17 October 1998


The wrong tank-mix can have dire consequences. Lucy de la Pasture discovers how to avoid problems this autumn.

EVERY sprayer drivers worst nightmare is a mixture that has turned to jelly in the tank. Apart from the difficulties of unblocking pipes and filters, the amalgam in the tank also poses a significant disposal problem.

Reading product labels is a good starting point for avoiding such disasters. However there are a few basic factors influencing compatibility, explains Richard Makepeace of Oxford Agricultural Consultants. These are water volume, formulation and the number of products in the tank.

"Growers applying chemicals in low water volumes are most at risk – the lower the water volume, the greater the chance of a problem arising," explains Mr Makepeace.

Formulation of products will also be a major factor in determining whether or not they will mix. Extreme formulation types, such as wettable powders and emulsifiable concentrates, are often not compatible (eg Benazalox (benazolin + clopyralid) and Fusilade (fluazifop-P-butyl)).

"Salt formulations are often the prime cause of incompatibility," says Mr Makepeace. "They tend to ionise in the water and are then able to destabilise the pesticide."

Two commonly-used salt formulations are manganese sulphate and mecoprop-P or CMPP. The 15% formulations of manganese sulphate in particular, often require constant agitation when in tank mixture. The ultimate incompatibility occurs when these two products are mixed together – a solid gunge is the result.

In situations where manganese needs to be applied to a crop in combination with mecoprop (CMPP), then using a chelated formulation of manganese or a specially formulated inorganic manganese sulphate will avoid the tank-mixing difficulties.

The third major influence on product compatibility is the number of chemicals being added to the tank. "When there are more than three products in a tank-mix, the chances of an incompatibility occurring will increase exponentially," explains Mr Makepeace.

However, compatibility is only part of the story when considering what to tank-mix – crop safety is also a major concern.

"Crop safety problems can occur because of the number of different wetting systems being put together in a tank mix," explains Ms Hayes. "It is often safer to use pre-formulated products in the autumn rather than making your own; for example, use Jolt instead of pendimethalin + IPU."

Equally, some active ingredients are well known for their ability to damage crops under the right conditions. Isoproturon is a prime example, especially when used on barley.

"Wait until crops are frost hardened before applying IPU or if they must be sprayed early, there is no likelihood of frost occurring for at least a week after spraying," advises Cambridgeshire consultant Caroline Hayes.

Cypermethrin (for control of BYDV vectors) is an emulsifiable concentrate (EC) formulation and can hot up tank-mixes, especially when in combination with IPU. The risk of crop damage can be reduced by applying separately to the herbicide.

As far as the number of products in a tank-mix is concerned, a useful rule of thumb is not to exceed three-way mixes in the autumn if safety is an issue and five-way mixes in the spring, advises Ms Hayes.

Another factor which may in some cases affect the efficacy of the tank mixture being sprayed is the pH of the water, according to Brian Lewis of Intracrop.

"Most water is alkaline (pH>7) which can create problems when mixed with a pesticide which is susceptible to breakdown or degradation in alkaline conditions," explains Mr Lewis.

The longer the pesticide is left unsprayed in the tank the greater the loss of performance, which can approach 50% in a few hours. Applying susceptible pesticides as soon as possible will limit degradation but this will still occur until the spray droplet has dried.

Worst mixes and mismatches

Crop What not to mix And Why

WW/WB CMPP + manganese Incompatible. Will form jelly in sulphate the tank

WW CMPP + graminicides Antagonism – the CMPP will pre- eg Cheetah S or Topik vent the graminicide from working. If the graminicide is applied first allow seven days before applying the CMPP or if applied in sequence with CMPP, allow 14 days before applying graminicide

WW/WB Grasp + sulfonylureas Antagonism – to avoid, apply, eg Lexus, Eagle, Grasp either five days before Quantum applying a sulfonylurea or 15 days after

WW Lexus + IPU Antagonism – IPU interferes with Lexus action on some weeds, eg blackgrass and chickweed

OSR Fusilade + Kerb May be oilseed rape varietal restrictions. Refer to distributor for advice on rates

OSR Pilot + oil + Butisan S Crop safety + cypermethrin

OSR Fusilade + Folicur + Crop safety. Causes transient Hallmark chlorosis of crop

OSR Fusilade + Benazalox Crop safety

WW/WB Eagle (after Feb 1) + Crop safety – fungicide can chlorothalonil or encapsulate the Eagle and form propiconazole may be released by addition of deposits in the spray tank which another pesticide when spraying a sensitive crop such as sugar beet

OSR Laser + MBC Both mixes are antagonistic and (eg Bavistin DF) or will reduce weed control Laser + Hallmark + Butisan S

Worst mixes and mismatches

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