13 March 1999

adding a little extra

Its only recently that some agrochemical manufacturers have recognised growers may include adjuvants in their spray tanks. David Millar lifts the cloak of mystery around these potentially-useful additives.

"A substance other than water with no significant pesticidal properties which enhances or is intended to enhance the effectiveness of a pesticide when it is added to that pesticide" – Control of Pesticides Regulations 1986 for Great Britain.

Thats the official line on adjuvants – otherwise known to users and their advisers as spraying oil, wetters, stickers, penetrants, spreaders, super-spreaders, mineral oils, vegetable oils, extenders, emulsifiable oils…

Confused yet? Theres more to come: surfactants, polymers, tallow amines, non-ionic spreaders, phospholipids, alkoxylated oils…

While the official Government definition might suggest adjuvants can be all things to all products, the reality is very different, says Peter Holloway, of the crop and environmental sciences department at IACR Long Ashton. For residual agrochemicals intended to stay on the leaf of target plants, the grower does not use a penetrant which might suit another pesticide which has systemic action and therefore needs to be taken up by the plant.

Dr Holloway sees a number of uses for adjuvants in todays arable farming, including enhanced biological activity of a product, for dose reduction, to minimise loss of agrochemical, to improve safety of use, and to lessen environmental impact.

Surfactants account for about half the UK adjuvant market, mineral and vegetable oils for about 16% each, polymers and polymer formers for another 11%, with phospholipids taking a further 3%. Surfactants for use in tank mixes come from a range of different chemical groupings, while mineral oils tend to be based on paraffins or aromatics, and vegetable oils either on oilseed rape, soya or sunflower.

There are some vegetable fatty acid derivatives based mainly on methanol or similar substances. Best known of the polymeric adjuvants is synthetic latex.

Agrochemicals already have their own adjuvants in the formulation but that is always a compromise taking into account variables such as length of storage, or the need for anti-freeze and anti-foam protection.

Duncan Webb, of Long Ashton, has been comparing the effect of adjuvants used with agrochemical on differing plants. On water repellent leaves such as those on peas, rape, cereals, wild oats, brome or fat hen, emulsifiers enhance the chemicals activity. However, most adjuvants come out little better than water when used with chemicals applied to wettable leaves such as those on sugar beet. Using a organosilicone spreader on such plants might even spread some active ingredient so well it comes off the leaves, although very often this means it is ending up on lower leaves not otherwise reached.

Richard Newman of Interagro highlights Arma, a recently-introduced alkoxylated fatty amine, for its all-round effectiveness with a range of herbicides, plant growth regulators and fungicides, including new chemistry such as strobilurins and quinoxyfen. Sulphonylurea herbicides can be used at half-rate with Arma against weeds such as knotgrass, field pansy or chickweed to give similar performance to a full dose.

For stem-based diseases such as mildew in cereals, a modified organosilicone such as Slippa can give 120-130 times greater spread than water and move pesticide down the stem of target plants.

Newcomers pack more punch

The list of existing adjuvants is a long one but there are still new generation materials coming through which are claimed to be even more effective.

DONT try this at home – diesel was one of the first oil-based adjuvants. Thankfully, for the environment, these additions to the spray tank have come a long way since then.

"Oils have been used to optimise the efficacy of agrochemicals for many years," explains Headland Agrochemicals marketing manager, Julian Berry. "They have two primary modes of action. First, they lower the surface tension of agrochemical droplets, helping them to spread out to give the greatest leaf cover. At the same time, they interact with the waxy cuticle of the leaf surface to increase the speed and extent of agrochemical penetration.

"Considerable research and experience over the years has shown that, regardless of their origin, oils with a carbon chain length of around 18 atoms give the most effective combination of surfactant and penetrant action."

Diesel was one of the first oil adjuvants. The fractional distillation process by which it and other petroleum oils are produced results in highly consistent mixtures of similar chain length hydrocarbons. Early mineral oils were, however, relatively harsh to plant tissues. Indeed some of them were used as herbicides in their own right.

During the 1980s vegetable oils were developed as alternatives which are very much gentler on the plant than mineral oils.

But they are naturally multigrade – comprising fatty acids of many different chain lengths. This makes them inherently less effective as adjuvants and liable to cause unstable emulsions in the spray tank.

More recently chemists have tackled these limitations by subjecting the oils to a process of methylation to even-up the chain lengths, producing more stable and effective adjuvants in the form of methylated seed oils or MSOs.

The new Headland adjuvant – Fortune – is distinctly different from both traditional petroleum-based mineral oils and the vegetable or seed oils that have been developed as mineral oil replacers in recent years.

"We use a specific blend of fatty acids selected from UK rapeseed to produce one of the highest grade C18 oils available," says Mr Berry.

"To this we add a number of special emulsifiers to ensure stable emulsions are created with water as well as oil-based agrochemical formulations, maximising compatibility and efficacy."

Headland claims that independent trials show the new adjuvant technology may widen the role of adjuvant oils by having wide application with a range of agrochemicals, including sulphonylurea herbicides, a range of different fungicides – especially triazoles and morpholines – growth regulators, micronutrients and insecticides.

"For example, in a conventional GS28-31 tank mix a traditional penetrant might also give some assistance to the growth regulator but it will do little, if anything, for any fungicides, other herbicides or micronutrients in the mix. Using Fortune will optimise the performance of all the ingredients," adds Mr Berry.

Hitting the right mix

LOWER cost of production per tonne and extra profit await adjuvant users if they choose the right spray tank combination for their winter cereals.

Control of mildew, stem base diseases and septoria with new chemistry and the top-line triazoles has been made more cost effective by the addition of adjuvants.

Perth-based CSC CropCare already knew that using adjuvants with triazole and morpholine chemistry in integrated cereal systems could improve both effective control and gross margin. The company wanted to check the effect with later generation fungicides.

Riband wheat, Pastoral winter barley and Prisma spring barley were used in the field trials with kresoxim-methyl + fenpropimorph (Ensign), quinoxyfen (Fortress), cyprodinil (Unix) and azoxystrobin (Amistar), as well as older products including prochloraz (Sportak), tebuconazole (Folicur) and fenpropimorph (Corbel). The adjuvants used were Slippa (an organosilicone wetter), Arma (an alkoxylated amine), Bond (synthetic latex), and LI-700 (a soya-based phospholipid).

The results show significant improvements in disease control and yield, according to CSCs Dr Keith Dawson, with the most effective results coming from the alkoxylated amine and organosilicone surfactants. Growers now have the possibility of either maintaining efficacy while reducing fungicide dose rate in the presence of an adjuvant, or of increasing the fungicide effect by maintaining the dose rate with the addition of an adjuvant.

He warns, however, that it is important to select the correct combination of fungicide and adjuvant to suit the particular disease target.

In the mildew trials with winter wheat, half-rate Ensign with Slippa produced better disease control and more than 1t/ha of extra yield.

In light of mildew resistance and subsequent trials, best economic results were obtained using a split dose programme with adjuvants to apply a full rate of Ensign.

On winter barley, Arma with half-rate Fortress gave comparable powdery mildew control and slightly-improved yield compared to 100g/ha of quinoxyfen. Similar results were obtained with spring barley.

Septoria tritici control in Riband was stepped up by adding Arma to Amistar at the 125g/ha rate and to Folicur at 125g/ha. In both cases the yield response was over 0.5t/ha.

The trials with stem base diseases showed that the lower rate of either Unix or Sportak produced the most cost-effective control of eyespot when used with Slippa. The efficacy of the reduced rate of both prochloraz and cyprodinil was increased to that of the full rate by the adjuvant, says Dr Dawson.

COUNTERPLAN TO NEMATODE NUMBERS

Low nematode numbers dont always mean low risk as Lucy Stephenson explains.

Dont be surprised when your adviser asks you to sign a disclaimer if a very low PCN count means you dont want to treat. Its a move that Barry Chapman, managing director of Spa Crop consultants is spearheading following more farmers suing their advisers after disastrous PCN damage.

The initiative will protect advisers from such claims and will benefit the grower by ensuring best practice is followed, claims Mr Chapman.

A checklist ensures the adviser not only interprets the results correctly but informs the grower every step of the way. Both parties sign the form, but growers going against advice to treat must also sign the disclaimer.

Advice to treat despite low nematode counts has not been heeded by some, who have then suffered severe damage to their crops. Bill Lankford of Rhône-Poulenc reinforces the point: "Treatment decisions based on estimated yield loss are unreliable And the risk of error in low counts understating yield loss is very hard to predict and so yield advice may be misleading."

PCN can multiply 40-fold with each potato crop so advisers need to make clear that treatment at low populations is not a yield treatment but a long-term management strategy.

The problem is that PCN distribution isnt uniform and a zero count doesnt mean there are none in the field; there could be up to 10m eggs/ha undetected. Advisers should recommend that samples with low or unexpected results be retested, says Mr Chapman.

Results of soil tests depend on the accuracy of sampling and counting. To reduce sampling errors, Mr Chapman suggests a maximum sampling area of 4ha walked in a W, with at least 40 cores taken to ploughing depth. The samples should weigh at least 1kg each and be clearly labelled. A zero result from a 4ha W should be followed by 1ha samples to warn of hotspots.

Known hot-spots should be sampled separately. But Craig Chisholm of DuPont reckons patch treating is a false economy in the long term because both soil and PCN move.

Mr Chapman would like to see a quality assurance scheme for laboratories so that growers and advisers can have more confidence in their results. But while the control strategy is based on accurate sampling and counting, other factors must be taken into account:

&#8226 water uptake has a strong influence on the impact of the infestation.

&#8226 planting date

&#8226 disease

&#8226 soil nutrient status affects root development

&#8226 soil type, rainfall and irrigation both affect the amount of water available to an infested crops damaged roots.

Treatment should consider variety tolerance. Mr Chisholm distinguishes between tolerance and resistance: "Resistant varieties reduce PCN multiplication by hindering development of females. Tolerant varieties are more able to withstand attack and maintain yield," he says.


% Blackgrass Control at four sites

Boxford Stambourn Scunthorpe Lincoln

Topik (0.125) + 97% mineral oil (1.0) 93.0% 78.0% 94.5% 94.5%

Topik (0.125) + Fortune (0.5) 97.5% 82.5% 97.7% 98.4%

Hawk (2.5) + 97% mineral oil (1.0) 92.5% 80.0% 97.0% 99.2%

Hawk (2.5) + Fortune (0.5) 96.3% 83.8% 97.7% 99.2%

Amazon (1.0) + 97% mineral oil (1.0) 94.3% 82.5% 96.8% 98.7%

Amazon (1.0) + Fortune (0.5) 96.3% 82.5% 98.4% 99.2%

Whip X (5.0) + 97% mineral oil (1.0) – – 97.7% 93.7%

Whip X (5.0) + Fortune (0.5) – – 97.9% 95.7%

Source: Levington Agriculture & NDSM Ltd