Over the years BCG has conducted numerous research trials investigating weed control options and their impact on crop performance. BCG’s five year water-use efficiency investigation, which concluded in 2013, left no doubt that weed control has the biggest positive influence on yield and profit (BCG 2012 Season Research Results, pp. 10-22). Yet, as discussed in the 2014 season summary (‘The year that was’ pp 5), weeds continue to challenge farmers, with “many more decisions on-farm being strongly influenced by concerns about weed resistance.”
Despite ongoing improvements in weed control across Wimmera and Mallee paddocks, new weeds have emerged over the past few years (such as flaxleaf fleabane and bifora), while many existing weeds (including rye grass and wild radish) are becoming harder to kill.
Attention has recently turned to the ways in which we kill weeds: timing, rates and types of herbicide applications, and cultural and mechanical measures (integrated weed management, IWM).
While the poor 2014 season in the southern Mallee and much of the Wimmera may leave growers and managers feeling less enthusiastic about tackling “more bloody weeds”, it’s important to look ahead and take advantage of any opportunities to improve the cleanliness of paddocks.
Herbicide control
To delay the onset of resistance, it is important to rotate herbicide groups, spray early (when weeds are small), use robust rates, and deliver a double knock to problem weeds. Replacing currently used herbicides with premier products may seem costly, but, in the long term, can pay dividends (because managing resistant populations can be much more expensive).
Late season weed control
As well as providing an opportunity to salvage some income from poor or failing crops, making hay can help reduce the paddock’s weed burden and conserve subsoil moisture for the subsequent crop.
At last year’s Main Field Day, Landmark agronomist Greg Toomey said cutting crops for hay was one means of capturing weeds before they have a chance to set seed. But he warned that certain export markets had thresholds for weed species in hay bales and growers should consider this when seeking a buyer.
The costs and practicalities of haymaking need to be understood; the time commitment, contractor costs and the way in which haymaking activities can have an effect on other farm activities.
A good understanding of the biological traits of target weeds (physiological development relative to crop growth stage) will also improve the efficacy of late season weed control tactics.
Crop-topping is another practice growers can use to control escapees and reduce grass weed seed set. Timing is the main issue (maturity of weeds vs. maturity of the crop), and any residues.
Last year, canola growers were able to spray ryegrass out of their crops pre-harvest thanks to the release of Nufarm’s weedmaster® DST® glyphosate. This product can be applied once the seeds have reached 20 per cent colour change, allowing growers to kill grass weeds before they set seed, although this is not appropriate when retaining seed as spraying at this stage could cause germination issues.
Pulse and legume crops can also be crop-topped (Paraquat) once they have reached maturity, sterilising grass weed seeds before they become next season’s problem. Generally, in the Mallee, this is not an option for chickpeas as being later maturing, weeds are often too advanced by the time spraying can occur.
Wheat can be crop-topped with glyphosate, but this is usually feasible only for early maturing varieties such as Axe. For most other varieties weed seeds become viable before spraying can occur.
Harvest weed seed capture
Narrow windrow burning is a practice Wimmera and Mallee growers have been trialling to combat herbicide resistant weeds or to delay resistance development.
Widely adopted in Western Australia, narrow windrow burning involves concentrating chaff and straw into 50 to 60cm wide windrows which are then burnt prior to sowing.
According to Australian Herbicide Resistance Initiative (AHRI) researchers, it is possible to capture 60 to 70 per cent of wild radish and annual ryegrass seeds at harvest. Concentrated these into narrow windrows and, when burnt in the right conditions, 99 to 100 per cent of weed seeds are destroyed.
Three years of BCG IWM research between 2009 and 2011 also found windrow burning an effective means of reducing ryegrass seed set (BCG 2011 Season Research Results, pp. 138-143).
To achieve optimum results from narrow windrow burning, AHRI recommends growers harvest low (beer can height), ensure windrows are narrow, select crops that are easier to burn, don’t stop the header, avoid grazing and burn as soon as possible.
In autumn, burn when there is a light breeze (10km/h or less) to fan the fire. Fires need to reach 500°C to kill wild radish and 400°C to kill ryegrass.
Other options to capture weed seeds at harvest time include chaff carts and the Harrington Seed Destructor. Farmers have had success reducing the weed seed bank with these machines, but they require a larger financial investment than other IWM strategies.
Summer weeds
It is now accepted wisdom that controlling summer weeds is vital.
Summer weed control should be a priority, even if it means there is less to spend on fertiliser. At the risk of being repetitive, let’s put it another way. Spending on summer weed control is a better investment than shelling out for extra fertiliser inputs and ignoring weeds. When spraying, prioritise paddocks according to the weed spectrum, the presence of any problem weeds and the likelihood of volunteers emerging (i.e. canola stubbles, then legumes).
While weed control can seem like a never-ending task, appropriate IWM strategies, will ensure improved productivity and profitability of the farm enterprise.
