Resisting weed resistance

UK - October 22, 2009



Propyzamide is one of the few herbicides that has no evidence of weed resistance any where in the world, even after decades of use, according to Dr. Stephen Moss, weed resistance expert from Rothamsted Research.

"Propyzamide is an important herbicide for oilseed rape as we just don't have very many other effective materials available.  In oilseed rape over 70% of fields now have fop/dim target site resistance and so this chemistry cannot be relied upon when used alone.  The whole weed resistance situation has become even more complex, as grass-weed herbicides such as trifluralin and IPU are lost.  The situation is further complicated by the remaining grass-weed herbicides used in cereals and rape all having a very high resistance risk.  The best of the wheat black-grass herbicides has succumbed to widespread resistance, with over 250 confirmed cases and rising."

Dr. Moss reports that when it comes to black-grass you are talking big numbers.  "A normal black-grass population can produce 10,000 seeds/m² and, whilst 70-80% of black-grass seed is lost each year, 3% is still viable after 3 years.  Just think about the numbers and you can see that high levels of weed control in the 90%'s are needed just to keep on top of the problem."

He points out that propyzamide must be used to best effect, not just to achieve high levels of weed control, but also to minimise any risk to water.  Propyzamide is usually applied in winter oilseed rape crops, established by minimal cultivation, during November through to December.  By then over 90% of black-grass will have emerged.  However its residual activity can last for several months if soil temperatures are not too high."

"Although fop/dim herbicide resistance is widespread, it wouldn't effect the whole weed population in the field and it can be beneficial to mix these graminicides with propyzamide."

Dr Moss is of the view that the use of propyzamide in oilseed rape, alongside cultural control methods, has become essential in any grass-weed management strategy.

Independent agronomist Ruth East is of the same view and values the contribution that Kerb (propyzamide) has made over the last three decades.  Advising farmers in Lincolnshire, she has had to contend with resistant black-grass for nearly thirty years.

"It is amazing to think that black-grass resistance was confirmed in our area in 1981.  Managing weed resistance successfully has involved careful planning and integrating chemistry with cultural and cultivation methods, such as spring cropping, ploughing and the use of stale seedbeds.  It has also meant careful use of the chemical tools we have.  We are now in a situation where most grass-weed herbicides in wheat suffer resistance to one degree or another.  Reliance on effective herbicides in other crops, such as rape, is now the norm."

Ruth uses Kerb as it has no known resistance and it introduces a different mode of action into the rotation.  "I am very pro-Kerb wherever there are grass-weeds - not just black-grass but also a range of brome species including smooth-brome, meadow-brome and rye-brome."

In oilseed rape, she recommends propyzamide in a weed control programme starting with a metazachlor- based herbicide, pre- or peri-emergence.  "Kerb is then applied normally mid-November when soil moisture and soil temperatures are correct for good performance and when the black-grass is small.  A graminicide can be added to enhance contact activity."

"In my experience Kerb does a great job, providing conditions are right.  This means making sure that black-grass isn't germinating from depth and that there is adequate soil moisture in the top few centimetres of the soil profile.  Because it is taken up by the roots, Kerb requires moisture, but not too much that you run the risk of any water issues.  We have seen the consequences that IPU has suffered by getting into watercourses.  This is a serious problem for all residual chemistry and all growers and advisors need to be fully aware of the risk and act appropriately and responsibly."

For further information, please contact Robin Bentley, Dow AgroSciences on 01462 426613 or 07976 452492 (mobile).