Edibles: Trials pinpoint pesticide residue levels

Bayer CropScience's initiative aims to help brassica growers in using its leading fungicide Nativo 75WG

Brassica growers aiming to satisfy supermarket demands for produce with reduced pesticide residues are being helped by an ongoing initiative being run by Bayer CropScience. Its objective is to determine the degradation rate on commercial crops of its leading fungicide Nativo 75WG and what residues, if any, remain at harvest time.

Dr Stephen Humphreys, the company's food industry manager, is overseeing the MiniMizer project. He said: "Our MiniMizer initiative looks to help growers manage residues in their crops as they attempt to meet supermarket residue requirements.

"We are gathering data that gives us a realistic indication of what any potential residues are likely to be after normal grower practice," he added. "Some retailers want 'zero' or limit of determination (LoD) residues and this will be a real challenge for many growers, not least because repeated lowering LoDs means the definition of zero keeps changing. We are therefore looking to help growers minimise residues in line with best practice, while ensuring produce quality is maintained."

The project, now going into its second year, has already shown that in most cases residues of the fungicide's two components, trifloxystrobin and tebuconazole, on the final sprayed crop sampling date were either below or around the LoD.

The trouble is that LoDs are being moved lower and lower by the Pesticide Residues Committee as analysis equipment becomes more sophisticated. For example, when Nativo was being developed a few years ago the LoD for tebuconazole was 0.05mg/kg but it has now been reduced to 0.01mg/kg or 1ppb — more or less equivalent to one drop of water in an Olympic swimming pool.

UAP senior vegetable agronomist Chris Wallwork, who strongly supports the MiniMizer initiative, said that he would like other agrochemical companies to follow Bayer's lead. It is essential that growers produce what their customers want and this initiative will help them do so economically, he added.

"It's very important that agronomists [and growers] making decisions on what and when to spray have good pesticide decline curve data," he said. "The more data we have on pesticide degradation, the better the chance we have of minimising residues, although it will always be an inexact science because degradation depends on weather conditions. If you guarantee a zero residue you may embarrass yourself because you may get it one year but not the next."

The MiniMizer work on headed cabbage was carried out at Leverton, Lincolnshire, by the Boston-based Allium & Brassica Centre (ABC). ABC agronomist Andy Richardson said the initiative "helps us understand" what happens to pesticides after application. This is important because detectable residues occur only in a very small number of cases "and we need to know why". Only then can agronomists and growers decide how to avoid them.

Two replicated trials were carried out on sites at Leverton in commercial savoy cabbage planted in July. Nativo, which is very widely used in the Boston area against a wide range of diseases, was applied at its full label rate of 0.4kg/ha in mid-August and late September. Crop samples for residue testing were taken at the product's harvest interval (21 days) and three times up to the 42nd day after the final treatment.

No trifloxystrobin residues were found in any of the samples taken in the two trials. That was also the case for tebuconazole in one of the trials but not in the second, in which a residue of 0.01mg/kg (the LoD) was detected in two of the nine samples. Humphreys pointed out that this is 100-fold lower than tebuconazole's maximum residue level (MRL) of 1mg/kg.

The trials in Scotland were carried out along similar lines on Brussels sprouts by a major brassica producer, except that there were three replicated trials on different sites. Furthermore, Nativo was applied to the crop either once (mid-September), twice (mid-September and mid-November) or three times (mid-September, mid-November and end of November).

"In all cases, samples were taken for residue analysis at different times after the last application," explained Humphreys. "In the majority of cases trifloxystrobin was not detected but in the few samples where it was, it was at or around the LoD.

"Where tebuconazole was detected it was again well below the MRL and declined with time," he added. "In the study where three applications of Nativo were made, neither residue (trifloxystrobin and tebuconazole) was detected in late-harvested samples in March."

The trials are being repeated on the same sites this season. The residues, or lack of them, will show if the different weather conditions (compared with 2008) affect the level and number of residues. Richardson believes that the results will further improve everyone's ability to decrease or minimise residues without compromising crop yield and quality.


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