Weed control in pastures

Weeds are one of the most common reasons pasture sowings fail. Paddocks can contain tens of thousands of weed seeds per square metre.  

Without intervention, species like annual ryegrass, barley grass and silver grass can overwhelm new pasture seedlings, especially in the critical first weeks after germination.

Flooding events can also trigger weed outbreaks. As outlined in the Seasonal pasture management page, floodwaters can spread weed seeds and create ideal conditions for germination.  

The presence of a green bridge—living plant material between crops or pastures—can also support pest and disease cycles, making weed control even more urgent.

Planning ahead

Weed control should begin at least one year before sowing and ideally be part of a three-year paddock improvement plan. This allows time to reduce seedbanks, improve soil fertility, and select appropriate pasture species.  

Things to consider when planning pasture renovation include:

• soil testing
• sowing rates  
• seasonal forecasts.

The Integrated Weed Management Handbook offers long-term strategies for both cropping and pasture systems, including herbicide resistance management and agronomic tactics.  

Types of weed control

• Spraytopping: Applying herbicide at flowering to prevent seed set. This is especially effective for annual ryegrass and silver grass.
• Silage or hay cutting: Removes biomass and seed heads before maturity. Early November hay cuts have been shown to significantly reduce seedbanks.
• Strategic grazing: Heavy grazing at the right time can reduce seed set and open the canopy for desirable species. Delayed grazing after sowing, as recommended in the Spring Pasture Management guide, helps protect young seedlings from weed competition.
• Cropping rotations: Using cereals or forage crops can suppress weeds and improve soil structure.
• Herbicide use: Targeted application based on species and growth stage. Always follow label directions and observe withholding periods.

Integrated weed control strategies

There are instances where producers may be able to incorporate both herbicide application and stock, a process called spray grazing, which is often used in pastures.

Spray grazing

Weeds are first sprayed with herbicides and then stock graze on survivors once grazing withholding requirements have been met.  

This can be an effective solution, as it may assist in meeting feeding requirements for stock while also controlling potentially hard to kill weeds due to stress or dust coverage.  

When considering this approach, keep in mind that though there is a low input cost compared to a spray application, grazing weeds may incur a greater loss in moisture and nitrogen. This is because stock take longer to achieve the same coverage across the paddock.

For information on grazing withholding periods check your herbicide’s label.  

Before putting stock on fallow paddocks to assist in weed control it is important to know the weed spectrum as there are weeds which can be toxic when consumed by stock. This is particularly important if they have been sprayed by herbicides which may make them more palatable.

In some instances, for example with the use of phenoxy herbicides (found in Group I), such as 2,4-D, the palatability of weeds increases after being sprayed. These herbicides mimic the effect of plant hormones and cause broadleaf plants to mobilise energy reserves to produce lethally abnormal growth. 

Always make sure to check the label before application, as regulations for these chemicals can be updated.