Grain growers are increasingly opting to use harvest weed seed control (HWSC) systems that collect chaff that is not burnt. But are they are capturing more than they bargained for?

This question is being addressed in a GRDC-initiated research project in which invertebrate species and mice are being surveyed in unburnt chaff lines, tramlines and chaff dumps.

The Western Australian Department of Primary Industries and Regional Development (DPIRD) is conducting the GRDC-invested research to provide insights into how these newer HWSC systems influence populations of pests and beneficial species.

"Initial results suggest that chaff in paddocks in the southern areas of WA is more likely to harbour invertebrates than chaff in the state's central and northern grainbelt," says DPIRD project leader Svetlana Micic, who discusses the research in a new GRDC podcast.

"The research follows anecdotal reports that retaining chaff in paddocks has increased numbers of some invertebrate pests that could damage subsequent crops."

Increasing populations

Anecdotal reports suggest that retaining chaff in paddocks increases mouse populations. Photo: Emma Leonard

Ms Micic says the researchers want to know if unburnt chaff increases invertebrate and mouse populations - so growers using non-burning HWSC chaff collection techniques know whether they are creating an environment that could foster more pests or beneficial insect populations.

The newer HWSC techniques that do not involve burning include chaff lining. This involves using a simple chute to divert the weed seed-bearing chaff fraction (from the sieves) into a narrow chaff line, which is left to rot or mulch while the straw is chopped and spread.

Chaff tramlining is a similar concept to chaff lining, but the chaff is diverted through a chaff deck on to permanent wheel tracks in a controlled-traffic farming system.

Chaff dumping involves collecting chaff using a cart towed behind the harvester. The chaff in the cart is then dumped, usually in piles in the paddock, and then either burnt, grazed or left to decompose.

The preliminary surveys, led by Ms Micic in 2019, suggest the geographical location of paddocks influences the abundance and composition of invertebrates associated with unburnt chaff.

In addition, there is an indication that the type of chaff may also influence invertebrate abundance.

"Pitfall traps adjacent to chaff tram lines captured fewer invertebrates than traps next to chaff dumps or chaff lines," Ms Micic says.

Assessing crop damage

Ms Micic says the project also assessed chewing or sucking damage to crops, and confirmed that plants grown close to chaff may sustain more invertebrate damage than plants located at least 20 metres from the chaff.

"However, we are yet to determine which pest species are responsible for the damage at the sites," she says.

"Another interesting outcome was that the composition of species adjacent to chaff was different to that of species located 20 metres from the chaff - possibly suggesting some species may not be attracted to chaff."

Ms Micic says the project tested the effectiveness of pitfall traps and chaff inspections as survey methods and found that simple pitfall traps located up to 20 metres from chaff were sufficient to determine whether a chaff system influenced invertebrates.

More information: Svetlana Micic, 08 9892 8591, 0427 772 051, svetlana.micic@dpird.wa.gov.au