19 September 1998

WIZARD WAYS WITH WEEDS

Break crops present a good opportunity for using different chemistry to keep on top of problem weeds. Lucy de la Pasture asks the experts for advice on autumn herbicide strategies.

TOPPING the pecking order in an HGCA study ranking the ability of weeds to compete with oilseed rape is cleavers, closely followed by chickweed, poppies and charlock.

"Late sown rape is generally much more vulnerable to weed competition because the rape will be less well grown going into the winter than earlier, well established crops," explains Dr Peter Lutman, of IACR Rothamsted.

A pre-emergence treatment is usually a very effective option for controlling many broad-leaved weeds, but what if youve broadcast the crop or just missed the pre-em boat altogether?

Post-emergence timing becomes critical, explains agronomist Dr Pat Turnbull. "Butisan S (metazachlor) is most effective used post-emergence when the majority of the rape crop is at the cotyledon stage and all of the weeds are also at cotyledon," she says. "If application is delayed so that weeds become larger (2-4 leaves), unless there is a very good seedbed and moist conditions, Butisan alone will not be enough."

Adding a low dose of Benazalox (benazolin+clopyralid) to the Butisan S can hot the mixture up but there is a narrow window of application. "Benazalox has a very small weed spectrum and as weeds get bigger this tank mixture will be less effective," explains Dr Turnbull. "Under these circumstances it would be better to move to Kerb (Benazalox, Kerb/Fortrol) mixes or Matrikerb (propyzamide/clopyralid) depending on the spectrum of weeds present."

Where either mayweed or charlock are the dominant species a specific approach is best – using Shield (clopyralid) to tackle mayweed and Fortrol on charlock. However, the crop should be sufficiently waxed to avoid damage when Fortrol is applied.

BASF has just launched Katamaran (metazachlor + quinmerac) for use this autumn at £50/ha, about £10/ha above treatment with Butisan S, but with improved activity on cleavers and poppies. "Katamaran can be used post-emergence of the crop, but best results on poppies and cleavers have been pre-emergence of those weeds," says Dr Turnbull.

The quinmerac component of Katamaran is more persistent than Butisan S and should provide an extended period of control. Cleavers are often stunted, preventing them from reaching the top of the crop canopy, even if they are not completely controlled.

Another option for post-emergence cleaver and chickweed control is Galtak (benazolin) which can be used from the three-leaf stage of the crop, but can be expensive when used at the full cleaver rate (£48/ha).

Where there is resistant blackgrass, a different strategy concentrating exclusively on blackgrass control is needed, advises Dr Turnbull. Trifluralin, either incorporated or applied as a surface treatment, will provide a good foundation.

While it is preferable not to use a graminicide at all in such situations, leaving Kerb to do the job alone can be risky since application needs to be delayed until soil is moist and soil temperatures are cooler for best results. If the following winter is dry, Kerb alone wont kill large blackgrass as it is impossible to get enough active into the plants via their root system.

SOWING method determines herbicide choice in winter beans. Most are established by broadcasting on to stubbles followed by ploughing down to 15cm (6in).

"Care must be taken not to plough beans down too deeply – shallow ploughing can be difficult with modern ploughs," warns Cathy Knott, of the PGRO. "Ideally, the surface should then be levelled and a herbicide applied."

Simazine has been the mainstay of herbicide programmes for many years and is inexpensive at about £7/ha (£2.83/acre). It can be applied safely until the end of February. "Application early in the season is the best strategy as weeds that have emerged at the time of application will not be controlled," explains Ms Knott.

Like other residual herbicides, good soil moisture is essential for activity and rain following application can improve weed control, whereas very dry weather may reduce it.

Winter bean crops which are drilled and not ploughed-in may require a different approach. "Simazine may not be safe for use in drilled crops as the seed must be covered by at least 7.5cm (3in) of soil and it is difficult to drill the seed this deep," says Ms Knott. "Remtal is a safe alternative under these circumstances but Opogard should not be used."

Kerb (propyzamide) is a useful alternative herbicide with additional activity on grass weeds allowing it to form part of a resistance strategy on fields where blackgrass is difficult to control.

Follow-up weed control may be required in the spring if cleavers are a problem.

"In the winter bean crop cleavers will set seed and should be controlled to avoid additional problems in following crops," says Ms Knott. "Basagran can be used safely on all varieties, however care should be taken when the crop gets near to flowering as the crop will be more susceptible to scorch."

AUTUMN weed control can give the comparatively non-competitive winter linseed a helping hand. Under current legislation, products with a full label approval for use in oilseed rape can also be used on winter linseed, but only at growers own risk.

However, this approach can be extremely dangerous, warns David Turley, of ADAS High Mowthorpe, in North Yorkshire. "Herbicides such as Butisan S (metazachlor) can be effective but can also cause serious damage to winter linseed, even at low rates."

There are no guarantees with winter linseed that the crop will emerge intact after a severe winter, being only tolerant but not resistant to frost. A cheap and cheerful strategy in the autumn is the best approach, believes Mr Turley.

"Trifluralin (Treflan) applied as a surface treatment can reduce the weed burden surviving until spring by up to 50% in situations where common weeds such as chickweed and speedwells make up a large proportion of the weed flora," he says. "Other weeds surviving treatment will be held back so that in the coming spring there is a reduced spectrum of smaller weeds left to control."

Where trifluralin has been incorporated into the seedbed, higher levels of winter kill have been seen in both ADAS and recent HGCA trials and also on a field scale.

Although winter linseed can tolerate quite high levels of weed, a follow-up treatment in the spring is advisable, particularly if the weeds are likely to lead to problems such as lodging of the crop.

"Ally (metsulfuron methyl) is most widely used with rates of 15-20g/ha sufficient to clear up most problems," says Mr Turley. "The weakness in this strategy is cleaver control but the addition of Eagle (amidosulfuron) to the armoury in 1998 has plugged the gap."

Timing of a spring herbicide can be tricky; the crop needs to have started growing again after the winter but any herbicide must be applied by the flower buds visible stage of the crop.

A PRE-EMERGENCE herbicide is critical for winter peas, according to the PGROs Cathy Knott.

"Weeds can come through the ground quickly if soils are still warm and autumn germinating weeds such as speedwells, pansy and annual meadow grass are not controlled by post-em tank mixes," explains Ms Knott.

The only pre-emergence label recommendation currently existing is for Senate (terbutryn + trietazine). Both Opogard (terbuthylazine + terbutryn) and Remtal (simazine and trietazine) can be used but are not supported by a full label recommendation and are at growers own risk.

"Avoid using anything that contains pendimethalin (Stomp and Monarch), as in a wet autumn this can cause damage and reduce plant populations," warns Ms Knott.

Spring options for follow-up, broad spectrum weed control are the same as for spring peas, either Pulsar with or without Fortrol (bentazone/MCPB+cyanazine) or Fortrol + Trifolex-tra (cyanazine + MCPA/MCPB).

Pre-emergence herbicides have no effect on cleavers which can be too large for effective control with Pulsar plus Fortrol in the spring. Basagran (bentazone) would be an option for cleaver control but its efficacy can be reliant on weather conditions and treatment is expensive (£67.50/ha).

Weeds in oilseed rape

Highly competitive

cleavers

poppy

chickweed

charlock

Moderately competitive

red deadnettle

mayweed

speedwells

annual meadowgrass

Poorly competitive

shepherds purse

fumitory

V. Poorly competitive

pansy

Source: HGCA