Pre-emergent herbicide performance in 2021 –how this happened and what to expect in 2022

Take home messages

• The late break and cool wet conditions during winter influenced pre-emergent herbicide control in 2021.
• Crop damage occurred through shallow sowing, not adequately separating herbicide from the crop seed and on soil types with low organic matter.
• Less soluble pre-emergent herbicides are safer to use for dry sowing.

2021 – A year of surprises?

2021 was one of those years of surprises. I took more calls about pre-emergent herbicide performance in 2021 than I have for some years. Partly this was due to the widespread use of products that advisors and growers had less familiarity with and part was due to the seasonal conditions that occurred.

Every year is a different, which means that pre-emergent herbicide efficacy can vary. Several key environmental factors influence the performance of pre-emergent herbicides. The main factors are rainfall patterns, existing soil moisture and temperature. Differences in these from year to year can change how pre-emergent herbicides behave.

Pre-emergent herbicide performance in 2021

Figure 1 shows weekly rainfall at Snowtown in South Australia, which encapsulates one of the key issues for 2021 across much of South Australia and Victoria. Summer was relatively dry, as was autumn with only a few small rainfall events. This meant that soils were still quite dry coming into seeding. The break was late with the first significant rainfall occurring at the end of May. Through June and July, there was above average rainfall, but spring was dry.

Figure 1. Weekly rainfall at Snowtown, SA for the first 9 months of 2021. Source Bureau of Meteorology.

The above average rains after the break meant that most pre-emergent herbicides activated well and controlled the first flush of annual ryegrass. The drier than average spring reduced late emerging ryegrass resulting in some excellent control of annual ryegrass with pre-emergent herbicides.

The higher than average rainfall during June and July reduced the performance of some of the more water-soluble herbicides, such as Butisan® and Luximax®. This is likely due to the herbicides being moved out of the weed root zone.

The dry soil at sowing meant the large rainfall events during June moved pre-emergent herbicides more quickly through the soil profile. This increased the risk of crop damage from pre-emergent herbicides. Selecting the correct herbicides for the soil type and seeding systems is essential in managing this risk. The more water-soluble herbicides with lower crop safety are the most likely to cause damage. Seeding systems where the herbicide is left sitting above the crop seed are also less safe.

Table 1 shows the solubility and soil binding characteristics of pre-emergent herbicides used for annual ryegrass control. Herbicides with higher water solubility and lower binding to soil, such as Butisan, are likely to move further through the soil with high rainfall events. Those with low water solubility, such as Sakura®, will move less far.

Table 1: Behaviour of some pre-emergent herbicides used for grass weed control.

*Boxer Gold contains both prosulfocarb and S-metolachlor

What happened with Overwatch® in 2021?

Perhaps the biggest talking point of pre-emergent herbicides 2021 was the behaviour of Overwatch (bixlozone). Overwatch was released in 2021 for the first time and, as often occurs with new herbicides, there was plenty to learn. The most obvious effect of Overwatch was the extensive bleaching of barley crops. Barley is known to be less tolerant of Overwatch than wheat, so more bleaching should be expected. However, environmental conditions in 2021 magnified this effect.

The large rainfall events after sowing moved the herbicide further into the soil profile allowing the barley crop to take up more herbicide. This led to the considerable bleaching observed. To recover from the bleaching, the crop needs to produce new green leaves, as the bleached tissue does not recover. The cool, showery and cloudy conditions through June and July slowed the growth of the crops and it took some weeks for new leaves to emerge. Once some sunny days arrived, most of the barley crops recovered.

Where crops did not fully recover, it was often the result of soil types or seeding systems that allowed more herbicide to reach the crop. In particular, crops sown too shallow, those sown too fast with treated soil being thrown into the next furrow, and those sown with disc seeders where herbicide was allowed to remain close to the crop seed were more likely to suffer damage. In addition, crop damage occurred in some soil types with low organic matter and in areas where there was overlap in spray application.

What to expect in 2022

2022 is likely to be a different year. The current forecast is for average to above average rainfall for January to March. There is likely to be more moisture present in the soil and the chances of a late break are lower. If the soil profile is damp before herbicides are applied, they are likely to move less in response to rainfall events. Under such conditions, Overwatch will be less damaging to barley crops and we will see less bleaching of the crop. An earlier sowing date will also allow the crop to recover faster from any bleaching that does occur.

What we have learned about Overwatch is that knife points with press wheels is the safest seeding system for barley crops. All other types of seeding systems are likely to result in crop damage.  Additional care should be taken in light soils and soil types with low organic matter to make sure the herbicide is kept well away from the crop row.

Mateno® Complete (a mixture of pyroxasulfone, aclonifen and diflufenican) will be released in 2022 and will offer an alternative to Boxer Gold and Overwatch for barley. The rate for use in barley will be 750 mL/ha applied IBS. As with Overwatch, knife points and press wheels will be the safest seeding system to use. All the components of Mateno Complete have low water solubility and will tend to stay closer to the soil surface. However, under conditions of dry soil and high rainfall, such as we saw in 2021, the herbicide may be moved into the crop zone and cause damage to barley.

Mateno Complete will be safer to use in wheat than barley. In wheat, it can also be used early post-emergent, at a similar timing to Boxer Gold, to achieve more extended control of annual ryegrass. The lower solubility of the herbicides in Mateno Complete means more rainfall after application is required to activate compared with Boxer Gold. This means the early post-emergent application of Mateno Complete will be most useful in higher rainfall regions.

What are the best products for dry sowing?

Using pre-emergent herbicides with dry sowing is challenging as there is no way of predicting when and how much rainfall will occur. A long period between sowing and getting sufficient rainfall to activate the herbicides can lead to some herbicide losses and a shorter period of persistence after the crop emerges. Of more concern is where there is a large rainfall event to start the season. As the soil is dry, large rainfall events will move the herbicides further into the soil profile, increasing the risk of crop damage.

As with all other uses of pre-emergent herbicides, soil type, soil organic matter, herbicide behaviour and seeding system need to be considered when choosing the appropriate pre-emergent herbicide. In terms of herbicide behaviour, trifluralin is the ideal pre-emergent herbicide for dry sowing. It has low water solubility and binds tightly to organic matter (Table 1). This means it has less chance of moving far enough into the soil to cause crop damage. Unfortunately, trifluralin resistance is common in annual ryegrass across South Australia and Victoria. This means the other herbicides with low water solubility, such as Sakura, Avadex Xtra and prosulfocarb, should be used. Herbicides with high water solubility and more mobility in soil, such as Butisan, Boxer Gold and Luximax, are less suited to dry sowing.

Another factor to consider is the tolerance of the crop to the herbicide. Where the crop is less tolerant to the herbicide, the risks increase with dry sowing. For example, the risks of crop damage from Overwatch to barley are much higher than the risks to wheat. Only the safest of herbicides are suitable for dry sowing where disc seeders are used.

The other aspect of dry sowing is managing the risk of the herbicides not activating in time to control weeds. This is most likely to happen with low solubility herbicides like Sakura. A way to manage this is to mix with a herbicide that needs less rainfall to activate, such as trifluralin or Avadex Xtra. Trifluralin requires less moisture as it works as a gas and turns into a gas on contact with water. Avadex Xtra is absorbed by the coleoptile rather than the roots, so does not need to be moved as far through the soil.

Useful resources

Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers

Contact Details

Chris Preston
School of Agriculture, Food & Wine
University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au