19 February 2000

Its gripping stuff

Whether youre wetting, sticking, oiling orpenetrating, theres more to adjuvants than meets the eye, as Tom Allen-Stevens found out at a recent adjuvant academy.

ALTHOUGH water is almost always used to help apply a chemical to a crop, without help it would actually do an incredibly bad job. Water droplets are water-loving: they bind to themselves with strong cohesive forces but are reluctant to bind to anything else. The last thing water wants to do is spread unwetted pesticide evenly on to the crop.

It is this anti-social behaviour that causes UK agriculture to consume 4,000t of surface acting agents, or surfactants, every year. These are just some of the adjuvants that help mix a pesticide to the water and spread the tank mixture on the crop. You spray them routinely every time you pour chemical in the tank: most formulations already contain surfactants.

But in an attempt to cut spray costs, or improve efficacy, many growers and consultants have been adding even more into the formulation. When and how you use adjuvants relies on understanding what they do. At a recent adjuvant academy in Peterborough, Newman set out to do just that with a group of agronomists from Banks Agriculture.

Wetters

They are the simplest of all the adjuvant types and work by reducing surface tension.

  "When a water droplet hits a leaf, it will want to bounce off. If it does stay on, any movement will make it roll down the axil of the leaf and the chemical will be lost. Non-ionic wetters move to the air:water interface of a droplet. They then compete with one another to force water molecules apart and spread the mixture out," explains Newman managing director David Cameron.

The amount of non-ionic adjuvant in a product is linked to water usage. Cutting product rates can, therefore, compromise its ability to work properly. Adding an adjuvant could rectify the situation.

Non-ionic wetters can be excellent adjuvants for fungicides, especially organo-silicone wetters which work in a similar fashion, but do a much better job, says Mr Cameron. But these super wetters also have an additional use, at a higher rate, as a soil wetter. This can be useful when using soil-applied insecticides for leatherjacket control, for example. Tests by SAC at Aberdeen have shown that control is improved by nearly three-and-a-half times if Silwet L-77 is added to a full rate of chlorpyrifos.

Newman technical director David Foster sets out the theory: "Water will always take the path of least resistance, commonly resulting in vertical striping through fissures in the soil. The addition of a wetter gives a more uniform, horizontal band as it spreads around the surface of the soil particles."

Cationic amine wetters can damage cell membranes, so are mainly used with lower rates of glyphosate. Glyphosate tends to be locked up by calcium cations in hard water areas and these wetters also help to reduce this. Dr Cameron argues that a water conditioner, such as X-Change, does a better job cancelling the need for these wetters at all.

Adjuvant oils

There are three:

&#8226 mineral

&#8226 vegetable

&#8226 methylated seed oils

Oils are widely used to assist the activity of graminicides – some formulations even recommend it. Oils soften the plant cuticle, making it more vulnerable to attack from the herbicide. This makes them a vital ally in the fight against blackgrass.

Due to their effect on the waxy leaf surface, crop scorch can be a problem with adjuvant oils. Most growers, however, if faced with high populations of large blackgrass plants, would probably rather take the risk.

The snag with mineral oils is that they are derived from petrochemicals, and therefore not very environmentally friendly. Vegetable oils are better in this respect, but the rising stars are the methylated seed oils, which are made from rapeseed and generally used at half the rate of a mineral oil.

Penetrating agents

Based on lecithin, these are similar to oils in that they help the pesticide penetrate the waxy cuticle of the leaf. They cause less damage, however, as they pass through virtually unnoticed.

Once through the leafs defences, their task is not over. The lecithin forms liposomes, like tiny bubbles, that encapsulate the pesticide and translocate it through the cells of the leaf. This means the pesticide is cunningly redistributed around the plant, independently of the plants own mineral movement mechanism, xylem and phloem cells.

So penetrating agents can be useful materials to transport growth regulator to stem and leaf bases.

Stickers

Much like glue, they stick the spray droplet to the leaf. Pinolene or Terpene-based agents contain gums or resins that often need sunlight to help them set, which can be a drawback. Pyrrolidones, meanwhile, contain a mixture of solvents to improve crop adhesion.

The third group are based on synthetic latex. This is a material commonly associated with bondage, and this is no less true with agrochemicals. The latex in these surfactant formulations binds the drops on to the leaf, which reduces run-off and increases rainfastness, but being non-ionic do not bind with the pesticide to prevent uptake.

A common use is with fungicides on potatoes, when a shower can undo the work of a well-timed blight spray. Some formulations have a tendency to completely close up the pesticide so that those with a vapour action, like pirimicarb, are less effective.

Newman claims one step further is to mix the spreading properties of a super wetter with the adhesion of latex – launched last year as Designer.


Desired Pesticide Typical Rate (%age Approx

property type Crop product of water vol) price /ha

Penetration Graminicides Various Toil 0.5%* £3 @ 200l*

Sulfonyl-ureas Torpedo 0.1% £5 @ 200l

Fungicides Cereals Arma 0.1% £5 @ 200l

Penetration and

  mobility Growth regulators (CCC) Cereals Li-700 0.5% £5@ 200l

Wetting Growth regulators (Terpal) Cereals Agral 0.04% £0.24 @ 200l

Soil wetting Insecticide Various Silwet L-77 0.1% £3.60 @ 100l

Rainfastness Fungicides Potatoes Bond 0.14% £3.60 @ 200l

Coverage and

  retention Fungicides Various Designer 0.125% £3.75 @ 100l

Acidification Any Any BB5 0.5% £1.50 @ 200l

Glyphosate additive Herbicide N/A Ethokem 0.5% £2.50 @ 200l

* If mineral oils are used, the rate is usually 1%. In resistant blackgrass situations, methylated rapeseed oils should be used at 1% and mineral oils at 2%.