All Winter Cereals – Late August 2020

BYDV – forward planning

Barley yellow dwarf virus (BYDV) has been seen in several winter and spring cereals this season. Aphids can be found on crops and volunteers although the rain may well have washed a few off. There is a risk of cereal aphids picking up BYDV from infected plants and spreading the virus to cereals being sown this autumn.

The loss of the neonicotinoid seed treatments has impacted the management of BYDV in cereals. As a result, there is a reliance on the use of pyrethroid insecticides for the management of aphids and the transmission of BYDV into cereals. Grain aphids in Scotland have resistance to the pyrethroid aphicides. Consequently, reliance on a pyrethroid aphicide spray is a risk as grain aphids may not be adequately controlled, elevating the risk of BYDV.

The most effective way of managing the ‘green-bridge’ of aphids on volunteers and stubble is to kill off any aphid host plants before sowing. Ideally stubble from the previous crop should be desiccated with a herbicide, ploughed in 7-10 days later and left for another 2 weeks before sowing the next crop. This will ensure that any aphid host plants will be dead and any aphids on them will have starved to death. Alternatively, stubble can be ploughed in and sowing delayed for 4 weeks to allow enough time for plants to die along with the aphids.

Both of these approaches will reduce the threat from walking aphids by killing off their host plants that could keep the aphids (and BYDV) ticking over. Aphids are quite resilient and can work their way up to the soil surface after ploughing, so killing off the plants is essential so that they won’t have anything to feed on underground. However, for aphids flying into the crop at crop emergence aphicide sprays are the only option.

A combined approach of tackling the ‘green-bridge’ for the walking aphids and using an aphicide to target the flying aphids is the only option available this autumn to reduce the risk of BYDV. Reducing the availability of BYDV reservoirs in cereals and volunteers/stubble, and in aphids overwintering on winter cereals will reduce the risk to spring cereals, where a pyrethroid aphicide spray at early crop emergence is again the only option currently available, and this may not take care of the grain aphid due to widespread resistance.

Because of the resistance issue in the grain aphid, try to avoid the use of pyrethroid aphicides on cereals for BYDV unless absolutely necessary – aphid colonies on the leaves in the autumn rather than individual aphids is the trigger for an aphicide treatment.

Pre-emergence weed control in winter wheat, barley, rye and triticale

If the unsettled weather continues it could be tricky getting pre-emergence herbicides applied. However, they do give the most cost-effective control and are particularly valuable in controlling annual meadow-grass – control is more difficult or impossible once it is established. Early sown crops are likely to have the greatest germination of annual meadow-grass. Once the crop is sown, if there are good spraying conditions with nice moist soil then it is best to get the herbicide on.

The key to effective control is to apply a residual herbicide as early as possible after sowing, ideally pre-emergence of weeds to moist soil. Application can be delayed a little due to weather, but as soils are still warm and moist, weeds are likely to emerge fairly quickly, so best not to leave it too long. There is a benefit to waiting in a slightly extended residual life of the herbicide, but best to get it on before the weeds germinate.  With residuals the greatest weed spectrum is generally pre-emergence of weeds and a higher dose is often needed post-emergence. Good spray cover is needed to get the best results from residuals. Studies have shown that keeping the boom low, forward speed down, water volume up and using low drift nozzles help achieve optimum performance by residuals.

It is particularly important at the pre-emergence timing to concentrate on control of grass weeds – either simply annual meadow-grass or more difficult and costly weeds such as bromes or black-grass. Product choice principally relies on the active ingredients flufenacet (note that in rye and triticale there are limited recommendations for flufenacet) formulated with either diflufenican or with pendimethalin.  Prosulfocarb is another versatile active in tank-mix where there are problem grass weeds.

For early drilled crops application rates need to be robust to give adequate persistence, reducing for later drilled crops. For brome control full label rates are recommended, particularly in winter barley where there are few follow up herbicides for grass weeds. Straight pendimethalin is sold either as old style, and inexpensive 330 g/l emulsifiable concentrate (EC) formulations or improved 400 g/l suspension concentrate (SC), or a slightly better 455 g/l capsulated suspension (CS) formulation. If using a straight pendimethalin product, it needs to go on pre-emergence. It can offer a cost-effective start to grass weed and broad-leaved weed control, particularly for later drilled winter wheats.

Chlorotoluron is only available in co-formulations. They have 250 g/l chlorotoluron, 40 g/l diflufenican and 300 g/l pendimethalin. They have a broad label for use on winter wheat, winter barley, rye and triticale. They are good on annual meadow-grass and a range of broad-leaved weeds. Note that they have a DRT (Drift Reducing Technology) buffer zone requirement. Three star nozzles must be used within 30 m of water, and within that 6 m from the top of the bank must be left unsprayed. This 6 m buffer zone cannot be reduced.

Where there is a serious grass weed problem, straight 500 g/l flufenacet can be used as a mixer in winter wheat and winter barley. Timing is pre-emergence to GS23, with a rate of 0.48 l/ha. A main use of a straight product is to top up the flufenacet level per hectare to improve control of difficult grass weeds such as larger annual meadow-grass, brome and black-grass.  Note there is no maximum amount of flufenacet that can be applied per hectare so long as each product label is complied with. Tank mixes are possible with diflufenican and pendimethalin – check the product label.

Activity of Autumn Residual Herbicides on Grass Weeds

*           As part of a programme of control. Do not rely on this product alone to give good control.

**          Always use in sequence or mix with product from a different chemical group to reduce chances of resistance development.

***        There is evidence that diflufenican at rates above 50 g a.i./ha applied pre-emergence to moist soils adds to annual meadow-grass control. It should not be relied upon nor is there a label recommendation for its use as a stand-alone product.