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Mild winter hampering grass-weed control

The mild and dry autumn experienced across the UK has led to a rise in the area of winter cropping. Alice Dyer looks at what this means for weed control this spring.

Many fields that were destined for spring drilling have gone into winter cropping thanks to a mild and easy autumn, higher crop pricing and good seedbed conditions, according to AHDB.

 

This breaks the growing trend of increasing areas of spring cropping, seen widely across the country in recent years to combat black-grass.

 

 

Dr Stephen Moss, independent weed consultant, says feedback from this autumn’s pre-em applications onto black-grass was generally good, but the mild winter could be hampering control.


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“The severity of the winter can have quite a significant effect on whether black-grass and other grass-weeds that were damaged by or survived pre-ems will struggle through or not. If you have a cold winter, many of those damaged plants get knocked on the head, whereas if it’s very mild those plants can recover.

 

 

“It was something we noted when we did a lot of work in outdoor containers which simulated field conditions quite well. It’s not so much a direct effect of the cold, but in cold winter you tend to get a lot of frost heave and the surface is frozen and then it thaws which uproots plants and they’re then very vulnerable to desiccation if it does dry up. The long and short of it is cold winters increase levels of control.”

 

 

However, a mild winter also means that crops are growing well and competition against weeds will increase, says Dr Moss.

“It’s a two-way sort. If you have a well-established crop it can suppress a lot of black-grass so there is a balance between the crop and the weed which always applies.”

In the field

 

In the South West, agronomist Mike Burge, Agri and General Solutions, says black-grass that has survived pre-ems may be a challenge to control this spring due to its size.

 

“With the mild autumn and winter black-grass has continued to germinate until very late in the season and pre and post-ems will have broken down quicker than usual due to soil and air temperatures being so high.”

 

The increase in winter cropping will mean there is a high potential level of black-grass around in the spring, especially if people were tempted into early drilling, says Mr Burge.

 

“Well timed applications of the correct herbicide will have the best effect on populations that are growing through the autumn weed control. Any delayed applications where weed populations are high will have a significant effect on yield.”

 

Applications of herbicides such as Atlantis (mesosulfuron 30g/kg + iodosulfuron 6g/kg) or Pacifica (mesosulfuron 30 g/kg + iodosulfuron 10 g/kg) need to be applied to a growing plant, says Mr Burge.

 

Delay

 

“If we have a cold spell in early spring it could cause problems as the black-grass is already a significant size. The cold spell will stall it and then we’d need to wait until it gets growing again and control will be compromised.”

 

Any areas going into spring cropping must be clean before drilling commences says Mr Burge.

“Any spring crops going in on black-grass infested land absolutely must have any existing plants sprayed off before cultivations or drilling commences, because transplanted black-grass is almost impossible to kill.”

 

Broad-leaved weeds are also continuing to grow through the winter. “Broad-leaved weed control this spring shouldn’t be much different to usual other than there being a higher than normal population of cleavers which don’t seem to have stopped growing, and will probably need some Flurox (fluroxypyr) adding with it.

 

“Docks also don’t seem to have stopped growing and will bolt away in the spring if left to their own devices. They should be controlled once active growth begins and leaves are the size of your hand. Thistles much the same and best control will come about a week before they flower.”

East Anglia

 

In the East of the country, resistant black-grass remains the focus for farmers this spring, according to East Anglian agronomist, Stephen Baldock, Prime Agriculture.

 

“We could still have a decent winter yet and it could change. However, for early drillings into fairly clean ground, generally weed control is very good, unless it has black-grass then some of those early drillings do have quite a lot of weeds.”

 

Where autumn drillings were delayed, chemical is still targeting black-grass but whether survivors will emerge depends on the weather, says Mr Baldock.

 

“It’s hard to know whether it’s going to live or die - if we get a decent frost or a decent winter on it I expect there will be some more that will succumb but if it carries on being mild then it probably won’t.”

 

The increase in winter cropping is unlikely to have a significant impact on weed populations this spring according to Mr Baldock.

 

“Winter barley drillings were certainly up a lump and winter wheat is up a little. However, most of it was drilled at a sensible time and sprayed well because the conditions were good, which is part of the reason such a large area went in I think. It was an open autumn and people carried on drilling, but they also carried on spraying and the seedbeds were generally good quality so weed control shouldn’t be too troublesome. Where people didn’t get pre-ems on then that’s more of a problem. If you’re drilling, you need to be spraying.”

 

Broad-leaves

 

Broad-leaved weeds are less of a problem because black-grass weed control programmes keep them under control says Mr Baldock, but grass-weeds are becoming increasingly challenging.

 

“If they’re susceptible weeds there are plenty of options available such as Broadway Star (pyroxsulam + florasulam) for wild oats, ryegrass or brome. The biggest challenge is where resistance is and then you don’t really have too many options.”

 

According to Dr Moss, the increased resistance to post-emergence herbicides is something growers must consider during their herbicide applications this spring.

 

“Going forward you haven’t got many options in terms of black-grass herbicides when you get into the spring. Performance of Atlantis and Monolith (mesosulfuron + propoxycarbazone) is falling away, but how badly depends on the individual farm. Look at past experiences - if you’ve seen poor control before down to resistance, the poor performance should be expected again. All the research we have done at Rothamsted suggests resistance is a one-way ticket.”

 

If weeds are unlikely to be controlled with post-ems, Dr Moss says crop destruction can be a solution to prevent a worsening weed problem.

 

“If you’ve got a really bad problem, you need to make the decision if you’re going to spray off the crop or spend money on post-em herbicides which may not work. It’s a tricky decision and has big financial implications but some farmers do have to bite the bullet and not throw good money after bad. If its localised grass-weeds that are fairly restricted, specific areas could be sprayed off a bit later on in the spring if necessary.”

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