Although broad-leaved weed resistance is relatively low on many growers’ list of concerns, a recently completed study indicates that overreliance on ALS inhibitor products is risky.
“To a lot of people it’s a minor issue,” says Lynn Tatnell, ADAS senior research scientist. “But relying on ALS inhibitors is a risky thing to do. ALS resistance is more of a problem in Europe where there is a lot more spring cropping and in cropping situations where the rotation is very limited.”
The four-year AHDB-sponsored trial, managed by ADAS, looked at control of poppies with known resistance to ALS inhibitor herbicides in winter wheat and oilseed rape (see panel for treatments).
An ALS inhibitor herbicide treatment alone was not effective at controlling the resistant poppies, says Mrs Tatnell. “An ALS and non-ALS mixture was more effective but highest efficacy was from a non-ALS programme. Oilseed rape is more competitive so there were less poppies in this crop than with wheat.”
With poppy producing such a large amount of seed – 20,000 seeds from one plant – unlike for black-grass, cultural control options are limited, adds Mrs Tatnell. “Putting a field down to grass will deal with black-grass in five years but poppies are there for life.”
Norfolk-based agronomist, Mike Thompson, says he has seen some poppy resistance to herbicides developing in his area in the last five years. “Sulphonylureas have been around for a while and it is not surprising that we are now seeing a scattering of broad-leaf resistance to them. It is the only group where I’ve seen a breakdown.
“The key thing is to know you’ve got it and identify it because dose, spray timing or inclement weather may be the problem.”
Mr Thompson says he has seen resistant poppies in spring cereals where sulphonylureas alone are being used for broad-leaved weed control. “In spring cereals where I’ve seen surviving poppies, the solution has been to introduce pendimethalin and the problem has gone away.”
BASF business development manager, Ian Ford, said: “We want to raise awareness of how to manage broad-leaved weed resistance in the UK so that we don’t find ourselves in the same situation as with black-grass.
“The trial results showed that we can still obtain excellent control of both susceptible and ALS-resistant poppies from a non-ALS herbicide programme, using well-timed applications of other herbicide modes of action.
“However, spray timing is critical; pre-emergence applications of pendimethalin provide the best control of poppies in cereals. Post-emergence sprays of contact-acting herbicides need to be applied early before the weeds are too big.”
Where ALS inhibitor resistance is suspected, AHDB advises collecting seed or plant samples for resistance testing and using alternative herbicides with lower inherent resistance risk.
To minimise resistance risk, avoid using ALS inhibitors as the sole means of broad-leaved weed control in successive years and use ALS inhibitors in mixture, sequence or rotation with herbicides with different modes of action, it advises.
Untreated; ALS inhibitor alone (metsulfuron); non-ALS and ALS inhibitor combination (flufenacet + pendimethalin followed by metsulfuron); and non-ALS inhibitor (flufenacet + pendimethalin followed by MCPA).
Oilseed rape treatments
Untreated; ALS inhibitor alone (imazamox); non-ALS and ALS inhibitor combination (imazamox + metazachlor; and non-ALS inhibitor (metazachlor followed by propyzamide + aminopyralid).