Residue Decay & Track It Below the MRL
Clears to safe levels
Residue falls by half each half-life — residue = initial × 0.5^(days ÷ half-life). Enter the initial residue, half-life and days to see what is left, whether it is below the MRL, and the days until it is.
Estimate residue at harvest
Next: residue is still above the MRL — wait until about day 8 from application before harvest, then re-check.
First-order decay is an estimate; real residue depends on chemistry, formulation, weather, washing and rinsing. Follow the registered PHI on the label.
Residue decay — key facts
- Decay
- initial × 0.5^(days ÷ half-life)
- Kinetics
- first-order (exponential)
- Half-life
- days to halve the residue
- MRL
- legal limit in mg/kg
- Days to MRL
- when it clears the limit
- Linked to
- pre-harvest interval
- Faster in
- sun, heat, rain, growth
- Privacy
- Runs in your browser; nothing uploaded
Residue decays on a half-life, so harvest timing is calculable
Once a pesticide is on the crop, sunlight, rain, microbes and plant growth break it down on a half-life — the residue halves over a fixed number of days, then halves again, tracing an exponential curve. Because that curve is predictable, you can work out exactly what is left after any number of days and, crucially, when it drops below the Maximum Residue Limit so the produce is legal to sell.
This tool gives the residue remaining, a safe-or-not check against the MRL, and the days to the MRL from the initial residue, half-life and days elapsed. Use it to plan harvest, schedule the last spray and stay compliant for market or export. Pair it with the Leaf Wetness Disease Risk, Spray Mix and Pre-Harvest Interval tools for a complete crop-protection plan. Always follow the label PHI and confirm with a lab test before sale.
Plan the harvest
Know the day residue clears the MRL.
Time the last spray
Work back from harvest to the final application.
Stay compliant
Check residue against the legal limit.
Model any product
Same decay maths for any half-life.
Frequently Asked Questions
How is the remaining residue calculated?+
By first-order decay, the same exponential maths as radioactive half-life. Residue = initial residue × 0.5^(days ÷ half-life). After one half-life, half is left; after two, a quarter; after three, an eighth, and so on. The tool also solves the equation backwards to give the days until the residue drops below the MRL you enter.
What is a residue half-life?+
The half-life is the number of days for the pesticide residue on the crop to fall to half its value, through sunlight, rain, microbial action, plant growth and metabolism. Each active ingredient has its own field half-life, which also depends on weather and crop. A short half-life means residue clears fast; a long one means it lingers. It is the single number that drives the decay curve.
What is the MRL?+
The Maximum Residue Limit is the highest residue of a pesticide legally allowed on a food, set by regulators (Codex, EU, FSSAI, EPA and others) in mg/kg. Produce above the MRL cannot be sold and may be destroyed. The calculator compares the current and projected residue against the MRL you enter so you know whether the crop is compliant and, if not, when it will be.
How is the pre-harvest interval related?+
The pre-harvest interval (PHI) is the legally required wait between the last spray and harvest, set so residues fall below the MRL by harvest. The days-to-MRL the calculator gives is the science behind a PHI: it estimates when residue clears the limit. Always follow the label PHI as the legal minimum — use this tool to understand and plan around it, not to override it.
Why does residue decay exponentially rather than linearly?+
Degradation processes act on a fraction of the residue present, not a fixed amount, so the more residue there is the faster it falls in absolute terms — the hallmark of first-order kinetics. That gives the familiar curve: a fast early drop that slows as the residue thins. Modelling it as 0.5 per half-life captures this far better than assuming a constant daily loss.
Where do I get the initial residue and half-life?+
The initial residue comes from a residue test on freshly treated produce, or from the label application rate and supervised-trial data. The field half-life comes from the product's registration dossier, extension data or published dissipation studies for that crop. Plug in values for your specific active ingredient and crop — generic figures only give a rough estimate.
Does weather change the decay?+
Yes. Sunlight, heat, rain and active plant growth all speed dissipation, so a residue may clear faster in warm, bright, growing conditions than in cool, dormant ones. The half-life you enter should reflect realistic field conditions. If conditions differ a lot from the study the half-life came from, treat the days-to-MRL as approximate and confirm with a residue test.
Can residue ever stay above the MRL?+
If the initial residue is already below the MRL, the crop is compliant from day zero and the days-to-MRL is zero. If it starts above, decay always brings it down eventually — the calculator gives the day it crosses the limit. The only way residue would not fall is repeated re-spraying, which resets the curve each time.
Are the figures exact?+
They're a sound model, not a substitute for a lab test. Real decay varies with weather, formulation, crop surface and application, and the MRL and PHI are legal limits you must meet regardless. Use the calculator to plan harvest timing and check likely compliance, then confirm with a certified residue analysis before sale where it matters.