Weed control bolsters profit in a variable climate

Low rainfall zones are often described as ‘marginal’ cropping regions. While average rainfall is low, these regions can also experience seasons that support very high-yielding crops.

Greg Rummery, Outlook Ag agronomist based at Walgett, says north-west New South Wales is a highly variable but far-from-marginal cropping region.

“Understanding and anticipating variability is underpinning very profitable farming enterprises in the region,” says Greg. “Businesses that invest in deepening and filling the soil ‘bucket’ are ready to take advantage of favourable seasons.”

The Walgett area has a long-term (125-year) average rainfall of 450 mm, and a range of less than 100 mm to up to 900 mm. In addition, the region experiences widespread flooding when any or all of the Macquarie, Castlereagh, Namoi, Gwydir or Barwon rivers overflow.

“The soils around Walgett are fairly deep and uniform grey and brown cracking clays, with inherently good fertility,” he says. “With investment in soil health and weed control, growers are storing more soil moisture and growing bumper crops whenever the opportunity arises. Over recent decades the cropped area in the region has expanded from around 150,000 ha to 750,000 ha.”

Fallow weed management stimulates a farming system transformation – less weed pressure, more soil moisture and nutrients, bigger crop yields and lower risk in a variable climate. Speaking at WeedSmart Week in Griffith recently, Greg said the WeedSmart Big 6 strategies are underpinning productive and profitable farming systems in low rainfall zones.

Expanding the soil moisture and nutrient bucket

Capturing and storing water from rainfall and flood events is limited by the soil profile’s water-holding capacity. Improving infiltration, addressing soil constraints such as compaction or pH barriers, and supplying adequate crop nutrition are essential to growing crops with the capacity to expand the effective root zone.

Greg says the typical ‘bucket size’ has expanded from 180 mm to 240 mm of water-holding capacity through no-till farming, stubble retention and a zero tolerance of weeds in fallow.

With 200 mm of starting stored soil moisture, the potential wheat yield ranges from 2.5 to 5.5 t/ha in the Walgett district, depending on the amount of in-crop rain. Greg advises his clients to prepare ahead of time for the season with a forecast 70 per cent chance of exceeding average in-crop rainfall.

“What this means for growers working in this highly variable environment is that one wet year can support at least two cropping seasons,” he says. “When the opportunity comes, we are ready with a sequence of a shallow-rooted crop such as chickpea, followed by a deeper-rooted crop such as wheat or barley, to utilise the stored moisture and drive profit.”

On the flip side, Greg says it is equally important to know when not to plant. For example, he says the advances in canola seed technology have been outstanding, and it is a highly profitable crop in the region in favourable years. But growers are disciplined about when to plant something other than canola.

Automation is changing the game for fallow weeds

The adoption of no-till farming and its historical reliance on knockdown herbicides has shifted the weed spectrum toward hard-to-kill summer weeds such as fleabane, sowthistle, awnless barnyard grass and feathertop Rhodes grass. In winter, black oats, sowthistle and herbicide-resistant ryegrass are the main challenges.

Greg says optical spot spraying technologies were readily adopted and have played an important part in improving fallow efficiency in the region. Coupling this technology with robotic platforms is providing the next step change for growers.

“We can now really focus on efficacy, without worrying about losing efficiency,” he says. “Robots that are regularly spraying small weeds while rotating and mixing products, travelling at low ground speed and applying higher concentration spray droplets are achieving excellent results, even in high stubble loads, that were previously difficult to attain.”

With the arrival of multi-resistant annual ryegrass in the region, winter fallow will be an important tool in keeping weed numbers low. Greg says the decisions growers make around short and long fallowing are key to keeping the cropping options open in this variable environment. The soil profile often does not completely fill during a short fallow; however, fallow efficiency drops off in a long fallow.

Growing the crop, suppressing the weeds

Each crop in the rotation needs to hit its KPIs – nitrogen from legumes, ground cover from cereals, biofumigation and soil structure from canola, and they all need to suppress weeds and be profitable. Intensive livestock operations to the north and south of Walgett provide the opportunity for growers to use on-farm storage of grain, particularly barley, to sell into an increasingly reliable and growing local stockfeed market.

While wheat, barley, chickpea and canola are the most common crops, a wide variety of summer and winter crops, including sorghum, sunflower, cotton, faba bean and forage, grow well in the Walgett area.

Pre-emergent herbicides are being increasingly used at planting, but they need rainfall or moist topsoil to activate. Many growers in the region have returned to using tyne seeders, which can improve crop establishment and crop safety when using pre-emergent herbicides.

Greg says spatial variation in seedbed moisture can make it difficult to establish competitive crops that can provide paddock-wide weed suppression. Developing precision agriculture tools to quantify and address this constraint is a high priority.

Soil nutrition is looming as a constraint to yield potential in many paddocks. Greg says nitrogen supply is always an important discussion topic as the full crop requirement is usually applied pre-planting or at planting. He is expecting deep-banded phosphorus to be required in the coming years, along with strategic management of potassium and zinc.