Resistant ryegrass challenges disc seeding systems
Disc seeding equipment is gaining popularity for its ability to handle crop residue, improved precision, faster sowing speeds, suitability for narrow row spacing and tendency to produce fewer clods when dry sowing.
These systems have an important downside, though, as very few pre-emergence herbicides are registered for use in conjunction with disc seeders where annual ryegrass (and other weeds) have resistance to in-crop herbicides.
Dr Chris Preston, Professor, Weed Management at The University of Adelaide, says that it is essential to follow the WeedSmart Big 6 tactic to mix and rotate herbicide modes of action when adding pre-emergence herbicides to a weed control program.
“Unfortunately, it is difficult to properly implement this strategy in a disc seeding system when there are so few pre-emergence herbicide options available,” he says. “There is also a very real risk of crop damage when using pre-emergence herbicides in a disc seeding system, and this has a knock-on effect on the crop’s ability to compete with weeds like annual ryegrass. Crop safety is #1 priority. The next priority is to avoid using single-molecule applications.”
The crop safety risk arises in disc seeding systems because herbicide remains in the soil above the crop seed in a minimal disturbance environment. In particular, single disc seeders tend to throw minimal soil from the crop seed row, increasing the risk of herbicide damage. Modifications such as wavy coulters can increase the amount of soil throw and make registered herbicide use safer.
Another option is to use a rate from the lower end of the rate range of the riskier herbicide and mix it with a safer herbicide product, such as tri-allate (Group 15 [J], e.g Avdex Xtra) in wheat, for example. Chris says the weakness in this approach is that most of these ‘mixtures’ will involve applying Group 15 [J or K] herbicides, putting considerable pressure on this single mode of action. An alternative is to use a weaker but safer herbicide pre-sowing and partner it with an early post-emergence herbicide.
Herbicides with registrations for early post crop emergence application against annual ryegrass are Boxer Gold (prosulfocarb plus metolachlor, Group 15 [J/K] in wheat and barley), Mateno Complete (pyroxasulfone, Group 15 [K] plus Aclonifen, Group 32 in wheat and barley) and Tenet (metazachlor, Group 15 [K]) in canola. These post-sowing options allow growers using disc seeders to include pre-emergence herbicides in their annual ryegrass control program, albeit with limited opportunities to mix or rotate away from mode of action Group 15.
The only option to use another mode of action is in a canola phase where Tenet can be mixed with clethodim (Group 1 [A]).
Early post crop emergence application of Boxer Gold or Mateno Complete needs to occur from 3-leaf in barley or 1-leaf in wheat, and the ryegrass also needs to be small for best results. The herbicides require rain to activate.
The high proportion of standing stubble in many disc systems is advantageous when getting pre-emergence herbicides onto the soil.
“Correct application is essential to ensure maximum benefit from pre-emergence chemistry,” says Chris. “Always use nozzles that produce a high percentage of large droplets to drive through stubble and onto the soil – that is, Very Coarse or coarser. Keep ground speed around 15–18 km/hr to allow the boom to stay 50 cm above ground level. Raising the boom height above 50 cm produces more smaller droplets, regardless of the nozzle size.”
The key to the success of weed control in disc seeding systems is to ensure that weeds are tackled throughout the weed growth cycle, reducing the pressure on pre-emergence herbicides.
To support the pre-emergence herbicide program in a disc seeder system, focus on low weed seedbank farming practices. Include other WeedSmart Big 6 tools like strong crop competition, using competitive varieties, early sowing and narrow rows, crop rotation strategies, including double breaks, stopping seed set with crop topping, swathing and cutting for hay, and implementing harvest weed seed control.
By implementing a diverse strategy, many growers who use disc seeders have successfully driven down the seed banks of important weeds, such as annual ryegrass, despite the limited options with registered pre-emergence herbicides.
