10 November 2001

Winter clean-up

Grass weeds, aphids and phoma are thriving. Sarah Henly seeks advice on getting crops clean and fit for the winter

STALE seedbeds usually offer a good opportunity to clean up on weeds. But not this autumn. Atypically, western growers who left uncultivated land to green up didnt have enough rain to chit weed seeds, while in the east, blackgrass tended to stay dormant until late September, emerging once the new crop was up.

Peter Gould, western regional technical adviser with UAP, is concerned that there is still a lot of blackgrass and annual meadowgrass at large this month.

"There was certainly less dormancy in local blackgrass populations than last season, though with less chitting because of the dry conditions, we werent able to kill as much as usual with glyphosate pre-drilling. Many areas received pre-emergence or early post-emergence herbicides, but despite that, we have been left with higher populations than usual."

Where minimum tillage has been introduced, which tends to be on the lighter chalks as well as heavier soils, the weed spectrum has changed, forcing management changes. Annual meadowgrass has become a greater problem, and the timing of glyphosate treatment more critical.

"It was hard to hold growers back because of fear of a repeat of last autumn. So some fields were sprayed off too early to control volunteers, brome and meadowgrasses. Brome came up again in such quantities that one or two crops of winter oats needed re-drilling. And even where growers played the waiting game, meadowgrasses have come back in earnest," says Mr Gould.

Early drilled cereals with rapidly emerging grass weeds required robust post-emergence treatments to deliver enough persistence. Mixtures such as Stomp (pendimethalin) plus Lexus (flupyrsulfuron-methyl), and Hawk (clodinafop-propargyl and trifluralin) plus Lexus, the latter with an oil adjuvant as recommended by Syngenta, seem to have worked well, though some crop scorch was inevitable in the unseasonably warm weather.

While some growers reduced seed rates to allow for drilling early, few have been brave enough, and consequently many crops are looking too lush already, warns Mr Gould. Mildew can be found easily in Consort and Claire.

East

Excessively high rainfall brought some winter wheat crops on too well. It has done the same for weeds, and delayed herbicide applications into the bargain.

Independent agronomist, Bill Goodacre, based in Suffolk, was surprised that stale seedbeds didnt help with blackgrass control. There was certainly enough moisture, but many seeds didnt germinate until late in September. This pattern of dormancy paralleled the situation in other parts of the country. Consequently, there was more blackgrass around than ever.

Where growers were able to travel, he recommended one of three pre-emergence options – Avadex (tri-allate), Crystal (flufenacet and pendimethalin) or trifluralin plus terbutryn – which nipped the threat in the bud. Elsewhere, there was a massive flush of blackgrass plants that needed a robust post-emergence mixture, such as Hawk plus Lexus with oil.

Growers still to spray may need to increase herbicide rates because of the sheer volume of weeds, he warns.

In contrast, oilseed rape is generally looking very clean.

Specific weed problems such as charlock may need attention now, depending on the amount of frosts experienced, suggests Mr Goodacre. To use Fortrol (cyanazine) successfully, the crop must have at least five leaves and be frost hardened. Then frost is ideally needed after application to break down the weeds cuticle.

South

Near the south coast, where phoma came in two weeks earlier than usual, a two-pronged attack is essential. But Bartholomews agronomist Laurence Power recommends the follow-up in mid-late January.

"In our experience, two applications of Punch C at near full rate, when thresholds are reached and in January to arrest the development of the disease sufficiently and obviate the need for later spring treatment."

The same principle can usually be applied to aphicides in cereals, but it may take at least three sprays at six-week intervals in the south to keep populations in check: "Ive never seen so many aphids in wheat crops – its the worst infestation for 15 years. The combination of early drilling, mild weather and lush growth has seen populations explode," says Mr Power. He wont relax his guard until February, when yield loss to BYDV subsides.

On blackgrass and ryegrass new pre-emergence herbicide, Crystal, has done a superb job. To his surprise, even cleavers has been checked.

North

In Yorkshire, independent agronomist Andrew Fisher is concentrating on meadowgrasses and the few emerged wild oats in early sown crops. Little has emerged yet in October-sown wheats.

With a cautionary note about frost damage, he is recommending "old faithfuls" such as isoproturon, diflufenican and mecoprop-P. To ensure larger grass weeds are killed, he suggests growers make up the maximum IPU Stewardship guideline rate of isoproturon – 1500g ai – using mixtures with diflufenican. A sniff of mecoprop-P can be added to take out big volunteer rape plants.

Before using herbicides, however, any manganese deficiency problems should be addressed. "We suggest growers use manganese sulphate powder rather than liquids or chelates. It should last right through the winter, and it can be tank-mixed with cypermethrin for aphid control where necessary."