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Seaweed on the Farm: Biostimulants, Soil, and the Cattle Feed That Cuts Methane

Fertiliser numbers from SAC, biostimulant crop trials, the Asparagopsis methane results and their bromoform caveat, what Queen's Belfast is finding in native seaweeds, and where hens, horses and Scottish policy fit.

By Jayson Byles, forager & chef · September 2026

Seaweed works on a farm in three ways: spread on the land as a potash-rich manure, applied to crops as a biostimulant extract, and fed to animals, where one tropical red seaweed cuts the methane a cow breathes out by a third or more. We hand-cut identified Scottish species on the Fife shore for kitchens and for trade, and farmers and feed formulators ask us about all three. This is what the trials show, what the regulators say, and where a cold-water seaweed fits.

Seaweed on the land: manure and soil

Cast seaweed has gone onto Scottish fields for as long as there have been fields by the sea. SAC Consulting’s grassland note gives typical values for fresh seaweed of about 15 per cent dry matter and, per tonne, 3.8 kg of nitrogen, 1.4 kg of phosphate and 13.5 kg of potash, with sulphur, magnesium and traces of boron, cobalt, copper, iodine, manganese, selenium and zinc. Only 5 to 15 per cent of the nitrogen is available in the year of application, around half the phosphate and 80 per cent of the potash. The usual rate is 20 to 35 tonnes a hectare against a 50-tonne ceiling for any manure, and because analysis varies with species, season and how long the heap has stood, SAC’s advice is to sample it first. Its March 2026 note, Seaweed: the free fertiliser, repeats the point for crofts and small farms. The Scottish Parliament’s 2018 kelp briefing notes storm-cast kelp still goes onto the machair of the Uists.

13.5 kgpotash per tonne of fresh seaweed, 80% available in year one (SAC Consulting)
16%average crop yield gain from seaweed extracts across 198 studies (meta-analysis, 2026)
38%less methane from grazing beef cattle fed Asparagopsis pellets (PNAS, 2024)
2 mg/kgrecommended maximum iodine in complete feed for dairy cows (legal limit 5 mg/kg)

Biostimulants: what the crop trials show

A biostimulant is not a fertiliser. It is applied in grams, not tonnes, and works on the plant’s physiology: rooting, nutrient uptake, stress tolerance. Seaweed extracts, mostly from knotted wrack, Ascophyllum nodosum, dominate the market.

  • A 2026 meta-analysis in Frontiers in Plant Science pooled 198 studies and 2,021 comparisons. Seaweed extracts raised yield by 16 per cent on average: 14 per cent in the field, 22 per cent under glass. Soil drenches gave 20 to 21 per cent, foliar sprays about 15; alkaline extracts and Ascophyllum were the most consistent. Evidence: pooled field and glasshouse trials.
  • A two-year open-field trial published in 2025 with a commercial Ascophyllum extract raised pepper yields by 28 and 46 per cent and aubergine by 81 and 108 per cent, mainly through more fruit per plant. Evidence: single-site field trial.
  • In April 2025 the James Hutton Institute in Dundee began SKE-NMT, an Innovate UK project with Algapelago Marine and the UK Agri-Tech Centre testing low-energy extracts of farmed sugar kelp as a way to hold yield while cutting nitrogen and phosphate. Evidence: under way, no results yet.
  • WWF-UK, CEFAS and the University of East Anglia ran a one-year spring barley trial at Holkham under reduced fertiliser with a biostimulant from Norfolk-farmed seaweed, measuring soil organic matter, grain quality and a life-cycle comparison with synthetic fertiliser. Evidence: single season; results due in 2025 had not been published when we wrote this.
  • A 2025 Journal of Applied Phycology study made monthly water extracts from cast UK Ascophyllum, Fucus serratus and Fucus vesiculosus, the wracks we cut, and found significantly heavier lettuce and beetroot seedlings at 1:50 dilution, strongest from weed harvested May to August. Evidence: greenhouse.

The honest summary: moderate, real gains that depend on extraction, dose and crop stress. The trade body UK Seaweed, launched at the Scottish Seaweed Industry Conference in Oban in November 2025 with WWF-UK funding, names crop biostimulants among the sector’s first markets. Gardeners will find the household version of all this in seaweed in the garden.

Seaweed as animal feed: a long record

The FAO’s review of traditional seaweed use records sheep and horses grazing seaweed beaches in Iceland, Ascophyllum meal milled there from 1939 and in Norway and Ireland by the tens of thousands of tonnes in the 1960s, and a working rule that seaweed meal could make up to 10 per cent of the ration for most livestock and poultry without harm. A 2016 review in Animal Feed Science and Technology collects 300 years of British accounts of sheep, cattle and pigs fed seaweed fresh, dried or ensiled.

Scotland has the living example. On North Ronaldsay in Orkney a stone dyke was completed in 1832 to keep the island’s sheep off the pasture, and they have lived on the shore ever since, eating kelp on the ebb and resting on the flood, on a diet the Orkney Sheep Foundation puts at about 15 per cent protein. Because seaweed holds under 5 parts per million of copper, the breed extracts copper so efficiently that a grass diet can poison it; a 2006 study called them the most copper-sensitive mammals known. A seaweed-heavy ration changes mineral balance, and any change needs a nutritionist.

The law treats whole seaweed as an ordinary feed material. The Catalogue of Feed Materials, retained in Great Britain, lists algae, dried algae and seaweed meal as entries 7.1.1 to 7.1.6, noting that both may be washed to reduce iodine. Iodine is the constraint: feed law sets a legal maximum for total iodine in complete feed of 5 mg/kg for dairy cows and laying hens and 4 mg/kg for horses, with recommended maxima of 2, 3 and 3 mg/kg, and brown seaweeds can carry over a thousand milligrams per kilogram of dry matter. That is why the Hebridean Seaweed Company on Lewis, which mills Ascophyllum for ingredient markets, and the rest of the trade sell it as a measured supplement, not a bulk feed.

Poultry and horses

For hens the evidence is mixed but respectable. A 2023 study in Animals on a commercial Russian flock fed bladderwrack meal at 0.1 and 0.25 per cent of the diet for 37 weeks reported eggs per hen rising from 244 to around 254 and egg weight from 65.2 g to 68.1 g. Evidence: one large farm trial. A 2020 Canadian study at 0.5 per cent Ascophyllum found little effect. What is not in doubt is iodine: a 1998 feeding trial with 40 hens showed seaweed raises the iodine in eggs within two weeks, which is a benefit or a problem depending on the dose, and the feed iodine limits apply.

Horses are where the caution is loudest. Seaweed is sold to horse owners for its iodine, and the UK feed company Forageplus notes that a 500 kg horse needs about 1.75 mg of iodine a day while some kelps run to 1,850 mg per kilogram of dry matter; goitre in foals and mares has followed heavy seaweed feeding in pregnancy. Its advice, a teaspoon or two a day at most and none on top of a fortified feed, is the right shape. We do not make equine supplements, and we would not feed our own kelp to a horse by the handful.

Running a feed trial, a biostimulant pilot or a soil project that needs one identified Scottish species? Ask for a sample pack and tell us what you are testing.

Cattle, methane and Asparagopsis

The headline use, solid at the centre with real questions at the edges. The tropical red seaweed Asparagopsis taxiformis stores bromoform, which blocks the last enzyme step in the rumen microbes that make methane. At a fraction of a per cent of the diet it cuts methane hard.

  • Grazing beef, 2024. A PNAS trial from UC Davis fed Wagyu-Angus steers on a Montana ranch pelleted Asparagopsis, about 193 mg of bromoform a day, for ten weeks. Methane fell 37.7 per cent; weight gain did not change. Evidence: field trial in grazing cattle, the hardest setting.
  • Pooled trials, 2025. A Journal of Dairy Science meta-analysis of 14 studies put the average cut in methane production at 47 per cent at a bromoform dose of about 28 mg per kilogram of feed, with larger doses giving larger cuts. Evidence: meta-analysis.
  • Feedlot beef, 2026. An eight-week University of Adelaide trial in 80 Angus cows cut methane by 49 to 77 per cent. Evidence: controlled feeding trial.
  • Dairy mechanism, 2026. A Swedish study in Microbiome fed Nordic Red cows Asparagopsis at 0.5 per cent of organic matter, recorded a 54 per cent drop in methane yield and showed the rumen community reorganising around propionate. Evidence: small mechanism study.

Now the edges. Bromoform is why it works and why regulators are careful. In October 2025 an EFSA-framework risk assessment concluded that bromoform is detectably carried into cows’ milk but a health risk from milk is unlikely, that transfer to beef and mutton is unlikely, and that the effects of long-term feeding on the animals remain uncertain. The Scottish Government’s 2023 review of farm technologies put beef trials at an average 56 per cent cut and dairy at 22, and flagged bromoform toxicity in rodents, iodine accumulation and bromoform’s effect on the ozone layer. A 2026 PNAS feature adds the practical limits: the species is tropical, supply is tiny against the global herd, and at higher doses cows eat less and give less milk.

Then the law. A product sold to reduce methane is a feed additive and needs authorisation. In Great Britain the only one on the FSA register is 3-nitrooxypropanol, Bovaer, assessed by the FSA in 2023 and on farms since 2024; it is not found in milk. No Asparagopsis product was on that register when we checked in September 2026. Swedish supermarkets already sell beef raised on it, and Defra has said it wants a mature market for safe, effective methane-suppressing feed products in England; for now a Scottish farmer cannot simply buy it.

StudyAnimals and settingSeaweed and doseResultEvidence level
PNAS, 2024UC DavisGrazing beef steers, Montana, 70 daysAsparagopsis pellets, ~193 mg bromoform/dayMethane −37.7%; daily gain unchangedField trial
J. Dairy Sci., 202514 trials, dairy and beefBromoform seaweeds and additives, ~28 mg/kg feedMethane production −47%Meta-analysis
Adelaide, 202680 Angus cows, 8 weeksAsparagopsis bromoform compoundMethane −49 to −77%Feeding trial
Microbiome, 2026SwedenNordic Red dairy cowsAsparagopsis at 0.5% of organic matterMethane yield −54%; rumen shifts to propionateSmall trial
J. Sci. Food Agric., 2025QUB / TeagascRumen fluid, 48 hSea spaghetti extract; bladderwrack; at 4%−43% (extract), −14% (bladderwrack); Asparagopsis control −93%Laboratory
Microbiol. Spectrum, 2024Rumen simulationScottish bladderwrack; knotted wrack; 2.5–5%−19 and −8 percentage pointsLaboratory
J. Dairy Sci., 2023Reading / QUB46 Holstein cows, 9 weeks330 g/day dried knotted wrackMilk iodine 166 to 1,886 µg/kgFeeding trial
Food Risk Assess Europe, 2025Consumers of milk and meatBromoform from Asparagopsis feedingMilk risk unlikely; meat transfer unlikely; long-term gapsRisk assessment

Swipe the table sideways for more columns

Seaweed in ruminant feed, 2023 to 2026: what was tested, on what, and how far the evidence goes. Laboratory results in the lower rows have not yet been repeated in live cattle.

Can a Scottish seaweed do the same?

Not yet, and probably not by the same route, but the work is serious and much of it is on species we cut. Queen’s University Belfast has run a three-year programme with Morrisons and its British beef farmers, with AFBI as partner, feeding seaweeds from the Irish and North Seas and aiming at a cut of at least 30 per cent, alongside the Teagasc-led SeaSolutions dairy trials that began on Northern Irish farms in 2022. The aim: a local seaweed that avoids bromoform, the ozone problem and shipping a tropical crop across the world.

The laboratory results point at phlorotannins, the polyphenols that give the wracks their taste. A 2025 QUB and Teagasc study screened seven temperate species at 4 per cent of the diet in rumen fluid: an extract of sea spaghetti, Himanthalia elongata, cut methane by 43 per cent at 48 hours and whole bladderwrack by 14 per cent, against 93 per cent for the Asparagopsis control; dabberlocks and carrageen did little. A 2024 rumen-simulation study using Scottish-harvested bladderwrack and knotted wrack found 19 and 8 percentage-point reductions. All of this is glassware; it needs live animals, then farms.

Two things will decide whether it leaves the laboratory. One is iodine: when Reading and QUB fed 46 Holstein cows 330 g of dried knotted wrack a day, milk iodine went from 166 to 1,886 micrograms per kilogram, enough for the authors to warn that children drinking it would exceed safe upper limits. QUB sees the same effect as an opportunity for iodine-deficient teenagers and pregnant women and is running dairy trials on that basis; either way the iodine limits force low inclusion. The other is consistency: a phlorotannin content that varies with shore and season is a formulation problem before it is a farming one, which is why we record the shore and the date on every bag.

Scottish policy

The Scottish Government’s April 2024 climate change action policy package committed to a pilot scheme with Scottish farms on the uptake of methane-suppressing feed products and envisaged support for their use in beef and dairy under Tier 2 of future agricultural payments. On the supply side, anyone harvesting wild seaweed for feed or biostimulant beyond personal use on Crown foreshore needs a Crown Estate Scotland licence, granted once NatureScot confirms no unacceptable environmental risk. We harvest under those rules and explain them in our guide to the law.

Three centuries in four dates

Kelp burning for soda ash was the first industrial use of Scottish seaweed, told in full in the history of seaweed in Scotland. In 1832 the North Ronaldsay dyke put a whole flock onto seaweed for good. In 1939 Iceland began milling seaweed meal for livestock. And in 2024 the first methane-suppressing feed additive reached British farms, with seaweed the candidate everyone hopes will follow. For the whole spread, from wound dressings to batteries, start with the other uses of seaweed; for the shore itself, our complete guide to seaweed foraging in Scotland.

Sources

Questions we get asked

Does seaweed really reduce methane in cows?

Yes, for one seaweed. The tropical red Asparagopsis taxiformis, fed at a fraction of a per cent of the diet, cut methane by 38 per cent in grazing beef cattle in a 2024 PNAS trial and by around 47 per cent on average across 14 trials in a 2025 meta-analysis. Cold-water seaweeds show smaller effects so far, mostly in laboratory rumen tests.

Is Asparagopsis approved as cattle feed in the UK?

Not as a methane-reducing additive. In Great Britain the only authorised methane-suppressing feed additive is 3-nitrooxypropanol, sold as Bovaer, cleared by the Food Standards Agency and on farms since 2024. No Asparagopsis product was on the FSA feed additive register in September 2026, and a 2025 European risk assessment asked for more long-term data on bromoform.

Can I feed seaweed to my chickens?

Yes, in small measured amounts. Dried seaweed meal is a listed feed material, and trials at 0.1 to 0.5 per cent of the diet have reported normal or slightly better laying. Seaweed raises the iodine in eggs within two weeks, and feed law limits total iodine for laying hens to 5 mg per kilogram of complete feed, with 3 mg recommended, so weigh it rather than scatter it.

Is seaweed safe for horses?

Only in small, measured doses. Seaweed is fed to horses mainly for iodine, and kelps can hold well over a thousand milligrams of iodine per kilogram of dry matter, while a 500 kg horse needs under 2 mg a day. Excess iodine has caused goitre in mares and foals. Feed law limits iodine for horses to 4 mg per kilogram of complete feed; ask a nutritionist first.

Can I spread seaweed on my fields in Scotland?

Yes. Fresh seaweed is a traditional manure worth around 13.5 kg of potash and 3.8 kg of nitrogen per tonne, applied at 20 to 35 tonnes a hectare within the 50-tonne manure ceiling. Sample the heap first because analysis varies. Gathering beach-cast weed in bulk from Crown foreshore for a business needs a Crown Estate Scotland licence; crofters have a traditional right.

This is a guide to the raw material, not agronomic, veterinary or regulatory advice. Feed inclusion rates, iodine limits and additive authorisation are the job of your nutritionist and the Food Standards Agency; we supply identified seaweed, with testing and a certificate of analysis available on request.

Need one identified Scottish seaweed for a feed trial, a biostimulant pilot or a soil study? Tell us the species, the form and the quantity and we will come back within a day.
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To understand the shore it comes from, start with our complete guide to seaweed foraging in Scotland.

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