This project examined the costs and benefits of defoliation strategies used in the defoliation of high input irrigated cotton crops. The project comprised of a field trial and a survey of irrigated cotton enterprises.

The field trial evaluated defoliation efficacy through ground rig and aerial application and measured crop damage from ground rig application.

Similar defoliation efficacy was achieved using ground rig and aerial defoliation for the first pass in this project.

Crop damage sustained through ground rig application must be considered in calculating the costs and benefits of application method. Based on a price of $12/ha for ground rig application and $16/ha for aerial application, cost per application by ground rig is lower than aerial application when damage is taken into account.

The benefits of ground rig application are lower drift risk and smaller down wind buffer zones than occur with aerial application. Area to be defoliated is a limiting factor for ground rig application. Arial application is an important method of defoliant application in large scale enterprises.

Efficiency drivers for both ground and aerial application were identified. These were boom width and speed of travel for ground rig application and volume for aerial application

The survey identified current defoliation practices used and the decision making drivers behind the practices used. From the current practices used by growers, a set of best practices for defoliation was identified. These practices achieve good defoliation efficacy while minimising spray drift risk and effects on the environment.

Best practices currently used by growers were identified as:

• Canopy management with plant growth regulator to enhance evenness and defoliant penetration.

• Sprayer set up which maintains efficacy while minimising environmental impact by using a coarser spray quality for example Turbo TwinJet®twinjet nozzles (standard or the air inducted version) for ground rigs and CP nozzles for aircraft.

• Droplet size – using bigger droplets (medium to coarse spray qualities) which maintain efficacy but minimise drift risk and the down wind no spray zone distances required.

• Using ground rigs to defoliate next to sensitive areas.

• Using a pesticide application management plan to manage spray drift risk in accordance with mandatory down wind no spray zones in accordance with new label guidelines.

• Defoliating as much are as possible when wind conditions are appropriate.

• Defoliating the crop in the least number of passes which reduces the drift risk and potential impact on the environment.

Decision making drivers for defoliation were identified

Timing of the first pass is primarily driven by crop maturity (top boll maturity and openness) with the timing of subsequent passes is more influenced by weather forecast (rain or a cold snap) and picking schedule

Application method (ground or air) is predominantly decided on by the area to be defoliated. Canopy density, weather forecast and soil moisture also play a part in the decision making process.

Sprayer setup is primarily driven by canopy density, label requirements and downwind buffer zones

Canopy density is the main decider for application volume, with products used and weather conditions as the other contributing factors.