Recent research by NIAB and Syngenta shows attention to detail and timing will help improve weed control this spring. Operators will also need to be prepared to select the right nozzle and rate to hit the target as the season progresses.
Overwintered grass weeds are set to be an exceptional challenge this season. A combination of the dry summer limiting stale seedbed control opportunities, earlier than normal autumn crop establishment and mild autumn conditions triggering prolific grass weed germination, have all increased the burden.
Going into the spring, farmers, agronomists and sprayer operators face the prospect of high weed populations of wild oats, ryegrass and some brome species, as well as larger plants to contend with.
Big competition
New research by NIAB and Syngenta reveals the overwintered and earliest emerged grass weeds have the greatest competitive effect on crop yields, as well as producing far higher numbers of viable seeds to perpetuate weed problems in future years, reports Syngenta Technical Manager and grass weed specialist, Pete Hawkins.
“The NIAB research really confirms the advice to target spring grass weeds as early in the season as possible. While there may be a temptation to delay treatment to allow later germinating grass weeds to come up, the trials show these later emerging weeds pose far less threat to the crop and produce a much lower seed return at maturity,” he explains.
Don’t hold off
Holding off grass weed treatments will result in more competition on crop yields and could prove more difficult to control – requiring higher herbicide rates and a greater challenge for application.
Pete advocates treating wild oats and ryegrass as soon as weather conditions enable consistent plant growth from the start of February. “Good growth of target weeds is important to translocate the herbicide active down into the root and achieve a complete kill,” he adds.
In recent seasons, after a good start to early spring growth, there have been prolonged dry periods in late spring that have curtailed growth that could compromise herbicide results. In these situations, it may be necessary to apply the grass weed herbicide treatment as a separate application, or delay treatment until better growing conditions resume.
Another good reason for applying early is that weeds are easier to target and hit with the spray while the crop is still relatively small and open, explains Pete.
“With the early Axial Pro treatments, Syngenta application research has shown using the 3D angled nozzle is most effective at hitting and retaining spray on the target grass weeds.
Applying in a water volume of 100 litres/ha with the 3D nozzle can ensure good coverage of the leaf, but without over wetting surfaces and leading to run off,” he advises.
Where crops have tillered and are larger at the time of application, however, the grass weeds may be shielded from the spray, he warns. “In these situations, we would recommend operators switch to the more upright spray pattern of an Amistar (GuardianAir) nozzle, to get more spray down into the canopy, and increase water volume closer to 200 litres/ha.”
Control the pressure
Growers should, however, avoid the urge to increase spray pressure in an attempt to push the spray into the canopy. It is actually counterproductive, because the higher pressure creates a greater proportion of fine droplets, which are less able to penetrate the crop canopy and are at risk of being lost as drift.
Using a larger sized nozzle or drift reduction technology at a slower speed will apply the required water volume more consistently and effectively.
Treating overwintered wild oats, for example, up to growth stage 29, an Axial Pro rate of 0.6 litres/ha is advised in good growing conditions, increasing up to the maximum 0.82 litres/ha where growing conditions are compromised. For larger weeds, up to growth stage 39, and for all ryegrass treatments, the higher rate of 0.82 litres/ha would be the default, he adds.
Each field’s weed burden needs to be individually assessed, he suggests. “Broad- leaved weeds are typically easier to control later in the season, without the same fears of crop competition and weed multiplication associated with grass weeds.”
Tank mixing with a compatible broad-leaved herbicide can achieve a complete spring clean-up of weeds in a single spray, which reduces time and cost, points out Pete.
Stick to the sequence
Operators do need to be aware of stipulations with herbicide sequences. Using Axial Pro first in a herbicide sequence that includes SU/ALS or hormone broad-leaved weed treatments, for example, ensures the grass weeds can be taken out at the optimum time. This is compared with a 21-day interval required after a ALS/hormone herbicide has been used before the grass weed control can be applied.
“Earlier targeting of wild oats, ryegrass and other grass weeds does give the flexibility to achieve control at lower application rates and increases the likelihood of better control overall. But as weeds increase in size, or where spring application conditions are less conducive, the rates may need to be increased accordingly,” he suggests.
Axial Pro can be applied at full rate right up to GS41 in wheat and barley. This compares with GS31 and at a capped rate with fenoxaprop in barley, for example. With these late season applications, where weeds are still within the crop using the Amistar (GuardianAir) nozzle is the best option, however when weeds have risen above the crop Pete recommends operators revert to an angled 3D nozzle at 100 litres/ha to best target the exposed leaves.
“These treatments are too late to prevent the yield loss from weed competition, but can be useful in reducing the weed seed return for future seasons,” he adds.
Sprayer operators also have an important role in identifying grass weed infestations in the crop as the season progresses. Mapping weed patches can help to build up a picture of problems and prove invaluable in decision making for integrated control strategies, including adapting crop rotations, establishment techniques, seed rates and drilling timing in subsequent seasons.
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