Comparing residual versus knockdown herbicides in sugarcane in the Mackay region

Key findings

• The runoff of three residual herbicides was higher than for the knockdown herbicides. Residual herbicides also persist longer in the environment.
• Reducing the amount of herbicide applied will reduce the losses in runoff.

About this case study

Weeds are a major contributing factor to losses in both production and profits for the sugarcane industry. Minimum tillage and green cane trash blanketing is considered best practice management in cane. Herbicides are used to control weeds; however, there are concerns about chemicals including herbicides being transported to the Great Barrier Reef. The photosystem inhibiting (PSII) residual herbicides are of greatest concern due to their potential to affect algae, seagrasses and coral.

Reducing the use of PSII residual herbicides, such as atrazine, diuron, hexazinone and ametryn, and using alternatives such as knockdown herbicides as part of an integrated weed management strategy, is considered best practice (Rolfe et al. 2007). Band spray application is also suggested as an improved practice.

This project in the Mackay region measured the amount of herbicides moved in runoff from a standard rain storm applied with a rainfall simulator.

Method

This study was undertaken on a sugarcane field with recently emerged ratoon cane. The site had a sandy soil (Sodosol), low slope and almost 100 per cent groundcover of trash and weeds.

The study involved treating plots with:

• bromide (a mobile salt tracer)
• residual herbicides – diuron, hexazinone and metolachlor
• knockdown herbicides – glyphosate and 2,4-D amine.

The application treatments were:

• blanket coverage (100 per cent coverage)
• band spray coverage (20 per cent, 50 per cent and 70 per cent coverage)
• no coverage (0 per cent coverage).

Rainfall was applied at 80 millimetres per hour for 40 minutes, one to two days after herbicide application.

Two to four plots of each treatment had rainfall applied using a rainfall simulator. Runoff rate and sediment concentration (almost nil) were measured through time. Runoff was sampled seven to 14 times for chemical analysis. Soil and trash samples were taken before the rain and analysed for the chemicals.

Results

• Residual herbicides (diuron, hexazinone, metolachlor) runoff more than the knockdown herbicides glyphosate and 2,4-D due to their inherent properties. Over time, these differences will become even greater because the knockdowns decay more rapidly than the residuals.
• Glyphosate and 2,4-D load in runoff was 40 per cent and 76 per cent less than the residuals for blanket spray respectively.
• Glyphosate and 2,4-D load in runoff was 68 per cent and 71 per cent less than the residuals for band spray respectively.

These knockdown products have lower toxicity in receiving waters than the residual herbicides (ANZECC/ARMCANZ 2000).