Best practice is critical when it comes to spraying herbicides both for immediate and longer-term results. At WeedSmart Week, agronomist Chris Davey explained how to optimise efficacy in four key steps from ‘tank to target’

WeedSmart southern region extension agronomist Chris Davey says there are four key factors around herbicides to consider as part of an optimal weed control strategy: the tank mix, spray application, the weather and the target.

Get these right and you can achieve greater weed control in the short term, while preventing weeds longer term by curbing weed seed-set and slowing herbicide resistance.

“It’s all about ensuring as much of that herbicide that you put in the tank hits the target and is taken up by the target, to do the best job possible,” he says.

1. Tank mix

Successful weed control, says Mr Davey, begins in the tank, and the objective should be to maximise the effectiveness of the active ingredient(s) in the solution. This can be achieved by using maximum label rates, but can also be impacted by what is mixed with herbicide in the tank. Firstly, water needs to be of appropriate quality so that it does not affect herbicide efficacy. ‘Hard’ alkaline water, or water with high pH levels, can remove glyphosate from the solution, while muddy water affects the action of paraquat and other similar products, he says.

When combining products in the tank, the mixing order is also important. Get it wrong and the mix can turn into sludge.  Along with this ‘physical antagonism’, ‘chemical antagonism’ can occur if incompatible chemicals are combined. For example, trials by YP AG in 2020 demonstrated a combination of glyphosate with metribuzin and propyzamide (herbicides commonly used with lentils) had less effective control against ryegrass than when glyphosate was used alone. Adjuvants such as wetters can also change droplet size and impact spray drift and efficacy, he says.

A GRDC technical guide, Mixing and Batching for Agricultural Chemical Application, features case studies with growers and spray contractors representing a range of operation sizes, crop mixes and rainfall zones. Summarising common agricultural chemical formulations and providing an overiew of common mishaps and how to avoid them, it is designed to increase spray efficiency and efficacy.

2. Spray application

After optimising the tank mix, the next objective is to ensure the maximum amount of active chemical hits the target when it leaves the nozzle. Droplet size (determined by nozzle choice), boomspray height and speed of operations are the key factors in reducing spray drift and maximising spray efficacy.

Sprayer speed and droplet size – so, nozzle choice – are two of the easiest things that we can do to reduce the physical drift, while boomspray height also plays a role in ensuring the spray hits the target.

Calibrating boomsprayers through a calculation using sprayer speed and nozzle output would help determine optimal chemical application rates by optimising the water volume applied, he says. Calibration would also help ensure that nozzles were operating as they should. “A general adage with nozzles is if you’re plus or minus 10 per cent from the manufacturer’s recommended output then it’s time to replace them. They are either running too fast or too slow,” he says.

3. The weather

When it comes to the weather, growers need to ensure herbicides are applied in conditions that will enable droplets to reach weeds efficiently. “While the weather is out of our control, we can make educated decisions about when we spray, knowing the conditions will impact how much of our spray is going to hit the target,” he says.

Current regulations prohibit spraying when hazardous ‘surface temperature inversions’ exist. Sprays applied in these seemingly benign weather conditions characterised by light winds, skies with little or no cloud cover, become trapped in a cool air layer and move in directions that are different to what the general weather pattern would seem to be indicating.

Spray applied at dawn, dusk and during the night is likely to be affected by hazardous surface temperature inversions. Growers also need to be mindful of Delta T – a measure of atmospheric moisture determined by the relationship between temperature and humidity. “Delta T is about droplet survival and if it’s too dry or too warm the droplet will evaporate, taking the chemical with it  and never making the intended target on  the ground,” he says.

4. The target

Mr Davey says that with tank mix, application technique and weather taken into account, the scene is set for optimal weed control. The only other factor to consider is the target itself.

If weeds are suffering from an internal or external stress, it may not respond favourably to herbicide, he says. “You want to make sure you are applying to actively growing and small weeds with no stresses,” he says. “Ryegrass, for example, is a classic. You throw a frost on it and it goes into survival or shutdown mode very quickly and doesn’t want to take up any chemical.

“So the objective here, and with the other three factors in mind, is to set the right parameters for the right weed in the right conditions so the chemical can do its job to the best of its ability.”

To help ensure these four steps are accounted for in a weed control strategy, Mr Davey encourages the development of a spray plan based on a consistent template for particular weeds and herbicides. “I encourage people to have a spray plan that can be understood by everyone so that you can very quickly infer what the best way to approach a certain weed or use a particular chemical is,” he says.

More information: Chris Davey, c.d@nextlevelagronomy.com.au

GRDC technical guide: Mixing and Batching for Agricultural Chemical Application