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Proposals to Amend the New Zealand (Maximum Residue Limits of Agricultural Compounds) Food Standards 2007

9 Proposal to exempt Ammonium thiosulphate

It is proposed that an MRL exemption is set for ammonium thiosulphate when used as a plant growth regulator for pomefruit and stonefruits. It is proposed that Schedule Two of the NZ (MRL) Food Standards 2007 be amended to include the following; this will be the resulting entry for ammonium thiosulphate in Schedule Two of the NZ (MRL) Food Standards 2007:

Amendment Rationale

The proposed MRL exemption represents a reassessment of a currently approved use pattern for ammonium thiosulphate. The rapid chemical breakdown of this compound means it is not suitable to be managed against a concentration limit, and therefore it can be exempted from requiring an MRL.

Chemical Information

Good Agricultural Practice

Ammonium thiosulphate is used as a blossom thinner/fruit reducer for pome and stone fruits. Application may be throughout plant flowering period, at 0.8-1.2kgai/100L.

Residues Information

Ammonium thiosulphate rapidly ionises with water to from ammonium ions and thiosulphate ions. The thiosulphate is then further oxidised to sulphate, these breakdown products are all absorbed by the crop. Distinguishing between naturally occurring sulphate and ammonium within the crop and that occurring as a result of ammonium thiosulphate treatment is virtually impossible. Levels of sulphate and ammonium ions will be within natural ranges when the fruit is harvested. It is not possible to determine residues resulting from the use of ammonium thiosulphate as an agricultural compound. Therefore an MRL exemption is proposed for ammonium thiosulphate.

Dietary Risk Assessment

No acceptable daily intake (ADI) has been specified for ammonium thiosulphate. However, it is indicated to be of low oral toxicity on the basis of high LD₅₀ values of 1-2g/kg (LD₅₀ values = median lethal dose, the dose required to kill 50% of a population of laboratory animals - usually rodents). Residues of sulphate ions and ammonium ions will be within the natural background range for fruit from treated orchards at the time of consumption. Because no detectable ammonium thiosulphate will be present on treated crops there is no dietary risk to consider.

Toxicological / Public Health Assessment

It has been determined that the use of ammonium thiosulphate as a blossom thinner/fruit reducer for use on pomefruit and stonefruit, according to the good agricultural practice specified above, is very unlikely to pose any health risks from consumption of the harvested commodity.

International MRLs

To meet New Zealand’s obligations under the Agreement on the Application of Sanitary and Phytosanitary Measures the proposed MRL exemption will be notified to the World Trade Organization. Any country may choose to comment if they believe the proposed MRL exemption represents a barrier to their trade.