Non-herbicide ways of managing blackgrass are becoming increasingly important. Andrew Blake relays some recent thinking on cultural controls.
With chemical sprays alone clearly unable to stop blackgrass infestations rising, agronomic trials are becoming more focused on the value of other methods, and useful pointers are emerging.
Agrii's Clare Bend highlights the problem. At the firm's Stow Longa trial site near Huntingdon, set up a decade ago, Atlantis (iodosulfuron + mesosulfuron) applied at any stage up to April consistently gave more than 90% control until 2008.
"In 2009, resistant blackgrass populations began to be selected, and ever since then acceptable control has been hard to achieve without extensive integrated control measures."
Rothamsted researcher Stephen Moss warns that high populations of resistant blackgrass can effectively rule out winter cropping.
"Cultural control becomes more important the greater your population and the worse your resistance. Too few farmers have resistance tests done and I'm concerned about some of the testing being done."
Good reference populations are essential to help interpret tests based on growing plants in pots or dishes, he explains. "Farmers and consultants should ask what standard reference populations are being used and if they don't get a straight answer, go elsewhere.
"Resistance testing is starting to get a bad reputation with some people, and I'm sure this is partly due to poor sampling and testing practices, and poor interpretation of the results."
Growers must use several non-chemical methods, including rotations, to reduce their need for herbicides and the risk of resistance developing, he urges.
Non-cereal crops, allowing alternative herbicides to be used, and fallowing help ease infestations, he says. "Fallowing in winter wheat rotations can give 70% control provided you prevent new seeding. But one year of fallowing won't be enough in severe infestations, and certainly won't eliminate blackgrass."
Fallowing, rotationally or as a well-planned one-off, can have dramatic effects on the viable seed bank, agrees Hutchinsons' Dick Neale. "But it must be actively managed for grass weed control. Fallow must not be used as a payment qualifier because that management flexibility is then lost."
Other cultural controls revolve around cultivations, drilling dates, sowing rates and the competitiveness of varieties.
When it comes to cultivation depth, Hutchinsons' research has a clear message, says Mr Neale. "Deep, non-inversion tillage loses control of the seed bank and creates the potential for high annual seed return.
"Ploughing can work when high seed returns are ploughed down, but can equally return seeds to the surface with subsequent use."
Growers must think about what they are trying to achieve through cultivations, says Dr Moss.
"Ploughing helps, especially rotationally, but good inversion is essential. I hear many people say that many farmers have lost the ability to plough properly because big tractors can 'force the job'.
"Don't reject ploughing simply on the basis that it brings old seeds back to the surface. That's only half the story - it often buries more seeds than it brings back up, and that half of the story is too often conveniently forgotten by those promoting non-inversion tillage."
On the worst fields at Stow Longa there was a clear benefit where crops were established using effective ploughing in 2011, says Mrs Bend.
However, where ploughing to a similar depth followed in 2012, quite a high population of still viable blackgrass seed was returned to the surface to germinate in the crop. "This supports current understanding that rotational, rather than annual, ploughing is worth considering."
For long-term grass weed control, Mr Neale believes shallow tillage to 50mm is the way forward. "Deeper soil loosening tillage can be used, but the integrity of the upper soil zone must not be compromised.
"Soil loosening and seed-bed preparation become separate operations – they cannot be combined." The former is slow, the latter fast, he explains.
Rolling has a big impact on seed germination, but often is used only after drilling, he adds. "Rolling during seed-bed preparation can strongly influence grass weed germination, but you need a proper tool – heavy rolls at a sensible forward speed. Light sets of finishing rolls bouncing along at 16-20kph will deliver nothing."
Farmers with bad blackgrass must consider delaying drilling, stresses Dr Moss. "By comparison, seed rates and varieties will help only marginally."
Herbicides tend to work better on crops sown later, he adds.
Delaying until mid-October offers the best chance of optimising blackgrass control while getting good crop growth to compete with the weed, says Mr Neale. "We've had up to 80% reduction in populations, and you have a good chance of maintaining acceptable seed-beds. Remember we've only tilled to 50mm so they shouldn't be a soggy mess that won't carry machines."
However, such tactics may disappoint in dry years, when blackgrass fails to chit in the stale seed-bed, warns Mrs Bend. Delaying drilling by two weeks in the dry autumn of 2011 achieved only 8% extra blackgrass control.
Results from the first year of a new Frontier trial near Newark are more promising, says the firm's Chris Harrold.
"We're seeing interesting comparisons between plots. For example, delaying drilling from mid-September until mid-October in 2011 resulted in 44% fewer blackgrass heads per sq m in the crop in June. This is a significant reduction given the lack of rain at the site last autumn."
Raising seed rates consistently improves blackgrass control - by as much as 20%, the firm's trials show. "But it needs planning to find the appropriate rate," says Mr Button. "Simply increasing it by 10% is of no value. You need to know the likely percentage establishment. That way savings in good, grass weed-free areas can be spent in areas which will benefit."
The influence of seed rate has varied, says Mrs Bend.
"In 2012, the improvement in overall blackgrass control from doubling the seed rate from 175 to 350 seeds/sq m was 10-15%. The benefit depends on seasonal factors and tends to be greater in dry springs. where the crop's ability to tiller is restricted.
"Small increases can also bring benefits, but the rates should be discussed with agronomists because unwelcome consequences, such as increased lodging, may result."
Variety competitiveness offers further fine tuning, affecting final blackgrass control by up to 25%, according to Hutchinsons.
Choice of oilseed rape variety is more critical than for wheat because delayed drilling is not an option, says Mr Button.
"Early establishment to get rapid ground cover that competes with the blackgrass is key. We favour semi-dwarfs D06 and D07 from Pioneer and Troy from DSV as they establish quickly, produce low interlocking leaves to blot out light at soil level, and do not fall over later."
The company is also exploring "companion cropping" – sowing a mix of vetches and clover – which among other benefits may help exclude light from emerging blackgrass, he says.
While other factors dominate wheat choice, the struggle against blackgrass is making a variety's ability to compete against the weed increasingly important, says Mrs Bend.
Although the ranking changes slightly from year to year, depending on the season, some useful information, apparently linked to tillering capacity, development speed and final height, has emerged.
"Varieties such as Hereward and Stigg compete poorly; whereas JB Diego and KWS Santiago are strong competitors.
"In the 2012 trial, the blackgrass headcount difference between good and poor competitors was more than 100/sq m. This, on average, equated to more than 1t/ha loss in yield – massive when normally variety selection might be made on the basis of a difference of 1-2% in yield on the CEL variety list."
Good field drainage is vital in the battle against blackgrass, warns Countrywide agronomist Simon Trenary.
"Drier ground harbours less blackgrass," he says. "The weed thrives in wet areas. Growers have done much to improve the structure of their top and subsoils, only to have the effects of all that work negated by blocked field drains and ditches."
Beware of spreading blackgrass when combining and baling clean fields after infested ones, he adds. "Small patches of weed seed can soon lead to populations all around the farm."