Archive Article: 1997/10/18
As the busy autumn spraying season gets underway, tank-mixing will no doubt be popular. But beware of the unwritten rules, warns Sarah Henly.
TANK-MIXING products which need to be applied at the same crop stage saves both time and application costs – if its done properly.
Provided the tank-mixing advice on all product labels is followed, spraying equipment is clean and tailored to the job in hand, there is nothing to worry about. But there may come a time when you need to spray right away, and theres no compatibility table to hand.
Or you may wish to make your own agrochemical concoctions. Theres no law against it, apart from restrictions on one or two of the more toxic chemicals. Provided the codes of practice are followed, including a COSH* assessment, theres no limit on how many products are mixed in the tank. Three is the norm.
Knowing which products mix safely takes experience. Get it wrong and the filters in the sprayer may not cope, or more importantly crops may suffer. Leaf scorch can occur if the crop has not hardened off and there are too many wetters in the mixture.
Even a three-way mix can be risky, say the experts, unless crops are sufficiently waxed up. Where sufficient moisture was available, the mild September weather allowed cereals to chit and grow unchecked. It takes a period of cool or frosty weather to encourage leaves to produce a protective, waxy coating.
Bob Pollard, arable specialist for Sherriff Crop Care based in Hertfordshire, would be wary of mixing an IPU-based herbicide with other products until crops have hardened off. The greatest scorch risk comes from mixing IPU with mecoprop or HBN-based herbicides or a solvent-based insecticide such as cypermethrin, he says.
Combining weed and aphid control may be a compromise anyway. "It may be easier and cheaper to apply a herbicide with an insecticide, but if aphids are present and you are waiting for the crop to grow to use a post-emergence herbicide, you could miss the critical aphid timing."
While crops are small and susceptible to damage, Mr Pollard feels happier recommending ready formulated products. For example, rather than use trifluralin and isoproturon he prefers Autumn Kite which contains the same two active ingredients but only one wetter. Emulsifiable concentrates such as trifluralin, used post-emergence in mixtures, increase dewaxing of the foliage considerably, he warns.
Mr Pollard applies the same principle to nutrients when tank-mixing them with agrochemicals, for two reasons. Some nutrient formulations contain wetting systems to improve their uptake, and these can interact, increasing the risk of both crop scorch and sedimentation.
"Usually if you use plenty of water and add the manganese to the tank first, you can avoid the products sedimenting out and blocking up sprayer nozzles. But I may, for example, recommend liquid manganese rather than manganese sulphate when tank-mixing is desirable."
Physical problems with mixing can be avoided to some extent if the sprayer is loaded in a certain order, depending on formulation (see table). But there are some exceptions.
"When mixing a morpholine fungicide with other fungicides such as a triazole, always add the morpholine last. But when tank-mixing it with propiconazole and chlormequat, the growth regulator should come last. And tebuconazole should always be added last in any mixture. Otherwise you can get sedimentation in the tank," says Mr Pollard.
The golden rule, according to Mike May, senior liaison officer at IACR-Brooms Barn in Suffolk, is to pour products separately.
Induction bowl
"Never put two products into the induction bowl together, or you could block the filters. Sulfonylurea herbicides are a particular problem – they come in small packs which are easily held in one hand so can be poured alongside others. But doing so can form a sediment or plastic layer on the pipes and filters. The sticky, solid result is difficult to clean off."
Sulfonylureas, which tend to be formulated as water dispersible granules, are more often used in the spring. Mr May has seen a bad reaction when metsulfuron-methyl plus thifensulfuron-methyl and the conazole fungicide, propiconazole were mixed. This is the reason the mix is specifically excluded on the label instructions.
Be sure to mix well, whatever the products being mixed, but particularly water dispersible granules, and powders which may need pre-mixing, says Mr May.
Matching filters to nozzles and products used can go a long way to preventing blockages. Tank hygiene is critical too. Following the approved guidelines ensures residues wont remain to affect the next spraying job.
Mr May has seen too many sugar beet crops killed because growers havent cleaned out their sprayer after applying sulfonylurea herbicides to cereals.
Sugar beet is one of the most sensitive crops to scorch damage. Ironically, four-way mixes are not uncommon in the spring.
Emulsifiable concentrate products such as some of the phenmedipham and ethofumesate herbicide formulations pose a serious risk when tank-mixed with oils or trace elements with wetting systems, stresses Mr May. Furthermore, emulsifiable concentrate formulations of phenmedipham should not be added to full tanks of water or crystalisation may take place, he adds.
If the mixture contains too many oils, albeit mineral or vegetable, tissue damage is likely. In general oils should be added to the tank after everything else.