Pre-harvest interval and re-entry interval: what they mean and where to find them

By Nikola Mojsa, MSc in Agricultural Engineering

What the pre-harvest interval is

During this period the active ingredient breaks down (SECPA, basic terms, in Serbian), so residues in the grain or fruit fall below the maximum residue level (MRL), expressed in milligrams per kilogram. For example, if you sprayed on 1 July and the PHI is 35 days, harvest can start on 5 August at the earliest.

The number of days depends on the product, but also on the crop. An insecticide based on lambda-cyhalothrin has a PHI of 35 days in small grains, 42 days in oilseed rape and 7 days in potatoes (Plant Protection Handbook, pesticidi.org, in Serbian).

The PHI runs to harvest, not to sale: days the produce spends in a store or cold store do not count towards the PHI, because in storage the product breaks down slowly or not at all (Agro saveti, in Serbian).

What the re-entry interval is

The re-entry interval is the shortest time from spraying until people may go into the sprayed field and work there. It is given in hours or days and depends on the product's toxicity (Jugpress, B. Karapandžić, agricultural engineer, in Serbian). For many products the label says “until the spray deposit has dried”, meaning until the sprayed layer on the leaf is completely dry.

The re-entry interval applies to every entry into the field: crop walking, checking pheromone traps or detasselling seed maize. A drone does not shorten it, because the product and the dose per hectare are the same as with a tractor sprayer, so the interval on the label applies after drone spraying too.

Source for the examples in the table: pesticidi.org (in Serbian).

Where to find the pre-harvest and re-entry intervals

Both intervals are stated on the product label, in the directions for use. The PHI is in the application table, in the row for your crop, next to the rate and timing; the re-entry interval has its own line below the table. If you no longer have the label, you will find the same data for every product and crop in the Plant Protection Handbook on pesticidi.org (in Serbian).

When reading the label, look out for three things:

• The crop's end use. For one maize herbicide the PHI is OVP for grain maize but 14 days for silage maize (Syngenta, weed control in maize, in Serbian).
• The OVP mark. It stands for “covered by the time of application” (Syngenta, oilseed rape protection, in Serbian): the product goes on early enough, for example a herbicide after sowing and before emergence, that enough days are certain to pass before harvest. OVP only applies if you sprayed at the time given in the “Time of application” column.
• Tank mixes. Two products in the same tank mean two intervals, and the longer one sets the harvest date.

The pre-harvest interval when we spray your field by drone

The PHI is set by the product label or by the advisory service's recommendation, and you choose the product, together with your agronomist. Nikola mixes it at the field, in the tank on the trailer, and the spraying date is the day from which you count both the pre-harvest and the re-entry interval.

So choose the product with the planned harvest date in mind as well. A drone can spray late in the season, in maize and sunflower taller than a metre and a half, when only a few days remain before harvest.

The same goes for desiccation (drying the crop before harvest): the earliest harvest date is set by the desiccant's PHI on the label. How to choose the right moment is covered in Soybean desiccation: when is the right time and Sunflower desiccation: when and how it is done.

After every job you get a report from the app with the number of hectares treated. Note the date and the product name with it: from those two details and the label you can work out the day you may enter the field and the earliest harvest date. How a job runs from your call to a sprayed field is described on the drone spraying page.