Effects of temperature on herbicides

To control properly control weeds is a major challenge for technicians and farmers seeking to optimize management and avoid losses. In several areas, the occurrence of control failures almost always raises the suspicion of resistance. But some simple details like the stadium of plants to application and effects of temperature cannot be ignored, as they can reduce the effectiveness of herbicides and cause greater damage in certain cultures.

The presence of weeds in several areas of allied cultivation the high capacity for damage they can cause to a crop must be seen with great attention. Several studies report that losses in productivity can reach 100% depending on population density and the infesting species. Furthermore, the effects of weed interference can be considered irreversible effects on development, causing a reduction in productivity even after its withdrawal. Good weed management in desiccation and stages initial stages of crop development are essential to guarantee high productivity. Another important window for weed management occurs during the summer-winter off-season, where technical assistance along with with producers should look carefully at the main weed plants to in order to prevent proliferation and dispersion, to keep the area clean in crops subsequent ones.

In the southern region of Brazil, the summer-winter off-season for autumn weed management is characterized by several changes environmental factors such as reducing incident solar radiation (fewer hours of light during the day) and greater daily temperature ranges, especially during months from April to August where cloudy days and temperatures are frequent. temperatures below 10oC, or even the occurrence of frost.

The effects of temperature on the effectiveness of herbicides depend of the herbicide molecule and species, however, tend to affect absorption, translocation and metabolic degradation of herbicides. At low temperatures (below at 15oC) and low solar radiation, the photosynthetic apparatus is less stimulated to carry out photosynthesis processes, resulting in a reduction in transpiration flow (stomata closed for longer) and photosynthesis (less carbohydrate production) in addition to affecting other processes physiological. Low temperature results in less water translocation and solutes, which directly affects the absorption and translocation of herbicides. Results report that the effectiveness of the herbicide clethodim for ryegrass control is greater for spraying carried out at temperatures above 20oC. Another important factor that should be observed, refers to changes in the composition and thickness of the cuticle in due to low temperatures. Under these conditions, the composition of lipids that form the cuticle are altered, affecting its fluidity, and consequently the percentage of intercepted herbicide that will be absorbed by the leaf surface (Figure 1).

With Based on literature information, low temperatures affect:

- diffusion through the cuticle and cell membrane;

- the thickness, chemical composition and viscosity of the cuticle;

- stomatal opening (decreases) which may affect absorption;

- the rate of metabolism, mainly with sugars and amino acids, due to reduced photosynthesis.

The interactions and barriers that affect the herbicide after its spraying until reaching the target site of action depend on the temperature due their involvement in various processes, which may result in less control and cause increased phytotoxicity to the crop (Figure 2).

It is also worth noting that the application of herbicides under conditions inadequate conditions contributes to the occurrence of control failures even in populations susceptible and, especially, when herbicides are applied in unfavorable temperature ranges and on plants with advanced development. Furthermore, the effectiveness and selectivity of certain herbicides may vary depending on the characteristics physical-chemical characteristics of each molecule and tends to be optimal for a given temperature range.

In the literature, it is reported that the ideal range for spraying Most herbicide molecules range from 20ºC to 30ºC due to favoring absorption and its fluidity in membranes. When comparing the temperature range ideal and the rate of photosynthesis, it is noted that the values ​​are on a very close, since the activity of the enzyme Rubisco (Ribulose 1,5 bisphosphate carboxylase oxygenase), responsible for carbon fixation in the plant, is great for temperatures in the range of 20ºC to 32ºC, thus favoring absorption, translocation and metabolism in the plant.

The reduction in the growth rate and the lower membrane fluidity can hinder absorption and translocation, resulting in slower and reduced weed control. For herbicides inhibitors of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPs) such as glyphosate, absorption and translocation of the molecule in grasses can be twice as high in applications carried out at temperatures close to 28ºC when compared to spraying at 17ºC, indicating a high potential for causing injury. For glyphosate-resistant horseweed, control at low temperatures (8°C-10°C) is higher compared to application carried out at temperatures above 20ºC-30°C, which can reverse resistance to the herbicide as long as the plant stage and dose are observed. The reasons for greater susceptibility are the lesser compartmentalization of the herbicide in the vacuole in response to cold and greater supply of the herbicide to remain in the cytosol and cause plant death.

For protoporphyrinogen inhibitor herbicides oxidase (PPO), the effectiveness in control is also favored with the increase of temperature. The result in controlling Ipomea sp. (viola or morning glory) and Amaranthus retroflexus (giant pigweed) was twice as large for applications carried out at temperatures close to 26ºC compared to spraying at 18ºC. In a similar way, it occurs with acetolactate inhibitor herbicides synthase (ALS) where the control of Lolium evergreen (ryegrass) with the herbicide flucarbazone-sodium was lower for temperatures between 10ºC and 20ºC compared to the effects observed in temperature of 30ºC. In another example, evaluating the effect of the interaction between herbicide and temperature, it was evidenced that the use of flumetsulam and metssulam in controlling Raphanus the radish (turnip) was bigger under higher temperatures.

In contrast, better levels of control for lolium rigidum (ryegrass) was found for sprays of glufosinate ammonium, an inhibitor of glutamine synthetase (GS), carried out at lower temperatures where the effect temperature is less important due to the smaller translocation of the molecule in the plant. Now, for herbicides that inhibit photosystem II and I (FSII and FSI), such as bentazon and paraquat, less weed control may occur without there is an increase in injury in crops at lower temperatures For the 2,4-D herbicide, the death of plants for sprays carried out in lower temperatures may be similar, where the onset of symptoms occurs more slowly when compared to applications at slightly lower temperatures. higher. However, care must be taken for applications in temperatures above 25ºC where the use of 2,4-D and dicamba herbicides tend to present greater potential for volatilization, reduce plant control weeds and increase the risk of injury to adjacent areas.  

Reduced absorption and translocation under mild temperatures can delay the onset and increase the duration of symptoms of phytotoxicity. In wheat cultivation, the application of clodinafop-propargyl in Temperatures below 5°C caused severe injury with symptoms visible for longer four weeks, causing a reduction in productivity. Low temperatures can negatively influence the degradation of herbicides by metabolism which, under this condition, tends to be smaller and cause greater phytotoxicity. That situation was evidenced in the wheat crop subjected to the application of sulfosufuron in post-emergence where there was greater phytotoxicity at lower temperatures at 20ºC. In a similar way, the application of diclofop-methyl in cultivars of barley at temperatures below 15ºC caused greater phytotoxicity when compared to spraying carried out at 25ºC due to the reduction in rates hydrolysis of the molecule and its conjugation with sugars. In corn, the effect of temperature during detoxification was observed for the herbicide rimsulfuron, which was highly selective for cultivation at temperatures of 25ºC-30ºC and reduced to 10ºC. Similar results were found for the herbicide iodosulfuron-methyl applied to oats at a dose of 20 g ai/ha where the tolerance was greater and directly proportional to the increase in temperature.  

For herbicides applied pre-emergence, temperature is a factor that contributes significantly to two possible effects:

1) increase the persistence of the molecule in the soil due to lower microbial activity and;

2) bigger control of plants by lower metabolism in the seedling phase.

The speed of biochemical and enzymatic reactions of microorganisms involved in herbicide degradation occurs in an ideal range temperature ranging from 25ºC to 35ºC where the cleavage (breakage) of the carbon tends to be larger (Figure 5). Results report that the time of half-life (t1/2) of the herbicide mesosulfuron-methyl was greater than 110 days and considered three times greater when compared to applications carried out at temperatures close to 10ºC. Other results also showed that, for the herbicides atrazine and s-metalachlor, the half-life was reduced by more than 50% when the application was carried out at temperatures above 25ºC. Situation similar was found in plants subjected to the herbicide metribuzin where phytotoxicity seven days after application was lower at a temperature of 16ºC compared to 30ºC. However, there was no reduction in injury and persistence was higher at lower temperature due to reduction in microbial metabolism.

Another important characteristic that must be considered is the temperature of water for herbicide application. Research work has demonstrated that the effectiveness of several herbicides in controlling weeds is impaired when using water at low temperatures. For example, horseweed control, caruru and viola string with the herbicides glufosinate ammonium, mesotrione, glyphosate and 2,4-D choline was lower when the water temperature was 5oC compared to a temperature of 22oC. Similar results were found for the herbicides diquat and for the premix of glyphosate with dicamba where the control was 10% lower, depending on the species. The main reason The lower performance of herbicides is related to lower dissolution in tank, since the water solubility of post-emergent herbicides is greater for temperatures close to 25oC. Furthermore, temperature has an effect on the seed germination, emergence and growth of seedlings, with greater susceptibility of plants to low temperature conditions due to limitation of metabolism. This effect was evidenced for sulfosulfuron, a herbicide used in pre-emergence of wheat, which showed increased efficacy at low temperatures.

After reviewing the literature, some points can be mentioned regarding weed control and the potential of herbicides to cause Phytotoxicity due to low temperature application:

• Pre-emergent herbicides: increased control and in the potential for crop injuries. Another expected effect is the increase in persistence in the soil due to reduced biological activity at low temperature.

• Post-emergent herbicides contact: reduction in the control of weeds of little significance since its absorption is limited and, effect on phytotoxicity in crops similar to applications performed in the ideal range.

• Post-emergent herbicides systemic: reduction in weed control effectiveness due to lower absorption and translocation to the growth points. Across cultures, there is a tendency to greater phytotoxicity occurs due to the reduction in the ability to metabolize molecules.  

AND It is also important to highlight that the effects of temperature on temperature control weeds become less evident when the dose increases because the amount of herbicide reaching the site of action may be large enough to cause damage. This behavior was verified for applications ammonium glufosinate and glyphosate where high doses cause high control of susceptible plants regardless of whether the application is carried out at low or high temperature. Therefore, it is recommended to observe the conditions of adverse temperature and solar radiation at this time of year to enhance performance of the herbicide molecule on weeds and minimize their effects on crops.

Joanei Cechin, Maicon Fernando Schmitz, Cristiano Piasecki, Jonas Rodrigo Henckes, Dirceu Agostinetto, Leandro Vargas, Embrapa Wheat

Article published in issue 232 of Cultivar Grandes Culturas, September, 2018.