THE pressure for growers to get across a large area in a short period of time has led to an increased use of complex tank mixes - but the efficiency gains of this practice can easily become unstuck if taking short-cuts results in not being able to spray the brew out.
FMC technical services specialist Stephen Pettenon said if there were many products in a tank mix, it became increasingly difficult to prevent adverse chemical reactions from occurring.
"If operators follow a few guiding principles it is possible to safely mix a complex combination of herbicides, insecticides and even crop nutrients," Mr Pettenon said.
"But it is also quite easy to end up with a tank of sludge that can not be sprayed out, if you don't take the time to get it right."
He said with several new products, such as FMC's Overwatch, Syngenta's Reflex and Callisto and Bayer's Sakura Flow, being released as suspension concentrates (SC), it was important to recognise that there was no guarantee that a desired combination could be mixed and sprayed out effectively.
Mr Pettenon said the first consideration was whether the tank mix was safe and if there were any biological antagonisms likely to arise.
"This is where one product impairs the efficacy of a tank mix partner or increases the risk of crop damage," he said.
"These antagonisms are relatively rare in pre-emergent situations, but where they occur they can also have implications for the evolution of herbicide resistance.
"The more common problem in tank mixes is the potential for the mix partners to be chemically incompatible, this can result in the formation of irreversible precipitate reactions or some components settling out of suspension and potentially causing blockages."
Tank mixing involves many products and so potential crop safety losses must also be a consideration.
The number one tip for complex mixers is for growers to take their time, as rushing is the most common cause of tank mix failures.
Products that are solid concentrate (SC) or water dispersible granules (WDG) need time to properly disperse and they also need sufficient solvent - that is water.
"Start by filling the tank to at least 70 per cent of its capacity with good quality water before adding any products," Mr Pettenon said.
"Each chemical must be added and dispersed fully before the next chemical is introduced to the tank.
"Keep water rates above 80 litres per hectare and ensure the agitation system is working well to improve the likelihood of keeping a complex mix in suspension."
It also helps to simplify the mix - keeping two or three products in a compatible mix is generally less challenging than achieving the same for a six or seven-way mix.
It's also important to be careful when choosing between formulations as not all products are created equal - some products are only available as a powder formulation (SC) and it is not possible for them to be produced as a more soluble, emulsifiable concentrate (EC).
Some formulations of the same active can behave vastly differently in tank mixes, for example, potassium (K salt) loaded glyphosates are often less compatible in a tank mix than isopropylamine (IPA) and monoethanolamine (MEA) loaded glyphosate products.
Mr Pettenon said K salt formulations have never been good mixers because the potassium ion had a high ionic charge and small molecular mass, so it had a high affinity to bind with other molecules.
"K salt formulations are known to cause flocculation issues if mixed with SC and WDG products and such combinations should be avoided," he said.
"There are some brands of potassium glyphosate formulations with complex surfactant systems that are mixing-friendly, provided agitation is maintained.
"Mixing order is crucial - start with correctly conditioned water and then add the least soluble formulation first, allowing time for each product to disperse before adding the next component."
When it comes to sprayer set-up, avoiding over-filtering and being careful when using transfer systems can help to minimise potential problems.
It is common for spray rigs to use filtration that is too fine for the nozzle size being used, but using the correct in-line and secondary filter for the selected nozzle can greatly reduce the chance of blockages.
Mr Pettenon said, for example, the standard 100 mesh filters on most spray rigs may not be the best choice for handling the mix.
"If using a single orifice nozzle that is 02 or greater in size, then using a 100 mesh filter, when a 50 mesh is adequate, will greatly reduce the area of passage and potentially increase the chance of blockages," he said.
"Pre-mixing some or all the products in a transfer system or nursery tank can have some advantages in time efficiency for refilling the sprayer.
"Problems can arise if the full mixture of chemical is added to a small nurse tank - for example, if the full load of components is added to a 1000 litre nurse tank destined for a 5000L spray rig refill, there is unlikely to be sufficient water in the nurse tank to allow for complete dispersion of the product."
Finally, if transfer systems are used it is important that the small tank contains only one of the spray mix components.
