May 16, 2024
Written by: Peter Newman
The best way to avoid things that make you fat is to stay away from scales, photos and mirrors.
And, if you happen to be a weed, the best way to avoid harvest weed seed control is to drop your seeds on the ground before the harvester gets there.
In 2016, a farmer from the central wheatbelt in Western Australia, and his agronomist, noticed that his wild radish was shedding it’s pods before harvest.
This farmer had used a chaff cart for harvest weed seed control for about 20 years.
The first measurements in the field by AHRI researcher, Dr. Mike Ashworth and his team, measured 86% of the wild radish pods had fallen from the plant at the beginning of the wheat harvest, and this compared to 0 to 3% pod shedding from other populations in the area.
Mike and the team then took some of the seeds of these wild radish plants and grew them in a common garden study over the next two years in Perth, confirming that this wild radish was not only dropping a lot of pods at the start of wheat harvest, but the height of the lowest pod was also very low compared to the control populations.
Yes, weeds can evolve to evade harvest weed seed control (HWSC).
No, we haven’t seen a lot of evidence of this happening.
And yes, there are things we can do to avoid this.
Field research 2016
In 2016, Dr. Mike Ashworth and the team travelled to the paddock near where the suspected early shedding wild radish was growing in a wheat field that was ready to harvest. They sampled 28 plants, keeping their remaining pods intact on the plant, and gathered seed pods from the soil surface from the area. They compared this to two other populations from nearby wheat crops from a neighbouring farm where harvest weed seed control had not been used. They also collected come control plants from nearby fencelines.
From this, they concluded that the early shedding radish population had shed 86% of its seed before the wheat crop was harvested.
Common garden study 2017
The next step was to take some of the seeds from collection in 2016 and grow them out in a common garden study in Perth.
A common garden study, as the name suggests, involves growing plants from a number of population in a similar environment, to compare them on an even playing field. This research is conducted in a randomised trial, where moisture, nutrient, pest and disease stress is absent.
It is important to note that these wild radish plants were grown in the absence of crop competition. Individual plants were grown 2m apart in rows that were 1m apart, using plastic to eliminate other weeds. Think commercial vegetable production.
The early shedding radish population was compared to three other populations from nearby fields or roadsides.
The table below shows that, at wheat maturity, the early shedding population shed a lot of seed, 60%, but the difference to the control populations wasn’t quite as stark compared to the field observations in 2016.
| Pods produced per plant | Pods shed per plant | % pod shedding | |
| Early shedding population | 5355 | 3220 | 60% |
| Average of three control populations | 3561 | 871 | 24% |
Common garden study 2018
Some wild radish seeds from the 2017 common garden study were retained and the study was repeated again in 2018. This time there were two additional control populations included from different parts of the state.
Once again, the results were a little different to the previous years, however there is significant evidence that the early shedding population is shedding more pods than the control populations at the start of wheat harvest.
| Pods produced per plant | Pods shed per plant | % pod shedding | |
| Early shedding P population | 8610 | 1048 | 12% |
| Average of five control populations | 3594 | 25 | 0.7% |
Flowering time
The researchers also measured flowering time of the populations. In the 2018 study, the early shedding population flowered six days earlier than the average of the control populations. This flowering time is not considered to be a big enough difference to explain the pod shedding.
At this stage, the researchers do not understand what mechanism the plants are using to shed their pods. This will be the subject of future research.
What contributed to this?
The grower purchased this block in the early 2000’s, and it had a very high wild radish seed bank. As a result, there were often surviving wild radish at harvest, meaning that selection with the chaff cart was always happening over a reasonable number of plants.
The farming practices of the early shedding population were quite diverse, growing a range of crops and using HWSC with very high-level efficacy.
What can we do about it?
Once again it’s that word DIVERSITY.
There are many growers who started with HWSC to tackle herbicide resistant wild radish, and they now have an almost non-existent wild radish population on their farms.
Computer modelling also shows us that where we have diversity, selection for changes in flowering time or plant growth habit are minimal (see last month’s edition of AHRI insight here https://www.ahri.uwa.edu.au/can-wild-radish-evade-hwsc/)
They use all of the Weedsmart Big 6 (link https://www.weedsmart.org.au/big-6/), and for wild radish the particular tools that work are crop rotation, mix and rotate herbicides, crop competition, and HWSC. They also know how important it is to spray small radish, and if necessary, spray them twice in sequence post-emergent.
It’s the combination of these tools that works, not any one tool in isolation.
We need to remember that when we started using HWSC for wild radish we weren’t sure how well it would work due to the long seed bank life of the weed. But, as it turns out, it worked really well because wild radish holds onto its seed at harvest (see AHRI insight https://www.ahri.uwa.edu.au/rules-of-thumb/) and it’s pretty easy to divert these seeds into a HWSC tool. And we also know that any weed control tool that is highly effective can result in the weeds adapting to avoid the tool.
Summary
We know that the gene mutations for traits such as dormancy, flowering time, or shedding seed are much more common in weeds than the genes for herbicide resistance, so we know that we can select for these traits. The trick is to keep using all of the tools in the tool-box, in combination with one another, to minimise the risk.
We know that HWSC is working extremely well for the majority of growers who use it, and it will continue to do so if they keep up the DIVERSITY. If we keep weed numbers low, and use all of the tools in combination, they will all keep working for a long time.
Paper
Posted in: AHRI Insight, Herbicide resistance mechanisms