Growers returning to oilseed rape this year are reminded to prioritise good establishment or risk storing up problems for following wheat crops.
Prices approaching £500/t and two decent seasons for rape are expected to see another increase in new-season plantings. Even a modest rise would take the national crop close to twice the area grown two years ago.
That expansion is likely to bring rape back into fields carrying significant blackgrass and ryegrass populations. UPL and Corteva are urging growers to treat weed management as a programme rather than relying on a single herbicide application.
Oilseed rape historically offered growers two important advantages: alternative chemistry and a highly competitive crop canopy, says ADAS weed scientist John Cussans.
“There are two reasons for that. First, there is different chemistry available in oilseed rape, which is like gold dust. Also, the crop itself is so suppressive. Oilseed rape was a godsend for cleaning up blackgrass at a time when rotations were very intensive. But over time, with the issues with cabbage stem flea beetle, the competitiveness of the canopy dropped away, and it exposed how important it was to get the herbicide element right.”
The consequences have sometimes been serious. Rather than reducing grass weed populations, poorly established rape has left some growers facing heavily infested wheat crops the following season.
Chemistry options
The principal herbicides available are Centurion Max, containing clethodim, and Kerb Flo 500, containing propyzamide, or AstroKerb, which combines propyzamide with aminopyralid. Both clethodim and propyzamide have unusual characteristics that make them valuable in resistance management, says Mr Cussans.
“In propyzamide, you have a root uptake herbicide applied post-emergence, which is quite unique. And then in clethodim you have a fop/ dim that you can’t bundle in with the other fop/ dims because resistance impacts it very differently.”
Clethodim remains relatively unaffected by the L1781 mutation found widely in UK grass weed populations, according to crop protection specialists UPL.
Studies of UK blackgrass populations found 85% of samples carried the L1781 genotype and remained susceptible to Centurion Max. Only 4% carried the R2078 mutation associated with resistance. A wider European study had also examined populations submitted because agronomists suspected resistance, says UPL technical sales manager Tom Wheelhouse.
“In a UPL study from countries across Europe of populations sent to us because agronomists suspect there is some resistance, we found that Centurion Max provided 94% control of blackgrass and 97% control of Italian ryegrass. It shows that where field performance has been an issue, it relates to factors other than resistance,” says Mr Wheelhouse. That puts greater emphasis on application.
UPL advises growers to apply Centurion Max by 15 October or when the crop reaches six leaves, whichever comes first. Blackgrass should be actively growing between the three-leaf and early tillering stages.
Growers should also observe a no-spray period for 10 days before and 14 days after treatment and follow restrictions on tank mixing. Water quality can also affect performance. Data gathered by independent weed specialist Stephen Moss found that including a water conditioner improved efficacy by an average 11% across six trials.
UPL also argues that formulation matters. Its analysis comparing Centurion Max with generic clethodim products found differences in mixing, spreading and drying characteristics, which can influence herbicide performance.
Timing pressure
The recommended strategy is to follow Centurion Max with propyzamide, giving growers different modes of action while spreading the risk associated with difficult autumn spraying conditions.
Growers increasingly had to balance optimum timing against the risk that suitable spraying opportunities might disappear, explains Hugh Guinan, field technical manager at Corteva.
“We are getting used to using propyzamide in less than perfect conditions,” he says. “In many cases, we need to compromise between efficacy, application conditions and date. What it means is that if conditions are suitable, we would advise getting propyzamide on, because the perfect date may never come.”
The decision is complicated because propyzamide persistence increases as soils become colder, while delaying treatment allows grass weeds to become larger and potentially harder to control.
“The sweet spot is usually late October to early December,” adds Mr Guinan. He recommends 850g ai/ha against high blackgrass populations, brome and ryegrass. The rate can fall to 750g ai/ha where populations are lower.
“Our experience shows that a Kerb Flo 500 or Astrokerb in sequence with Centurion Max is the best combination for maximum efficacy across the programme, and also resistance management of both actives,” says Hugh.
Canopy effect
Chemistry alone, however, will not restore rape’s rotational value. Crop establishment remains central to the strategy because a vigorous canopy suppresses weeds and reduces the pressure placed on herbicides. That means drilling decisions, soil conditions and management of cabbage stem flea beetle damage all have implications beyond the rape crop itself.
“One of the features of oilseed rape when it was considered a cleaning crop was just how suppressive the canopy was,” says Mr Cussans. “All of the elements involved in establishing the crop and getting it through any cabbage stem flea beetle damage, until it forms a decent canopy, are an integral part of good weed management alongside the herbicides.”
The potential expansion in rape therefore presents growers with both an opportunity and a risk. Strong crops, combined with well-timed clethodim and propyzamide treatments, could again reduce difficult grass weed populations before wheat.
But simply putting rape back into the rotation will not clean fields. Its value depends on establishing a competitive crop and protecting the remaining effective chemistry, says Mr Cussans. With the national area rising again, getting those decisions right could determine whether rape becomes part of the grass weed solution – or leaves an expensive problem for the crop that follows.