Periods of lack: Factors that influence your estimate

19/11/2015

CWhen discussing pesticide residues in fruits or other agricultural products, the term Lack Period is immediately considered, which corresponds to the time necessary for the residue of a pesticide in a fruit or edible product to reach a concentration below Maximum Residue Limit (MRL) allowed by the country or market in which it will be consumed. This parameter is determined using dissipation curves, for which an agrochemical is applied at its maximum recommended dose and its concentration in the product (fruit, vegetables, etc.) is measured over time. Thus, by having the maximum concentration of the pesticide applied in the fruit (Initial Deposit), the daily loss of the pesticide residue (Dissipation rate) and the MRL, it is possible to estimate the Lack Period (CP).

Much is talked about the factors that would be affecting the PC, however it must be clear that of the three parameters used to determine it, only the initial deposit can be handled by the producer, the dissipation rate being a totally variable process . Now, of all the factors that are interacting in the dissipation of a pesticide (figure 1), there are four that have more important effects in the estimation of a Lack Period, and correspond to the size of the fruit at the time of application, species , application technique and agro-ecological zone. The formulation of the pesticide, in general, does not produce significant changes in the estimation of the PC, with the exception of some formulations that allow the controlled release of the active ingredient, such as micro-encapsulated (Angioni et al., 2011).

The size of the fruit is an important factor

The size of the fruit is an important factor when fixing a PC, since the initial deposit that leaves the application of a pesticide will depend on the size of it. In figure 2 you can see a relationship between the diameter of the grape berries and the concentration of the residue. It can be seen that as the diameter of the berry increases, the concentration of the pesticide residue decreases to approximately 8 mm. This relationship is explained given that as the berry, or the fruit, increase its diameter the specific surface increases, but also its weight due mainly to cell division and expansion (Pinta) and therefore the concentration of the pesticide is decreasing, because the quantification of the residue of a pesticide is determined based on the fresh weight and is expressed in mg of the pesticide per kilo of the fruit (mg kg-1 or ppm). Therefore, when the fruit approaches its final size, the surface remains constant and the change in weight is lower, which means that the concentration practically does not change (figure 2).

When applying a pesticide at the beginning of the fruit formation period, and as already indicated, it will result in a higher initial deposit, it could be thought that the CP would be higher (higher initial concentration of the pesticide), however this is not always This is the case, since when applied in the initial stage of fruit development, it has a very high growth rate, which translates into a greater dissipation of the residue (degradation + dilution), and therefore a lower CP (Table 1 ).

However, the relation between fruit size and PC presented for apple (table 1) is not something that can be extrapolated to all fruit species, since it can depend on factors such as type of fruit (shape, pubescence, growth rate), pesticide (physical-chemical properties) and agro-climatological conditions. The interaction of these factors can in some cases generate changes in the behavior of the pesticide residue and this is an aspect that should be studied in more detail.

The fruit species also affects the period of lack

The species is another factor that has an effect on the estimation of the CP, which could be related to the shape of the fruit, type of surface (skin, type and quantity of waxes, etc.), speed of fruit growth, among others . This, as already indicated, produces a direct effect on the initial deposit of a pesticide, but not always on the rate of waste loss.

Figure 3 shows the dissipation curves for Lambda-cyhalothrin and pyrimethanil in apple and grape. When comparing the values of time necessary to lose the 50% of the product initially deposited in the fruit (TD50) of both products, no differences were observed, however if the PC was affected since if there was a greater deposit of pesticide in the grape that in the apple, this given the size of the fruit (greater specific surface of the grape compared to the apple). From the above it turns out that the PC to reach a residue of 0,01 mg kg-1 is higher in grapes than in apples.

Little information on the technical factor of application

Currently there are several pesticide application systems, which seek to increase the efficiency of the application in terms of saving water use, without losing the ability to deposit the pesticide in the right place. The existing information regarding the effect of the application technique on the PC of pesticides is very limited.

In general, the few published works show that there is no effect of the application technique on the dissipation of pesticide residues (daily loss), but in the initial deposit. In the 2 table, the results of Giles and Blewett (1991) are presented, comparing the dissipation of Captan in strawberries when applied with a conventional system versus electrostatic system. As shown in the 2 table, the dissipation of the residue was similar in all treatments, observing TD50 values within or very close to the variation range (standard error of the mean), but there was a significant change in the initial deposit when applied with electrostatic

Upon seeing these results, a change in the PC can be seen when using an electrostatic system, given the significant increase of the initial deposit but not in the daily loss of the captan residue (TD50) or dissipation rate.

On the other hand, these results would suggest the possibility of reducing the dose of the pesticide as a way to shorten the PC and / or product savings, however it is a risky extrapolation given that there is no clear or direct relationship between waste in the fruit and biological effectiveness on the pest to control. This is more critical in the case that the pest to control requires that the pesticide must be applied with a high volume of water (greater coverage), something that with the electrostatic system is not possible to achieve.

The agroecological zone affects the rate of dissipation

So far, all the points that have been addressed have an effect mainly on the initial deposit of the pesticide in the fruit, however, the agro-ecological zone where the pesticide is used would have its effect mainly on the waste dissipation rate. If you think carefully about this point, the question immediately arises: How is it possible that a product that is used in grapes? of table in Copiapó have the same PC as when that product is applied in the Metropolitan Region or in Talca? This is more critical in the case of crops such as tomatoes or grape vines, which are grown practically throughout the country.

In Figure 4 you can see the dissipation curves of pyrimethanil and tebuconazole in Pink Lady apple, in three different agro-climatological zones. The results show that in the Casablanca site the dissipation of the two molecules was slower than in the other two zones, resulting in that the TD50 for both fungicides was achieved 2 to 6 days later compared to Temuco or San Clemente.

According to the above, if the initial deposit was similar, the PC would be totally different depending on the area, so for tebuconazole the PC to obtain a residue of 0,01 mg kg-1 would be around 65 days in Casablanca, 30 days in San Clemente and 40 in Temuco. For the case of pirmetanil it would be 57, 46 and 50 for Casablanca, San Clemente and Temuco, respectively.

Many answers but there are still questions

The results presented here have been obtained through research projects carried out within the framework of the FONDECYT 1120925 project, which has made it possible to elucidate many of the existing concerns regarding the determinants of the dissipation of waste in fruit products and how they relate. However, there are still many unknowns to clarify, to be able to elucidate the magnitude of the effect of each of them on the persistence of waste.

While it is true that the results presented in this article have already been evaluated for more than one season, there are still points to be confirmed or clarified to have a clearer vision and thus reduce uncertainty. For example, determine which are the climate parameters that have the greatest effect on waste loss, or if there is a variation in the dissipation of pesticides in the same area year after year (seasonal effect). These questions and others are those that are intended to respond within this last year of execution of the aforementioned project.

Source: Redagricola.com