Azolla vs Duckweed Protein Content: Not the Same Number

Almost every page answering this question puts two percentages next to each other and declares a winner. The problem is that the two percentages were not measured on the same scale, so the comparison they support is weaker than it looks.

Here are the reference numbers first, because that is what the question asks for. Fresh Azolla averages 20.6 percent crude protein of dry matter across 15 samples, ranging from 13.9 to 28.1. Dried duckweed averages 27.8 percent across 18 samples, from 15.5 to 35.6.

On those figures duckweed leads by about seven points. Both figures were computed with a nitrogen conversion factor of 6.25 that suits neither plant, and misfits them by different amounts. That is where this gets interesting.

The two numbers are not on the same scale

Crude protein is not a protein measurement. It is a nitrogen measurement multiplied by a factor that assumes protein is 16 percent nitrogen. When a plant carries nitrogen outside its amino acids, the factor overstates it.

What the plant-specific factors are

For Azolla, this has been measured directly. A study that ran nitrogen and total amino acids on the same biomass found nitrogen at 37 to 41 g per kg of dry weight and amino acids at 208 to 244 g per kg, with the amino acids holding only 82 to 88 percent of total nitrogen. The resulting protein to nitrogen ratio was 4.9 to 5.0, and the authors state plainly that the 6.25 estimate is unsuitable for Azolla biomass.

Duckweed has since been given its own factor: 5.78 plus or minus 0.02, consistent with a proteome dominated by RuBisCO.

So the standard assay overstates Azolla by roughly 28 percent and duckweed by roughly 8 percent.

What happens when you rescale

Run the two reference averages back through their own factors and the picture shifts. This is arithmetic on published averages rather than a measurement, but it is the arithmetic every comparison table skips.

Duckweed at 27.8 percent implies 4.45 percent nitrogen, which at 5.78 gives about 25.7 percent protein. Azolla at 20.6 percent implies 3.30 percent nitrogen, which at 4.9 gives about 16.1 percent.

The gap does not close. It widens, from about 7.2 points to about 9.6.

One honest caveat. The Azolla factor was derived on indoor material from one production system, and the reference table averages field-grown samples from many. Treat the rescaled figures as a direction of travel, not as decimals. The rest of the chain between a nitrogen assay and the protein an animal deposits is covered separately in what a crude protein figure does not tell you.

Azolla makes its own nitrogen, duckweed takes yours

This is the mechanism that explains why the studies below disagree with each other, and almost nobody comparing the two crops states it.

Azolla lives in symbiosis with a nitrogen-fixing cyanobacterium. In the continuous-harvest experiment above, under ambient carbon dioxide, fixation by that symbiont accounted for all of the nitrogen in the biomass, at an average productivity of 90.0 to 97.2 kg of dry weight per hectare per day, with no nitrogen fertiliser applied. Raising carbon dioxide to 800 ppm lifted biomass production by 36 to 47 percent and did not lower protein content.

Duckweed has no such arrangement. Its protein content tracks nitrogen in the water, and it tracks it hard. In FAO-documented pond farming, protein rose from 17 to 37 percent of dry matter as water nitrogen went from 5 to 10 mg per litre.

That single difference sets the shape of both ranges. Azolla’s band is narrow and set mostly by species. Duckweed’s band is wide and set mostly by the water you put it in. Which is why the honest answer to “which has more protein” is a question back: what is in your water. The full account of which levers actually move duckweed protein content sits in its own article.

Why the same-pond studies disagree

The most useful evidence is the handful of studies that grew both crops in one place under one protocol. They do not agree, and the pattern in how they disagree is the finding.

In earthen ponds analysed by a single laboratory, Azolla africana reached 28.9 percent crude protein against 25.6 for Spirodela polyrhiza, with Azolla also carrying more crude fibre, 12.2 against 8.7 percent. Azolla ahead.

In concrete tanks in Assam over 30 days, Azolla pinnata reached 24.2 percent against 22.55 for Lemna minor, and grew faster too, doubling in 5.55 days against 8.15. Azolla ahead again.

Then put both on municipal wastewater. In Egyptian treatment ponds run side by side, duckweed averaged 26.9 percent protein against 25.6 for Azolla filiculoides, while Azolla produced more biomass, 1.3 against 0.9 tonnes of dry weight per hectare per month.

Nutrient-poor water, Azolla wins on composition. Nitrogen-rich water, duckweed catches and passes it. That is exactly what a nitrogen-fixing fern competing against a nitrogen-scavenging angiosperm should do, and it means a single ranking cannot be transferred between sites.

Digestibility reverses the ranking anyway

Composition is the part everyone compares. Digestion is the part that decides the outcome, and it points the other way.

One trial fed both crops fresh to Nile tilapia for 70 days in outdoor concrete tanks. Lemna at 31.11 percent crude protein returned apparent protein digestibility of 96.18 percent against 62.06 for Azolla at 22.06 percent crude protein, in small fish. In larger fish the gap widened to 92.29 against 38.52. Dry matter digestibility followed, 60.08 against 48.55. Feed conversion in static water came out at 1.99 against 2.76, and daily weight gain at 1.76 against 0.37 g.

Ruminant data points the same direction. In situ, crude protein degradability at 48 hours was 61.6 percent for Lemna trisulca and 51.81 for Lemna perpusilla, against 43 percent for Azolla pinnata.

The likely cause is on the Azolla side of the ledger. The same study that measured its conversion factor found soluble and total polyphenols of 20.9 to 69.1 g per kg of dry weight, very high against ordinary foodstuffs, and names them as the probable reason for the low inclusion rates reported in feeding trials. Lignin in the reference table runs to 11.4 percent of dry matter. In the earthen pond study, Azolla also carried more cyanide and tannin than Spirodela, fresh and sundried alike.

None of which makes duckweed uniformly digestible. Standardized ileal digestibility of crude protein runs 72 percent for Lemna, 69 for Wolffia and 39 for Spirodela, the last dragged down by ash. Genus matters on both sides of the comparison, and it matters more than the difference between the two crops. The practical version of that for aquaculture is in duckweed in fish feed.

Amino acids: where Azolla has a defensible edge

Fair is fair. Compared with soybean meal, the Azolla biomass in the continuous-harvest study carried a higher proportion of every essential amino acid except histidine, with methionine at 91 to 97 percent of the laying hen recommendation and 68 to 72 percent of the Nile tilapia recommendation.

The earthen pond study found Spirodela’s essential amino acid index superior to Azolla’s on every count except methionine. That is the same signal read from the other side: methionine is where Azolla is relatively strong and where duckweed is usually short.

Relatively strong is not the same as sufficient. Reference-table methionine for Azolla is 1.4 percent of protein, which is a low absolute number for a monogastric ration. The duckweed half of that argument is set out in the duckweed amino acid profile.

Inclusion ceilings, which is where the argument usually ends

A systematic review of Azolla feeding trials puts protein at 21 to 26 percent of dry matter and recommends inclusion below 15 percent in poultry, 5 percent in broiler chicken and below 25 percent in fish.

At those inclusion rates, a two or three point difference in crude protein between the two crops moves almost nothing in the finished ration. A 30 point difference in protein digestibility moves a great deal. If you are choosing between them for a real diet, the ranking that matters is the digestibility one, and the composition tables are the wrong place to look for it. The wider provenance and inclusion question is covered in duckweed as animal feed.

The water that decides the comparison is often the water that disqualifies the crop

Look at where the high numbers in this article came from. Municipal wastewater ponds. Manure-fertilised tanks. Septage-fed systems. The FAO field data that showed duckweed protein doubling came from ponds enriched with animal waste.

Duckweed’s advantage over Azolla is specifically an advantage on nitrogen-rich water. The cheapest nitrogen anywhere is a waste stream. Which means the condition under which duckweed wins this comparison is frequently the condition under which the biomass cannot be sold as food or feed at all.

SERAPH treats that as a hard line rather than a judgement call. Biomass grown on wastewater, manure or contaminated water never becomes food or feed here, and the production and remediation streams are separated from the first sensor reading rather than at the point of sale. The same document states our maturity: TRL 3, proof of concept, with outdoor field validation still ahead.

Read against the firewall, Azolla’s independence from water nitrogen is a genuine advantage on clean, food-safe water, where a duckweed operator has to buy nitrogen that a nitrogen-fixing fern does not need. On effluent it is no advantage at all, because the nitrogen is already there and free, and the destination has changed. Where that destination leads instead is duckweed wastewater treatment.

We should be direct about our own position in this comparison. SERAPH works on Lemnaceae, about 37 species across five genera, and not on Azolla. Every figure above is drawn from published literature, mapped in a knowledge base of 256 indexed duckweed papers across 31 research areas in 5 domains. We have no field-validated protein figures of our own to put into this table, and we are not going to invent any.

How to run the comparison for your own site

Which conversion factor produced each figure, and whether it was the same one.

Whether both crops saw the same water, because if they did not, you are comparing two water qualities and not two plants.

Whether the figure is whole biomass or a protein concentrate, since extracts and crops are not comparable and get mixed up constantly in this particular comparison.

Whether a digestibility number is attached, and in which animal.

What inclusion ceiling applies, because it usually caps the difference into irrelevance.

And whether the water is food-safe, because that question can end the comparison before composition ever gets a say.

FAQ

Which has more protein, azolla or duckweed?

On the reference tables duckweed leads, 27.8 percent of dry matter against 20.6 for Azolla. Both are computed with a 6.25 factor that suits neither plant, and same-pond studies split both ways depending on how much nitrogen the water carried.

Why do azolla and duckweed protein figures vary so much between sources?

Because crude protein is a nitrogen assay, and the plant-specific conversion factors differ: 4.9 to 5.0 for Azolla against 5.78 for duckweed. The usual 6.25 inflates Azolla by roughly 28 percent and duckweed by roughly 8 percent.

Does azolla need fertiliser to reach its protein content?

No. Under ambient carbon dioxide, fixation by its cyanobacterial symbiont supplied all the nitrogen in the biomass, at 90.0 to 97.2 kg of dry weight per hectare per day with no nitrogen fertiliser. Duckweed protein content, by contrast, tracks nitrogen in the water.

Is azolla or duckweed protein more digestible?

Duckweed, by a wide margin in the one trial that measured both in the same fish. Apparent protein digestibility in Nile tilapia was 96.18 percent for Lemna against 62.06 percent for Azolla in small fish, and 92.29 against 38.52 in larger fish.

How much azolla can you put in a poultry or fish diet?

Trial reviews recommend below 15 percent of the diet in poultry, 5 percent in broilers and below 25 percent in fish. High polyphenol content is the likely reason those ceilings are low.

Can azolla or duckweed grown on wastewater be used as feed?

Not under SERAPH’s policy. Biomass grown on wastewater, manure or contaminated water never becomes food or feed, and the two streams are separated from the first sensor reading.