Long-term planning for grass-weed control – Arable Farming
After a challenging year, many farmers and agronomists may feel their grass-weed control has taken a step backwards ...
The focus now turns to autumn strategies to restore progress made over the last 10 years. Arable Farming asked three experts for their views.
Growers and their advisers have worked hard to get on top of grass-weed problems over the last decade. But this season a combination of factors – including the wrong weather at key timings – has affected weed emergence, seedbed quality, and herbicide timing and efficacy. As a result, weed control has, in many cases, been very variable.
While control of grass-weeds from pre-emergence programmes was surprisingly good in some cases, other growers struggled to apply post-emergence herbicides for brome and wild oats, for example, in the challenging March weather. This, combined with a lack of crop competition, saw high weed headcounts develop.
NIAB weed specialist John Cussans says: “Some farmers have done quite well; others have had a frustrating year struggling against a combination of factors. The weather was very dry after harvest, and before the main drilling window in October, so there wasn’t much of a chit of grass-weeds.
“Those that chose to delay drilling into November couldn’t get decent seedbeds, and there were some crop safety issues with large volumes of rain falling just after pre-emergence herbicides were applied. Finally, the weather in January and February was drier and warmer than usual, which meant winter crop canopies were not suppressing the weeds that survived pre-emergence herbicides as well as expected.
“The lesson is that we must focus on the crop canopy to suppress any surviving plants. The crop is finishing pre-emergence herbicides’ work to translate good plant control into good head control.
“I would encourage farmers to look at anything that will improve the crop’s outcome, such as drainage or resolving soil compaction. Those sorts of things are good general practice but should also be considered [apart of] weed control.”
Dick Neale, technical manager at Hutchinsons, says: “To stand still in black-grass control terms, growers need to be achieving 97% to 98% control, and this hasn’t been achieved this year.
“Farmers have worked hard and have been very successful in knocking the population of black-grass down, but they may have become a little complacent.
“We have seen about 90% of control of plants in the autumn but only about 60% control of black-grass heads by the spring. The weather has encouraged the survivors to tiller their socks off.”
It has been a similar story for many spring crops; the mild dry weather in February encouraged some farmers to sow earlier than usual.
“Looking at the seedbeds in February, I couldn’t argue against the drilling decision then. However, February is still mid-winter. Crops went into the ground, but black-grass had not germinated. Then heavy rain in March encouraged the black-grass to germinate once the crops emerged,” says Mr Neale.
Mr Cussans adds: “The only thing I can say with absolute certainty on weed management is: ‘never base your decisions on what happened last year’.
However, if we ever see autumn conditions like last year, where it was bone dry, we want farmers to leave weed seeds on the surface.”
There is no point cultivating in these conditions because it is just protecting the weed seeds from the weather, he says.
“The grass-weed species in the UK do not like long, hot, sunny days, so it is best to leave the seed exposed. These lessons must be remembered, but farmers must take this autumn as it comes.”
Growers are encouraged to start planning their weed control for next year by looking at the weed burden in fields now.
Mr Neale says: “Growers have got to be honest with themselves and identify those fields that are a red zone. If they are a red zone, it has to go into a spring crop. That can be a difficult decision, especially if the price rises again. It influences growers’ decisions, but it should not when grass-weeds are concerned. This happened last year-wheat was £280 per tonne, and some growers chose to sow winter wheat in borderline fields.
“Many fields have a background level of black-grass. It may only be 25 heads per sq.m, which would be difficult to see in the crop. However, those heads are returning 2,500 seeds/sq.m.
“Cultural controls, such as delayed drilling and a good pre-emergence herbicide programme, should give 95% control, which still means 125 plants/sq.m are uncontrolled. Those are the numbers we are dealing with.”
He adds that this is why many farmers were surprised to find very high levels of black-grass in fields they thought did not have a significant problem.
Mr Neale encourages growers to assess grass-weed populations in their fields and plan herbicide programmes accordingly. If black-grass plants are well-tillered, then this indicates that they are autumn germinating. Therefore, more emphasis should be placed on pre-emergence programmes.
If the population has fewer tillers and is smaller, then it is likely to be spring germinating, so herbicides in the autumn may do much less to reduce that population. The same is valid for identifying which brome species are in fields and adjusting the cultural controls to suit their particular growth habits.
The 2022/23 season was the first year the new pre-emergence herbicide active Luximo (cinmethylin) was widely available. Along with aclonifen, this has given growers new herbicide options within their programmes. Although a welcome addition to the arsenal, Luximo has also added complexity to the relatively straight forward programmes the industry has relied on for several years.
“We do not want to be hostage to weed management on-farm by spending a fortune,” says Mr Cussans. “We have gone from an era where you could easily put a herbicide programme together by starting with a flufenacet-based herbicide and adding products like Avadex depending on the risk. Now, there are lots of different ways to skin a cat.
“Not many farmers can use all the available chemistry at once to deal with their grass-weed problem, partly because of crop safety but mainly because of cost.
“We have seen improvements inefficacy with the new chemistry, but it does not remove the need for existing products. There is a synergy between existing products where farmers know their place and how they will perform.”
The addition of tried and tested herbicides continues to add value. While the new chemistry is working well in many cases, irrespective of the grass-weed and the programme, the addition of Avadex (tri-allate) can still provide a consistent and considerable step-up in control, according to Hank King, UK country manager for Gowan.
He says: “Last autumn, some farmers decided to reduce their overall use of Avadex on marginal fields where they questioned its value, and to move that proportion of their herbicide budget into more expensive new products.
“It is clear now that many were left regretting that decision, given the season that followed, and are now left with a greater problem going forward. In many incidences where that is the case, I would expect those farmers to return to using Avadex.
“This emphasises that there is no one product that will control grass-weeds by itself, and all of the available herbicides are important. Farmers should be looking to responsibly use all the cultural methods for control as well as modes of action they have at their disposal.”
As well as building a pre-emergence strategy, Mr King encourages farmers to look at the product’s activity and how this might impact follow-up treatments.
A good example is Avadex’s ability to inhibit surviving weed plants by stunting their development of leaf waxes as they grow through the treated soil area. This can leave them considerably more vulnerable to follow-on applications.
Looking at trials this year, there has not been a single program that has consistently been the best across all sites and situations, says Mr Neale.
“One thing we can conclude from this year, is that all the actives are important in controlling black-grass. But, for some products, it is not the case that always adding them to a programme will achieve value for money.”
Growers should continue to plan for the long-term when seeking to manage their grass-weed populations – even newer and more effective herbicides will only provide an incremental advantage. This needs to be combined with the existing cultural and chemical control options that have been successful in reducing black-grass populations on many farms in recent years.
Although it has been a difficult year for grass-weed control, the longer term direction of travel is positive. New challenges lead to lessons being learned, and with a good understanding of the issue plus a range of options, farmers can get back on the front foot in the coming year.
Email Us