Necessary care to avoid drift and volatilization problems

Weeds compete with crops for water, light and nutrients, compromising expected productivity. Furthermore, the occurrence of herbicide-resistant weeds worries producers and researchers due to the limited control possibilities. In Brazil, more than 50 species that are harmful to soybean cultivation have been reported, including horseweed (Conyza canadensis, C. bonariensis, C. sumatrensis), the black walleye (bidens pilosa) and the caruru (Amaranthus palmeri, A. hybridus, A. retroflexus e A. viridis), with resistance to different herbicide modes of action. In the USA the number of cases of herbicide-resistant weeds is even higher, with more than 160 cases to date.

In 2016, aiming to improve weed control in soybeans and cotton, cultivars resistant to the herbicide dicamba, a synthetic auxin from the benzoic acid group, specialized in difficult-to-control dicotyledons, were launched in the United States. As it fits as an alternative for controlling weeds in soybean and cotton crops in Brazil, it is expected that this technology will expand in use here in the coming years.

Developed more than 50 years ago, the herbicide dicamba has already been widely used in Brazil. It was recommended for the control of dicotyledons in crops such as sugar cane and corn in past decades. However, its use was restricted due to problems of high volatilization and drift in sensitive crops. Today, new formulations have less volatility, but still require care regarding application, which ranges from preparing the syrup, the application itself and cleaning the hydraulic circuit of the sprayers after use.

In late 2017, several reports of drift were reported in the US. Around 2.700 cases are related to the herbicide dicamba, affecting more than 1,4 million hectares of soybean area, according to the North American regulatory body.

Volatilization, drift, contamination in the tank, thermal inversion and misuse were the main suspicions raised by researchers and authorities in the country at the time.

Since then, there have been several studies regarding herbicide volatilization and drift, including research into weather conditions during application, droplet size and spray tip designs, product mixes, and cleaning of tanks and spray systems. pulverization. Companies and researchers have been refining recommendations for herbicide application practices ahead of new, expanded scale adoption.

 

WEATHER CONDITIONS AND APPLICATIONS

Applications of phytosanitary products must respect certain meteorological conditions. In general, it is recommended to avoid applications with wind speeds greater than 10km/h, relative humidity below 55% and temperatures above 30°C, to reduce the risk of drift. However, in the main areas and seasons of soybean and cotton cultivation in Brazil, weather conditions outside those recommended are frequent, requiring technologies that allow work to be made more flexible beyond general limits.

It is important to remember that drift and volatilization are different phenomena. The term volatilization indicates the movement of the molecule in vapor form away from the target. Some formulations present greater volatility, as the vapor pressure of the active ingredient is higher. The herbicides clomazone, trifluralin and dicamba are known for their high volatility. The term drift, however, refers to the movement of the droplets formed by the spray tips away from the target, and the smaller the droplets, the greater the susceptibility to drift. Although there are some factors that can make up the “potential risk of drift”, the percentage of the volume applied with drops smaller than 100 micrometers is one of the most popular indices internationally used to qualify susceptibility to drift.

Therefore, investments in improving formulations to reduce volatilization are very important for applications with lower air humidity, as verified in several studies.

The concentration of auxin herbicides suspended in the air decreases when relative humidity is high. North American researchers observed a reduction in concentration of up to 5,7 times when relative humidity went from 20% to 50% within a period of two days after application.

Increasing environmental temperature also influences the volatility of auxin herbicides. Researchers at the University of Tennessee found that low temperatures (below 15°C) practically eliminated the volatility of dicamba in the air.

Wind speed directly affects the drag of spray droplets, influencing the range of herbicide volatilization. Therefore, wind limits are more stringent for the selection and use of the moment and application technology, and it is advisable not to apply due to the risk of thermal inversions or gusts of wind during spraying.

Even in the absence of wind, herbicide particles remain concentrated and suspended in the air and a gust of wind or thermal inversion can move this mass suspended in the air to an area sensitive to the herbicide, causing serious damage. This reinforces the importance of taking care with the formulations, preparing the mixtures and applications.

APPLICATION VOLUME

The application of volatile products that require drift reduction can be safer with spray droplets classified as extremely coarse or ultra-coarse. For the same application volume, in general, when the droplet size increases, the coverage will be lower. Therefore, to achieve sufficient coverage to safely control the target, volumes greater than 100L/ha have been considered for applications of auxin herbicides.

In the USA, application volumes of at least 140L/ha are required. Researchers at the University of Arkansas observed that dicamba applications showed better control results with volumes greater than 180L/ha. Given experience and studies on the subject, it is recommended that operational adaptation be made to use application volumes that result in sufficient spray coverage to control weeds. This volume will depend on factors such as the selected droplet size, which must be, for safety, at least extremely thick; the stage of development of the weeds and the crop; weather conditions; equipment adjustment and use of adjuvants.

Checking the coverage of targets is extremely important, allowing, even before applications, to check any gaps in the necessary coverage and make the necessary adjustments to calibrate the sprayers.

RECOMMENDED TIPS FOR APPLICATIONS

Sometimes overlooked, spray tips are the most important components of sprayers, as they are responsible for the formation of droplets and their distribution over targets. Therefore, once the target is known and the product is defined, getting the spray tip model and working pressure right is essential to achieving the application objective. The combination of cutting-edge model and working pressure results in droplet size and uniformity having an extremely influence on target coverage and drift.

Numerous spray tip models are available with wide variation in droplet sizes and purposes of use. Remembering that, within the range recommended by the manufacturer of the selected tip, the lower the pressure, the larger the droplet size produced.

However, in addition to the concern with drift, the choice of spray tip and the droplet size produced, attention must be paid to the control effectiveness of the target in question. Studying the effectiveness of dicamba in relation to droplet size, researcher Thomas Butts from the University of Arkansas and collaborators observed that a droplet size of 395 micrometers in volumetric median diameter (MDV) presented the best weed control, but with significant increase in drift. As for the tip that produced a DMV close to 620 micrometers, the researchers observed control of greater than 90% of the weeds, with a significant reduction in drift. According to data from the Center for Study and Development of Application Technology - NEDTA, at Unesp in Jaboticabal, there are several models of spray tips in Brazil capable of producing DMV drops even larger than the models studied in Arkansas. Thus, Brazilian recommendations may make use of spray tips with safer droplet production in relation to size, more commonly seen in models with air induction.

TANK MIXTURES AND VOLATILIZATION

Mixtures in the sprayer tank between phytosanitary products and adjuvants can significantly decrease or increase the droplet size produced and volatilization.

In tank product mixtures, the addition of adjuvants can be useful depending on the desired objective. Adjuvants can be classified according to their function, such as humectants, spray conditioners (pH, sequestrants, stabilizers), foam reducers, evaporation reducers, drift reducers, spreaders, adhesives, penetrants and activators. When mixing herbicides associated with adjuvants, it is possible to adjust and stabilize the mixture in terms of acidity (which interferes with volatilization), the size and uniformity of spray droplets, among other effects on the physical-chemical characteristics of the mixture.

Reducing the pH of the syrup below 5 considerably increases the volatility of dicamba. Products such as ammonium sulfate adjuvant and glyphosate potassium salt are examples of potential pH reducers and present risks of increased volatility when associated with dicamba, according to researchers at the University of Tennessee. The researchers observed that the mixture of dicamba and glyphosate increased the presence of the herbicide in the air by up to nine times, with a reduction only occurring at temperatures below 15°C. It is therefore recommended to avoid mixtures between phytosanitary products or adjuvants, or both, which could reduce the pH of dicamba mixtures, especially in situations of lower relative humidity and higher temperature.

SPRAYER CLEANING

The ideal before and after applying phytosanitary products is to always wash the tank and the spray system, including hoses, filters and spray tips. Product residues and dirt accumulated in the sprayer tank pose a great risk of poisoning the crops to be sprayed. Spraying with auxin herbicides, in turn, requires extra care.

As it is an herbicide that mimics the plant hormone auxin, low concentrations of this hormone can cause major effects. Therefore, residues remaining in the tank or at other points of the sprayer that result in very low concentrations of auxin herbicides in the spray can cause serious damage to cultivated plants that are sensitive to these herbicides. Correctly cleaning the tank, as well as the spray system, is a crucial step to avoid damaging crops in subsequent applications. Several studies have compared the cleaning of dicamba tanks and spray systems using various cleaning products and water. The process of triple washing the tank with cleaning products or just water was enough to remove the amounts of dicamba that could cause damage in subsequent applications. It is important, therefore, to wash the tank with water or another product with at least 10% of the tank's capacity three times, to ensure the safety of a sensitive crop in applications subsequent to the application of an auxinic herbicide.

CONCLUSION

Volatilization and drift problems with auxin herbicides in the USA have highlighted the importance of correct use of application technology. Today, Brazil has the opportunity to avoid, even before the launch of new cultivars, the problems observed in the USA. Researchers and companies have been working towards this.

The indications in this article highlight the reduction of risks and the benefits that the adoption of good agricultural practices brings. Spreading knowledge about application technology is a fundamental step to answer basic questions about phytosanitary treatment: when and how to apply?; how much product is needed to promote target control?; and how to minimize the risks of contamination? With this focus, the chances of reaching the target and obtaining safe and economical results from the treatment performed will be greater.

Pedro Henrique Urach Ferreira and Marcelo da Costa Ferreira, NEDTA/FCAV/Unesp - Jaboticabal