Call to action/take home messages

• Harvest weed seed control (HWSC) can reduce weed seed inputs into the seed bank.
• Many common annual weeds have high levels of seeds present at wheat crop maturity.
• Over 95% of weed seeds were destroyed after passing through the integrated Harrington seed destructor (iHSD^®).

Introduction

A large proportion of Australian cropping paddocks contain weeds that are resistant to some of the herbicides used for their control. This both reduces the herbicide options for their control and limits crop yields if the weeds are unable to be controlled in a timely manner. Many annual weeds in Australian cropping regions retain their seed at maturity and thus are able to be captured by the harvester. Harvest weed seed control (HWSC) is a suite of management practices all of which target the seed of weeds at harvest time. HWSC systems include narrow windrow burning, chaff lining, chaff carts, bale direct and seed destruction.

The integrated Harrington seed destructor (iHSD^®) is the first system developed to destroy weed seeds during the harvesting process. The iHSD does not crush or grind the weed seeds; rather it is an impact mill where the seeds are broken through numerous high speed impacts. An advantage of this system over other HWSC is that all harvest residues are retained and spread across the paddock.

Two factors influence the level of control provided by the iHSD. Firstly, the weed seeds must enter the front of the harvester; and secondly, they must be destroyed by the HWSC system used.

Seed retention

On average 93% of ryegrass seed was retained at the time of wheat crop maturity. The amount of ryegrass seed above 15cm differed between two experiments (Table 1). A greater percentage of ryegrass seed was found above 15cm in wheat crops with low numbers of large ryegrass plants with high seed production (Broster et al. 2015). Wild oat seed retention was also found to be high, however if harvest was delayed for 28 days seed retention had fallen to 39% (Walsh and Powles 2014).

Seed retention levels for flaxleaf fleabane were high although there was a large range in the proportion of seeds above 15cm in a wheat crop, while sow thistle had much lower and more variable seed retention and more of the retained seed was above 15cm (Table 1).

Table 1. Average seed retention for weed species at wheat harvest and percentage of retained seed above 15cm

References: a - (Broster et al. 2015), b - (Walsh and Powles 2014); c - (Widderick et al. 2014)

Seed destruction

To evaluate the efficacy of the iHSD on different weed species, a specific number of weed seeds were added (larger species were dyed for subsequent identification) to 2kg samples of wheat chaff. 11 weed species were tested at two different times. The samples were then introduced to a iHSD test stand by spreading the wheat chaff evenly across a conveyor belt. The chaff was fed into the mill at 1.5kg/sec equivalent to that processed by a twin-mill iHSD system with a wheat harvest rate of 35t/hr (Walsh et al. 2017).

All of the tested weed species had more than 95% of seeds introduced into the iHSD destroyed. For the majority of species seed kill was greater than 99%, annual ryegrass at 96% seed kill was the lowest (P<0.005) (Table 2).

Table 2. Number of weed seeds placed in wheat chaff and percentage destruction of seed from 11 weed species using the iHSD test stand (adapted from Walsh et al. 2017)

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. The work of Allison Chambers and Charles Sturt University students in assisting with the test stand operation and sorting through processed chaff samples is much appreciated.

References

Broster JC, Walsh MJ, Aves C, Powles SB (2015) Harvest weed seed control: ryegrass seed retention levels in south-eastern Australia wheat crops. Proceedings of the 17th Australian Agronomy Conference. Hobart, Tas. (Eds T Acuna, C Moeller, D Parsons and M Harrison) pp. 566-569. (Australian Society of Agronomy, Hobart)

Walsh MJ, Broster JC, Powles SB (2017) iHSD mill efficacy on the seeds of Australian cropping system weeds. Weed Technology.

Walsh MJ, Powles SB (2014) High seed retention at maturity of annual weeds infesting crop fields highlights the potential for harvest weed seed control. Weed Technology 28, 486-493.

Widderick MJ, Keenan MD, Walsh MJ (2014) Harvest weed seed control: is there a role in northern region farming systems? Proceedings of the 19th Australasian Weeds Conference. Hobart, Tasmania, Australia. (Ed. M Baker) pp. 153-156. (Tasmanian Weed Society)

Contact details

John Broster
Charles Sturt University
Locked Bag 588, Wagga Wagga, NSW, 2678
Ph: 02 6933 4001
Mb: 0427 296 641
Fx: 02 6933 4001
Email: jbroster@csu.edu.au

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