Weed control still to do despite autumn herbicides working well

Autumn herbicides have worked well this season, but there's still plenty of weed control left to do, not all of which has a blackgrass bias, as Louise Impey finds out.

The good news for most cereal growers is that there was greater use of pre-emergence herbicides last autumn, says agronomist Andy Scott of LW Vass, Bedfordshire/Hertfordshire, adding that most of them have worked very well.

"The conditions were spot on at application," he recalls. "Seed-beds were of the right quality and there was enough moisture. So in most cases they achieved what they were designed to do, which is to considerably reduce the grass weed burden."

However, the sudden arrival of winter put an abrupt end to field operations and left Atlantis supplies sitting in the spray shed, he says.

"Very little has happened since November. That means we've still got the majority of post-emergence blackgrass sprays to apply."

The next two months will be critical, he believes. "The situation with blackgrass and herbicide resistance now means that most growers are left with a one-hit opportunity. They need to get it right, for the sake of future crops."

Spring applications of Atlantis are more variable and can be difficult to time, points out Mr Scott. "Rising soil temperatures are needed, which for the last two years has meant waiting until April. And after the very cold winter we've just had, the soil is going to take a while to warm up."

Air temperature is irrelevant, he stresses, so a couple of milder days won't be enough. "You need soil temperatures of 6C and going up."

The other issue is that Atlantis no longer has a comprehensive tank-mix list, he remarks. "It will give other weeds, such as cleavers and wild oats, a good knock, but you may need to come back again if they're present in numbers or emerge later."

He finds that cleavers usually receive enough of a check from Atlantis to contain them until the flag leaf spray.

Wild oats are another weed that Atlantis has an effect on, he says. "If they're over-wintered oats, Atlantis will deal with them. Don't forget that 100ml of Topik can always be added to that spray, which will also help on the blackgrass."

Any later-emerging wild oats can then be treated separately with Axial.

"Wild oats have a nasty habit of suddenly coming through the crop in April, once the soil has dried."

His other main weed target on the lighter, chalky land is poppies. "For this situation, I'll be recommending the use of Presite. It can be mixed with Atlantis, which is fine as you are only using sulfonylurea chemistry for broad-leaved weeds."

Independent agronomist Andrew Wells, based in Nottinghamshire, has a similar situation.

"Autumn residuals have worked well and where Atlantis did get applied by mid-November, it's done what was expected of it."

However, wheat drilled at the beginning of October hasn't been sprayed yet and will need either Atlantis or Broadway Star, depending on the weed spectrum, he says.

"It's the same dilemma as last year. You have to get a balance between weed size and soil temperature - it's always preferable to target small blackgrass, but you need to have rising soil temperatures and sustained growth for Atlantis to work.

"We've seen enough trial results to know that there's always a period of poor control in January and February."

In the absence of blackgrass, Mr Wells will be using Broadway Star for ryegrass and brome control.

Non-blackgrass situation

Neither blackgrass nor Atlantis feature on the farms that ProCam Fieldcare agronomist Nigel Scott looks after up in County Durham and North Yorkshire, so his spring plans are rather different.

Only where ryegrass populations are very high does he recommend the use of a pre-emergence herbicide. "There has to be a severe problem for us to go to that expense. An early post-emergence treatment of chlorotoluron (CTU)/diflufenican (DFF) is a more cost-effective option."

Providing seed-bed conditions are good, that mix works well on annual meadow grass, ryegrass, wild oats and broad-leaved weeds, all of which are typical of the region, he explains.

"On non-CTU tolerant varieties, we'll either use a pendimethalin/Picolinafen mix or pendimethalin/DFF," he continues.

The loss of straight CTU has been a blow, he admits. "It was very useful for ryegrass, as we could tailor rates to suit the problem."

His preferred spring options are either Axial for wild oats and ryegrass, Pacifica for ryegrass and brome or Broadway Star for all three grass weeds, he says. "Axial is the most cost-effective while Pacifica is the best choice where there's brome as well as ryegrass, although it's harsher and more expensive. Broadway Star is good on brome, but doesn't control annual meadow grass."

Where an autumn spray wasn't used, Mr Scott will opt for Othello. "It works very well and will be needed where the weeds are getting bigger. Remember that we can't use straight CMPP at this stage anymore."

Otherwise, he's left with Pacifica.

"It's not a 'get out of jail free' card. It does control wild oats, ryegrass and brome, but it needs care with application and it will take expenditure up to £50/ha."

Cleavers will have been thinned out by the autumn residual, he points out. "My preference is to use an early spring treatment, at the T0 timing, of either Sekator or Hunter. It's cheaper to go early."

Case study: Michael Donnachie, Dorset

Dorset grower Michael Donnachie is also unconcerned about blackgrass, but finds that other grass weeds always need a spring follow-up.

He starts the programme in the autumn with a pre-emergence treatment of Liberator, which deals with the most pressing weed problems.

"Sterile brome is our main target. It's become a much bigger problem since we switched to min-till, so a pre-emergence herbicide is essential."

Wild oats often need a follow-up treatment too, he says, as they continue to germinate throughout the winter and into the main growing season. Axial has been effective on them in the past two years.

For the continuing brome problem, he plans to use Broadway Star this spring, although he has used Atlantis in previous years.

"Atlantis will hit wild oats and small cleavers as well, but the issue with it is getting the timing right. When it works well, it's very good. "

Getting good control of brome in wheat is very important, adds Mr Donnachie, as he has winter oats and winter barley in the rotation, both of which have limited herbicide options.

Where grass weeds aren't a concern and he needs to control cleavers and speedwell he will switch to Ally or Starane, with either Topik or Axial added where wild oats need controlling.

Blackgrass evident at all growth stages

Don't go by drilling date alone when making decisions about spring grass-weed control, as trial sites show blackgrass at a wide spread of growth stages.

This reflects the two distinct drilling and spraying phases during the autumn, points out Bayer CropScience's herbicides development manager Gordon Anderson-Taylor. Growers with early drillings might be surprised at how far some plants have moved on.

"September drillings are now tillering – ranging from one to three tillers – which means control will start to become difficult where resistance is present or a pre-emergence programme hasn't been used.

"These fields should clearly be a priority, as these plants will be very big if not tackled until March or April as happened in last season."

Meanwhile, for October sowings, growth stage appeared to be quite delayed for the time of year following cold weather since drilling (ranging from one leaf to one tiller – most are from one to three leaves).