Pesticide usage in Scotland: arable crops and potato stores 2020

Appendix 4 – Survey methodology

Sampling and data collection

The sample of farms used for this survey was the same as that for the Arable Crops 2020 survey. Using the June 2020 Agricultural Census⁽¹⁰⁾, a sample was drawn representing arable cultivation in Scotland. The country was divided into 11 land-use regions (Figure 8). Each sample was stratified by these land-use regions and according to holding size. The holding size groups were based on the total area of arable crops grown. The sampling fractions used within both regions and size groups were based on the areas of relevant crops grown rather than number of holdings, so that smaller holdings would not dominate the sample.

Data relating to pesticide use in potato stores were collected from all potato growers encountered in the arable sample, either during an on-farm or telephone interview, or via e-mail. In instances where the potato land was let, and storage was on a separate holding, the potato grower was contacted individually to obtain storage details. Data were collected for all potatoes stored by these growers, not just for those crops grown on the holdings sampled. Therefore, the sample of stored potatoes relates to a greater area of potato cultivation than that for which field pesticide treatments were collected in the 2020 arable pesticide survey report. In total, data were collected from 81 growers. The crops grown by these growers represent 33 per cent of the total 2020 potato crop census area.

The data collected included the areas of seed and ware crops grown, quantities of potatoes sold and stored, storage type, storage method and post-harvest pesticide applications at crop lifting and during storage. Fungicidal seed treatments applied prior to planting are included in the arable crop report.

Raising factors

National pesticide use was estimated by ratio raising. This is a standard statistical technique for producing estimates from a sample. It is the same methodology used by the other UK survey teams and has been used for all historical datasets produced by the Pesticide Survey Unit, allowing comparability over time. The sample data were multiplied by raising factors (Table 10). These factors were calculated by comparing the sampled crop area to the areas recorded in the Agricultural Census within each region and size group. An adjustment (Table 11) was made to the ware fraction to correct for the potatoes grown as seed that were then designated as ware. A second adjustment (Table 12) was made to align the survey estimates of total tonnes stored with the estimated tonnage of Scottish potato stocks held in store at the end of November provided by AHDB Potatoes. This represents a change in methodology from previous surveys (see next section).

Due to the low numbers of potatoes grown and sampled in some geographic regions, stored data were amalgamated into four regions to allow more robust estimation of pesticide use: the North (Highlands & Islands, Caithness & Orkney, Moray Firth and Aberdeen), Angus (the main potato growing area in Scotland), Central (East Fife, Lothian, and Central Lowlands) and the South (Tweed Valley, Southern Uplands, and Solway).

Changes from previous years

There has been a significant change in survey methodology which must be taken into account when comparing data from this survey and previous surveys. During the statistical estimation of national pesticide use on stored crops from the sample surveyed (see raising factor section) an adjustment is made to align the survey estimates of total tonnes stored with production estimates provided by AHDB potatoes. In the 2016 and 2018 surveys total storage tonnage was estimated from ADHB total potato production data, which were then adjusted by a standard estimation of the proportion of crops routinely held in store. This year, for the first time, AHDB was able to provide estimated tonnage of Scottish potato stocks held in store at the end of November. This is significantly less than the historic estimated proportion of potatoes held in store. AHDB reported that 95.7 per cent of GB seed production was held in store at the end of November. This percentage was applied to the 2020 Scottish Seed production figure (obtained using data from the Scottish Seed Potato Classification Scheme) to calculate the Scottish seed and ware storage figures. Had this new method been employed in 2018, this would have reduced the 2018 storage estimates by 23 per cent for seed potatoes and 19 per cent for ware potatoes. It is thought this new method will provide more accurate estimates of potato storage than the previous method which may have slightly overestimated Scottish Storage in the past. This change brings us in line with the method used for the UK Pesticide Use in Potato Store report. It should be noted however that the AHDB end of November stock figure is based on grower stock and excludes storage by processers etc. Please note whilst the new methodology impacts the estimates for the tonnes stored it does not impact the per cent of crop treated. No comparisons have been made in this survey to tonnes treated in previous surveys, discussion only relates to the per cent of crop treated.