Reducing uncertainty in chemical use

MRLs, WHPs and uncertainty

“The app is only as good as that data,” Alison said.

Internationally, export markets have no tolerance for breaches of maximum residue limits (MRLs). A breach in just one grape sample affects the reputation of the entire table grape industry.

Producers perform a delicate balancing act in high pest and disease pressure years, managing food safety requirements and quality control processes, to ensure fruit complies with export MRLs.

The Australian Table Grape Association (ATGA) is working in partnership with Cherry Growers Australia and Summerfruit Australia Limited on a Chemical Use for Export Toolkit.

The project, funded by the Victorian Government through its Food to Market program, involves modelling the breakdown of chemical residues to determine withholding periods to meet different export MRLs.

ATGA's Alison MacGregor, who worked on the project, said it was a timely project for industry, to reduce uncertainty for growers, particularly in high disease pressure years when they may need access to late season treatments.

Alison said that it was important that producers avoid taking unnecessary risks when it comes to meeting MRLs for different markets.

“What makes this tricky is that MRLs are different in all different markets, and sometimes we’re not sure how long we need to wait to meet a particular MRL,” Alison said.

“Differences between the MRLs in our export markets are not necessarily about health. Maybe that market has not used the chemical before, so they haven’t set an MRL. Or, maybe, the chemical is used commonly on other commodities, which means they need to ensure the acceptable daily intake (ADI) for that chemical is balanced across other fruit and vegetable categories, which means that their government needs to ensure the acceptable daily intake (ADI) for that chemical is balanced across other fruit and vegetable categories.”

“This is also frustrating for growers,” Alison said, “particularly if they can’t use a product after flowering simply because we don’t know how long it will take to break down to be undetectable in a market that has not yet set an MRL.

“These chemicals have been assessed as safe in Australia. They're proven to work well, and might be ideal in an integrated pest management program, with no impact on beneficials. But we can’t use them on export fruit if we don't know the wait period to achieve no detectable residue.”

The ATGA MRL app has been the industry’s go-to for guidance on extended WHPs to meet export MRLs, but it can be hard to source information to supplement the data behind the app.

“Sometimes the app recommends that a chemical is not used after flowering. Sometimes such a long WHP is necessary to avoid any trace of residue being detectable at harvest. In other instances, a long withholding period has been recommended simply as a precaution, because there is uncertainty around persistence of the chemical.”

Modelling the breakdown of chemicals

The project team used a model to derive eXtended Withholding Periods (XWHPs) to help determine how long a grower must wait to meet an export MRL.

“For table grapes, we selected nine chemistry options, and modelled the XWHP to to confirm the use patterns to meet export MRLs," Alison said. “In total, 25 chemical/crop combinations were modelled across grapes, cherries and summerfruit.”

“We chose chemicals that growers would be wanting or needing to use late season,” Alison said. “For example, growers might need fungicide options later in the season, and could get nervous about making that selection. So we chose chemistry to try to fill the gap, to give us access to options, late season, that will not compromise any Australian or export MRLs.”

The model used by the project team is a globally recognised tool – the OECD MRL calculator – used internationally to conduct risk assessment of residues. While it is usually used by government regulators to set their MRLs, the project team is using the model in reverse. Instead of using it to estimate an

MRL based on residues after a fixed WHP, they are using it to estimate WHPs that achieve target MRLs.

The actual data cannot be shown because some was provided in confidence by the chemical registrants.

Alison said he project is critical to provide "much-needed clarity about extending withholding periods".

“Identifying XWHPs will help producers make safe decisions: safe for the consumer, safe for our markets, and safe for business,” she said. “In some instances previously, we didn’t know whether a grower would have to wait just a few more days, or much longer, to meet an export MRL.

“Now we know that in some cases, we only have to wait a few more days – so this is really valuable! We now have XWHPs for some of the products we used to avoid, which gives producers more confidence in meeting export MRLs.

Alison acknowledged that although the project was beneficial, it was just the beginning.

“We’ve really just touched the tip of the iceberg,” she said. “This was a pilot – we’ve only examined nine chemistry options for grapes as part of this project, but there are still many more to cover.”

Estimating The Xwhp For Chemical X

The project team followed a multi-step process to estimate the XWHP for target markets. This example demonstrates how the project team estimated the XWHP for Chemical X, an unnamed product.

Source

The project team sourced data from the registrant chemical company, the APVMA and the World Health Organisation.

Sort

The team sorted the data, because it wasn’t all suitable. They excluded about half the data either because the treatment conditions were quite different from how Australian growers use Chemical X, or because a trial had too few sampling dates or if there were too few test results at a sampling date.

Model

They entered the suitable data into the model. The model estimates the highest likely residue after spraying (after 1 day, 3 days, 7 days, 14 days, 21 days etc.) and uses statistics to apply a safety buffer to estimate a level that is higher than any likely residue on each day (e.g. 7 days after spraying).

Result

Using the modelled values (including the safety buffer), the team established a decay curve to plot the predicted highest residue for each day. This curve is used to predict the XWHPs needed to meet each target MRL.

Is your MRL test relevant to your spray program?

Most laboratories offer a selection of multi-residue tests. Growers need to provide test results to auditors and agents as part of quality assurance programs for domestic or export markets. The test required by domestic retailers focusses on a specific range of chemicals. In a high disease pressure year, growers might need to apply fungicides that are not included in the basic screening required for domestic markets.

Export markets tend to screen for a much broader range of chemicals than are required for the domestic market test, including chemicals we don’t even use in Australia, and also a lot of the newest chemistry that is not screened for in a generic domestic market test.

Growers often talk about C3, C5 or C6 tests – some labs give their comparable tests other codenames.

Each screen includes a different array of chemicals. For example, the C6 screen includes many more chemicals than a C3 screen. Prices vary –including more chemicals in the multiresidue screen usually means the test price goes up, but more comprehensive screens also increase certainty for exporting growers that fruit won’t be rejected by a critical export market.

Be proactive – check that your MRL test complies with the requirements in your export market. v

Australian WHP (days) and extended WHPs (XWHP) for nine chemicals and key grape markets for 2022/23 season

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