Feed-Grade Amino Acids: Methionine, Lysine, Threonine, Tryptophan, Valine, Isoleucine, Arginine, How They Work and How to Choose

Feed-grade amino acids are amino acids, their salts and analogues added to feed to cover the indispensable amino acids feed protein leaves short; the EU Register lists them in functional group 3c: methionine, methionine hydroxy analogue, lysine, threonine, tryptophan, valine, isoleucine, arginine and histidine. Among the 3c entries, only L-lysine sulphate carries an EU legal maximum, 10 000 mg additive per kg complete feed (12 % moisture); every other dose follows the animal's digestible amino acid requirement.

Guanidinoacetic acid (GAA), sold alongside them as a creatine precursor, is a 4d zootechnical additive in the EU with a legal range of 600 to 1 200 mg/kg complete feed, while the US lists it as a food additive for poultry feeds at no more than 0.12 % (21 CFR 573.496). The amino acids themselves are generally recognized as safe in US animal feed under 21 CFR 582. EU entries are tied to production strains, and older ones such as DL-methionine 3c301 (Implementing Regulation (EU) No 469/2013) are past their printed expiry with no renewal act found as of 3 November 2026.

Every value below is compiled from EU implementing regulations, the EU Register of Feed Additives, EFSA opinions, the eCFR and named studies, checked on 3 November 2026; see the methodology.

What Are Feed-Grade Amino Acids?#

A feed-grade amino acid is a crystalline, granulated or liquid form of an amino acid, salt or analogue that a feed mill adds to compound feed, premixes or water to meet the animal's amino acid requirement. Under Article 2(2)(a) of Regulation (EC) No 1831/2003, a feed additive is a substance, micro-organism or preparation, other than feed material and premixtures, intentionally added to feed or water to perform a listed function, and group 3c sits in category 3 among the five feed additive categories of Article 6(1).

EU functional group 3c and the US GRAS listings#

In the EU, feed amino acids form functional group 3c, "amino acids, their salts and analogues", in category 3 nutritional additives of Annex I to Regulation (EC) No 1831/2003. A 3c entry is therefore never a zootechnical additive or a growth promoter, and it is generic: it names the production strain, form and minimum content, never a holder.

In the US, the same molecules are generally recognized as safe (GRAS) in animal feed under 21 CFR 582, for example 582.5475 for methionine and 582.5411 for lysine. Neither the EU entries nor the US listings set a withdrawal period for any amino acid, MHA or GAA.

Two neighbours of the group sit elsewhere in EU law. Implementing Regulation (EU) 2023/2628 reallocated GAA from 3c to 4d "other zootechnical additives", and EFSA (2016, 2022) concluded that GAA is converted to creatine and does not enter protein synthesis. Several amino acids also hold category 2b flavouring entries, a sensory use separate from 3c.

Why diets need crystalline amino acids#

Feed protein supplies amino acids in proportions that rarely match the animal's requirement, so the amino acid in shortest supply relative to need limits protein deposition, and crystalline amino acids fill that gap at lower crude protein.

Lower crude protein is also an environmental technique: the EU BAT conclusions for intensive pig and poultry rearing (Implementing Decision (EU) 2017/302) list low-protein diets with crystalline amino acids under BAT 3 for reducing nitrogen excretion. The reduction has a floor: the Kansas State University (K-State) Swine Nutrition Guide reports that late-finishing pig growth fell below 13 % crude protein (Soto et al. 2019).

Feed-Grade Amino Acids List: Members, EU Entries and US Status#

The members below are the amino acids, analogues and the creatine precursor sold to feed mills, with the newest EU entry, its expiry and the US section side by side; the two pages in the second table explain how to formulate with them. Minimum contents in the table are purity specifications of the additive, not doses.

GAA categories by act: 2016/1768 chickens for fattening, weaned piglets, pigs for fattening (feed); 2023/2628 chickens reared for laying and for breeding (feed and water), chickens for fattening (water); 2025/2505 weaned piglets and pigs for fattening (water), turkeys for fattening and reared for breeding (feed and water); 2026/1019 chickens for fattening, chickens reared for laying or reproduction, turkeys for fattening and reared for reproduction (feed and water).

The second table maps the two formulation pages to their EU basis.

How Do Feed Amino Acids Work?#

Feed amino acids work by supplying the indispensable amino acids that feed protein does not cover, so the diet meets the requirement for each one without excess crude protein. Nutritionists express that requirement per unit of digestible lysine and set the other amino acids as ratios to lysine, the ideal protein concept.

Protein synthesis and indispensable amino acids#

Absorbed amino acids are built into body and egg protein, and an animal can only deposit protein as fast as its scarcest indispensable amino acid allows. In methionine, the L-isomer enters protein synthesis directly, while the animal converts the D-isomer of DL-methionine to L-methionine; EFSA (2012) concluded that the methionine sources are effective sources of methionine for protein synthesis.

Requirements shift with conditions and diet. In the 2026 review by Choi and colleagues, the tryptophan requirement of pigs rose mainly under direct immune challenge rather than at weaning alone. Leucine, valine and isoleucine share transporters and degradation enzymes, and diets with blood cells (high in leucine, low in isoleucine) gave an estimated SID isoleucine to lysine ratio of 0.59 against 0.54 without them, overestimating the requirement.

Methionine precursors and the creatine route of GAA#

Methionine hydroxy analogue carries no amino group, so the animal first oxidises it to the keto acid and then transaminates it to L-methionine. The EFSA FEEDAP Panel (2018) rated HMTBa at 75 % of DL-methionine on an equimolar basis, citing microbial competition in the small intestine and lower-efficacy oligomers.

Guanidinoacetic acid works through creatine, not protein. The liver methylates GAA with S-adenosylmethionine through the enzyme GAMT to form creatine, a step that consumes methyl groups, and the EU GAA acts require the label to draw attention to vitamin B12 and to methyl donors other than methionine.

Evidence from meta-analyses#

Meta-analyses by Choi and colleagues (Animal Bioscience, 2026), covering 50 threonine, 71 tryptophan and 29 valine experiments, found that optimal ratios to lysine for modern pigs exceed the NRC 2012 values. The threonine optimum for daily gain rose with body weight, the valine optimum in nursery pigs reached 0.70 against 0.63 to 0.67 in NRC 2012, and the tryptophan optimum (0.16 to 0.20) stayed roughly constant.

For methionine sources, a meta-analysis by Lemme, Li and Dorigam (Animals, 2024) pooled 95 data sets from 25 publications and found that 65 parts DL-methionine replace 100 parts MHA product by weight without loss of broiler gain or feed conversion. The authors are employed by a DL-methionine producer, and the weight basis differs from EFSA's equimolar basis.

Which Form and Source for Which Diet?#

Form decides more of the cost than the molecule: lysine comes as HCl with at least 78 % L-lysine, as sulphate with 52 % or 55 % and a legal maximum, or as a liquid base with 50 %, and methionine activity comes as DL-methionine, liquid MHA or its calcium salt. The table sets each form decision against the legal content in the EU acts and the basis a buyer needs to state.

Active content per kilogram, not product weight, makes two offers comparable, and lysine sulphate also adds sulphur to the dietary total. Liquid MHA is acidic and corrosive and goes in by pump, while the 3c310 preparation is corrosive to skin and eyes and its act requires the label to state storage conditions and heat stability. The choice between methionine sources turns on the basis of comparison, equimolar or by weight, and bioefficacy and cost on each basis are weighed in DL-methionine vs MHA.

Selection by Species and Diet#

Species changes three things: the requirement level, whether the amino acid survives digestion, and which EU entries name the animal, since most 3c entries cover all animal species while GAA entries name chickens, turkeys and pigs only. The table pairs each species group with its amino acid facts and the programme page.

Amino acids for poultry#

Broiler requirements are set as digestible amino acids: the Ross 308 specifications (Aviagen, 2022) give digestible lysine of 1.32 %, 1.18 % and 1.08 % of diet for starter, grower and finisher 1, a breed-guide basis, not a legal or NRC value.

GAA entries for poultry rest on Implementing Regulations (EU) 2023/2628 (Alzchem Trostberg GmbH) and 2026/1019 (Evonik Operations GmbH), which name chickens and turkeys in feed and water, each with the bird categories written in the act. In the US, 21 CFR 573.496 permits GAA in poultry feeds only.

Amino acids for pigs#

For grow-finish pigs the K-State Swine Nutrition Guide sets SID lysine at 1.08, 0.88, 0.78 and 0.70 % of diet across the 55 to 130, 130 to 175, 175 to 220 and 220 to 285 lb bands (25 to 129 kg), on a diet with 1,150 kcal NE/lb. K-State warns that formulating low-protein pig diets on DE or ME instead of NE leads to fatter carcasses.

GAA reaches pigs under two EU acts. The feed use for weaned piglets and pigs for fattening under Implementing Regulation (EU) 2016/1768 has a printed expiry of 2026-10-25; renewal decision not found. Implementing Regulation (EU) 2025/2505 authorises GAA for weaned piglets and pigs for fattening in water for drinking at 200 to 600 mg/l, never at the same time as feed use. The US GAA regulation has no pig use.

Cattle and other ruminants: why free amino acids are degraded#

Rumen microbes degrade free amino acids before they reach the small intestine, and recent EU acts oblige feed business operators to ensure the additive is rumen protected when fed to ruminants. The obligation appears in Implementing Regulation (EU) 2025/160 for threonine 3c412 and in Implementing Regulation (EU) 2026/353 for histidine 3c353 and 3c354, not in every amino acid entry. EFSA (2018) found that HMTBa degrades less in the rumen than DL-methionine but still needs protection.

For lysine sulphate in ruminant diets, NRC (2005) sets maximum tolerable sulphur at 3 g S/kg DM (concentrate) or 5 g S/kg DM (roughage).

Fish and crustaceans#

One methionine source is written for aquaculture only: DL-methionyl-DL-methionine 3c306, a dipeptide with at least 93 % content for fish and crustaceans under Implementing Regulation (EU) 2015/1408, printed expiry 2025-09-09. Other 3c entries, such as L-histidine 3c352 (Implementing Regulation (EU) 2020/1090), name all animal species, fish included.

Dosing Logic: Requirements, Legal Maxima and the GAA Range#

Amino acid doses are requirement-driven, because EU acts set no maximum content for 3c amino acids except L-lysine sulphate at 10 000 mg additive per kg complete feed, while GAA carries a legal range of 600 to 1 200 mg/kg. The table separates the legal limits from the requirement values and gives each its basis.

The two halves of the table are never compared. Legal values are per kg complete feed and bind the feed business operator; requirement values are per cent of diet, met by feed protein plus additive, so they are not additive doses. The sulphur rows are ceilings for the whole diet, so the sulphate maximum alone does not settle a lysine sulphate inclusion.

Regulation Summary: EU 3c Entries, GAA in 4d and US Status#

Each EU amino acid entry is tied to the production strain named in its implementing regulation, while in the US the same amino acids are GRAS listings under 21 CFR 582 with no numerical limit and no withdrawal period.

EU entries are strain-specific: expiry and renewal status#

A new production strain needs its own EFSA opinion and implementing regulation, so one amino acid sits under a series of acts with different expiry dates, as lysine does from Implementing Regulation (EU) 2017/439 (sulphate 3c323, printed expiry 2027-04-02) to 2026/93 (sulphate 3c330, to 2036-02-05). Only the GAA entries name holders.

For DL-methionine 3c301 and the MHA entries 3c307 and 3c308 under Implementing Regulation (EU) No 469/2013, each reads: printed expiry 2023-06-12; the EU Register records a renewal application and still lists the entry pending the Commission decision (Article 14 of Regulation (EC) No 1831/2003). For L-threonine 3c410 under Implementing Regulation (EU) 2016/1220: printed expiry 2026-08-16; the EU Register records a renewal application and still lists the entry pending the Commission decision (Article 14 of Regulation (EC) No 1831/2003). For L-valine 3c370 under 2015/1114 (printed expiry 2025-07-30), L-arginine 3c361 under 2016/972 (printed expiry 2026-07-08) and the GAA feed use under 2016/1768 (printed expiry 2026-10-25), renewal decision not found. How Article 14 renewals work is explained under EU feed additive regulation.

An EFSA opinion does not change status by itself: for three 2026 lysine opinions (EFSA Journal 2026;24:10033, 9922 and 10118) no implementing regulation was found as of 3 November 2026. In Great Britain the FSA register, updated 15 July 2026, still lists GAA as 3c372 and gives 3c301 and 3c307 to 3c310 the GB status "Authorised".

US status: GRAS listings and one food additive regulation#

In the US, methionine, MHA and its calcium salts, lysine, threonine, tryptophan, valine, isoleucine, arginine and histidine are generally recognized as safe in animal feed under good manufacturing or feeding practice (21 CFR 582). The ten US sections for the members are listed below.

• Methionine: 21 CFR 582.5475
• Methionine hydroxy analog and its calcium salts: 21 CFR 582.5477
• Lysine: 21 CFR 582.5411
• Threonine: 21 CFR 582.5881
• Tryptophane: 21 CFR 582.5915
• Valine: 21 CFR 582.5925
• Isoleucine: 21 CFR 582.5381
• Arginine: 21 CFR 582.5145
• Histidine: 21 CFR 582.5361
• Guanidinoacetic acid: food additive, 21 CFR 573.496, poultry feeds at no more than 0.12 % of complete feed

The GAA regulation dates from 81 FR 86269 (30 November 2016), amended by 86 FR 37038 (14 July 2021), and requires at least 97 % GAA.

Use conditions in the acts: water, rumen protection, endotoxins, flavouring use#

Where an act allows use in water for drinking, the label must warn that supplementation should take into account all essential and conditionally essential amino acids to avoid imbalances. EFSA (2025) reiterated its concern about simultaneous use via feed and water, and the GAA acts forbid it.

Implementing Regulations (EU) 2022/1493 (L-methionine 3c305ii), 2020/1090 (histidine 3c352) and 2021/709 (histidine 3c352i) cap endotoxin exposure at 1 600 IU per m3 of air, a limit the other acts do not carry. Flavouring entries such as L-valine 2b17028 and DL-isoleucine 2b17010 (Implementing Regulation (EU) 2018/249) carry a recommended maximum of 25 mg/kg complete feed, and they sit with the other feed flavourings.

Rumen protection, the third condition, applies only where an act states it, as for threonine 3c412 and histidine 3c353 and 3c354.

Sourcing Checklist for Feed-Grade Amino Acids#

Source a feed-grade amino acid in eight checks, from the identification number and strain on the label to the origin that decides the import duty.

1. Match the identification number and production strain on the label to the implementing regulation.
2. Compare price per kg of active amino acid: L-lysine HCl 78 %, sulphate 52 or 55 %, liquid base 50 %, isoleucine 90 to 93.5 %, MHA 88 % (liquid) or 84 % (calcium salt) HMTBa.
3. Check the sulphate maximum and the total dietary sulphur before choosing lysine sulphate.
4. Ask for a certificate of analysis by EN ISO 17180 for additives and premixtures above 10 %, or by Regulation (EC) No 152/2009 Annex III F for feed below 10 %.
5. Ask for endotoxin data (European Pharmacopoeia 2.6.14) on fermentation products and for the user-protection terms of the act.
6. Check impurity limits: 1,1'-ethylidene-bis-L-tryptophan at no more than 10 mg/kg in tryptophan, histamine at no more than 100 ppm in histidine.
7. Check the origin, because lysine and valine from China carry EU anti-dumping duties.
8. Ask for FAMI-QS or GMP+ certification.

Implementing Regulation (EU) 2025/1330 of 10 July 2025 sets 47.7 % (Meihua group), 53.1 % (other cooperating companies) and 58.2 % (Eppen and all others) of the CIF price on Chinese lysine, and Implementing Regulation (EU) 2026/1824 registers imports during an absorption reinvestigation. Implementing Regulation (EU) 2026/319 of 12 February 2026 sets 31.3 % (CJ (Shenyang) Biotechnology), 42.2 % (other cooperating companies) and 53.8 % (Bayannur Huaheng Biotechnology and all others) on Chinese valine. The 2025/1330 act also cites Eurostat average EU import prices for lysine from China of EUR 876, 1 342, 1 679 and 1 159 per tonne for 2020, 2021, 2022 and 2023 (all forms), dated trade figures rather than the price of any offer.

Feed Amino Acid Questions#

Three questions about feed amino acids come up beyond formulation and status.

Is there a downside to amino acids?#

The downsides come from formulation errors rather than the molecules: formulating on total instead of digestible amino acids, mixing up the equimolar and weight basis for MHA, and ignoring the sulphur from lysine sulphate. Each one hides a shortfall of the limiting amino acid, or a sulphur excess, behind a diet that looks balanced on paper.

Which amino acids are essential for animals?#

For pigs, NRC 2012 sets standardised ileal digestible requirements for lysine, methionine, methionine plus cysteine, threonine, tryptophan, valine, isoleucine, leucine, histidine and phenylalanine, and the Ross 308 broiler specifications add arginine. The amino acids in those two sets fall into five groups.

• Lysine, the reference amino acid (lysine = 100)
• Sulphur amino acids: methionine and methionine plus cysteine
• Threonine and tryptophan
• Branched-chain amino acids: valine, isoleucine and leucine
• Histidine, phenylalanine and, for broilers, arginine

Are synthetic amino acids allowed in organic feed?#

No, with one exception: Implementing Regulation (EU) 2021/1165 permits only L-histidine monohydrochloride monohydrate produced by fermentation, for salmonids when their feed does not supply enough histidine.

Annex III of that act, adopted under Regulation (EU) 2018/848, holds the full list of feed additives in organic production. The EU BAT conclusions (Implementing Decision (EU) 2017/302) also mark low-protein diets with crystalline amino acids as not applicable to organic production.