Archive Article: 1997/12/13
Did you manage to get an autumn herbicide on winter wheat when you wanted to – and has it done the job? Sarah Henly seeks an update on weed control.
CHALLENGING is a fitting word to describe the battle against weeds during this extended autumn.
Time was on growers sides with spraying windows available among the extremes of dry and wet, and mild and frosty. But herbicide programmes have had to be adjusted to prevent crop damage following their lush growth after the generally early start.
In general, September drilled wheats emerged evenly where moisture was conserved by rolling immediately after sowing. But those dry seedbeds now support patchy crops with plenty of space for weed encroachment. Fortunately the dry conditions also did little to encourage weeds.
Some growers decided to wait for more weeds and moisture to appear before applying a herbicide, only to be caught out by early frosts then uninterrupted rain. Many were forced to wait until this month to spray. Others got on early and now fear a second flush of weeds.
So will the result be poor weed control this winter? Here are theviews of four members of the Association of Independent Crop Consultants from across the country.
SOUTH
IN THE south, Peter Cowlrick of Chichester Crop Consultancy believes there may be some "mopping up" to do next spring. Fields which contain grass weeds as well as cleavers will be the priority.
Fortunately most early drilled crops have received at least one autumn residual herbicide, albeit different to the usual approach. There were noticeably fewer grass weeds about in early October, probably because they struggled to emerge in the dry. However Mr Cowlrick was keen to get a residual on as early as was practical to control meadow-grasses and broad-leaved weeds.
He considered seedbeds too dry and soil temperatures too high to use tri-allate (Avadex) or trifluralin against blackgrass successfully. Similarly, he was cautious of using a full rate of isoproturon, for fear of rapid crop growth and luxury herbicide uptake.
"Most growers needed to go in early with an aphicide for BYDV control and to achieve early weed control, so I recommended a third of the full rate of isoproturon with diflufenican to widen the broad-leaved weed spectrum.
"In blackgrass and wild oat infested fields, I planned to apply the remaining two-thirds, possibly with a contact acting product, along with the second aphicide in November. But the weather has turned wet and spraying opportunities have been few and far between. I think some follow-up sprays will inevitably be spring applied," explains Mr Cowlrick.
For the second hit in early drilled crops, growers must now be prepared to use a contact acting herbicide to control large grass weeds, for example fenoxaprop-ethyl (Cheetah R) or clodinafop-propargyl and trifluralin (Hawk), he stresses. He will contemplate recommending the new sulfonyl-urea, Lexus Class (flupyrsulfuron-methyl plus carfentrazone-ethyl) as a possible anti-resistance strategy against blackgrass, provided wild oats are not a threat.
EAST
CAROLINE Hayes, who works with the North Herts Farmers buying group, also advocates spraying grass weeds as early as possible in the autumn. She too was wary of using full rate isoproturon with an aphicide when crops were steaming ahead. So she tried a new approach suggested by weed specialist Jim Orson of ADAS Boxworth.
Mrs Hayes asked growers to go against every spraying principle and apply just 1 litre/ha of IPU in the early morning dew, when blackgrass plants were at the one leaf stage only. Aided by the moisture, the low rate spray is supposed to roll down the spear-shaped leaf and into the crown, killing the growing point.
It didnt quite do the trick, but the technique meant blackgrass plants which would otherwise be tillering were only at the two leaf stage by mid-November. At that time, a second flush of blackgrass was appearing in September drilled wheats, so Mrs Hayes was able to recommend an across-the-board residual treatment based on three-quarter rate isoproturon and diflufenican (Javelin).
"Where blackgrass wasnt sprayed with a low rate of isoproturon early, I have in some cases had to recommend a more expensive mixture incorporating a contact herbicide such as Puma or Hawk," she says.
She expects the only spring problems to be spring-germinating wild oats and cleavers, which havent yet been fully tackled. The former will be controlled using clodinafop-propargyl (Topik) or Cheetah R, depending on the best buy at the time.
Diflufenican did a holding job on autumn emerging cleavers, as the isoproturon did on the blackgrass. But fluroxypyr (Starane), or metsulfuron-methyl (Ally) if other broad-leaved weeds are also present, will be recommended with the first growth regulator treatment.
WEST
IN THE Cotswolds to the west, Brian Keen will increasingly be recommending some spring applications of the sulfonyl-urea herbicide amidosulfuron (Eagle). Not only cleavers, but increasingly fools parsley threatens many winter cereal crops in his region.
Mr Keen hopes his growers wont have to return to control the main weed target on the clay soils – blackgrass. Poorer control in recent years, where resistance is developing, has encouraged greater use of Avadex at sowing. This year he fears it may not work as well as usual, due to the dry conditions at the time of application.
"I recommended that Avadex treated crops had a follow-up treatment in late October or early November with a mixture of IPU and trifluralin or diflufenican, depending on the weed spectrum. In theory this sequence should sort out blackgrass and broad-leaved weeds. However in late October it was too dry for IPU to work effectively," he explains.
Most growers delayed spraying, not only to wait until soils were moist, but also to avoid damaging crops during the incredibly cold snap in late October, when an air temperature of -7íC was recorded at nearby RAF Benson. Early November was wet and windy, delaying applications further. By mid-November, only 40% of Mr Keens cereal area was treated, compared with the usual 70%
His recommendations still apply, at least for part of December. "IPU works best when the soil temperature has fallen and seedbeds are moist. But unless the weather improves soon, well look back on this season as a difficult one."
NORTH
ITS not often that northern growers have things easier than in the south. But Andrew Fisher, agronomist for Yorkshire Arable Advice, considers this has been a straightforward autumn.
"We havent had the large volumes of rain in Yorkshire and County Durham that they have in the south and east. Much spraying was done in early November when it was dry and mild, and most should be completed before the weather closes in this month," he predicts.
Fortunately few fields in his patch contain blackgrass, so there isnt the need for an early "Rolls Royce" approach. Annual meadow-grasses and chickweed are the main threats in winter wheat. Those were tackled with a mixture of isoproturon and diflufenican.
"IPU was the obvious choice. By late October, its price had fallen to about £14 for a 5-litre can, and we used it with Oyster or Amulet at a sufficient rate to achieve control of all weeds, with the exception of volunteer oilseed rape," explains Mr Fisher.
Volunteer rape has become more troublesome in cereals in recent years, and this autumn it is at its worst in Yorkshire, despite the good harvest. Seed that has laid dormant for several years found conditions right for germination.
Fortunately, conditions were also right earlier on in the autumn to mix a little mecoprop in with isoproturon without risk of scorch. So controlling the volunteers hasnt been a problem. But paying attention to rape management after harvest would avoid having to take chances, he stresses.
All that remains to be done in the spring is to finish off the cleavers which were softened up by the diflufenican, probably with Starane. All in all, it should be a less challenging year than usual for northern growers.