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Herbicide resistance strategies

Oilseed rape is often seen as an opportunity to control pernicious grass weeds such as blackgrass and bromes. Volunteer cereals may also need controlling especially if there has been no opportunity to control them with glyphosate before establishing the oilseed rape. However, there are restrictions in place for the use of all of the grass weed herbicides either to reduce the buildup of resistance to the products or to prevent them getting into the water supply.

The table shows the main herbicide options for control of volunteer cereals and grass weeds in oilseed rape.

S = susceptible

S(R) = Resistance is increasing

S(R)* = see note below on Centurion Max

MR = moderately resistant

R = resistant

Carbetamide and Propyzamide are widely used to control blackgrass as there is no known resistance to these products. They are also valuable products for controlling Bromes. However, these products need to be applied once soil temperatures are below 10o C and falling. There are known water quality issues with these products and it is important to follow the stewardship guidelines to prevent these products reaching the water supplies (see below for guidance from the Voluntary Initiative).

Restrictions on using ACCase herbicides

The remaining active ingredients in the table are all ACCase inhibitors and are popular products for post emergence control of grass weeds and volunteers in oilseed rape. They are divided into 'fops' and 'dims' based on the chemical grouping of the active ingedient. Fusilade Max and Falcon are both 'fops' whereas Lasser and Aramo are 'dims'. Blackgrass, has developed target site resistance to both 'fops' and 'dims' and resistance to these products is now widespread. Italian ryegrass has also developed resistance to these products but it is less common. Wild oats have developed resistance to 'fops'.

The current Statory Conditions of Use to prevent further resistance developing means that any one of these products can only be applied once to any oilseed rape crop. There is also a restriction on applying a second product containing an ACCase inhibitor to a crop as this will increase the risk of resistance development; only use a second ACCase inhibitor to control different weeds at a different timing.

Note on Centurion Max

Dr. David Stormonth of Interfarm explains that, "One of the special attributes of Centurion MAX is that although it is an ACCase inhibitor product, it is active on strains of black-grass that are resistant to other ACCase products. In a paper given at the Crop Protection in Southern Britain conference, Dr. Stephen Moss of Rothamsted Research reported on six field trials with clethodim in rape where it gave significantly better levels of control of black-grass than tepraloxydim, even though the black-grass populations were known to show resistance to this group of herbicides. He concluded that the results indicate that of this herbicide group, clethodim could make a significant contribution to UK growers of oilseed rape."

In winter oilseed rape it can be applied from the 4 true leaf stage of the crop. One application can be applied per crop.

Don't misuse it

Dr. Ellerton of Hutchinson's advises growers and advisors to stick to a number of key guidelines that will protect Centurion MAX in the longer term.  "Firstly don’t cut the dose rate. Always use the label rate of 1 l/ha which is the effective dose. Secondly don’t use Centurion MAX as the only method of controlling grass-weeds, particularly black-grass. Use cultural control where you can in the rotation – such as stale seedbeds - and always use other active ingredients with different modes of action such as metazachlor, carbetamide and propyzamide in the programme."

"Thirdly don’t waste it purely to control volunteer cereals. Use it where you get the most value from it. In fact CRD have effectively stopped its early use on volunteers by imposing the earliest application date of the 4 leaf stage of the rape crop. This means that, abiding by the label, it really shouldn’t be used on early flushes of volunteers. Centurion MAX is very good on volunteers and it will take them out later but it is best to use another graminicide early on. Fourthly use Centurion MAX later on in the autumn when the grass-weeds are there at the right stage and always either mix it or sequence it with a residual."

Protecting water supplies

Unless extreme care is taken to protect water from propyzamide and carbetamide there is a serious risk that their use will be restricted.

How to stop propyzamide and carbetamide reaching water

All pathways matter. Follow basic water protection advice:-

• Take care when filling and cleaning the sprayer.

• Use 6m grass buffer strip, or 5m no-spray zone, beside water courses.

• Discuss cultivation and spray timing with your BASIS registered adviser.

• Manage soils and tramlines to avoid surface run-off or erosion.

• Do not apply when soils are cracked, dry or saturated.

• Do not apply propyzamide or carbetamide if heavy rainfall is expected within 48 hours of application.

How best to use the products

• Propyzamide and carbetamide work best when applied to cold moist soils, but this must be balanced with the need to protect water. Soils do not need to be completely saturated. Where practical, avoid use if  drains are flowing or are likely to flow in the near future.

Dose – Propyzamide

• Only use full rate of propyzamide (840g ai/ha) for severe Blackgrass populations. A lower 700g ai/ha dose will control all other susceptible weeds. Lower rates can give equivalent control when applied in  combination with other herbicides.

Dose – Carbetamide

• Use the full rate of carbetamide (2,100g ai/ha) when targeting  blackgrass: however if annual meadow grass is the problem then  1,500g ai/ha should be sufficient.

• Always check the required dose with your BASIS-registered adviser