Some farmers across the region have begun summer spraying following rainfall on December 21, with much of the Wimmera and Mallee receiving up to 20 mm. Reports from Birchip through to Berriwillock suggest weed pressure has been less than expected after the pre-Christmas rain, which has come as a relief.
Western Ag Agronomist Spencer Weir noted that summer weed germinations have been sporadic in the Wimmera and Mallee region. He’s mainly finding a few weeds on hay stubbles or light loamy soils. However, he expects to find more consistent germination after this week of hot weather.
The investment in summer spraying has proven well worth it, based on the results seen this year. Following a decile 1 growing season in 2025, achieving average yields of around 3 t/ha this harvest, emphasises just how valuable moisture retention strategies are.
“It was well highlighted this year and last which growers were diligent and had summer weeds under control early, because they had crops that handled the dry spring, compared to those who didn’t,” Spencer said.
Timing is everything
While summer rainfall creates opportunity, it also brings risk. Past research at BCG has consistently shown that controlling summer weeds is critical for retaining valuable soil moisture, which in turn supports stronger crop yields in the following growing season. In 2020, BCG trial work demonstrated a 1:5 economic return on investment from summer weed control, with yield increases of 0.7t/ha on sandy soils 0.9 t/ha on clay soils [1].
The most effective strategy is to start early and target small weeds while they are actively growing and easier to control. Research indicates that summer weeds should ideally be sprayed within two to four weeks of germination to maximise moisture retention [2].
“Targeting younger weeds result in cheaper knockdowns, compared to larger stressed weeds that requiring more expensive, double knock applications – not only costing yield but increasing chemical usage.” Spencer commented.
BCG’s water use efficiency research has also shown that spraying summer weeds within 10 days of a significant rainfall event resulted in the greatest subsequent winter crop yield. Importantly, even weed control carried out up to three weeks after a rainfall event still delivered a yield benefit compared to leaving weeds unmanaged. Some control is always better than none.
Spraying in the heat
With temperatures across the Mallee and Wimmera expected to exceed 40 degrees this week, growers are reminded that temperature plays a major role in herbicide performance. Avoid spraying when temperatures exceed 28 °C, particularly when using phenoxy herbicides [3]. The best time to spray is late afternoon or evening, or very early morning. This timing gives herbicides the greatest opportunity to translocate throughout the plant before it shuts down under heat stress [3].
Prioritising paddocks
Paddock prioritisation is critical. Start with paddocks that had established weeds prior to the recent rainfall, as these weeds will be larger and already drawing heavily on stored soil moisture and nitrogen [3]. Regrowth from harvested crops is currently one of the biggest competitors for stored soil water and nitrogen.
In paddocks that were clean before the rain event, weed control should be prioritised based on overall weed density. Correct weed identification is essential to ensure the most effective control strategy is used.
Uncontrolled summer weeds can rapidly draw down stored summer rainfall and nitrogen reserves [1]. Taking early action reduces the amount of herbicide required, improves herbicide efficacy and increases overall kill rates. Growers are encouraged to capitalise on the first suitable opportunity for control rather than risking a missed pass and higher costs later.
ChatBCG Sources:
All sources can be found in ChatBCG when searched.
- BCG 2010 Season Research Results: Crop Management in Wimmera and Mallee, Page 26
- Summer Weed Control is Critical!, Page 1
