It is widely understood that frosts can have a negative effect on the efficacy of herbicides. Earlier this year, BCG conducted a trial to better understand how the application of grass selective herbicides are affected during frosty conditions. Â
With larger farming programs, growers are now torn between spraying during frosts when weeds are small (then coming back if necessary), or delaying the application until conditions are more conducive.
With herbicide resistance a constant consideration, growers want to make sure they get the most out of each spray. Will the reduced efficacy be a handicap for those weeds?
Between 31 May and 15 June a replicated trial was established at Sunnyside (10km west of Birchip) in a commercial canola crop.
Six treatments were applied at three timings (2-4 leaf, 4-leaf and 6-leaf canola). The treatments included:
- Untreated control
- Select® (Clethodim 240) at 300ml/ha
- Select® at 300ml/ha plus Verdict® (haloxyfop) at 75ml/ha
- Select® at 600ml/ha
- Select® at 600ml/ha plus Verdict® 75ml/ha
- Select® at 600ml/ha plus UAN at 50L/ha (to enable the ryegrass to take up more of the chemical)
Hasten (1% v/v) and Liase (2% v/v) were also added to each herbicide treatment.
At the site, there were four below zero temperature events at the trial site (Figure 1).
The weather station temperature gauge is located two meters above the ground, and using the rule of thumb that ground level will be approximately two to four degrees cooler, then there would have been 14 frost events at crop height during this time.
Figure 1. Minimum and maximum temperatures at two meters above ground level at the Birchip West trial site from 27 May to 18 June.
According to best practice, for every frost event you should wait a day before spraying. Therefore, if you have two frost events you need to wait two days before spraying. Frost slows the plants metabolism and reduces the plants ability to translocate the herbicide to the active site which can reduce efficacy, especially in plants that already have some resistance.
The issue many growers faced this season was the frequency of frost events were hampering the ability to undertake timely and logistically efficient sprays.
Delaying applications will mean weed size may be larger when the next opportunity presents, but also that the crop growth stage has increased and/or use of other chemicals (eg. atrazine) may not be applicable.
In the trial there was a significant difference between the untreated control and spray applications (Figure 2). If abiding by best practice, growers would not have been able to spray during this time, which would have resulted in ryegrass numbers the same as the untreated/control.
Figure 2. Mean number of plants per square meter after the three applications of the six treatments P<0.001, CV=12.3% (data log transformed).
This paddock had very high ryegrass numbers (up to 200 plants/m2), which would test most herbicides. With such a population, secondary germinations were likely.
Spray applications occurred during the middle of the day to early afternoon, which is preferable during periods of frost.
Weed counts conducted on 25 September showed that there was no effect on treatment application timing, regardless of the frosts. It was evident there was a ‘rate’ response. The best response to killing the ryegrass was to use higher rates of Select with a mixture of Verdict, this supports the continued advice that growers should use higher rates when tackling weeds, especially ryegrass.
While Select at 600ml/ha plus UAN at 50L/ha showed impressive results on reducing ryegrass numbers, there was some negative crop effect with burning of leaves. The treatment did not significantly perform better than Select 600ml/ha on its own, but it showed trends that need to be investigated further.
Overall, growers should assess long-term (weekly) weather forecasts when planning sprays in frosty conditions to take into consideration temperature as well as logistics.
If spraying in frosty conditions cannot be avoided, aim for the warmest part of the day (10:00am to 3:00pm) for the chemical to be taken up from the plant, and use robust rates.
It is always preferable to spray weeds when small, with high water rates. Using pre-emergent herbicides will buy some time to get herbicides out, and applying a grass herbicide may still reduce the numbers enough to give the crop a chance to compete. It is then imperative a second or follow up spray is applied.
As we head into harvest, there are still effective practices that can reduce weed numbers. Harvest weed seed management is becoming a widely adopted tool to lower weed populations in subsequent years.
Assess paddock weed populations now and make a plan for 2018. Resistance testing is another great way to gain an understanding on whether a herbicide is going to be effective, which could save you thousands of dollars long-term.
For more information about this trial please contact the BCG office on 03 5492 2787.
