Pellet Durability: A Metric to Handle with Care
Published: July 29, 2025
By: Marc Perel, Independent Consultant in Feed Technology | Premix | Additives / FeedSphere Solutions
Summary
A good Pellet Durability Index (PDI) alone doesn’t guarantee feed quality. But poorly interpreted, it can give a false sense of compliance. The real challenge is to align performance goals with real-world conditions. That’s where both technical insight and human intelligence truly make the difference.
Among the many quality criteria in animal feed production, pellet durability is often cited as one of the most critical. And rightly so: fragile pellets tend to generate too much dust and fines, which can impact feed intake, zootechnical performance, and even trigger complaints from farmers.
It’s no surprise, then, to see a growing number of posts, articles, and expert opinions stressing the importance of durability and how to improve it.
But behind this apparent consensus lies a more complex reality. This article offers a technical perspective on how pellet durability is measured, its limitations, and how best to interpret and integrate it into a broader, coherent quality management approach.
It’s a measurement, not a guarantee
Pellet durability is fundamentally a predictive indicator. It assesses how well pellets are likely to withstand mechanical stress once they leave the cooler, through transport, storage, and distribution, all the way to the animal’s feeder.
In other words, it doesn’t tell us what will actually happen in a given farm, but provides an estimation based on lab simulation.
And yet, despite showing compliant results during in-plant quality control, a batch can still lead to complaints a few days later: fines in the silos, crushed pellets at delivery, dust in the troughs… So what happened in the meantime?
The answer often lies in the handling and logistics chain:
- Intra-plant transport: worn-out conveyors, aging bucket elevators, long transfer lines, and harsh drop heights.
- External logistics: delivery trucks (mechanical or pneumatic), condition of the screw unloaders, unloading speed and handling.
- On-farm storage: silo height, drop angles, seals, pipework, and feed distribution equipment.
These are all too often overlooked, yet they play a major role in the final physical integrity of the pellet. Meanwhile, most discussions around durability focus almost exclusively on formulation or the pelleting process itself.
A quick historical note: where PDI comes from
The Pellet Durability Index (PDI) remains the most widely used method to assess physical strength. It was developed in the early 1960s by Kansas State University and standardized in 1969 (latest version: ASAE S269.5 in 2012).
The procedure is as follows:
1. Weigh out 500 g of pre-sifted pellets.
2. Place them in a rotating drum with internal fins, turning at 50 rpm for 10 minutes.
3. After rotation, sift the material again to separate broken particles.
4. The PDI is the ratio between the final weight of intact pellets and the original sample weight.
It’s a simple, economical method that has become standard in the industry, but not without its limitations.
Known limitations of the PDI
Several factors can compromise the accuracy and repeatability of PDI results:
- No pre-sifting before testing, introducing a bias.
- Sample mass variability: a difference of even 10–15% can skew comparisons.
- Final sifting procedure: whether manual or automatic, with variable time, vibration amplitude, and sieve mesh size (the recommended mesh being ~80% of the pellet diameter).
Another issue is low discriminatory power: visually different feeds can produce very similar PDI values. To improve sensitivity, some modify the method by adding steel balls or bolts to the drum (as per ASAE variant). But that approach reduces standardization and limits cross-plant/device comparison.
Alternative or complementary approaches
To address these shortcomings, other methods have been developed:
- The Holmen method, which uses an air jet in a mini cyclone chamber to simulate erosion by turbulence. It’s faster (~30 seconds), more reproducible, and arguably more representative of real handling stress.
- Mechanical devices with rotating knives (e.g. QuickTest), which exert higher mechanical impact.
In terms of cost, these alternatives range from USD 2,500 to 3,500, depending on the model and location, versus ~USD 1,500 for a traditional tumbling box.
One value doesn’t fit all
Crucially, comparing durability values across different sites or instruments is meaningless without strict methodological standardization.
The PDI is primarily an internal monitoring tool. It’s useful for:
- Assessing the impact of a formulation change.
- Monitoring the stability of the pelleting line.
- Detecting deviations (e.g. die wear, steam flow irregularities).
- Correlating with customer satisfaction or field feedback.
But outside of its specific measurement context, a PDI value has little meaning.
Best practices for meaningful PDI usage
1. Sampling matters
As with any analysis, clearly define where, when, and how samples are collected. Write down the procedure, and follow it consistently.
2. Timeliness is key
PDI should be measured shortly after production. Waiting several days reduces relevance, and the opportunity to act is lost.
3. Train your operators
They must understand not only how to perform the test, but why it matters. The PDI should be tracked jointly by production and quality teams.
4. Cross-reference results
PDI should be compared with field data: customer complaints, silo observations, fines at unloading. Review quality thresholds if needed.
5. Use complementary indicators
o Fines content: simple but effective.
o Hardness: particularly relevant in poultry, turkeys, rabbits, shrimp…
o Average pellet length: often overlooked, but useful to monitor knife wear and settings.
In conclusion: a compass, not a GPS
Pellet durability is an essential quality indicator — but it doesn’t tell the whole story.
The PDI is a useful compass, as long as its limitations are understood and it’s embedded in a broader quality system.
Interpretation should always be contextual, supported by other metrics, and aligned with real-world feedback. Understanding mechanical stress throughout the handling chain, listening to field observations, and training teams on these aspects is just as important as the lab result.
A PDI value in itself won’t make a feed good or bad. But misused, it can foster a false sense of confidence. Only when combined with technical judgment, practical field insight, and a bit of humility does it become the powerful tool it’s meant to be.
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