Grass weed control in wheat can be markedly improved by better use of residual herbicides this autumn, according to the latest evidence from an integrated crop management research programme.
Application timing, spray settings and multi-functional adjuvants all have equally important roles to play in making the most of crucial residual chemistry, agronomy company Agrii insists.
“As residuals have become more and more vital in controlling black-grass and ryegrass, so it has become more and more essential to use them in the most effective ways possible alongside all the cultural controls in our armoury,” pointed out Agrii regional technical adviser, Will Foss. “That way we maintain their activity, not to mention our freedom to use them.
“The key weather factors affecting residual activity are moisture – which determines herbicide uptake – and temperature and sunlight which affect persistency. So, as well as giving time for the most effective pre-planting control of grass weeds, we find invariably later-autumn sowing also means better residual herbicide efficacy.
“Our work suggests October drilling just as soil temperatures begin to decline into ‘magic’ single figures results in the best persistency. Combine this with firm, fine seedbeds and pre- and peri-em residuals with a range of solubilities and persistencies and you have the best recipe for dealing with problem weeds regardless of soil moisture levels.”
While Mr Foss agrees that flufenacet needs to be at the heart of most black-grass and ryegrass control programmes, he is adamant that partnering it with complementary actives is essential both for the greatest efficacy and to guard against resistance development.
As well as a different mode of action, diflufenican is especially valuable here for its lower solubility and much greater persistence.
Late October sowing
Excellent black-grass control was secured from residuals at Agrii‘s Black-grass Technology Centre at Stow Longa last season. However, at its full 240g/ha rate pre-em flufenacet alone gave less than 15 per cent control from late October sowing. This was more than doubled by a co-formulation with diflufenican and flurtamone (Movon) delivering the same amount of flufenacet and boosted to over 80 per cent with the addition of tri-allate (Avadex 15G).
“Movon at 1-litre/ha is more expensive than the popular flufenacet/DFF combination, Liberator at 0.6-litres/ha, but across the 31 comparative trials we’ve conducted over the past nine years it has delivered 6 per cent better control of black-grass ears,” reported Mr Foss. “Based on our Stow Longa data and a 500 ear/m2 black-grass population, half as much DFF again and the flurtamone really helps here, giving an average 0.3t/ha wheat yield advantage worth £54/ha at current values.
“Six years of our trialling shows Avadex 15G continues to have a substantial performance advantage over the liquid formulation (Avadex Factor) with its lower tri-allate content. Reduced losses through volatilisation, though, means the later the application date the smaller the difference between the granule and liquid.
“The final icing on the pre-em cake we’ve found to be prosulfocarb,” he added. “Including Wicket at 2-litres/ha in late-October with the Movon, Avadex 15G stack at our Brackley iFarm in Northamptonshire last year lifted overall control of black-grass heads to within a whisker of 100 per cent. Interestingly too, even with this five active stack a delay of just seven days in drilling here was vital in ensuring the best result.”
Where the most robust approach is needed – to cater for earlier-than-ideal drilling or the most challenging infestations, for instance – Mr Foss recommends increasing the Wicket rate to 3-litres/ha and topping-up the residual programme with a peri-em spray 7–14 days after the pre-em.
For this he suggests an extra 120g/ha of flufenacet co-formulated either with diflufenican (0.3 litres/ha Liberator) or picolinofen (0.5-litresl/ha Pontos); the latter giving valuable extra contact and better broad-leafed weed activity and avoiding a total DFF dose of more than 120g/ha limit to reduce the potential risk to following crops of OSR or sugar beet.
Alongside firm, fine seedbeds and drilling to an even depth, of course, sufficient chemical activation in the soil is essential to residual performance. In this crucial respect – as well as in ensuring the greatest crop safety – Agrii trials have consistently shown the importance of including the specialist long-chain oil adjuvant, Backrow.
Indeed, 42 trials with the adjuvant over the past 9 years in a wide range of pre- and peri-em situations have shown an average increase in grass weed head control from 62 to 71 per cent. Based on an untreated population of 500 ears/m2 of black-grass, this 9 per cent improvement is calculated to be worth an extra 0.45t/ha of wheat yield.
It works by binding to clay particles in the soil to more consistently maintain a lethal dose of active herbicides in the weed germination layer. According to the most recent Stow Longa research this is particularly valuable in reducing the impact of a dry autumn on herbicide performance and maximising crop safety.
“Our study of late-drilling residual strategies last season showed that Backrow can be almost as effective as 1in of rainfall within a week of application in improving pre-em performance,” Mr Foss added.
According to Agrii, Backrow also has a positive influence on spray quality, noted regional technical adviser, David Felce, who also underlined the importance of a whole host of application tactics in making the most of residual herbicides.
“Soil’s almost infinite combination of different-sized, shaped and distributed aggregates makes it one of the most difficult ‘canopies’ to target with sprays,” he observed. “So everything we can do to increase both spray coverage and targeting can make a huge difference.
“For this we need the best balance between smaller droplets which cover a greater area with more spots of chemical, and larger droplets which deliver more of them on target. Halving the size of a droplet means eight times the number of spots for the same volume of liquid. But it also increases the drift risk, reducing targeting.
“By ‘engineering the water’, our work with Backrow shows we can substantially reduce both smaller and larger spray droplets, narrowing the spectrum considerably for the maximum overall efficacy. And we can do this with both flat fan and air induction nozzles and at a range of water volumes.”
Although it will reduce output by a third, Mr Felce is adamant that 200-litres of water/ha is much better than 100-litres/ha for pre- and peri-em spraying, highlighting Syngenta trials showing black-grass control of 85 per cent compared with just 50 per cent at the lower water volume.