Farmers are now well versed about the benefits of keeping their paddocks weed free both during the growing season and over summer.
Over the years BCG has conducted numerous research trials investigating weed control products and strategies and their impact on crop performance. The five year water-use efficiency investigation, which concluded last year, left no doubt that weed control has the biggest positive influence on yield and profit (BCG 2012 Season Research Results, pp. 19).
Yet while control has markedly improved across Wimmera and Mallee paddocks, weeds remain a challenge. The past few years have seen new weeds emerge (such as flaxleaf fleabane and bifora), while many existing weeds (including rye grass and wild radish) are becoming harder to kill due to increasing herbicide resistance.
Attention has now turned to how we kill weeds – the timing, rates and types of herbicide applications, as well as cultural and mechanical measures (integrated weed management, IWM).
While the poor season experienced by many growers in the southern Mallee and much of the Wimmera can leave land 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.
Late season weed control
Frosts, insects and below average late winter rain has severely hindered the yield potential of many crops in our region. As a consequence, some growers have made the decision to cut oilseed and grain crops for hay.
As well as providing an opportunity to salvage some income from poor or failing crops, making hay can also help reduce the paddock’s weed burden and conserve what moisture is in the subsoil for the following year’s crop.
At this 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 also need to be understood.; the time commitment, contractor costs and how haymaking activities impact 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 that growers can use to control escapees and reduce grass weed seed set. Timing is the main issue (maturity of weeds versus maturity of the crop), along with residues.
This year canola growers can spray ryegrass out of their crops pre-harvest with 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.
Pulse and legume crops can also be crop-topped (Paraquat) once they’ve 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 only feasible 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 are 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 that 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. These can be concentrated into narrow windrows and, when burnt in the right conditions, 99 to 100 per cent of weed seeds are destroyed.
A three year investigation into IWM strategies undertaken by BCG from 2009 to 2011 also found windrow burning an effective means of reducing ryegrass seed set (BCG 2011 Season Research Results, pp.138).
To achieve optimum results from narrow windrow burning, AHRI recommends growers:
- harvest low
- ensure windrows are narrow
- select crops that are easier to burn
- don’t stop the header
- avoid grazing
- burn as soon as possible
In autumn, burn when there is a light breeze (10km/h or less) to fan the fire. Use the Pocketfire app (available from the App store) to check conditions. 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
In a year when maximising opportunities is vital, summer weed control will be extremely important.
To minimise expenditure spray early and use a double knock if necessary. If rain falls during harvest, it can mean getting straight into the sprayer before the harvester has even cooled.
To avoid getting overwhelmed prioritise paddocks according to the weed spectrum, the presence of any problem weeds and the likelihood of volunteers emerging (ie. canola stubbles, then legumes).
While weed control can seem like a never-ending task, with appropriate IWM strategies, the productivity and profitability of your land can not only be maintained, but can improve.
