Why thirsty weeds are hard to kill

Written by: Peter Newman

Unfortunately, large parts of Western Australia, South Australia and Queensland are as dry as a London newspaper! This edition of AHRI insight looks to address the issue of spraying stressed weeds.

We had to go back to 1995 to find some relevant research, but it was worth it. The GRDC funded research conducted by Dave Minkey and John Moore at DAFWA in the 90s is pure gold! They set out to determine what factors influence herbicide rate. They looked at moisture stress, relative humidity, temperature, and days since 5mm of rainfall. As you would guess all of these factors had an effect, but it appears that moisture stress had the biggest impact. The rate of glyphosate to kill moisture-stressed wheat was more than 10 times that of wheat growing with adequate moisture.

In a dry year, we’re often faced with the whole kit and caboodle – moisture stressed weeds, high temperatures, low relative humidity and long durations between rainfall events. There’s no quick fix, but by understanding the driving factors, it can help us to pick the best time to spray weeds.

This research also reminds us that we’re likely to see surviving weeds at harvest, which is why we believe that perhaps this year is the year to adopt chaff lining at harvest.

Dr Dave Minkey is now the Executive Director of the WA No-till Farmers Association (WANTFA).  The Dr part of his title was achieved by completing his PhD at AHRI studying ants and their predation of weed seeds. But once a weedie always a weedie and big thanks to Dave for his help to drag out this old research that’s still extremely relevant today. Dr Dave Minkey is now the Executive Director of the WA No-till Farmers Association (WANTFA).  Over the range of conditions encountered in this research, the glyphosate 450 rate for 90% control ranged from 150mL/ha to 3L/ha.

That’s a 20-fold difference in rate to kill the same sized weed, purely as a result of seasonal conditions.  In other words, if conditions are cool and wet with high relative humidity at spraying the lethal dose of glyphosate to kill wheat could be as low as 150mL/ha. But if ryegrass is being sprayed and conditions are hot and dry, that rate could be 3L/ha or even higher. This was also observed for a range of leaf uptake herbicides.

1. Moisture stress

It comes as no surprise that moisture stress has arguably the biggest impact on herbicide efficacy, but what will surprise you is the sheer size of the effect.  For these trials in 1995, Dave and John sowed wheat and then used a rain out shelter to remove 50% of the rainfall from some plots. The rain from the shelter was then used to irrigate other plots and these were compared to normal rainfall, six weeks after seeding.  We must note that glyphosate 360 was used in this research, they didn’t have the fancy new glyphosates with fully loaded surfactant packages back then.Table 1: ED90 (herbicide dose to kill 90% of the population) and ED50 for wheat sprayed with glyphosate for low, medium or high soil moisture treatments (gai/ha = grams of active ingredient/hectare).

Figure 1: Log scale dose response curves of glyphosate rates for wheat at three different levels of moisture stress. RWC = Relative Water Content (%) of wheat leaves.

2. Plant species

This research also investigated the rate of glyphosate required to control different plant species.  Keep in mind that this was back in the good ol’ days, before glyphosate resistance.  In simple terms, ryegrass is about twice as hard to kill with glyphosate as wheat.

3. Relative humidity

Two field sites in Newdegate and Katanning were sown to wheat and experienced almost identical growth, climatic conditions and relative water content of the wheat leaves at spraying (95%).  The only measurable difference was the relative humidity at spraying.  The rate of glyphosate to kill wheat at Katanning with low RH was about 60% higher than the rate needed at Newdegate with high RH.

4. Temperature

The mean degree days (average of max and min temperature) from germination to spraying was 11.9oC in 1995 and 16.2oC in 1996. This roughly doubled the rate of glyphosate required to kill 90% of the ryegrass population (ED90).

Why is it so?

When plants are moisture-stressed they…

  1. Develop a thick, waxy cuticle on their leaves that is a barrier to herbicide uptake. After a rain event, the weeds freshen up as they start to grow again but the waxy cuticle remains, so even though rainfall helps, it doesn’t completely undo the harm that has been done and these weeds will remain relatively hard to kill. If conditions improve and new leaves emerge with a normal, thin cuticle, the weeds can become more susceptible to a herbicide.
  2. Translocation is slow during moisture stress so translocated herbicides struggle to reach their site of action.

Low relative humidity at spraying reduces droplet survival on the leaf and reduces the amount of herbicide that the plant takes up.

What can we do?

Firstly we need to be acutely aware of just how big an effect moisture stress and seasonal conditions are on herbicide efficacy.

Second, we can only do our best to choose the best possible conditions to spray weeds in a dry year. The best-case scenario is to wait for a rain event to remove moisture stress/wash the dust off the leaves and then wait even longer for new leaves to emerge that do not possess a thick waxy cuticle. This isn’t always possible, and by waiting we may once again find ourselves in a situation where moisture/heat stress returns. Spraying during high relative humidity helps, but it only helps a little compared to spraying after rain. It’s important to get your adjuvant and spray quality right. Some adjuvants/spray quality are better than others under these conditions. Consult your agronomist to get this right.

Thirdly, we can expect surviving weeds in a dry year and we should aim to do something about them. Spraying out failed crops in spring is a good option or even crop topping where possible. Harvest weed seed control is a must and we believe that perhaps the best fit (if you are not already practising HWSC) is to adopt chaff lining. It’s low cost, has high residue retention, and it’s easy to adopt. One farmer recently commented to Peter Neman that he let some wild radish set seed in the 2006 drought and he’s still battling that population now. The cheque book may be stapled shut but there are low-cost options that could make a big difference in the future.


We all hate a dry season and agronomy decisions are challenging during these times. This AHRI insight aims to help make the best decisions at the lowest cost for growers experiencing dry times. Hats off to Dave Minkey and John Moore on some excellent, timeless research. Let’s hope we don’t need to drag it out again anytime soon.


  • As mentioned earlier in the post, chaff lining is worth considering, particularly in dry conditions. The below video is from WeedSmart’s Diversity Era HWSC 101 online course, explaining chaff lining in detail. You can do the whole course for free here

  • Case Study: WeedSmart did a case study on Michael and Marnie Fels. They have adopted chaff lining and are having good success with it. Find out more here
  • You can find out more about the costs associated with harvest weed seed control from one of our previous AHRI insights here
  • WeedSmart is running a series on what you can do in dry conditions. Chaff lining is a cost-effective weed management tool you might like to consider. Here’s how it works:

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