Summary

The Australian Pesticides and Veterinary Medicines Authority (APVMA) has completed a review into use patterns for paraquat for crop topping in pulses. To prepare for a potential change in paraquat use patterns, this project tested reduced paraquat rates and alternative products in combination with, or instead of, paraquat in lentil and faba bean crops. Results showed that paraquat-based treatments were still the most effective at crop desiccation and reducing seed set in ryegrass. However, when lower rates of paraquat were trialled, paraquat plus Slasher® was the most effective combination.

Background

Crop topping is widely used in pulses in South Australia to reduce annual ryegrass seed production and desiccate crops to ensure even ripening prior to harvest. Paraquat has been the backbone of this strategy because it rapidly desiccates weeds late in the season, reducing weed seed set and the weed burden in subsequent crops. Growers need practical alternative herbicides or strategies that reduce reliance on full label rates of paraquat but still deliver effective control of weed seed set. The review did not recommend reduced paraquat rates for crop topping but the with-holding period has been increased from 7 to 14 days. Pre-sowing knockdown rates of paraquat have been reduced by approximately half.

Research Aims

The core objectives of the project were to:

• Evaluate the effectiveness of reduced rates of paraquat for crop topping in lentil and faba bean
• Assess alternative products, including Sharpen® and Slasher®, for reducing annual ryegrass seed set
• Determine impacts on ryegrass seed viability, crop maturity and grain yield
• Provide practical guidance on timing and product choice for South Australian pulse growers.

In The Field

Pot trials investigated application timing and efficacy of control of ryegrass seed set. Crop topping treatments were applied at early flowering and post-flowering of ryegrass.
Field trials were also conducted in lentil and faba bean crops at low and high rainfall sites in South Australia in 2024. Herbicide applications were timed to coincide with ryegrass at early to mid grain fill stage and when pulse crops were at appropriate maturity to optimise crop safety and reduce risk of loss of grain yield and quality.

Field trial treatments included:

• Full and reduced rates of paraquat (Group 22) applied standalone
• Sharpen® (700g/L saflufenacil, Group 14) applied as a spike with reduced rate of paraquat
• Sledge (25g/L pyraflufen-ethyl, Group 14) applied as a spike with reduced rate of paraquat
• Slasher® (525g/L nonanoic acid, Group 26) applied standalone or in combination with paraquat or glyphosate
• Glyphosate (540g/L glyphosate, Group 9) applied standalone or in combination with Sharpen®.

Ryegrass seed set, seed viability, crop yield and grain quality were measured and weed control was assessed both visually and by collecting ryegrass seed to test germination.

Results

For crop desiccation, the most effective alternative to paraquat at full rate (0.8L/ha) was paraquat at 0.4L/ha plus Slasher® at 3.5L/ha, achieving similar levels of brownout 14 days after treatment (14DAT) without impacting yield or pod drop. Paraquat at 0.4L/ha plus Sharpen® at 34g/ha performed similarly in some cases but was less consistent across sites. Treatments including glyphosate, Sledge® or lower rates of Slasher® were less effective at crop desiccation.

For reducing ryegrass seed set, paraquat-based treatments remained the most reliable options, with the exception of paraquat plus Sledge®. Reduced paraquat rates lowered seed set compared to untreated controls, but the efficacy was more variable. Paraquat at 0.4 L/ha plus Slasher® at 3.5L/ha achieved 100 per cent ryegrass seed set control. Paraquat at 0.4L/ha plus Sharpen® was also highly effective at reducing ryegrass seed set. Glyphosate or Slasher® alone were not as effective at reducing ryegrass seed set.

Seed viability of faba bean was not affected by any treatment. However, at the low rainfall site, lentil seed viability was negatively affected when Sharpen® was included in a treatment.