SETAC Spotlight: Residue studies for bird and mammal risk assessment: controlling the uncontrollable

Residues studies for wild bird and mammal risk assessment (B&M RA) are performed to refine the residue level (i.e. used to refine the residue per unit dose, RUD) or decline rate (i.e. DT₅₀ used to calculate time weighted average factor (fTWA)) on potential food items. Historically, it has been known for residue refinements to be changed or rejected during the EU peer review process. Our experience in monitoring such studies has raised repeating or unusual issues with each new study.  Newly revised EFSA B&M guidance (EFSA 2023) provides clarification on performing these studies, but does it help to answer all issues? Will residue decline studies be as useful in future considering the new endpoint screening required to decide whether or not the time-weighted average approach can be used in the RA?

In this poster we considered potential solutions to these issues, and references to any useful information provided within the new draft guidance.

Weather

Weather can cause erratic residue results. Rain, for example, can result in increased degradation rate of residues, or can increase measured residues due to a more rapid uptake in plants. Predicting weather events over the days or weeks required for the initial stages of a residue study can be difficult however, and extreme weather events seem to be becoming increasingly common due to climate change. EFSA (2023) is not particularly helpful in this matter, giving no specific guidance for evaluating weather during residue studies.

We recommend:

• For above-ground matrices, no rain for 24 hours after application unless quicker rainfastness has been proven. Thereafter, only light rain accepted before the DT₅₀.
• Definition of (s)light/heavy rain according to WMO (world meteorological organization) 2018
• Rainfall to be measured locally using a method suitable for determining hourly rates (comparable across trials).

Site requirements

It can often be difficult to find sufficient suitable trial sites for a regulatory submission, depending on the specific GAP. EFSA (2023) states that sites must be 100 km apart, but 20-100 km is acceptable if sites proven to be sufficiently different. A minimum of four trial sites are required for plants per category per zone, but only two for invertebrates (but more may be needed if DT₅₀s are not consistent).

We recommend:

• Follow EFSA (2023) for number of trial sites, but consider logistics.
• For RUD refinements, refer to EFSA (2023), but note that the default database has now been expanded.
• Plan the ideal number of sites, but consider adding extra, because things can and do go wrong.
• Pre-sample soils and consider climate of chosen sites in advance.

Matrices

Finding naturally occurring matrices for refining the B&M diet (if not crop) can be a challenge: e.g. low numbers of earthworms and invertebrates. There is also no clear guidance on how to measure residues in weed seeds e.g. should a rain cover be used? What to do about seeds germinating? What seeds can be used as surrogates for weed seeds? Residues in plants for desiccating herbicides also poses a challenge: is wet weight appropriate for the risk assessment? Are desiccated plants no longer attractive to birds and mammals? EFSA (2023) provides some more guidance on matrices for invertebrates, but little is provided for the other matrices.

We recommend:

• Pre-sample arthropods and earthworms. Irrigate site prior to treatment to ensure site has sufficient organisms.
• Irrigate crops/weeds to standard agricultural practice if weather threatens survival (but avoid application timing).
• Use surrogate crop plants where ‘weeds’ not naturally available.
• Crop seeds of suitable size can be used as a surrogate for weed seeds – easier to find on soil surface. Be aware of germination times and choose a sampling strategy beforehand (e.g. when to stop sampling seeds due to germination).

A general point to note before placing any residue trials for birds and mammals is that EFSA (2023) now requires an assessment of whether it is appropriate to use an fTWA <1 in the risk assessment. This assessment should always be considered as a first step. For more information see our poster on “Time-weighted average factor (ftwa) assessments for birds and mammals under the new bird and mammal guidance: experiences to date” (Brooks et al. 2023).

If you would like to discuss any of the topics raised in this article, feel free to contact us.

The poster is available for free download.

You can find all of the other posters that CEA presented at SETAC Dublin here. You can also find all of our publications from previous conferences and links to journal articles we have authored on our library page.

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