- PDF Version (114K) - Print Friendly - Help

Maximum Residue Limits (MRLs) for Agricultural Compounds in Food: The Purpose and Procedure for Determining MRLs

3 How are MRLs set?

There are two stages to determining an appropriate MRL. First, good agricultural practice (GAP) must be determined. Second, evidence that the residues arising from appropriate GAP are unlikely to pose any human health risks is required. Once the appropriate MRL has been determined, other matters are taken into account such as avoiding trade restrictions, maintaining consistency and meeting international obligations (see criteria in section 2.2). The MRL is then set at a level that supports GAP and allows for the monitoring of compliance with GAP.

3.1 Determining the Appropriate Good Agricultural Practice

Use of agricultural compounds is quite different when being used in or on animals as opposed to plants. Animals are often treated individually when suffering specific conditions (although they may also be treated as a group), whereas for plants, a grower’s entire crop is usually treated. Additionally, farmers can be more flexible when withholding animals from slaughter, whereas plants must be harvested at a certain times of the year (e.g. when appropriately ripe). This means that determining GAP and therefore MRLs for animals and plants follows a slightly different process.

3.1.1 Agricultural Chemicals

Plants are very dependent on the season and food-producing plants are usually harvested at a specific time of the year (e.g. when ripe). Agricultural chemicals are an important component of pest and disease management programmes and may need to be used at various stages of crop development. For agricultural chemicals, GAP encompasses a range of application rates and frequencies up to the highest authorised use, applied in a manner that is effective but leaves the smallest residue practicable in the crop at harvest.

When registering an agricultural chemical, data are considered from residue studies that represent the range of growing conditions in the field. In each residue decay trial, samples of the crop are taken and analysed at regular intervals from the time of the application of the agricultural chemical until harvest. These data are presented in a graph similar to the one in Figure 1 below (the values used in this diagram are for illustration purposed only and are not intended to reflect a real situation):

The withholding period (WHP) between the last application of the agricultural chemical and harvest is established to suit the particular pesticide and its efficacy profile in order to allow the longest interval between treatment and harvest that is compatible with good protection of the crop until harvest. In this way, residues are minimised without compromising efficacy. The residue trial data can then be used to determine what maximum residue levels are likely to remain in the crop or plant at harvest. The proposed MRL is based on these expected levels.

For example, if after application, an insecticide’s residue decays until after 14 days, it is no longer present on the crop in sufficient amounts to control insect pests, then to protect the crop, the product needs to be applied every two weeks at an appropriate rate until harvest. The minimum WHP for that product on those fruit is likely to be 14 days and the fruit should not be harvested any less than 14 days after the last application of the insecticide. Using the hypothetical graphs in Figures 1 and 2, as the withholding period is 14 days, the residue after this time is 0.15 mg/kg. It should be noted that normally a number of residue decay trials are undertaken to provide confidence of the likely residues in the crop at harvest (see Figure 2 below). The proposed MRL is therefore based on a slightly higher value to take into account variations. In this case it would be likely to be set at 0.2mg/kg.

This value would only be breached if a product was not being used appropriately (e.g. the crop was harvested too soon and thus the WHP was not followed, or the application rate of the insecticide was too high).

In addition to this, there are times due to grower practices which mean a WHP may not be based on its efficacy. In such cases GAP may be dictated by this practice. An example is picking of strawberries during the harvesting period. As strawberries are only ripe for a short period of time, they need to be picked frequently. Hence, a short WHP may be required.

The MRL is the regulatory limit by which compliance with GAP can be monitored. However, it is not the tool by which the use of the agricultural compounds is controlled. This is done by placing controls on the agricultural chemical’s registration (which is one of the main tools used by NZFSA to ensure their appropriate use). Compliance with the MRL follows naturally from compliance with the application rate, timing and WHP.

From the above, it can be seen that there is usually no direct relationship between the MRL for an agricultural chemical and its Potential Daily Exposure level (PDE_(food)) determined from toxicological studies. However, the MRL is part of a system intended to keep residues of agricultural compounds as low as reasonably achievable and intake of those residues below a level likely to cause harm. See section 4.1 Protection of Public Health for an explanation of how health considerations are managed.

3.1.2 Veterinary Medicines

Because food-producing animals are not ‘harvested’ at set times of the year as plants are, there is more flexibility in determining GAP. Therefore the MRL for a veterinary medicine can be simply set at a level that will ensure the PDE_(food) for that compound will not be exceeded. The WHP is then set to ensure compliance with that MRL. So, whereas agricultural chemical MRLs arise out of GAP set on the basis of efficacy, for some veterinary medicines, GAP is dependent on compliance with the MRL, which is dependent on the PDE_(food). However, it is more usual for veterinary medicine MRLs to be set in a similar way to those for agricultural chemicals. The major difference is that good practice in the use of a veterinary medicine is that a WHP is set as short as possible to allow maximum flexibility for the farmer to send stock to slaughter when the market offers the best return on meat, or to ensure the least disruption to the milking regime (the discarding of quantities of milk can create environmental concerns).

Some veterinary medicines are flushed out of the animal’s system so quickly that WHPs of days make no sense as sometimes residues have dropped below the level of detection within hours. Additionally, some veterinary medicines are identical to substances naturally occurring in animals. Residues of these substances can only be detected indirectly as transiently elevated levels in the animal.

The decline in residue levels in animals are determined from at least one trial, involving several animals, run under controlled conditions. From the resultant residue decay curve, a WHP can be determined to ensure that a proposed MRL is unlikely to be breached. However, when assessing residues in animals, the potential for livestock to be exposed to compounds applied to animal housing or remaining in forage crops and other sources of animal feeds is also considered.