Cultural tools essential to manage the weed seedbank

Any weeds that evade in-season herbicide applications pose a significant risk to future crop yield, and spreading them further at harvest increases pressure on next year’s herbicide program.

Countering these high-risk weed populations with non-herbicide, or cultural, tools is central to an integrated weed management program that results in a low weed farming system.

End-of-season cultural tools are limited to harvest weed seed control, hay-making and green manuring with cultivation. Each has its place, and a farming system without any of these tactics is vulnerable to a larger weed seed bank and the spread of herbicide-resistant weed seed within the paddock at harvest.

By way of example, WeedSmart’s extension agronomist Chris Davey cites a 20-to-25-year scenario on the Yorke Peninsula and similar regions in South Australia and Victoria where widespread clethodim resistance in annual ryegrass has resulted from the cumulative effects of several crop rotation, herbicide use and seasonal factors. For many years, harvest weed seed control tactics such as narrow windrow burning, chaff decks, chaff lining and impact mills had provided effective end-of-season control that kept clethodim-resistant ryegrass numbers low.

“Recently, we saw large infestations of clethodim-resistant ryegrass in some paddocks where growers decided to not use a late-season cultural tactic such as harvest weed seed control,” says Chris. “It can be very risky to cease using an effective tactic if you don’t have a plan to swap in an equal or better tactic.”

This underscores the importance of the three non-herbicide pillars of the WeedSmart Big 6 needed to keep herbicides working well long-term. These are ‘diverse farming systems’, ‘crop competition’ and ‘harvest weed seed control’. Prioritising these three strategies significantly widens the herbicide program’s options, often keeping older chemistry effective longer and allowing strategic use of newer and more varied modes of action.

Growing evidence suggests diverse cropping rotations can be very profitable, partly because some weed control tactics are built in rather than adding to input costs.

Greg Condon, WeedSmart extension agronomist in NSW, says one example is including a crop or sequence of crops that effectively stop weed seed set late in the season.

“In Southern and Central NSW, growers are including hay in their rotations as a profitable crop that practically eliminates ryegrass seed set in that year,” he says. “This can be the first stage in a break crop phase to tackle high ryegrass numbers. It can be followed by another hay crop, or a pulse crop then canola to achieve a triple break on ryegrass with three years of seedset control. The double break pulse-canola rotations have shone in long-term farming systems research in our area, and strategic hay ahead of this sequence extends the impact on ryegrass numbers.”

Planning a crop rotation that includes highly competitive crops, such as barley, or varieties, break crops for soil health, opportunities to stop weed seed set in-crop with hay or croptopping, crops suited to harvest weed seed control and a double or triple break sequence that allows diversity in herbicide products and use patterns is very cost-effective.

“When the rotation provides built-in diversity in timing of operations, such as sowing and harvest, integrates different pre- and post-emergent herbicide modes of action, includes opportunities to use a pre-sowing double-knock tactic and optical spot spraying in fallow, it may be possible to be more strategic about which paddocks need harvest weed seed control,” says Greg.

In the western region, WeedSmart extension agronomist, Peter Newman, says growers in the medium- and high-rainfall zones generally have large weed seed banks and are embracing more diverse systems, including double-break phases and haymaking.

“These are the farms that first developed and adopted harvest weed seed technologies,” he says. “While some are being more strategic in their use of these tools, there are growers who have experienced a weed blow-out as a direct consequence of not implementing harvest weed seed control in one season.”

Peter says there are growers in some lower-rainfall regions that are maintaining low weed numbers even in simple cereal-dominant rotations.

“These low-diversity systems are working well for growers because they are very diligent with preventing weed seed set during winter fallows,” he says. “These farms are usually very large-scale continuous cropping enterprises, often using robotic platforms equipped with optical spot spraying technology to ensure weed numbers are kept very low going into the wheat crop.”

Where a diverse crop rotation enables several herbicide and cultural ‘hits’ on the main weed species and in seasons conducive to delivery of a highly effective herbicide program, there may be minimal late-season survivor weeds.

When contemplating the cost-benefit of harvest weed seed control, growers can use the WeedSmart HWSC calculator to compare the available tools and consider whether strategic use of this tactic is an option in some paddocks.