US studies highlight droplet impact
US researchers attending the GRDC Northern Research Updates outlined their work on better understanding the correlation between spray nozzles and droplet size, and herbicide and pesticide effectiveness.
Since 1996, glyphosate has been the predominant post-emergence herbicide for weed control in maize, soybeans and cotton in the US. Due to this, glyphosate-resistant weeds have become increasingly prevalent in glyphosate-resistant crops, forcing many growers to use other herbicides.
Herbicide programs that relied primarily on glyphosate often used carrier volumes as low as 60 litres per hectare or less.
Other herbicides used in row crops often require a higher labelled carrier rate compared with glyphosate and growers sometimes considered this a burden – more water to be transported, more refills and more potential for mixing errors.
When it comes to pesticides there is the added concern of pesticide drift. Both drift and efficacy can be managed by spray quality, which is influenced by application decisions such as nozzle selection, operating pressure and the spray solution components.
A series of studies in the US have measured the influence of carrier volume on droplet size and weed control using four different post-emergence herbicides with four different modes of action for weeds in soybeans.
Roundup® PowerMAX (glyphosate at 1.4L/ha), Liberty® (glufosinate at 0.96L/ha), Cobra® (lactofen at 0.54L/ha), and Weedone® (2,4-D at 1.4L/ha) were applied at different carrier volumes. The four herbicides are an EPSP synthase inhibitor, glutamine synthase inhibitor, PPO inhibitor and synthetic auxin, respectively.
The four herbicides were each sprayed with appropriate adjuvants and were each applied at five carrier volumes (40, 60, 80, 120 and 160L/ha).
Droplet size of each treatment was evaluated at a wind tunnel facility using a diffraction laser. Weed control ratings were recorded at three field sites across Nebraska, US, at 14 and 28 days after treatment.
The sprayed plots were 1.3 metres wide and 5m long. Planted across each plot were rows of non-herbicide-resistant maize and soybeans, velvetleaf and grain amaranth. Treatments were replicated four times at each site.
Except for amaranth, the performance of the systemic herbicides (glyphosate and 2,4-D) on weed control was not influenced by different carrier volumes. The amaranth exception was no surprise because, in the US, amaranth has a lot of genetic diversity.
However, there was a clear interaction between the effect of carrier volume and the contact herbicides glufosinate and lactofen. Control of velvetleaf increased from 52 to 83 per cent and 37 to 85 per cent respectively for these two contact herbicides, as carrier volume was increased from 40 to 160L/ha.
Control of the amaranth by glufosinate and lactofen increased from 56 to 80 per cent and 81 to 100 per cent respectively. This was not too surprising since the Cobra® label recommends 120L/ha and the Liberty® label recommends 160L/ha.
The researchers concluded that as operators start using products other than glyphosate for weed control, it will be important to fully understand how to maximise their efficacy.
Similar studies were conducted across Nebraska in 2013 to investigate spray quality. Four locations had four replications each. In each plot conventional maize, conventional soybeans, amaranth, velvetleaf, flax and quinoa were planted.
In the studies, Roundup® PowerMAX was sprayed at 0.7L/ha (glyphosate), Clarity® at 0.25L/ha (dicamba), FirstRate® at 4 grams/ha (cloransulam-methyl), Flexstar® at 0.5L/ha (fomesafen) and Select® MAX at 0.19L/ha (clethodim).
These herbicides were used with the recommended adjuvants and all applications were made at 80L/ha.
It is important to note that the rates used are about half the recommended rate for each of these products. The use of these rates for weed control is strongly discouraged. This was done for research purposes only, to observe differences in weed control using different application methods under rates that had marginal control.
Each herbicide treatment was sprayed through an XR1003 flat-fan nozzle at 300 kilopascals, an XR11002 flat-fan nozzle at 276kPa, a TT11002 nozzle at 276kPa, an AIXR11002 nozzle at 276kPa and an AI11002 nozzle at 276kPa.
The five nozzles when tested in water at these pressures should give, respectively, fine/medium, fine, medium, coarse and very coarse spray qualities.
For each of these nozzles and spray solutions, the droplet size was determined using a low-speed wind tunnel. The droplet sizes were determined using a 24 kilometre per hour wind speed across the nozzle and droplet sizes were measured using a Sympatec laser diffraction instrument.
As expected, certain herbicides performed as well as or better with large droplets while other herbicides performed poorly when droplet size was too large.
There were clear interactions between spray solution and nozzle type, confirming the need to not only consider the ideal droplet size when making a spray application, but also the most appropriate nozzle type. Further work in this area will be necessary to fully understand the interaction.
For the Roundup® PowerMAX, 0.7L/ha was effective at controlling all five species in the study. Greater than 90 per cent efficacy was observed using all five nozzles.
Because of the high level of efficacy, it was difficult to make comparisons between spray qualities.
Since Roundup Ready® systems have been widely adopted, the recommendation for glyphosate applications in these crops has generally been toward using nozzles and pressures that would give larger droplets. The findings of this research would support this recommendation as larger droplets provide less potential for drift while delivering equal or better efficacy.
However, when increasing Roundup® PowerMAX concentration in the spray solution from 0.7L/ha to 1.4L/ha, the droplet size is likely to decrease.
Clarity® applications tended to have a similar trend as Roundup® PowerMAX in respect to the use of the different nozzles. There was little difference between the nozzles and therefore, the nozzles producing the largest droplets provided a superior application, as the drift potential was lower. This result is not really a surprise as dicamba (Clarity®) is a systemic herbicide as well.
FirstRate® was similar to Clarity® and Roundup® PowerMAX in that there was not much difference in weed control between nozzles that produced different droplet sizes.
The results from FirstRate® were more dependent on species. For example, the AI11002 nozzle provided the greatest control of the amaranth while the XR11003 flat-fan nozzle provided the greatest control of the velvetleaf.
For both Flexstar® and Select® MAX, the larger droplets had less efficacy on the weed species for which control would be expected. An interesting observation was that the XR11002 flat-fan nozzle was not nearly as effective as the XR11003 or the TT11002, which both give slightly larger droplets, indicating that the droplet size can get too small for maximising efficacy.
This is a critical finding because these products are important as tank mixture partners for post-emergence applications with glyphosate to control glyphosate-resistant weed populations in soybeans.
Pesticide efficacy and drift
Drift and efficacy are two critical components of any pesticide application. The problem is that for many different pesticides, the two components work in conflict with one another.
For many pesticides, the smaller the droplet the greater the efficacy. However, the smaller droplets are the ones most prone to drift. In cases where the pesticide is most effective with small droplets, there is a need to find a balance between pesticide efficacy and drift potential.
When applying a pesticide, wind speed and direction, distance to nearest susceptible species and the toxicity of the pesticide should be carefully evaluated and considered.
Further, intensive scouting and detailed record-keeping should be undertaken to ensure operators are aware of how they made the application and the results so future applications can be maximised.
While there are trends that can be followed, such as minimising pesticide drift, following the label rates to ensure adequate carrier rates, and using the correct nozzles and pressures, much of what is required to maximise pest control is achieved through tweaking the system based on scouting and observations.
University of Nebraska-Lincoln
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Greg Kruger is from the University of Nebraska-Lincoln in the US; Andrew Hewitt is from Lincoln Ventures Limited; and Bradley Fritz and W. Clint Hoffmann are from the US Department of Agriculture’s Agricultural Research Service.
GRDC Project Code ICN00012
Region Overseas, North