Impacts of dry conditions on herbicide effectiveness and weed management decisions

Take home messages

• Grass weed seed banks are likely to be high and to have greater dormancy in 2017 as a result of spring conditions in 2016.
• The efficacy of pre-emergent herbicides applied dry in 2017 will depend on the solubility and persistence of the product used and rainfall patterns at the break.
• Frost can severely impact the efficacy of Group A post-emergent herbicides, such as clethodim.

It has been dry, what does that mean for weed management?

Unseasonally dry conditions prevailed across most of the Eyre Peninsula in May and June 2017. This has impacted on the ability to sow crops, but has also delayed the emergence of dry sown crops. Now that mid-winter has set in, there are a lot of questions about what to expect this year from herbicide performance to weed emergence. There are really three main things to think about in making the most of weed control in 2017:

1. The impact that 2016 had on weed seed banks and seed dormancy?
2. The effect of dry conditions on the persistence of pre-emergent herbicides?
3. The effect of frost on post-emergent herbicide performance.

Weed seed banks and behaviour

In 2016 a particularly soft and extended spring was experienced. This was ideal for producing larger than average grain yields, but was also good for producing larger than average weed seed set. In many weed species, seed that is set during cooler, wetter periods tends to have greater seed dormancy than seed that is produced during hotter, drier conditions. So not only has there been greater than average weed seed set, a greater proportion of that seed than usual will have a higher dormancy.

In addition, there is increasing evidence that grass weeds in particular are changing their germination pattern in response to farming practices. This has been most notable for weeds such as brome grass and barley grass, where many populations from cropped fields now show greater dormancy than those from fence lines and pastures. This has also occurred, to a lesser extent, with annual ryegrass from continually cropped fields. The dormant weed seeds typically require cold and/or dark conditions to allow them to germinate.

The extended dry period during May and June will have reduced the level of dormancy in grass weeds, and therefore, a greater proportion of them will germinate once sufficient rain falls. For late sown crops, this means a large germination of weeds around or shortly after sowing. For dry sown crops, there are likely to be weeds emerging from about five days after sufficient rain falls. Given the large seed banks from 2016, there will be continued emergence of grass weeds through the late winter in response to rainfall events.

Pre-emergent herbicide behaviour in dry sown crops

Some crops have been sown dry, either on the calendar or in anticipation of rainfall that did not eventuate. Where pre-emergent herbicides were used, the question is have those herbicides disappeared? The answer depends a lot on what has occurred since seeding. Where the soil was dry and remained dry until the first substantial rainfall event, the herbicides will still be present. If the soil has been moist for some of that time, some or most of the herbicide may have dissipated.

The key factors to breaking down most herbicides in soil are moisture, temperature and soil organic matter. As conditions since application have, on the whole, been cool and dry, then there will have been little breakdown of herbicides. To understand the possible fate of herbicides through this period, you can start with their half-life in soil (Table 1). These values are averages and, of course, will vary greatly depending on soil type, soil organic matter, rainfall and temperature conditions. However, the herbicides with longer field half-lives are likely to have broken down less than herbicides with shorter field half-lives. So herbicides like Sakura^® and propyzamide are likely to have more of the original amount applied still present in the soil than products such as Arcade^® or Butisan^®.

Table 1. Average field half-lives of some commonly used pre-emergent residual herbicides.

^* Boxer Gold^® contains both prosulfocarb and S-metolachlor.

The second issue to consider is how rainfall will impact on herbicide performance. The more water soluble herbicides are the more likely they are to perform better where there is patchy rainfall around the break. These herbicides need less moisture to activate. Products, such as Butisan^® and Boxer Gold^® will be better than Sakura^® and propyzamide as they have higher water solubility (Table 2). Trifluralin is a special case, as it turns into a gas on exposure to water. Therefore, its low water solubility does not affect its ability to control weeds in patchy breaks and trifluralin is one of the better products for patchy breaks.

However, if there is too much rainfall then crop damage may occur as the herbicides are washed down into the crop root zone. This is a particular problem when herbicides are applied to dry soil. Where the soil is dry, herbicides will not have interacted with the various soil components (organic matter and clay) and can move much faster and further in response to a large rainfall event than they would if the soil was already moist. The most water soluble herbicides will move fastest through the soil and movement will be greater in sandier soils. Even if the crop was dry sown weeks ago, some damage may occur with a large opening rainfall event.

Dry seasonal conditions may also impact on soil residue carry over affecting re-cropping intervals for 2018. Dry sowing means that the re-cropping interval should be calculated from the break of the season and not from when the herbicide was applied. However, as temperature and moisture are required to break down herbicides in soil, rainfall amounts in spring 2017 and autumn 2018 are going to be the most important factors in assessing risk to planting susceptible crops in 2018.

Table 2. Water solubility of some commonly used pre-emergent residual herbicides.

^* Boxer Gold^® contains both prosulfocarb and S-metolachlor.

Finally, crops will be emerging in cooler soil and will grow more slowly due to the lateness of the break. This means they will not be as competitive against weeds. Over the past five years our research has shown that competition from crops is an essential factor in good pre-emergent herbicide performance on grass weeds. The combination of a competitive crop with pre-emergenent herbicides will result in up to 50% less grass weed seed set compared to a poorly competitive crop. Therefore, even if the pre-emergent herbicides from dry seeding applications are still present, the reduction in crop competition due to cooler soils may mean they do not work as well.

Frost and post-emergent herbicides

Another impact of a dry winter will be an increased number of frost events. Frost can reduce the performance of many post-emergent herbicides. Group A herbicides in particular are greatly affected by frost. Frost can also reduce the ability of the crop to detoxify herbicides resulting in crop damage.

Our research has identified frost as a particular issue with our use patterns of clethodim. Clethodim works best on annual ryegrass at the three-leaf stage. However, it has become common to apply clethodim later in winter to larger tillered annual ryegrass. One of the problems that arises from this use pattern is that the risk of frosty conditions is greater during this period, which can reduce the performance of this herbicide. Our studies of the effect of frost on annual ryegrass, shows that twice as much clethodim is required for the same level of control of susceptible annual ryegrass after three nights of frost (Figure 1)^ɸ.

^ɸIn commercial situations, it is important that growers read the label carefully and only apply product that follows label recommendations.

Figure 1. Response of susceptible (left) and resistant (right) populations of annual ryegrass treated with clethodim at the three leaf stage in the absence of frost or with three days of simulated frost either before clethodim application or after clethodim application.

With populations of annual ryegrass resistant to clethodim, the problem can be even worse, with dramatic reductions in efficacy under frosty conditions (Figure 1). Even though many farm populations of annual ryegrass now have resistance to clethodim, this resistance is rate responsive and use of the top label rate can provide some suppression of the resistant ryegrass. Applying clethodim to large tillered resistant populations of annual ryegrass under cold weather conditions makes it much harder for clethodim to work.

Useful resources

GRDC Soil Behaviour of Pre-emergent Herbicides in Australian Farming Systems Manual

Maximise Clethodim Performanc: Impact of Frost

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 author would like to thank them for their continued support.

Contact details

Christopher Preston
University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au