New insights in integrated weed management and new herbicides

Author: Christopher Preston, Sam Kleemann, Peter Boutsalis and Gurjeet Gill (School of Agriculture Food and Wine, University of Adelaide) | Date: 07 Feb 2017

ɸExtra technical comment by Protech Consulting Pty Ltd

Take home messages

  • Crop competition is an excellent partner with pre-emergent herbicides for control of grass weeds.
  • Sowing on time to maximise wheat competition with annual ryegrass helps reduce weed seed set and increases crop yield.
  • New pre-emergent herbicides need to be used appropriately to maximise weed control.

Competition and pre-emergent herbicides are made for each other 

As grass weeds become increasingly resistant to post-emergent herbicides, more reliance is being placed on pre-emergent herbicides for weed control. One issue that arises with the use of pre-emergent herbicides is the emergence of weeds after the herbicides have dissipated. At this point there remains no further effective controls until the end of the season. Obtaining early crop ground cover before the pre-emergent herbicide has dissipated will reduce the competiveness and seed production of later emerging weeds. This can be achieved by increasing the competiveness of crops, particularly cereals. 

There are several ways to increase the competitive nature of cereal crops, such as increasing the seeding rate or reducing the row spacing. However, this study has been investigating an alternative approach to increasing the competitiveness of wheat crops. As wheat is sown later it grows more slowly as the soil temperature decreases going into winter taking more time for canopy closure and giving weeds a greater opportunity to use resources. Earlier sowing, when soil temperatures are warmer, results in more rapid growth and faster canopy closure. This provides an opportunity to increase the amount of competition against weeds without having to significantly change other aspects of the farming system.

Over the past three years, a series of trials at Hart and Roseworthy have been run in collaboration with the Hart Fieldsite Group to look at the role of competition from early sowing to aid pre-emergent weed control. In the trials, wheat was sown about one month apart with several different pre-emergent herbicide options used. Data on weed numbers, crop establishment, weed seed production and crop yield were collected.

In 2014 at Hart, LongReach Scout wheat was sown on 4 May and 2 June 2014. There was an additional knockdown herbicide treatment applied between the two sowing times. Pre-emergent herbicide treatments used at each time of sowing were: nil, Sakura® (118g/ha) and Sakura® (118g/ha) + Avadex Xtra (2L/ha)ɸ.

ɸNote: when used for commercial purposes please always adhere to rates listed on label which states: Incorporated by sowing (IBS) rate of 3L/ha or 3.2L/ha depending on weed species and rate of 1.6 to 2.4L/ha is when tank mixing with TriflurX.

In 2014, the delay in sowing did not lead to a reduction in the number of ryegrass plants present in crop (Table 1). Where no pre-emergent herbicide was used, there were more annual ryegrass seed heads produced from the early time of sowing. In contrast, where effective pre-emergent herbicides were used there was no difference in the number of seed heads produced. Wheat yield from the first time of sowing (TOS1) was 4.15t/ha and from the second time of sowing (TOS2) was 2.93t/ha.

The 2014 season in SA ended with an exceptionally dry spring period, although there was enough stored early moisture to allow most crops to finish well. There had also been abundant winter rainfall that helped the pre-emergent herbicides to work well. 

Table 1. Annual ryegrass present in crop and seed heads at maturity for two times of sowing (TOS) at Hart in 2014. For each measurement, different letters indicate significant differences in means.

Pre-emergent herbicide  Plant counts (8 Aug) (per m2 Head counts (10 Oct) (per m2
   TOS1 TOS2  TOS1  TOS2 
 Nil  59 a 77 a  350 a  164 b 
 Sakura® (118g/ha)  8 b 8 b  39 c  41 c 
 Sakura® (118/ha) + Avadex Xtra (2L/ha)ɸ
 3 b 3 b  32 c  9 c 

ɸNote: when used for non-research purposes please always adhere to rates listed on label which states: Incorporated by sowing (IBS) rate of 3L/ha or 3.2L/ha depending on weed species and rate of 1.6 to 2.4L/ha is when tank mixing with TriflurX.

This trial was repeated in 2015 sowing on 30 April and 27 May in 2015, and the pre-emergent herbicides used were: nil, Boxer Gold® (2.5L/ha), Sakura® (118g/ha) and Sakura® (118g/ha) + Avadex Xtra (2L/ha)ɸ. Once again there was an additional knockdown herbicide application between the two sowing times. Conditions were much drier in 2015, particularly in spring, leading to lower annual ryegrass seed head numbers. However, the effect of early sowing on annual ryegrass numbers were similar (Table 2). Again, there was no significant effect of herbicide treatments on crop yield. Yields were lower in 2015; however, the early TOS (2.2t/ha) still had significantly greater yield than the later TOS (1.5t/ha).

ɸNote: when used for non-research purposes please always adhere to rates listed on label which states: Incorporated by sowing (IBS) rate of 3L/ha or 3.2L/ha depending on weed species and rate of 1.6 to 2.4L/ha is when tank mixing with TriflurX.

Table 2. Annual ryegrass present in crop and seed heads at maturity for two times of sowing (TOS) at Hart in 2015. For each measurement, different letters indicate significant differences in means.

Pre-emergent herbicide  Plant counts (10 Jul) (per m2 Head counts (16 Oct) (per m2
   TOS1 TOS2  TOS1  TOS2 
 Nil  18 a 6 b  45 a  44 a 
 Boxer Gold® (2.5L/ha)  3 bc 1 bc  5 bc  9 bc 
 Sakura® (118g/ha)  1 bc 2 bc  3 bc  13 bc 
 Sakura® (118g/ha) + Avadex Xtra (2L/ha)ɸ
 0 c 1 bc  0 c  15 b 

ɸNote: when used for non-research purposes please always adhere to rates listed on label which states: Incorporated by sowing (IBS) rate of 3L/ha or 3.2L/ha depending on weed species and rate of 1.6 to 2.4L/ha is when tank mixing with TriflurX.

Despite the difference in seasons, delaying the time of sowing and including an additional knockdown application did not reduce the number of annual ryegrass emerging in crop. This supports other data that indicates the behaviour of annual ryegrass is changing with greater levels of dormancy and later germination under selection pressure from continuous cropping farm practices. This behaviour makes delayed sowing a less useful option for control of grass weeds in continuously cropped fields.

The trials have also shown that in drier seasons, Boxer Gold® used early can be effective. However, in wetter seasons, such as 2016 (data not shown) Boxer Gold® does not have sufficient persistence to be successfully used as the only pre-emergent herbicide. The better strategies in wetter seasons involve more robust and longer persistence pre-emergent herbicide mixtures or following up a pre-emergent herbicide application with a post-emergent Boxer Gold®. It is also important to sow a cultivar of wheat with the correct maturity for the early sowing, sowing time. This may mean changing to a longer-season cultivar. 

New herbicides

The cropping season 2017 should see the availability of two new pre-emergent grass herbicide products. These could provide some increased flexibility for grass weed control. However, it will be important to use them under the correct circumstances to get the best out of the herbicides.

Butisan, active ingredient metazachlor, is a new pre-emergent grass herbicide for canolaɸ. It is a Group K herbicide and the active ingredient is similar to S-metolachlor in behaviour (Table 3). Compared to other pre-emergent herbicides available for use in canola, metazachlor is more water soluble and has lower binding to organic matter. This means metazachlor is likely to be more mobile within the soil than other pre-emergent herbicides. This will be an advantage where there is limited moisture at or shortly after application. Less rainfall will be required to activate the herbicide and it is more likely to move into and control weeds in the crop row. However, on light soils with low organic matter, the herbicide can be easily moved too deep by heavy rainfall to control weeds. Metazachlor has a relatively short half-life. In our trials, it struggled with very large annual ryegrass populations, as a result of its short persistence. With larger annual ryegrass populations, a mixing partner will be essential to obtain high levels of control.
ɸProduct currently not registered.

Table 3. Solubility and binding to organic matter (KOC) characteristics of pre-emergent herbicides for grass weed control in canola compared with S-metolachlor.

 Pre-emergent herbicide Trade name  Solubility  Koc (mL/g) 
 S-metolachlor Dual Gold*  480  226 
 Metazachlor Butisanɸ   450 45 
 Propyzamide Rustler®   9 840 
 Trifluralin Triflur-X®  0.2  15,800 
 Simazine Gesatop®  130 

*Dual Gold is only registered for toad rush control in canola.
ɸProduct is currently not registered.

Arcade®, active ingredient prosulfocarb, has been registered for the control of annual ryegrass in wheat and barley. Prosulfocarb is a Group J herbicide that is already widely used as the main component of Boxer Gold®. Arcade®, due to the lack of the S-metolachlor component, will have different behaviour to Boxer Gold®. It will not be as water soluble as Boxer Gold® as the water solubility of prosulfocarb is more similar to pyroxasulfone, the active ingredient in Sakura®. The lack of water solubility means that Arcade® is not suited to post-emergent use.

Arcade® has a rate range of 2.5 to 3L/haɸ however, our trials indicate that every bit of 3 L/ha will be required to control annual ryegrass. Arcade will provide the opportunity for a range of mixtures with other pre-emergent herbicides. Good mixing partners will include trifluralin and triallate (Avadex Xtra). There is also the opportunity for mixing different ratios of Arcade and S-metolachlor. This will be particularly useful in light soils where damage to wheat from Boxer Gold® can occur.
ɸLabel states 2.5L/ha is for suppression of annual ryegrass, while 3L/ha is for control of annual ryegrass.

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. The authors also thank Sarah Noack of Hart Fieldsite Group for managing trials at Hart and Peter Hooper for helpful discussions.

Contact details

Chris Preston
University of Adelaide
0488404120 
christopher.preston@adelaide.edu.au