Humidity and air pressure conditions directly interfere with the evaporation and drift of pesticide spray droplets, a climatic characteristic that also requires attention when spraying.
The application of pesticides is greatly affected by psychrometric conditions, and the factors relative air humidity, temperature, wind speed and direction are those that most affect this operation in the field.
Relative humidity is the relationship between the amount of water vapor in a sample of humid air and the maximum amount of vapor that the air could retain at the same temperature (Vieira, 2009). The existence of water in the atmosphere and its phase changes play a fundamental role in several natural physical processes, such as the transport and distribution of heat in the atmosphere, evaporation, evapotranspiration, among others. Water vapor in the atmosphere is equally important as a condition for the occurrence and phytosanitary control, and is also a determinant of the quality, storage and conservation of agricultural products (Pereira et al.
The meteorological factors relative air humidity, temperature and wind speed are those that have the greatest influence on spraying. Together, they increase the risk of drift, evaporation and volatilization, reducing the average life of the drops. Xu et al (2010) determined through sequential images that the time for evaporation of 300µm droplets was 2 seconds, and for 600µm drops it was 4 seconds. A andvaporization was calculated by the time interval and the total number of sequential images from the beginning of droplet deposition until evaporation. The authors observed thatWhen the droplet diameter increased from 300µm to 600µm, the evaporation time increased by 279% without the surfactant and 452% with the surfactant, while the total average wetted area was 166% without the surfactant and 229% with the surfactant. The average wetted area was greater on the top of the leaf than on the bottom of the leaf, while the average time for evaporation was slightly shorter on the top of the leaf than on the bottom of the leaf.
Understanding the physical process of spray droplet evaporation and knowledge of leaf surface interaction patterns with the target is essential not only to improve the efficiency of pesticide application, but also to minimize off-target contamination (Xu et al.
The use of adjuvants is a recommended practice in many situations, as they promote changes in the spray mixture, making it possible, for example, to minimize meteorological effects that can compromise the efficiency of Carbonari phytosanitary treatments. et al (2005). However, most of the problems arising from the use of these additives in the syrup originate from the lack of knowledge of their action and the implications of their use on the control efficiency of the target recommended in the application, Antuniassi (2006), including in relation to the droplet spectrum generated .
Work was carried out to compare the effect of temperature and relative humidity with the water vapor pressure deficit in spraying and its effects on evaporation, volatilization and drift of spray droplets. The study was carried out at the Machinery and Mechanization Laboratory (Lamm), linked to the Institute of Agricultural Sciences at the Federal University of Uberlândia, campus Monte Carmelo, Minas Gerais.
Temperature and relative humidity data were obtained from a 30-year historical series of climatological normals, published by the National Institute of Meteorology (Inmet).
Historically, the recommendation for ideal weather conditions at the time of spraying is a temperature between 20°C and 30°C, relative humidity greater than 55% and wind speed below 10km/h. From the relative humidity of the air and the water vapor saturation pressure, the water vapor pressure was estimated.
Adopting the temperature and relative humidity ranges used as a reference for applications, the pressure deficit considered suitable for spraying pesticides varies between 1.052,0 and 1.910,0 Pascals, for temperature conditions between 20ºC and 30° C and humidity above 55%, respectively.
The data were analyzed using descriptive statistics to represent the behavior of pressure deficit, temperature and relative humidity in five municipalities, with strong agricultural suitability, in different regions of Brazil.
The application of pesticides has guaranteed the protection of plants with high production and quality, but the increase in the use of pesticides has raised public concerns about the exposure and contamination of workers, as well as negative impacts on vulnerable ecosystems.
The application of pesticides is a complicated operation, due to the use of various types of equipment and spraying methods, physical properties of chemicals, types of crops, growth and multiplication habits of weeds, insects and pathogens, skill of operators, conditions weather, safety, operator ergonomics and environmental regulations and the consequences arising from the application of pesticides.
In Central Brazil, the Brasília region has great agricultural potential in the production of soybeans, corn, beans, wheat, among others. The ideal time to spray depends on wind speed, relative humidity and air temperature. The analysis of climatological normals indicated that the largest pressure deficit obtained was 1.850,46Pa (Graph 1). With these results, spraying could be carried out throughout the day, with acceptable evaporation values, considering results of work that demonstrate good biological effectiveness, in applications carried out under these conditions.
In the state of Pará, the municipality of São Félix do Xingu is an important producer of black pepper and banana, two crops that are heavily attacked by insects, pathogens and that suffer from competition with weeds. In this region, the maximum deficit was 977,31Pa, the relative humidity in the region is quite high, favoring the presence of pathogens. Therefore, spraying can be conducted throughout the day with relatively low evaporation values.
Decision-making based solely on temperature would result in considerable insecurity for the applicator, as the maximum temperature always presented values above 30°C (Graph 2). However, the average relative humidity of the air is high, so the pressure deficit indicates low droplet evaporation. With the addition of the right adjuvant, conditions would be even better.
The municipality of Barreiras, in the state of Bahia, is a producer of corn, soybeans, beans, coffee, cotton and sugar cane. Spraying is intensely practiced as one of the most effective alternatives for phytosanitary control.
In São Félix conditions, as evaporation is relatively low, after correct selection of the tips, drift will be reduced, as the drops will show a small reduction in size towards the target. However, when comparing with the conditions of Barreiras, it appears that, due to the high evaporation, the selection of the tips must consider that the droplet will greatly reduce its size towards the target and will therefore be more prone to drift, in speed conditions. of smaller winds, similar to that described by Yu et al (2009a) and Gil et al (2008). The data shows that in the months of July, August, September and October the pressure deficit is 1.930,54Pa; 2.542,17Pa; 2.909,04Pa and 2.067,14Pa, respectively. This is a period of high temperatures and low relative humidity (Graph 3).
The farmer must look for alternatives to increase the lifetime of the drops on the target when evaporation is high and one solution is the addition of adjuvants. These are added to the spray solution with the aim of improving the efficiency of foliar sprays, reducing the impact of environmental factors and allowing more efficient cuticular penetration, facilitating wetting on water-repellent surfaces and the contact of the solution with the cuticle on hairy surfaces, that can keep the drops suspended, corroborating Carbonari's statements et al (2005) and Antuniassi (2006).
In Minas Gerais, the Triângulo Mineiro region is considered the state's agricultural breadbasket. There are areas of pastures for livestock, soybeans, corn, sugar cane, coffee, cotton, beans and fruits in general.
The most critical pressure deficit for spraying pesticides occurred from July to November and was below 1.909,25Pa, considered a difficult condition to spray (Graph 4). Unlike Barreiras, the weather conditions are milder and allow applications throughout the day, when the tips are selected correctly.
Conditions for spraying in Passo Fundo, in the southern region of Brazil, are most critical in the months of November, December and January, with deficits of 1.109,21Pa; 1.232,85Pa and 1.115,40Pa (Graph 5). In comparison with the other regions studied, it presented an average potential for droplet evaporation.
Although the water vapor pressure deficit is a function of temperature and relative humidity, it is humidity that has most increased the rate of water evaporation. Depending on the region of the country, the variation in temperature is small, such as Brasília, São Félix do Xingu, Barreiras and Uberaba, while in Passo Fundo the temperature showed a variation of 10°C between the winter and summer months, contributing with the increase in the deficit.
The vapor pressure deficit above 1.909,25Pa increases the evaporation of the droplets and in these situations the applicator must choose to work with larger droplets. The phenomenon of evaporation when applying pesticides would be greatly reduced if applications were carried out under low pressure deficits, or using tips that produce large drops. However, the drift caused by lateral winds appears as a problem when the evaporation rate is high, even winds considered suitable for spraying can promote large drifts, in cases where large droplets have high evaporation and long falling time and stay on the surface. the target, Yu et al (2009b) and Nuytten et al (2006) also reached similar conclusions.
Drift is a phenomenon that occurs due to runoff, due to the excess or coalescence of drops, and the action of the wind. In both cases, it represents risks of environmental contamination, phytotoxicity of neighboring crops and even symptoms of acute or chronic poisoning in humans.
Using the pressure deficit to define the timing of applications can reduce drift losses. However, this concept is not yet widely known and would be difficult to adopt in the field. An alternative is the automation of machines, equipped with sensors that would act by altering the spectrum depending on psychrometric conditions.
Evaporation intensifies the effect of exodrift, as it reduces the size of the droplets, increasing the time to reach the target and the susceptibility to lateral displacement.
Evaporation is controlled by the pressure deficit; application based exclusively on temperature and relative humidity, separately, leads to sprays with a high evaporation rate,
Cleyton Batista de Alvarenga, Paula Cristina Natalino Rinaldi, Federal University of Uberlândia; Robson Shigueaki Sasaki, Federal Institute of Minas Gerais
Article published in issue 161 of Cultivar Máquinas.
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