The threat posed by potential resistance to key residual herbicide, flufenacet, could bring harvest weed seed management further to the fore in the UK
Already popular in Australia for tackling herbicide-resistant ryegrass, harvest weed seed management could have a similar role against that weed in the UK too. But farmers and researchers are interested in seeing if it could help with black-grass control as well.
In Australia, difficult ryegrass populations prompted the search for alternative weed control techniques, says Dr Stephen Powles of the University of Western Australia.
“At harvest, any weed seeds left on the plant usually end up in chaff after combining. Most harvest weed seed management techniques rely on keeping chaff separate from straw to manage it separately.”
Chaff lining diverts the chaff from the main straw chopping process and places it in a line behind the combine, while chaff tramlining drops two rows of chaff in the tramlines. The compaction and concentration of chaff creates a more hostile environment for seed and consolidates it in lines rather than spreading seed across the field.
In Australia, they also cart chaff away and dump it on non-agricultural land, but this is not feasible for most farms in the UK. Likewise, narrow windrow burning is not an option in the UK.
“Seed destruction is a more thorough and, you could say, elegant solution. Chaff is passed through a rotary mill so weed seeds are ground into dust. The major downside of this is the cost of buying and running the equipment but it is effective,” says Dr Powles.
“The ryegrass life cycle means it is particularly susceptible to all these techniques. Its strength is it quickly develops tolerance to herbicides, but its weakness is that it is short lived in the seedbank and it stays attached to the plant until harvest. Any control method used should target the weeds’ inherent weaknesses.”
From his knowledge of the UK, Dr Powles thinks these techniques offer more for ryegrass control than black-grass, but he is aware of researchers working on adaptations to improve performance against black-grass.
One such researcher is Dr Paul Neve, formerly of Rothamsted Research, now at AHDB. Like Dr Powles, he thinks chaff lining, tramlining and seed destruction are of most interest to UK farmers and he is looking at how to use them for black-grass control.
Dr Neve says: “The big question is how much seed has been shed at the time of harvest. Some initial work we’ve done on black-grass suggests that, on average, no more than 10-20% of seed remains on the plant at wheat harvest time. However, if you pick earlier-harvested crops, such as winter barley or oilseed rape, that figure will be higher, but we haven’t researched the exact quantity yet.
“There are question marks about the applicability of the seed destructor to UK conditions and crops. Our crops tend to have more chaff and often have higher moisture levels which could also affect things.”
To investigate these differences, Rothamsted scientists borrowed a cage mill from colleagues in Australia. A cage mill is tool from the coal mining industry which was used for the initial development of seed destruction equipment.
“The plan is to run through various chaffs with a known quantity of weed seeds added. Whatever comes out the mill will then undergo germination tests to check the level of weed destruction,” he says.
Rothamsted hopes to demonstrate the cage mill at Cereals 2020.
Frontier crop production specialist Dr Paul Fogg is another researcher looking at harvest weed seed control.
Working in close partnership with Primary Sales and Essex farmer Jeremy Durrant, he has so far been impressed by the results of chaff-tramlining using a ‘chaff deck’ modification to the combine harvester. Work has taken place on three other farms too, all with successful results.
“Everything we do for weed control should be about reducing the amount of seed,” he says.
“Some 95% of seed taken in by the combine header ends up in the chaff, so it makes sense to manage chaff carefully. On existing combines, most chaff, along with associated weed seeds, is distributed across the whole header width via radial spreaders or mixed with the straw. Chaff tramlining consolidates the seed into two strips behind the combine wheels.”
Trials of the chaff deck began in harvest 2017, with measurements taken in a small number of fields.
However, Mr Durrant was so confident in the system he used it across the whole farm immediately because of the anticipated benefits. The chaff deck has been used in cereals, beans, oilseed rape and linseed without any issues.
“In 2017, the background back-grass population was 200-250 heads/sq.m. In the following crops we witnessed six times more black-grass in the tramlines than the rest of the field,” says Dr Fogg.
He is keen to stress this system does not need controlled traffic farming to work. The GPS technology on most combines allows farmers to keep to the same tramlines year after year.
“The next step is to look at further interventions. Consolidating seed into tramlines is a good start but there are most likely additional steps to improve control. For example, we are investigating mowing black-grass heads in early June to remove the main tiller. Other heads grow up to replace it, but they will shed seed later so the chaff lining will be more effective. Clearly, this would not be practical over a large area. However, on the worst affected areas it would be. It’s a bit like patch spraying with glyphosate, except you get to keep the crop. Early results are encouraging.”
Long-term, Dr Fogg thinks harvest weed seed control equipment will become a standard addition to combine harvesters. Reducing weed seeds at harvest is not a complete solution but it will make a useful contribution to the control programme.
The real benefits of the system is when you start to look at the efficacy across a rotation which combines early and later harvested crops.
“We need 98% control of grass-weeds to keep the problem in check and this is another method which should be cost-effective. Indications are a chaff deck will cost from £10,000-£12,000 but spread over the whole farm, say a 1,000-hectare area, over 10 years, this will give a cost of £1-£2 per hectare per year,” he says.
Bayer’s Ben Coombs agrees these techniques are likely to become more common in the UK, especially for ryegrass control.
“The principles behind it make a lot of sense. If a large number of weed seeds are in the chaff then it is sensible to manage it separately rather than spread it across the field,” he says.
“I have always emphasised it is better to focus on managing populations rather than percentage control. Like most techniques, this has the potential to make a huge difference when weed levels aren’t out of control.
“If farmers are already using a varied rotation, drilling wheat later with a robust pre-em such as Liberator (flufenacet + diflufenican) and spraying out the worst patches, then adding this on top will help push population levels down.”