14 August 1999

Clobbered by Christmas

Its been a dire year for blackgrass. In the first of a series looking at different tactics against this pernicious weed, Lucy Stephenson discovers how Aubourn Farming clobbers the weed – and all before Christmas…

NO MATTER how bad your weed problem is, dont let blackgrass control hold up drilling, says Bridget Carroll of Aubourn Farming. Successful crop establishment has to take priority – because thats what sets the yield potential.

It might be thought that this principle sits rather uncomfortably alongside a need to tackle blackgrass – particulary if youre facing resistant populations. Cultural control of the weed appears to be the only effective answer.

Ms Carroll disagrees – and reckons that crop competition from early establishment is underrated. "We drill very early and at very low seed rates. This is quite contrary to some guidelines which recommend later drilling to control blackgrass. But we have often found blackgrass control easier in early drilling because the farm is on top of the workload, and the drier conditions make it easier for sprayers to travel."

Aubourn uses a plough and press one year in three. Ploughing is a valuable option; total inversion gives better control than minimum cultivation, because if blackgrass seeds are buried over 10cm below the surface they generally wont emerge.

The rest of the time only discs and drill combination are used. This is because seedbeds can be more difficult to form on ploughed land and can be dry, and also to keep establishment costs down.

A non-selective herbicide such as glyphosate is used, if theres the opportunity of a good weed chit. Avoid cloddy seedbeds as they give blackgrass the chance to emerge from depth, warns Ms Carroll. Fine seedbeds also get the crop off to a good start.

Autumn is the critical period for blackgrass control. "Most blackgrass germinates in the autumn. We aim to control it by Christmas because the chances of killing bigger plants in spring are vastly reduced," she explains.

This has been most noticeable in the last three years; mild, wet winters have fostered blackgrass survival and recovery so that on many farms the problem has become worse than ever.

A pre-emergence herbicide 24 hours after drilling is a must in any blackgrass situation, says Ms Carroll. She alternates using Prebane (terbutryn) and Avadex (tri-allate). "Avadex granules are volatile so we roll after applying if we can."

Ms Carrolls strategy is to follow a pre-emergence herbicide with up to three post-ems as necessary. It has even worked on populations with five-star enhanced metabolism resistance to fops and dims.

The early post-em must be applied before the wheat reaches 3-leaf stage; before November when the blackgrass will have begun to tiller. Cheetah (fenoxaprop-P-ethyl) can give control on resistant blackgrass at first leaf, she says.

"If you get in early enough, the plants are actively growing and can be killed, because more chemical is taken in. The combination of moisture and warmth at this stage makes a considerable difference."

But beware – this tactic may not work for all populations with enhanced metabolism resistance: "Everyones experience will be different because each population is different," she says. And if the resistance mechanism is target site rather than enhanced metabolism then even very young plants will be unaffected, she stresses.

Post-emergence

Where the blackgrass situation is severe, the early post-emergence spray combines a contact herbicide with a residual. The contact is Lexus (carfentrazone-ethyl plus flupyrsulfuron-methyl) at 20g/ha, and the residual is either IPU or Stomp (pendimethalin) at 2 litres/ha. "The residual is only a gentle spray as the crop is also at a vulnerable stage," adds Ms Carroll. Trifluralin or diflufenican is added as a broad-leaved weed killer where necessary.

The crop is monitored frequently to assess requirements for the following post-em spray. If plants are dead theres obviously no point in adding a contact herbicide in the November top-up, she says. The contact herbicide in later post-ems can be tougher – and more expensive; usually clodinafop.

With a low level problem, a pre-em and one post-em would keep the crop clean, but background populations in some fields on the Nevile Estate are high.

"The residual should last longer than 4-6 weeks, but the problem is so severe on these fields that we have to keep gently hitting it. Three sprays sounds extreme but a lot of growers will need to adopt a policy like this to control the blackgrass," she says.

The spray quality needed to control blackgrass most efficiently changes as the plant develops. "As the blackgrass comes through its as slender as a pin, but as it grows it begins to feather out, so the spray quality you need to hit it is different each time," explains Ms Carroll.

Only fine spray with some sideways drift will cover the first shoot adequately, at 100-200litres/ha. Flat fan and twin jet nozzles can deliver this pattern.

As the weed grows bigger, more volume and more penetrative spray is required; Ms Carroll recommends a 200-300-litre/ha nozzle. "This is where application can go wrong because if the spray is too coarse it bounces off the leaf," she warns.

Twin jet nozzles can deliver quality spray at up to 500 litres/ha, but low drift and air inclusion nozzles should be avoided because they produce a coarse droplet size so there is a lack of penetration, says Ms Carroll.

"Ensure your nozzles are new. The highly sophisticated products we have now wont do a good job if you put them through 10-year-old spray technology. Accept that hiring a contractor to spray the right quality at the right time can be money well spent."

Water pH can also affect the efficiency of chemicals: "In Lincolnshire the pH is high and the calcium ions in the water can latch on to the agrochemicals and neutralise them. We put in buffers at an extra cost of only 20-30 pence per acre," she explains.

The Aubourn approach is not suitable for every farm. It must be well planned, so that weeds can be sprayed immediately they are ready. Again, use a contractor rather than miss a spray window if there are problems, adds Ms Carroll.

Planning should begin in July. Produce a weed map and judge the severity of populations. Divide this into three categories; different levels of infestation call for different treatment regimes (see table).

Note the success of each treatment in every crop. "Get tested for resistance if you feel its necessary – but an in-field assessment of the problem is just as straightforward," says Ms Carroll.

Make a five-year plan, making use of mixed chemistry, and diversifying your rotation. Take the opportunity to use alternative chemistry in broad-leaved crops, she advises.

Early control is just as important in other crops. For oilseed rape, a pre-em of trifluralin and/or Butisan (metazachlor). Contact and residual products should go on post-em as soon as the crop has reached the 3-leaf stage. "Laser (cycloxydim) is the only reliable contact graminicide. A Kerb (propyzamide) plus Laser mix makes this a three or four-pronged attack on the weed," says Ms Carroll.

In winter barley infestations can be severe: "It is becoming a real problem because available products give poor control."

She suggests Prebane or Avadex as pre-em, and then IPU and Tigress Ultra (diclofop-methyl plus fenoxaprop-P-ethyl). "But dont hesitate – it must go on when the blackgrass is really small; 1-2 leaves."


Situation Products Rate £/ha Comments

(litres/ha) (RRP)

Blackgrass problem Prebane or 3.0-5.6 8.6-16.1 Use pre-em – high rate except on light soils

extremely severe Avadex Excel 15kg 21.9

Lexus 20g Apply at one leaf of blackgrass. If miss timing change to

+ Stomp 2 26.9 Hawk/Lexus/oil ASAP followed by IPU/Trifluralin. Some concerns over effectiveness on wild oat control with Hawk/Lexus/oil mix

IPU 5 18.3 Apply 3-4 weeks later – may combine with following spray if

+ Trifluralin 2.3 blackgrass surviving

Hawk 2.5 24.0 Apply if blackgrass surviving – before tillering

+ Oil 1

Blackgrass problem Prebane or 3.0-5.6 8.6-16.1 Consider pre-em for increasing problems

increasingly difficult Avadex Excel 15kg 21.9

to control

IPU 2 Dont add more than 1,000g IPU to Hawk; do not mix IPU with + Hawk + Oil 2.5 + 1 29.2 Lexus. Add Hawk/oil if blackgrass emerged

Blackgrass still Prebane 3.0-5.6 8.6-16.1

easy to control or

IPU 2.0-2.5 5.2-6.5 Adopt residual approach. Only use this if good control still being

+ Puma 3.0-5.0 18-30 achieved with fenoxaprop. Apply at one leaf blackgrass

(fenoxaprop-P

-ethyl plus IPU)

Late-drilled IPU 5

wheat + Trifluralin 2.3 18.3

or

IPU 2

+ Hawk + OIl 2.5 + 1 29.2 Add Hawk + oil to IPU if blackgrass emerged

Winter barley Prebane 3.0-5.6 8.6-16.1

or

Avadex Excell 15kg 21.9

Tigress Ultra 1.0-1.25 18.6-23.3 Apply highest recommneded dose of Tigress

+ IPU 3.4 7.8-10.4 Ultra and IPU that crop will stand. Avoid frosts following application

Source: Simon Knight, Bridget Carroll, Aubourn Farming