Archive Article: 1997/09/06

6 September 1997

With no herbicides specifically approved for linseed, David Millar plots a course of weed control action

with ADASs Martin Froment.

CONTROLLING weeds in the expanding winter linseed acreage isnt just a case of reading a label and opening a can.

There are a number of questions the grower needs to ask first. Am I on light or heavy soil? Is the site exposed and will the seedling crop be stressed? Are there cleavers present? If the crop fails, what can I sow as a following crop?

There may be no specific herbicides for winter linseed but a number of products are available to the grower – at their own risk – based on approvals for spring linseed or for winter oilseed rape.

Watch out, however, says Martin Froment, of ADAS Bridgets, Hampshire, for labels which denote only an approval for spring linseed rather than linseed.

He started work last year on a series of weed control experiments with differing herbicides likely to be used by growers. In practice, maybe 80-85% of linseed growers use Ally (metsulfuron-methyl) in the spring, but that still leaves growers of the winter variety to decide whether they should also treat in the autumn and if so, what to use.

"If you drill a crop like winter linseed at the end of September and decide not to spend much on herbicide, then by the time you get to February or March some of the weeds could be quite big and vigorous," says Mr Froment. "Follow that with a difficult spring crop with few spray days and those weeds are even bigger."

He is quite clear that growers with a lot of autumn weeds will need to consider a herbicide treatment before winter. Researchers agree that linseed, both winter and spring, competes poorly with weeds and, with high seed costs for the winter crop, you need to protect seedlings.

Ally cant be used in the autumn, and there are clear restrictions on its use in spring – it must be after 1 February and the plants must have the first two true leaves unfolded.

This means that, for the autumn, growers have to look again at older chemistry, some of it less reliable than the newer sulfonylurea. Among the cheapest available is trifluralin (Treflan).

It was used fairly extensively in spring linseed when it was introduced to the UK, but growers were often unhappy with the additional management involved in incorporating the herbicide.

It has label approval for winter oilseed rape and its incorporation is recommended for both this crop and for use with spring linseed.

The ADAS trials compared incorporation with surface treatment by the trifluralin. Assessment of plant vigour in the treated crop, both in November and in March, showed that incorporation clearly knocked plants back more than a surface treatment, pre-emergence (table 1). The effects were even clearer after the more-stressed crops – where weed control was by incorporation – had been through the winter.

"In some circumstances you can incorporate trifluralin without any problem at all," says Mr Froment. "You have to be aware that on the lighter soils there is more risk of damage so consider your dose rate and application method very carefully."

The Bridgets trial is on light chalk soil but he suggests a similar trial on heavier clays could produce a different result showing little difficulty with incorporated trifluralin.

However, Mr Froment is unconvinced that winter linseed is going to prove sufficiently attractive for growers on clays, who will probably get better margins from oilseed rape, which traditionally yields more on eastern clays than on southern chalks.

For crop growers with a wheat/wheat/rape rotation, the inclusion of winter linseed to extend the rape break might be attractive for some East Anglian growers.

Better margins on light soils suggest most linseed crops will still be grown in those areas where spring linseed has been popular.

Other autumn pre-emergence herbicides were compared in the Bridgets trial (table 2). Metazachlor (Butisan S), knocked crop vigour badly, as did a linuron plus lenacil mixture (Seppic-Lin) which is non-approved for the UK but used in France. Linuron plus trifluralin and trifluralin alone did well.

Early post-emergence bentazone plus bromoxynil and clopyralid (Basagran + Vindex), which is one of the most expensive to linseed growers and very effective in spring, also had less than welcome effects when applied. Cyanazine (Fortrol) post-emergence killed the crop.

The results showed the difficulty in using these early post-emergence treatments in the autumn – the crop needs to be growing vigorously and reach critical growth stages quickly to get the best from the herbicide. In practice, the crop may be growing slowly and temperatures falling.

A number of crops in the southern part of England were wiped out or severely damaged last winter but, while stress from herbicide application may have contributed to some damage, there were many untreated crops which also failed to survive through to spring. Conditions of low or zero snow cover on exposed fields freeze-dried a number of crops, while frost heave damaged roots.

Mr Froment says growers should think carefully about the weeds they have and the rate of herbicide to be used. Trifluralin alone and in mixture with linuron will give good control of choking chickweed in the autumn, he points out. So why not use a low rate then and come back in the spring as necessary with something more effective and with the added advantage that costs are minimised until crop survival is assured.

Ally is the only sulfonylurea currently approved for use on linseed in the spring, but could be joined next year by Eagle (amidosulfuron) if its approval for linseed comes through in time. Although Eagle has a narrower spectrum of control than Ally, it is particularly effective against cleavers and, in cereals at least, works well even in the low temperatures of early spring.

"In our work, we did see a slight reduction in the number of cleavers from using trifluralin but nothing much. If you have a field with a lot of cleavers, I wouldnt grow linseed in it until Eagle has been approved," adds Mr Froment.

ADAS is now committed to further work with weed control in winter linseed, having recently secured funding from the HGCA to investigate both broad-leaved weeds and grass weeds control at the Bridgets, Boxworth and High Mowthorpe research centres.

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