Get prepared for grass weed control in wheat

Article taken from the Agronomist & Arable Farmer

Reports suggest that pre-emergence herbicides have done a good job on grass weed this autumn so now it is time to capitalise on this success with sprays in the spring.

The first stage in planning spring herbicides is to check the effect of autumn sprays and assess the weed spectrum.  For many people, pre-emergence sprays have delivered pleasing results.

“It’s the best residual activity we’ve had for many years,” says Chris Martin, Agrovista technical manager covering Northern England and Scotland.  Waiting until conditions got wetter in October was the key to success:  “Anybody that held back drilling and got the moisture has got really good control although people who drilled earlier in dry conditions haven’t done so well.”

This is backed up by his colleague, Tim Bullock, who advises on crops in Gloucestershire and Wiltshire:  “As long as the seedbeds were good then pre-ems have done a good job, especially where they were teamed with stale seedbeds.”

Chris Martin also thinks that the good control is partly down to increased attention to detail on residual herbicides.  “The pre-em spray is becoming more important in black-grass control to take the pressure off Atlantis WG (meso-sulfuron + iodosulfuron); a lot of residual went on this autumn with 240g/ha flufenacet plus pendimethalin, diflufenican and perhaps prosulfocarb.”

These reports from the field are backed up by early results from herbicide trials conducted by Bayer CropScience.  At Chishill in Cambridgeshire, researchers have tested the efficacy of Liberator (flufenacet + diflufenican) on its own and with tank-mix partners.

“Liberator is giving 80% black-grass control which is at the high end of the normal range,” says Gordon Anderson-Taylor, Bayer development manager.  “This is what we’d expect as there was a good seedbed and good moisture levels when we drilled on 10th October.

Looking at mixing partners, prosulfocarb was the strongest in a pre-em stack, adding 15% additional control of black-grass.  The trials also brought out the importance o f spraying Liberator at the true pre-em timing rather than early post-em.

In the trial, Liberator’s efficacy on black-grass dropped from 80% for a pre-em spray to 60% at the one-leaf stage and 35% at the three-leaf stage.

For annual meadow grass the decline was even more precipitous; control reduced from 100% at pre-em to 30% at one-leaf and 15% at three-leaf.

Spring Sprays

So now the question is how to maintain the good levels of control seen so far and give the crop the best chance of yielding well.

The choice of product for spring is largely dictated by the grass-weed problem on the farm – Atlantis WG is the first choice when black-grass is the main enemy whereas Pacifica (mesosulfuron + iodosulfuron) is stronger against brome or when there is a mixture of different grass weeds.

Once the target has been identified, timing is everything; smaller plants are far more susceptible to the herbicide’s active substance.  Against black-grass it really is a case of the sooner the better.  As long as the plant is actively growing and spray conditions are good, it’s worth getting on.

The benefit of spraying early is shown by interim results from the Chishil trial, where applications of Atlantis (0.4kg) + Liberator (0.3L) + biopower (1L) applied at one-leaf of black-grass on 4th November appear to be outperforming the same treatment at the three-leaf stage on 8th December.

However, the trial is still in progress and efficacy at the later timing is not complete.

Focus on black-grass control

Avoiding the temptation to delay post-em sprays is essential for good black-grass control.

“The successful pre-em programme is not a reason to hold back on the post-em,” explains Ben Giles, Bayer technical manager.  “Some people may like to wait so they can hit some of the spring-germinating broad-leaved weeds and wild oats but this will compromise black-grass control.”

“If you delay, the black-grass will be bigger, sprays are likely to knock it back but not kill it, meaning that it will shed seed later in the season and build up problems for next year.”

“There are other options for broad-leaved weed control later in the season.  Post-em Atlantis WG sprays are there to hit black-grass; any activity against other weeds should be treated as bonus.”

For those concerned about broad-leaved weeds, there is the option to get more protection from adding a residual partner to Atlantis WG.  Liberator contains diflufenican, which controls emerging broad-leaved weeds.

“A residual partner will be a good ideas for many people but the timing might be a little tight, so be careful,” says Ben Giles.  “A maximum of 03L/ha of Liberator can be used at this stage of the season and it must be applied before the wheat crop reaches GS 23rd or 31st March.

Going relatively early in the spring is also the approach of Chris Martin of Agrovista: “We need to get the black-grass while it’s still small.  In the current wet conditions I’d expect it to be at least mid-February before people can travel on all types of land.”

“Spray timing is always a compromise but I think that you’re better off spraying onto a smaller plant in cooler conditions.  Milder weather can increase the efficacy but will give you bigger plants to deal with.”

Tank Mixes and Sequences

Applying post-em sprays as soon as conditions allow means that grass weed control will be out of the way well before the fungicide programme starts.  Ben Giles again: “Keeping the two programmes separate means that the active ingredients will have the most effect on the target weed or disease, as well as reduce the risk of damage to the crops.”

Sequences can also be an issue particularly for those using liquid particularly for those using liquid fertiliser.  Wheat needs a minimum of seven days after any spray to allow the leaves to re-wax and in most cases leaving it 10 days is probably a better option.  A sequence which is tighter than this could potentially lead to crop damage.

Drastic Measures

Despite the national picture of a successful pre-em programme, some people will still have some patches in some fields with very high black-grass populations.  Even at this early stage, it may be worth spraying off these patches with glyphosate.  Spraying off is about saving money in the long-term as black-grass infested wheat will yield less and return many seeds to the seed bank creating further problems down the line.

Peter Brumption, AICC Agronomist in Lincolnshire and Nottinghamshire is keen on the practice.  “I’ve seen some farms where this happened last year and we got good results – the following crops are much cleaner this time round.”

“Spraying off is a big decision, so take if field by field or even by part field.  The most important thing is to get into the crop and get an idea of black-grass numbers before making the call.”

Making the decision now gives time to get a spring cereal properly and get a strong yielding crop.  Most growers will be limited to a spring cereal because autumn applied residuals in the soil mean that crops like rape, beans or linseed may not establish well. 


18 March 2015

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