Dave’s Concise Stubble Spraying Guide

These are general principles that apply to summer weed control and pre-seeding spray operations in standing stubble.

The three main factors we have to deal with are:

1. Stubbles ‘catching-up’ too much of the spray mix on vertical surfaces,
2. Shadow effect behind standing stubbles due to forward movement, and
3. Turbulence, dust, and weed-seed or residue distribution caused by sprayer wheels.

Spray Operators have the ability to mitigate each of these factors by their own actions.

The first two factors of stubbles ‘catching-up’ too much of the spray mix and that of the shadow effect are both part of the same problem. Your spray droplets have a forward momentum due to your own forward speed. The higher the speed, the more severe the problems.

As to wheel tracks, that has a narrative all of its own. Spinning wheels on your boom-sprayer displace air. The result is usually to displace smaller droplets away from the wheel tracks; we have all seen this! That’s where nozzles with bigger droplets and greater momentum are a good choice behind the wheels, regardless of what else is on the boom.

Dust churned up from wheels will deactivate some products (e.g., Glyphosate) and the wheels may stir up and redistribute weed seeds and residues too!

5 Actions for Spray Operators to Take

1. Stating the obvious, slow down. Spraying speeds in light stubble should not exceed 20 km/h. In moderate stubble, we suggest 16 km/h and less in heavy stubble.
2. Angle your spray backwards a moderate 10 to 15 degrees. This will help compensate for your forward motion achieving a more directly downward path. This is easily done with new products recently available.

Regarding the tilt back, we are suggesting a moderate angle here, not an aggressively angled nozzle body that would require lowering the boom to maintain droplet velocity. We suggest a nozzle like ASJ Spray-Jet’s Compact Fan Air TILT (stubble JET) with a 13-degree angle built in that you can use in conventional horizontal nozzle bodies. You can check them out here.

1. If you are using air-induction nozzles, keep your pressure to the upper end of your manufacturers table of recommendations to achieve maximum droplet momentum.

The danger with air-induction nozzles generally is not with high pressure (due to their very low %age volume of small droplets), it is with LOW PRESSURE. Under 200 kPa or so, the fan angle of most air-induction nozzles will decrease affecting your evenness of spray pattern. Thus decreasing the momentum of your larger droplets. This may lead to poor kill and resistant survivors, all bad stuff! Regardless, always consult the manufacturer's full technical specifications when making your nozzle selection.

1. Increase water rates in line with the density of the stubble and dusty conditions. Higher rates are also required with contact herbicides.

The water rates are governed by label recommendations and we suggest that generally speaking, the more the better. GRDC’s Bill Gordon has given some useful guidelines in this video. Check out Bill’s comments here.

Another dependable reference on spray volume is Dr. Jorg Kitt’s “Spraywise – Broadacre Application Handbook” published by Nufarm (Australia). This comprehensive handbook standard issue to most Australian Spray Operators as part of their Level 3 Training Course.

“…if 50% of the ground is covered with stubble only half the spray volume will hit the soil directly; the other half will strike stubble. To optimize applications in this situation it is important to produce droplets with sufficient velocity to increase bounce and splatter. The applicator should use higher water volumes (70-100 L/ha) and a coarse to very coarse spray quality. In a clean bed spray quality and water volume have little effect. A coarse spray quality and water volumes above 50 L should be sufficient to achieve efficacy.”

Tom Wolf & Jason Deveau comment on their sprayers101 website that; “These volumes are low for North America (Australia uses nerve-rackingly low volumes) but otherwise this is solid gold."

1. Use a different nozzle configuration in your wheel tracks. It may even be necessary to go one nozzle either side as well in dusty conditions. We suggest one or more air-induction Twin Fan Ceramic nozzles behind each wheel with a higher output than the rest of the boom. Other considerations are nozzle placement to avoid updraft from the sprayer (Thanks Barry Haskins).