with Glenn Shepherd, agronomist, IMAG Consulting, Dubbo
In the interests of conserving soil moisture, fallow paddocks are ideally weed-free. The difficulty lies in incorporating sufficient diversity into fallow management to ensure the chemical options remain viable in the long term.
Glenn Shepherd, agronomist and director of IMAG Consulting in Dubbo, NSW, says growers still have opportunities to take an integrated approach to fallow management using the WeedSmart Big 6 as a checklist.
“The Big 6 still applies in the fallow phase, and it is a useful way to ensure that you are building as much diversity as possible into your weed management program,” he says. “Fallow water use efficiency drives profit in the western plains region of NSW and many other dryland cropping regions in Australia. Research has shown we can produce 60 kg/ha of wheat per millimetre of deep soil moisture stored. This makes summer fallow control critical to producing profitable crops, particularly in seasons with low in-crop rainfall, which 2023 has proven to be.”
Keeping fallow paddocks weed-free relies heavily on herbicide tactics, and that can only be sustainable if other measures are in place to control any plants that evade the herbicide applications.
“In the western plains region, our fallows are usually over summer, but most of the strategies apply equally to winter fallows,” says Glenn. “It is essential to increase the diversity of herbicide modes of action used and to ensure that the spray application is as effective as possible.”
The WeedSmart Big 6 strategy to grow more crop and fewer weeds offers a range of herbicide and non-herbicide tactics that growers can deploy against weeds growing in the fallow.
What Big 6 non-herbicide tactics can I deploy in the fallow?
While there are few options to add diversity within the fallow phase, the fallow itself provides added diversity to the overall weed management program. The key is to use different tactics in the fallow that are not used in the cropping phase.
Although harvest weed seed control is usually associated with the cropping phase, it can also be considered a ‘start of fallow’ tactic that reduces the amount of weed seed entering the seed bank. In many regions, some weed species are expanding their growing season and often germinating in the ‘wrong’ season. For example, annual ryegrass and sowthistle germinate almost year-round in some areas.
Crop competition is usually completely lacking, removing one of the best forms of weed control. Green or brown manure crops can be an effective option for weed control and soil health. The potential impact on soil moisture and the timing of cover crop termination are vital considerations.
In fallow, stopping seed set in weeds that escape herbicide application will be best done with mechanical tools such as strategic tillage or chipping. Tillage is particularly useful if combined with other tasks, such as incorporating lime, gypsum or redistribution of surface nutrients.
Burying weed seed is often a useful re-set tactic for the seedbank, but be sure to consider the biology of individual weed species. For example, fleabane is known to be a surface germinator, with poor seed survival at depth, whereas buried annual ryegrass seed can still have some viability if brought to the surface in subsequent years.
How can I best implement the Big 6 herbicide tactics in fallow?
While some fallow herbicide treatments are considerably cheaper than others, it pays to look beyond the budget for the current season. A treatment that introduces another mode of action might increase the cost, but the benefits may lie in preserving the cheaper chemistry into the future.
Environmental conditions in summer can be very harsh, and it is often difficult to properly implement tactics like the double-knock. For example, in Warren, NSW, seven consecutive days of temperatures under 30 degrees C between November and February happens in about 50 per cent of years, and on average, only three times in those years. Consequently, weeds are often stressed, and applying both herbicide knocks in the right conditions is difficult.
The best approach is to be ready to take advantage of every opportunity that arises. Using batching systems to refill in the paddock will keep the sprayer going in the small window of opportunity to get the job done. For example, halving the sprayer refill and travel time from 40 to 20 minutes equates to an extra 384 ha sprayed within the 7-day window for a 30 m, 4000 L spray rig operating under best management practice.
How can I make optimal use of fallow spray technology?
In addition to batching stations to improve efficiency, optical sprayers also offer cost savings that allow growers to consider more diverse herbicide options.
Provided the weed burden is 30 per cent or less in a paddock, many products are now registered, or on permit, for optical spot spraying technology (OSST). These include products from the mode of action Groups 9, 10 and 22 (knockdowns), Group 4 (phenoxys and pyridines), Group 14 (spikes) and Group 1 (grass selectives). The cost savings are apparent when spraying only 15 per cent of the area, making a $64/ha product just $10/ha. Many growers have added a dual line to their optical sprayer to do a blanket spray and a spot spray application in one pass.
Other important technologies are now available to growers to minimise spray drift risk while potentially expanding safe spraying windows in summer. These include weather forecasting and monitoring towers such as WAND, very coarse nozzles, low drift adjuvants and improved water management and refilling efficiencies.
Improving on-target pesticide application (SOS)