Deposition of Pesticides on the Soil Surface
Results1.1 Crop cover in 8 crops in the 1998-1999 growing season1.2 Deposition of spray liquid on the soil below four crops 1.2.1 Winter wheat, spring barley, sugar beet and potato 1998/1999 1.2.2 Winter wheat 1999/2000 and 2000/2001 1.2.3 Spring barley 2000 and 2001 1.2.4 Sugar beet 2000 and 2001 1.2.5 Potatoes 2000 and 2001 1.3 Interaction of spray quality and spray formulation on deposition of spray liquid on the soil 1.1 Crop cover in 8 crops in the 1998-1999 growing seasonThe first part of the project included measurements of crop characteristics in some agricultural crops that were not included in the activity with measurements of deposit of spray liquid on the soil below the crop. Results of these measurements are shown in the following tables (7-14). The crops included were grown according to normal practice. The crops included were fodder peas, spring oilseed rape, silage maize and spring barley with undersown grass for subsequent seed production and finally two grasses grown for seed production.
When a cereal crop is undersown with grasses for subsequent seed production the degree of soil covered by the crop increases as the undersown grass increases the total soil cover. The spring barley with undersowing in table 10 was not compared with spring barley without undersowing so the difference cannot be quantified. The increased soil cover in cereal crops is dependent on the type of grass undersown and the way the grass is undersown. Soil cover from ryegrass will generally be larger than from meadow grass. When the grass seed is established in the same row as the cover crop a minor influence on soil cover will be seen as when the grass is established in the centre between the rows of the cover crop as it is possible with some sowing equipment.
The two grasses were perennial ryegrass (Lolium perenne L.) and meadow grass (Poa pratensis L.). Perennial ryegrass was established either undersown in spring barley or established in a pure stand in the autumn. Meadow grass was established as an undersowing in winter wheat. Measurements in both grasses started in the autumn when the cover crop was harvested. Perennial ryegrass established undersown in spring barley has a higher coverage in the autumn after harvest of the cover crop than perennial ryegrass established in a pure stand in august. The following spring, the year of seed harvest, there was still a higher crop cover in the perennial ryegrass undersown in spring barley than in the autumn sown crop.
The two meadow grass crops followed was undersown in winter wheat. There are other recommended ways to establish meadow grass for seed production. Concerning the crop followed in our investigation, it was a rather open crop after harvest of the winter wheat (table 13) but reaching a high ground cover in the following spring. The meadow grass crop that was used for a second harvest was also established from an undersowing in winter wheat. After the first years seed harvest, the straw was removed, and a close cutting was performed. The data for crop cover in the autumn reflects only the “green part” of the crop. The soil cover in the autumn was close to 100% but a large proportion of this cover comes from dead leaves and straw. Burning is often used in meadow grass as an autumn treatment when the crop is used for a second harvest. This leaves the field with no soil cover for a period but the grass typically recovers within a short period and the crop cover in the late autumn will typically be as high as in unburned fields.
This part of the project was only included one year because a literature review on the same topic was published by Becker et al (1999) during the first year. The data in the paper comes from more than 2000 individual trials carried out under practical conditions by BASF between 1993 and 1996. in Germany, Belgium, the Netherlands, Denmark, Sweden and Great Britain. This review is very comprehensive and includes results on crop cover assessments at various growth stages in the most important agricultural crops. Besides the cereal crops it includes values on beets, potatoes, silage maize, fodder peas and oilseed rape.
The review also includes estimated crop interception of spray in the same crops. The estimated interception by the crop is obtained assuming that the interception is correlated to the degree of crop cover. In the paper it is suggested that the interception factor should be derived from the area somewhere above the mean value. The range between the mean and the mean + the standard deviation is given in the paper as representative estimates of interception values assuming a correlation between plant cover and intercepted spray. Assuming that the proportion of the spray, which is not intercepted, is lost on the soil values on soil deposition can be calculated from the interception values. These values are shown in the following figures as Becker min and Becker max. Figures 6-8 show the plant cover measured in our project in 1999 in fodder peas, spring oilseed rape and silage maize compared to the values collected by Becker. The figures also include the calculated soil deposition values based on Becker’s figures.
1.2 Deposition of spray liquid on the soil below four cropsIn this part of the project parallel measurements of crop characteristics and measurements of deposit of spray on the soil surface below four crops was performed through the growing season in three years. The four crops involved were winter wheat, spring barley, sugar beet and potatoes. The crops were grown according to normal good agricultural practice including plant protection. In the first year, the spray liquid used consisted of tap water and the tracer but without any further additions. This solution was chosen in order to avoid problems in the subsequent analysis of the samples, which eventually could arise from added surfactants. A spray liquid of water without any further additions has a very high surface tension and the deposition of the droplets on difficult to wet targets will be reduced compared to sprays including adjuvants. From this it follows that the soil deposit of spray on the ground below the crops is higher than can be expected for most formulated pesticides. The results from the 1998-1999 season therefore can be regarded as a worst-case situation concerning pesticide formulation. 1.2.1 Winter wheat, spring barley, sugar beet and potato 1998/1999The results from the 1998-1999 growing season is summarised in Tables 15-18. The deposit of spray on the ground shown in the tables is calculated as a percentage of the applied dose. Objects were also placed just above the crop canopy but these objects were primarily used to test whether the actual applied dose was within the expected value. Due to different factors the values found on these objects cannot be expected to be equal to 100% of the applied dose per area unit. When the falling droplets are approaching a horizontal target as these samplers are, a proportion of the spray droplets will follow the air current around the horizontal object. Spray drift is a second factor reducing the theoretic dose to values below 100% although the extent is probably limited in these experiments where a shielded sprayer is used. Another loss comes from evaporation during the travel from nozzle to target. This part is not quantified but theoretical considerations suggest that it can be of significant importance under some climatic conditions with high temperature and low humidity (Reichard et al, 1992; Kaul et al, 1996). The deposit value on horizontal samplers at the top of the canopy is from these reasons expected to be below 100% of the applied pr area unit. The values found at the canopy top varied in a Dutch study from 80-90% of the applied dose (Van de Zande, pers comm).
The values for soil deposit of spray liquid at the early growth stages in the two cereal crops (Tables 15 and 16) when the crop cover is limited is below what is expected if one assumes that soil deposit values should correspond to 100 - % crop cover. The results should probably be explained by the way the droplets move when they are approaching the soil level. The droplets are not only falling vertically but also moves in the horizontal direction by turbulence created by the driving speed and the natural wind. Droplets that also has a horizontal movement can be caught by the erect leaves of the crop which acts as a filter before the droplets reach the soil surface.
However at later growth stages when the ground cover of the crop exceeds 50% the values are better in accordance with expectations as the sum of % crop cover and % soil deposit exceeds 100%. Values above 100 is expected as the deposition of the droplets on the target is influenced by a number of factors that decrease the deposit values on the crop below the crop cover values. The 1999 results in the two row crops, sugar beet and potatoes (Tables 17 & 18), are influenced by the methodological problems discussed in the M & M section concerning the early observations when the crop cover is limited.
Before the 1999/2000 and 2000/2001 growing season, the effects of various additives to the spray solution on the subsequent tracer analysis was investigated and none of the tested adjuvants interfered with the tracer analysis. The non-ionic surfactant was therefore added to the spray solution in both seasons. The surfactant reduces the surface tension of the spray, which is then more comparable to a spray consisting of water and a pesticide that typically includes surface-active ingredients. In the following tables, results from the last two growing seasons are shown for the four crops.
1.2.2 Winter wheat 1999/2000 and 2000/2001The assessments of crop characteristics and soil deposit values for winter wheat is shown in tables 19 and 20 for the two years. Special attention to the crop development in the autumn should be given. The crop development in the autumn 1999 was limited with a maximum crop cover of 15% whereas the winter wheat in the following seasons had a maximum crop cover of 61% in the autumn. Winter cereals are not fertilised in the autumn and large variations in crop development in the autumn can occur due to effects of the preceding crop, organic manure etc. Despite the vigorous growth in winter wheat in the autumn 2000 the crop development in the spring 2001 did not deviate too much from the former years crop. In both years soil deposit values decreased during the growth season and values below 10% of the applied dose was registered in both years for a period in the late spring.
Our results from these two seasons are compared to Becker’s values in figures 9 & 10. Concerning plant cover in relation to growth stage, values below the Becker mean values were found at early growth stages in both years whereas the plant covers at late growth stages were above the Becker mean values. Comparing our actual measured deposition values with the values based on Becker show the relation in figure 10. The soil deposition values measured in our study exceeded the Becker max values at some of the early growth stages and were close to or below the Becker min values at late growth stages. The deviation between our results and the values calculated from Becker’s estimates reflects the corresponding differences in crop cover between the two investigations.
1.2.3 Spring barley 2000 and 2001The spring barley (tables 21 and 22) did not reach a 100% crop cover at any time during the growth season and the soil deposit values never came below 10% of the applied dose. The cultivar chosen for the study (Barke) is ranked as an average cultivar concerning characteristics such as plant height and plant cover.
It can be seen that the crop development during these two growth seasons was rather parallel. Leaf area index measurements were taken in the last part of the 2001 season but it can be seen from the figures that the soil deposit values are not well related to this parameter. The comparison to the Becker values is shown in figure 11 and 12. The plant cover results from the two experimental years are close to the mean values given by Becker. Concerning the soil deposition values however, the measured values are closer to the estimated max values given by Becker.
1.2.4 Sugar beet 2000 and 2001The results from the two last growth seasons are shown in tables 23 and 24. Two different methods were used to evaluate soil deposit in sugar beet in these experiments for reasons discussed in M&M were the methodology is described. Generally, however the values were obtained by a combination of experimental values and calculation considering the values from the first part of the season.
In the last part of the season at high crop cover values, the same methodology as in the cereals was used. At the time when the method was changed an assessment using both methods is included in the tables. Due to the low number of plants per area unit and a slow development in the early part of the season sugar beet constitutes a very open crop in the early crop stages where the weed control is carried out. High crop coverage and low soil deposit values are seen in the end of the season where control of fungi attack and pests can be relevant. There were large differences between the two years in plant cover at a corresponding growth stage in the early part of the season and this is also reflected in the soil deposition values found.
1.2.5 Potatoes 2000 and 2001The investigations in potatoes in 2000 and 2001 (Tables 25 and 26) were carried out in Bintje, a cultivar used for production of potatoes for human consumption. The canopy development is not as vigorous as in those cultivars that are used for industrial purposes such as Dianella, which was used in the 1999 investigation.
The investigation in potatoes in 2000 and 2001were carried out as in sugar beets using different methods at early and late growth stages and with one overlapping assessment where both methods were used. At the time of weed control there is a limited plant cover and hence a high soil deposit of spray liquid. Control of fungi attack in the last part of the season is very intensive in potatoes. At the time when the crop canopy had closed in Bintje soil deposit values from 4-18 % of the applied dose was found. Although the spray liquid was used without any surface-active additives in the very dense industrial potato Dianella in 1999 a soil deposit value of only 4% of the applied dose was seen in that cultivar. The measured plant cover values in the two study years was somewhat below the Becker mean values at early growth stages but raised above the Becker values at late growth stages (figure 15 and 16). This was reflected in the soil deposit values where the measured values in our study was above the max values calculated from Becker at early growth stages and fell to levels below the Becker min values at late growth stages.
1.3 Interaction of spray quality and spray formulation on deposition of spray liquid on the soilThis activity was included in the project for two years in 2000 and 2001 in order to investigate how much change in pesticide formulation and application technique can influence the deposition on the soil surface below dense crops. A large number of investigations have shown that formulation and spray quality affects deposition and retention on crop and weeds (Holloway et al., 2000; Webb et al., 2000; Taylor & Chambers, 2002). Only a few of those have included soil deposition measurements (Table 17). The investigation included a total of four experiments, two in winter wheat and two in spring barley. The results are shown in figures 17-20.
The four formulations of the spray solution represents the range of many real spray solutions concerning surface tension of the spray liquid. The two application techniques chosen represents the same way realistic spray quality characteristics although the fine spray quality is the normal recommended and used whereas the very coarse spray quality included represents an extreme with a small actual use at the time.
sIn winter wheat there was a strongly significant influence of both formulation and spray quality on soil deposit values in 2000. Using water without any additives soil deposit values were more than doubled changing from the fine spray to a coarse spray. Using the coarse spray, addition of surfactant to the spray solution reduced the soil deposit values with a factor three. There was also a significant interaction between formulation and spray quality in this experiment. Such an interaction is described in literature where the influence of spray quality is reduced when the surface tension of the spray liquid is reduced, as it is the case where the surfactant is added and partly where the oil additive and formulated pesticide Amistar is added.The differences in 2001 in winter wheat were generally much smaller (see y-axis). However there was still a significant influence of formulation and the interaction between spray quality and formulation was also significant. 51 The spring barley crop used for the application was not as dense as the winter wheat and the values found regarding soil deposits of spray liquid were generally at a much higher level. In the trial in 2000 there was a small but significantly increased soil deposit going from fine to coarse spray. There was a greater and highly significant influence of formulation with water without any additions giving the highest values and with the surfactant addition giving the lowest values. No interaction between spray quality and formulation were seen in spring barley in 2000 or in the following experiment in 2001. In 2001 there was a small but significant influence of spray quality and a larger and highly significant influence of formulation again as can be seen in figure 20. The results of a three-way ANOVA test (Table 27) summarises the influence of spray quality, formulation and year on soil depostion of spray liquid in winter wheat and spring barley. The large influence of year could in both crops to a large extent (seen in an analysis of covariance) be ascribed to a difference in plant cover at the treatment time (see Table 4). From the table it can be seen that spray quality is more important than formulation in winter wheat whereas the opposite is the case in spring barley. Why there is such a difference between these two cereal crops remains a question. In general however these experiments documents that other factors than plant cover/growth stage is of importance when the soil deposition of spray liquid is to be estimated. This is especially of importance at late growth stages/high plant cover where the influence of spray quality and pesticide formulation on plant/leaf deposits indirectly can be very important in changing the absolute values of soil deposition of spray liquid. |
