12 December 1998


Another difficult autumn with limited spraying opportunities has left many growers in a quandary about weed control. Sarah Henly finds out whats being recommended across the country.


RICHARD Cartwright of Chichester Crop Consult-ancy sees three distinct crop scenarios this season, each requiring a different approach to weed control.

The first is where growers got cracking with drilling in mid-September and wheat was up and away by later that month. The second is late September drilled crops which emerged by mid-October, and the third is the winter wheat still emerging or at the one-leaf stage.

"Growers normally finish drilling around here by 20 October, but later sowings arent a problem because the warmer weather we get on the south coast allows crops to catch up. This year the high rainfall in October put everything on hold, and although the ground is taking it surprisingly well, its still too risky in places to get on," explains Mr Cartwright.

Fortunately most of the early drilled wheats received a herbicide between the wet spells, and weeds have already curled up. Some of the second group were sprayed between showers in early November. And in late drilled crops, weeds shouldnt be a threat until the spring.

But weed control policies didnt exactly go by the book. Planned Avadex (tri-allate) applications were not possible, new chemistry played a part, and some of the old favourites got a look in thanks to being more competitively priced than in recent years.

Where blackgrass appeared at high populations, Mr Cartwright recommended a sequence or combination of Lexus Class (carfentrazone-ethyl and flupyrsulfuron-methyl) and Hawk (clodinafop-propargyl and trifluralin). Although pricey, he hopes it will leave little work for the spring, with the exception of some broad-leaved weeds.

In blackgrass situations where Hawk was used early without Lexus Class, broad-leaved weeds such as charlock, cranesbill and rape may have become large. There, treatments may consist of Hawk with Quantum (tribenuron-methyl) rather than contact materials which tend to scorch the crop in frosty weather.

Where blackgrass populations were low, and meadowgrasses and broad-leaved weeds the main targets, cheaper options such as isoproturon and chlorotoluron sufficed as the basis for mixtures. Apart from being better value than last year, the latter has the edge over isoproturon on wild oats and gives more persistent activity.

Those two stalwarts were used mainly in mixtures with either Panther (diflufenican), Stomp (pendimethalin), or as Puma X (isoproturon and fenoxaprop-ethyl), depending on the main weed targets.

"Wheat crops after oats generally receive a three-way mix of isoproturon, diflufenican and fenoxaprop, while Stomp – down in price this season – is our preferred option for downland soils where leaching is a high risk. If it remains wet, isoproturon will be used less because of its high water solubility," he explains.

Typically, isoproturon would be used in the spring with low rates of Ally (metsulfuron-methyl) to control a range of broad-leaved weeds, or Duplosan (mecoprop-P) more specifically for volunteer rape, groundsel and runch. But alternatives next spring may include Eagle (amidosulfuron) early or Starane (flyroxypyr) later, particularly where cleavers threaten.


THE wetter west was decidedly wetter this autumn than most. Since last January, 965mm (38in) of rain has fallen in Herefordshire – much since harvest time.

Andrew Goodinson, agronomist and director of Technicrop estimates that at least 20% of the planned wheat area is yet to be drilled in north Herefordshire. Late lifting of potatoes and sugar beet has made drilling impossible in some fields.

"Usually, even after root crops, 85% of winter cereals would have received an autumn herbicide by mid-November. Were down below half that this year. But we hope to be left with only a quarter of crops unsprayed by the spring," he explains.

Theres no urgent need to get on later drilled crops because weeds, like crops, are late getting started. Growth is slower and there seems to be less meadowgrass than usual. An exception to this would be fields where blackgrass is present, which must be tackled as soon as ground conditions allow.

"Our main weed worries are meadowgrasses, ryegrasses, volunteer rape or potatoes and cleavers. Control of annual meadowgrass and ryegrass is more cost effective if carried out in the autumn, but can be dealt with in the early spring along with wild oats if they appear," stresses Mr Goodinson.

Some growers did however manage to spray early drilled wheat following rape or peas in October, mainly with residual products. Where volunteer rape was the most abundant weed, the addition of 0.5litre/ha of Duplosan to the residual herbicide component gave good control.

A number of crops follow herbage seeds or grass leys in the rotation, hence ryegrass is a major problem. In such situations, chlorotoluron forms the mainstay of Mr Goodinsons autumn programme, provided the variety is tolerant.

Chlorotoluron has come down to £22/ha this season based on £3.20/litre. Although that compares with the price of isoproturon, he would rather use the less soluble option where the variety is suitable.

Where annual meadowgrass and broad-leaved weeds such as pansy and speedwell are the main targets, a number of different strategies can be adopted, advises Mr Goodinson. 2-2.25litre/ha of isoproturon partnered by 40-50g/ha of diflufenican costing about £16/ha will give effective control. Alternatively pendimethalin and isoproturon, formulated as Jolt, at 4litre/ha for about £24/ha is also reliable provided weeds are small.

Where autumn treatments havent yet gone on and grass weeds threaten, its not too late to apply one of those mixtures. But check cut-off dates since some can be used only until the end of January, he warns.

Follow-up treatments will be based on Eagle for early cleavers control, or Starane for cleavers and volunteer potato control later in the spring, concludes Mr Goodinson.


NORMALLY all sprayed up by mid-November but only 60% done, is how Simon Draper sums up the weed situation. The consultant with Independent Agronomy in Suffolk is concerned about blackgrass, particularly on the poorly drained sands in his region.

"The coastal sand belt stays wet a long time and its risky to travel at the moment. Some growers havent even got their wheat in yet. But where they have and theres blackgrass, they must go whatever the state of the soil, using low ground pressure tyres," he says.

The most common autumn approach is isoproturon and trifluralin, however the blackgrass became large before some growers had a chance to spray. They were advised to use Hawk with oil at 1.5-2l/ha, despite costing £5-7.50/ha (£2-3/acre) more.

"At least Hawk is cheaper than it was last year. And it has worked fairly successfully on sites where we see blackgrass with partial resistance. Isoproturon is effective only if it goes on to cotyledon stage blackgrass," he warns.

But isoproturon will be sufficient for weeds other than blackgrass, in mixtures with diflufenican for a range of broad-leaved species. Leaving broad-leaved weed control until the end of February is now a feasible option, says Mr Draper. Weed growth will slow and there is a wide range of products which control the common spring weed spectrum.

He commonly recommends Harmony M (metsulfuron-methyl and thifensulfuron-methyl), possibly with mecoprop if it is warm and weeds are growing again.

Where spring germinating wild oats are expected, he may recommend Avadex at two-thirds of the full rate in January or February. It was too wet to get it on when the crop had one leaf – the usual policy.

Mr Draper fears this could be a more expensive year for growers. "Delays in applying herbicides could cost more money because larger weeds require higher rates to control them. And although you could argue that it may be cheaper because drilling and weed emergence has been delayed, late drilled crops are bound to yield less," he says.


WITH up to a quarter of the wheat area still to drill, the north has been worst hit by the wet weather. And only half of whats in is fit to spray, according to Bryan Pickles, technical manager in the north east for Fieldcare, part of the Procam group.

"With one or two exceptions there were no sprays applied in November, and I would estimate that just 20% of the drilled acreage has been given a herbicide," he reports.

Capping caused by heavy rain on the lighter soils has delayed crop emergence but also weed emergence. Certainly there are fewer and smaller weeds than usual by now, but Mr Pickles would like to see residual herbicides applied to grass weeds before they get much bigger.

"The weed priorities are barren brome, blackgrass and high populations of annual meadowgrass. Where brome has not yet emerged but is expected, its not too late for Avadex Excel followed by isoproturon. Otherwise the latter alone will have some effect, although using it where it remains wet could be environmentally unsound," he stresses.

Resistant or suspect populations of blackgrass must receive a full programme, ideally including Avadex Excel then Stomp or another pendimethalin product. Trifluralin may suffice where the blackgrass is yet to or just emerging, says Mr Pickles.

For sensitive and lower populations, he will recommend Lexus with a residual component other than isoproturon, or Hawk alone.

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