EXPERTS ASSESS ADJUVANT VALUE
By Andrew Blake
THERE is little doubt that spray adjuvants can boost pesticide performance. The difficulty growers face is identifying when to use them and when to leave them out of the spray tank.
Adjuvants do not have to jump the "efficacy" hurdle which applies to other products coming through the official approvals system. That means users must rely on makers claims or additional trials to assess their value.
Agronomists in particular need to know how the wide range of oils, wetters, spreaders, stickers and other pesticide enhancers perform when used with herbicides and fungicides.
For the past six years a group from eastern region membership of the Association of Independent Crop Consultants has carried out experiments involving adjuvants.
The AICC-funded trials, which are still under way, focus on controlling blackgrass in wheat. It is an area where dose, timing, mixtures and adjuvants all have a big impact on the outcome, says AICC member John Clarke of Suffolk-based Independent Agro-nomy.
In the early days the main area of investigation was how the newly introduced Cheetah R (fenoxaprop-ethyl) would perform in mixes with other contact herbicides, in tandem with residuals, and with or without adjuvants.
More recently the work, reported by Cambs-based Caroline Hayes and Herts-based Jamie Mackay, has examined differences between Cheetah R and the newer Cheetah Super (fenoxapropo-P-ethyl) and compared the effects of various wetters with Topik (clodinafop-propargyl).
In 1994 there were two trial sites – at Thaxted, Essex, and Bury St Edmunds, Suffolk – each with a different spread of susceptible and resistant strains of the weed. Each had four replicates and were sprayed at the end of January, control levels being assessed in May.
"We screened a whole range of additives," says Mr Clarke. They included oils, non-ionic wetters, acidified penetrant materials, and glycol-based Galion. "We even included some of the newer silicone materials. But we didnt use the vegetable oil Codacide as the method of use (mixing it with the concentrate) is different."
In general the trials back Cibas view that for inclusion with Topik, mineral oils "give us the edge" over other additives.
Mr Clarke believes a key to good blackgrass control is to slow the rate at which herbicide solution dries on the leaves. "Its all about getting the chemical into the plant."
By acting as an anti-evaporant, rather than as a spreader, oil prolongs the period available for absorption, he explains. "Keeping the plants wet is the answer."
This theory is reinforced by relatively poor results from spraying in dry conditions. "Even in February you can get dry periods when this group of herbicides dont work so well."
Adjuvant price is another factor that needs to be taken into account. "Oils cost about £2/ha – some of the more expensive additives on the market can be three times that," he cautions. "Non-ionic wetters can be even cheaper than oil, but theyre not very good with these particular fops and dims."
Droplet size is also important, especially when tackling monocot weeds, he advises. "Youve got to be at the fine end of medium spray quality to get the coverage."
The herbicide work has yielded useful information (see box). By contrast using adjuvants with fungicides is "a minefield", says Mr Clarke.
"Its really a matter of defining your target. The problem is that with fungicides you are rarely dealing with just one disease and one part of the plant. So there is no general answer. It all depends on what you are after. In contrast this recent work with fops and dims is in a very specific subject area." *
• Scope for cutting dose of Cheetah R, but depends on weed growth, weather and blackgrass susceptibility:
– avoid cutting rate when well tillered (GS28-32).
– marked differences in day/night temperatures and dry spells reduce efficiency of Cheetah.
– susceptibility can only be determined by testing plants grown from collected seed.
• Cheetah R more reliable than Super at reduced rates.
• Topik seems less dose flexible than Cheetah.
• Some populations more susceptible to Topik than Cheetah at present, but situation "evolving".
Blackgrass dying back after treatment with a mix of contact and soil-acting herbicides with oil. The oil is thought to help by slowing evaporation of the mix from the leaves, so increasing absorption.