October 1, 2018
Making the most of residuals
Grassweed control in wheat can be markedly improved by better use of residual herbicides this autumn, according to the latest evidence from Agrii’s integrated crop management research programme. And not just by using extra flufenacet or more complicated herbicide stacks or sequences either.
Instead, application timing, spray settings and multi-functional adjuvants all have equally important roles to play in making the most of crucial residual chemistry, the research-led agronomy company 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 grassweeds, 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 Will 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.
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% 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% with the addition of triallate (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% better control of blackgrass ears,” reported Will 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 triallate 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%. 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 – Will 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 120 g/ha of flufenacet co-formulated either with diflufenican (0.3 l/ha Liberator) or picolinofen (0.5 l/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 nine years in a wide range of pre and peri-em situations have shown an average increase in grassweed head control from 62% to 71%. Based on an untreated population of 500 ears/m2 of blackgrass, this nine percentage point 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 1” of rainfall within a week of application in improving pre-em performance,” Will Foss explained.
“Highlighting the importance of soil moisture, simulated rainfall through irrigation increased average black-grass head control in June with our favoured pre-em combination from 79% to 89%. This improved to 92% control when followed by a peri-em, against 84% with no added moisture.
“Including 0.2 l/ha of Backrow with both the pre-em and peri-em lifted non-irrigated control levels to the same 92% overall. So the adjuvant with both applications proved as effective as an inch of rainfall.
“Add to this the marked reductions in crop chlorosis and improvements in populations we’ve recorded by using Backrow with liquid Avadex in deliberately shallow-drilled wheat and the value of using a specialist adjuvant as part of the residual programme is clear. Not least given the very small additional cost involved.”
We should not ignore the very positive influence Backrow has on spray quality either, notes Agrii regional technical adviser, David Felce who also underlines 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, David 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% compared with just 50% at the lower water volume.
In the same way, he notes that doubling the boom height from 50cm to 100cm increases spray drift by a factor of 10, reducing black-grass control from 81% to 66% in trials.
“While the latest ultra-low drift nozzles give less soil coverage than I like by virtue of their very coarse droplet size, Defy 3D nozzles are a good choice, offering valuable flexibility to operate at a greater boom height with less loss of efficacy,” said David.
“Their bigger proportion of medium-sized droplets has given 93% black-grass control in trials against 85% for ordinary flat fan nozzles at the same 200 litres/ha. In my experience, the best results come from an alternate forward and backward facing nozzle arrangement.
“Finally let’s not forget spraying speed. Doubling forward speed quadruples turbulence, so it’s the enemy of accuracy. I always advise 10-12 kph as the best compromise between efficacy and practicality.
“Remember, small plot trial results tend to be based on the most accurate pedestrian spraying at less than 4kph. This puts a far greater onus on residual choice, adjuvant use and all the other elements of best application practice to get the same results in the field.”