20 October 2001

Troublesome trio resist arrest

With grass weeds continuing to fend off some of our most familiar products, Sarah Henly explores the herbicide resistance battleground

OXFORDSHIRE boasts some of the most advanced science students around, but it also has some of the most scientifically advanced blackgrass and ryegrass plants. Many field populations show an element of resistance to herbicides, which complicates control programmes.

Laurence Sim, agronomist and AICC member based at Goring on Thames, found the first case of ryegrass resistance 15 years ago, and has many growers with resistant blackgrass. He has had to learn to deal with the problem, so is as experienced as anyone can be.

"We still have resistant ryegrass on the farm where it was first picked up, and probably always will have. Its almost the perfect weed because it germinates at any time of the year in moist soil, the seed is shed before the cereal harvest, and it is persistent. But if we tackle it from several angles, we can keep it from taking off like blackgrass has in many areas. Fortunately we have a few years headstart with this weed."

Resistance prevention strategies should already have been put into place where a problem is suspected. However, Mr Sim recommends growers have plant material checked to confirm if resistance is indeed the cause, and which type it is.

"You need to know whether you have enhanced metabolism or target site resistance, and to which herbicides. Each blackgrass population is unique in terms of cross-resistance, and the same is no doubt true of ryegrass. A test can cost anything from £35 to £150, but it may save you wasting money on unsuitable herbicides."

Even where resistance isnt the main cause of herbicide failure, there is still a need to adopt a practical approach to slow its development. Ploughing regularly, delaying sowing using the stale seedbed technique, and using rotations to broaden the use of herbicides, all help to reduce blackgrass numbers. Cultural methods of control are less useful with ryegrass because of its germination habit and vigour, says Mr Sim.

"Sloppy husbandry can make all the difference between acceptable and unacceptable levels of control. We regularly achieve 95% control of resistant blackgrass populations after ploughing when other husbandry factors are up to scratch, but if theyre not, the weed comes back with a vengeance."

Producing a fine, firm seedbed to allow for maximum germination is important, as is careful spray timing and application. Resistant weeds must be tackled early, when they are at their weakest, which is usually as soon as they have emerged, though a two-pronged attack – pre-emergence and early post-emergence – is often most effective, says Mr Sim.

Mixing and matching herbicide active ingredients is arguably the best way to prevent resistance development. Using a product that is partially detoxified by the weed, or has no affect on it at all, only increases the selection pressure, and should be avoided where an alternative is available, he advises.

The choice of herbicides for blackgrass control in winter wheat is reasonably good. Where resistance is confirmed, Mr Sim usually starts the programme with a pre-emergence application of tri-allate (Avadex Granular) at the full rate of 15kg/ha. This season, he will also try the new pre-emergence spray-on alternative, Crystal, containing slow-acting flufenacet and pendimethalin.

He follows that with a mixture of pendimethalin (Stomp) and flupyrsulfuron-methyl (Lexus) where TSR occurs, or a clodinafop-proparyl plus trifluralin (Hawk) based mixture where only EMR is present. The partner is usually either Lexus or isoproturon (eg Tolkan). Mr Sim recommends maximum rates across the board, though the manufacturers suggested rates often depend upon the degree of resistance found.

In winter barley with resistant blackgrass, the follow-up choice is more limited. He favours Stomp plus an isoproturon product at the one leaf stage of the weed.

For Italian ryegrass control, the choice is severely limited. Commonly, Mr Sim recommends pre-emergence terbutryn (eg Alpha Terbutryn), which appears little affected by resistance, followed by Hawk. He will start some programmes with Crystal this season.

Crystal is marketed for blackgrass, meadowgrass, and loose silky bent control, but BASF product manager Andrew Jones claims it suppresses ryegrass and brome when used at the full rate of 4 litres/ha.

"Crystal should ideally be applied before weeds start to emerge, and followed up with an early post-emergence spray where resistance is present, for example, Stomp plus Lexus. This season, with plentiful seedbed moisture, it should offer good control of problem weeds with less application hassles," says Mr Jones.

Since Crystal works out more expensive than Avadex when used at full rates, Mr Sim will try it at 3 litres/ha where grassweed populations are likely to be moderate. Though currently less affected than most by EMR, like others it could become less effective over time.

"Its prudent to use mixes based on older products such as isoproturon and chlorotoluron for as long as they offer acceptable levels of control, to minimise the build-up of resistance to our premium herbicides. And where possible, choose active ingredients with different modes of action to the fops and dims, to reduce the selection pressure for TSR," he stresses.

In break crops such as oilseed rape and beans, theres the opportunity to control ryegrass with propyzamide (Kerb) or carbetamide (Carbetamex). Even the dim, cycloxydim (Laser), is effective provided TSR is not an issue.

Unlike their blackgrass and ryegrass counterparts, some resistant wild oats can be controlled using dim herbicides. The dim group must bind at a slightly different site to the fops because wild oats exhibit TSR to the fops only.

Dr Moss is relieved about that. "Wild oats fortunately arent as successful at resisting herbicides, and indeed at spreading, as blackgrass and ryegrass. There is little movement of genes between plants because they dont produce pollen. Thats why there are relatively few cases and no huge increase in the problem."

EMR in wild oats is restricted so far to the fops, tralkoxydim, flamprop-M-isopropyl (Commando) and imazamethabenz-methyl (Dagger).

For control of resistant populations, Avadex is probably the preferred pre-emergence choice, followed by isoproturon, chlorotoluron or difenzoquat (Avenge), to which no resistance has been found in the UK so far. Using a full rate while weeds are still small should do the trick, says Dr Moss.

"Some of the older herbicides are likely to be withdrawn from the market in the near future for a variety of reasons, putting greater reliance on newer herbicides which ironically are often not only more expensive, but may also pose a higher resistance risk. There is no room for complacency."