicides.”