7 September 2001

Take steps to tackle weeds

This week we restart our

popular baseline advice series,

offering key management

information to help rebuild

arable profits. Oilseed rape

weeds are our first target

WEEDS in oilseed rape fall into two categories – those that are a direct threat to the crop and those that are an indirect threat but should be treated for the benefit of the rotation.

There are also unsightly weeds, such as poppies, which do not damage the crop, but are often treated for cosmetic purposes, says Bob Bulmer of Dalgety Arable.

"The direct threats are volunteer cereals and broad-leaved weeds, such as cleavers, mayweed, charlock, shepherds purse and sow thistle to name a few. The indirect threats are the grassweeds, which should be controlled in oilseed rape to prevent a rise in populations.

"Oilseed rape also presents a chance to use different chemistry on grass weeds, which is particularly important if resistance is a problem."

Establishment method

If the crop has been established by direct drilling or Autocast herbicide options are immediately limited, says Dr Bulmer.

"There are depth restrictions with the pre-emergence and early post-emergence herbicides, so you are really just left with the graminicides."

That should not be a problem where establishment has been good, as the crop is very competitive and weeds will be smothered.

Herbicide choices

There are four main pre-emergence/early post-emergence active ingredients to choose from – trifluralin, metazachlor, quinmerac and chlomazone.

"Chlomazone is new for this year, and is being sold as Centium. It is really a mixer product, and can be added to one of the others to extend the weed spectrum."

Both trifluralin and chlomazone have to be applied pre-emergence. "The other two can go on early post-emergence, but their activity is better pre-emergence."

On light soils, trifluralin should be avoided, says Dr Bulmer. "Use Katamaran (metazachlor+quinmerac) where poppies and cleavers are particular threats. Opt for Butisan (metazachlor) for other broad-leaved weeds, but add Centium to control cleavers and shepherds purse."

Medium and heavy soils should first be assessed for grass weed problems. "If grass weeds are a concern, use trifluralin and add Centium where cleavers exist. This treatment should then be followed with Kerb or Carbetamex.

If the grass weed pressure is low, Katamaran is preferred by Dr Bulmer for poppies and cleavers, Butisan for mayweed, and a Butisan/Centium mix to control cleavers and shepherds purse.

Other considerations are volunteer cereals. There are a number of graminicides on the market (Falcon, Fusilade, Checkmate, Laser), but Dr Bulmer highlights Aramo (tepra-loxydim) as having advantages with target-site resistant blackgrass.

Grassweed options

The next step is to deal with grass-weeds, with the chosen graminicide.

"That is usually in September, depending on weed growth. Think about the chemistry if you are trying to avoid resistance."

Late October or November is Dr Bulmers recommendation for the Kerb/Carbetamex follow up. "Kerb is preferred for wild oats. There is also Fortrol, which can not be applied beyond February. It controls shepherds purse and charlock, but the crop must be vigorous and well waxed at the time of spraying."

Dose rates

Do not be tempted to cut rates if using Katamaran, warns Dr Bulmer. "There is not much flexibility with this product."

The same applies to Kerb/ Carbetamex, as a full dose is needed for good control of blackgrass, ryegrass and brome. With meadow grass there is more leeway.

He suggests a rate of 1-1.5 litres/ha for Butisan and adds that the Centium rate depends on the need for cleavers control – use 0.33ml for cleavers or 0.25ml if they are not a target.

"The rate of graminicide depends on weed size. But bear in mind that there are few farms where grassweeds are not an increasing headache." &#42

10-POINT PLAN – OSR WEED CONTROL

1 Establishment method – reduced herbicide options with Autocast and

direct drill.

2 Variety – hybrids at lower seed rates more vulnerable to early weed

competition.

3 Weed targets – direct (volunteers and broad-leaved weeds) and

indirect (grass weeds) threaten crop.

4 Soil type – separate weed control plan for light soils and medium/heavy soils.

5 Pre-emergence options – new active chlomazone available as mixer

product this year.

6 Grassweeds – use trifluralin where grassweeds are problematic, but

not on light soils.

7 Graminicides – Aramo preferred for target-site resistant blackgrass

8 Kerb or Carbetamex follow up – use where needed in October/November.

9 Wild oats – Kerb is the best option.

10 Costs – remember grass weed resistance and make use of

opportunity provided by oilseed rape.