Rules of thumb

Rule of thumb: A useful principle having wide application but not intended to be strictly accurate or reliable in every situation.

“Then they measured my right thumb, and desired no more; for by a mathematical computation, that twice round the thumb is once around the wrist, and so on to the neck and waist, and by the help of my old shirt, which I displayed on the ground before them for a pattern, they fitted me exactly” – Jonathan Swift, Gulliver’s Travels.

Where would we be without rules of thumb in agriculture? No, they are not 100% scientifically accurate, but they can be very helpful in decision making.

Dr Michael Walsh from AHRI has developed a new rule of thumb from his research into weed seed retention at harvest – Australia’s main cropping winter weeds shed seeds at about 1% per day during harvest.

Our common harvest weed seed control tools rely on the weeds retaining seed at harvest time. In his research, Michael found that ryegrass, wild radish, brome grass and wild oats all retained at least 75% of their weed seeds at the first opportunity to harvest. There were differences in seed retention between species, but not as great as expected.

This research also highlights why annual ryegrass is a herbicide resistance world champion, setting more seed than the other species. Wild oats, on the other hand, was a distant fourth in the weed seed set stakes.

_KCP2128Dr Michael Walsh has dedicated the bulk of his 25 year career in agricultural research to harvest weed seed control and is delighted to see the level of adoption steadily increasing around Australia.

The first step in harvest weed seed control is to get the weed seeds into the front of the harvester. In 2008, Michael, with the help of grain growers and grower groups, set up nine sites throughout Western Australia to measure the seed retention of the four major weeds of Australian cropping.


Screenshot 2014-12-02 11.26.38

Samples of annual ryegrass, wild radish, brome grass and wild oats were taken from wheat crops at the first opportunity to harvest and then every week after that for four weeks.  2008 was generally considered to be a good rainfall year in Western Australia, however the sites were spread across 650 kilometres resulting in considerable growing season variability.

Seed retention

Averaged across all sites, all four weed species retained at least 75% of their seeds above beer can cutting height at the first opportunity to harvest.

weed seed retention graph

Wild radish retained nearly all its seed, 99%, at the start of harvest, and continued to retain 79% of its seeds four weeks later.  While the other species did not have quite this level of seed retention, all are good targets for harvest weed seed control.

Seed shedding – 1% per day

The rule of thumb of 1% per day is not precise, but adequate, with the main exception to the rule that wild oats shed seed at 1.5% per day.

Table 1

Annual ryegrass sets a lot of seed – wild oats don’t

This data is a reminder of why annual ryegrass is such a successful weed – it sets a lot of seed.  The weed density at the sites was similar for the three grass species, 25 to 30 plants /m².  Wild radish density was 2 plants /m².

Figure 1 v2

Seed set per plant

People often ask, ‘how many seeds does one ryegrass plant set?’  The answer to this question is how long is a piece of string?  However, this data gives a more specific answer.  Annual ryegrass is the king of the grass weeds, but wild radish set more seed per plant than all of them put together.

Figure 2

The above information forms the basis of paper recently published in Weed Technology, a Journal of the Weed Science Society of America. The authors have just received the “Outstanding paper of the year” award for this publication.

More weed seeds into the front of the harvester

Harvest weed seed control is effective, but is limited by how many weed seeds are collected into the front of the harvester.  Some of the techniques aimed at collecting more weed seeds include;

  • Swathing – this can bring harvest forward by a week or so, minimising the risk of weeds shedding / lodging prior to harvest as well as maintaining a high seed retention for the swathed weeds.
  • Crop lifters are being used by some growers to pick up weeds that have fallen down between crop rows. We are yet to measure the effectiveness of this, however it seems plausible.
  • Sowing at narrow row spacing increases competition from the crop, forcing weeds to grow tall to chase sunlight. Narrow row spacing may also help ‘hold up’ the weeds in the crop canopy to improve the chances of getting them into the front of the harvester. Research has commenced to measure the effect of crop competition on weed seed height at harvest.


Yes, it is true that all weeds shed their seed as harvest progresses.  This research shows that seed shed varies less between species than we may expect. The rule of thumb of 1% of seed shed per day gives growers an indication of how effective their harvest weed seed control will be.

The recommendation is simple – ‘harvest the weediest crops first’.


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