Feed is costed per kilogram of complete diet, so an additive is worth what its active nutrient is worth. That explains why two drums labelled “lysine” can differ by a quarter in the lysine they deliver, and why comparison per tonne is the wrong one. The examples below come from our own feed-grade range.

Why the purity figure is the wrong comparison

An assay of 98.5% describes the salt, not the nutrient. A product can be 98.5% pure and still yield only four-fifths of its weight as the amino acid the animal needs. The figure that sets the inclusion rate is the active nutrient content, and it has to be calculated rather than read off the certificate.

Lysine: one name, two counter-ions

L-Lysine hydrochloride (CAS 657-27-2, C₆H₁₄N₂O₂·HCl, MW 182.65) is the form most mills already run. Feed grade assays at 98.5% or better, but lysine accounts for only 146.19 of that 182.65 molecular weight, so the free base is about 80% of the salt and delivered lysine works out at roughly 78.8%.

L-Lysine sulphate (CAS 60343-69-3) is a fermentation product specified at a minimum of 55% L-lysine, so a tonne carries about 550 kg of lysine against about 788 kg for the hydrochloride. The two should not be close on price per tonne, and the only honest comparison is price divided by lysine content.

The counter-ion is not idle: a hydrochloride inclusion supplying 2 kg of lysine brings in roughly 0.48 kg of chloride, and chloride is a term in the dietary cation–anion difference that poultry rations are managed against. The sulphate grade contributes sulphate in its place, which is why formulators reach for it when chloride, not the amino acid, is the binding constraint.

Threonine and methionine: what the secondary rows are for

For a single amino acid the headline assay is largely fixed by the market, and differentiation moves to the rows beneath. L-Threonine (CAS 72-19-5, C₄H₉NO₃, MW 119.12) is sold at 98.5% or better with loss on drying ≤1.0%, residue on ignition ≤0.5%, pH 5.0–6.5 and heavy metals ≤10 ppm. Threonine sits among the first three limiting amino acids in a maize–soy diet, so small differences in delivered content are not academic. Loss on drying only means something if the assay is quoted on the same basis: 98.5% alongside 1% moisture leaves at best 97.5% on a dry basis. Fermentation-derived threonine also carries residual biomass and sulphate salts that land in the ash figure, so residue on ignition is the quiet specification, and the heavy-metals limit is the gate that keeps the route acceptable to export markets.

DL-Methionine (CAS 59-51-8, C₅H₁₁NO₂S, MW 149.21) is the one case where a racemic mixture is not a defect, because animals convert the D-enantiomer and use the 50:50 blend essentially as fully as the pure L-form. The commercial grade sits at 99%.

Choline chloride: the carrier is part of the product

Choline Chloride (CAS 67-48-1, C₅H₁₄ClNO, MW 139.62) needs the same arithmetic. The choline cation is 104.17 of that 139.62, so pure choline chloride is 74.6% choline, and a “60%” grade — 60% choline chloride on a vegetable carrier — delivers about 45% choline. The carrier itself then takes up premix space.

The physical properties cause more trouble than the analysis. Choline chloride is strongly hygroscopic and deliquescent, which is why it is normally kept out of the vitamin premix: moisture migrating from the choline fraction degrades vitamins A, D, E and K and thiamine in the same bag, and the loss shows up only when the premix is assayed.

Trace minerals: name the hydrate and the ligand

Zinc is where the hydrate matters most. Zinc sulphate (CAS 7733-02-0) is bought as the heptahydrate, which is 22.7% zinc; the anhydrous salt under the same name is 40.5% zinc, and zinc sulfate monohydrate (CAS 7446-19-7) sits between them at 36.4%. A specification reading “zinc sulphate” and nothing else has not said how much zinc is in the bag.

Zinc methionine sulfate (CAS 56329-42-1, C₅H₁₁NO₆S₂Zn, MW 310.66) runs at roughly 21% zinc with the metal held by a methionine ligand. That binding is the point: chelated zinc is less exposed to phytate and other antagonists than a soluble salt, but 21% is well below any sulphate grade, so inclusion rates and premix space must be recalculated rather than substituted one for one. Ferrous lactate (CAS 5905-52-2, C₆H₁₀FeO₆, MW 233.99) follows the same pattern for iron at about 24% in the ferrous state, and since ferrous iron oxidises to the far less available ferric form, how that state is held through storage matters.

The smaller nutrients, where the form decides the function

Betaine (CAS 107-43-7, C₅H₁₁NO₂, MW 117.15) is quoted as the anhydrous compound, while the hydrochloride grade carries only about 76% betaine, so the two are not interchangeable as methyl donors. Vitamin D3 (CAS 67-97-0, C₂₇H₄₄O, MW 384.64) is measured in IU, and one microgram of cholecalciferol is 40 IU, so a potency quoted in IU and one quoted in micrograms must be converted before comparison; crystalline material is sensitive to light, oxygen and acid, which is why feed use is normally a stabilised beadlet or spray-dried powder on an antioxidant and carrier. Nicotinamide (CAS 98-92-0, MW 122.12) is the B3 form chosen where solubility matters, and a different CAS from nicotinic acid.

Amino acids that exist in two oxidation states are the clearest case of all. L-Cysteine (CAS 52-90-4, MW 121.16) is the free thiol; L-Cystine (CAS 56-89-3, MW 240.30) is the disulphide dimer formed when two cysteine units oxidise and join. On a molar basis a kilogram of cystine supplies about a kilogram of cysteine equivalents, but only cysteine carries the reactive thiol, and cystine is the less soluble of the two. Guanidineacetic acid (CAS 352-97-6, MW 117.11) is the direct creatine precursor, traded as that compound rather than as a salt.

L-Hydroxyproline (CAS 51-35-4, MW 131.13) is the exception that proves the rule: food grade at 99% or better with loss on drying ≤0.5%, and here the assay is the useful value, because hydroxyproline comes from hydroxylation of proline on collagen and appears almost nowhere else in animal tissue. Its content is the usual way of confirming a collagen peptide really contains the protein claimed.

What to put on an enquiry

• The active nutrient content, and the basis it is quoted on — as is, dried, or as the salt.
• The counter-ion or ligand, with the CAS of that exact form, and for a mineral the hydration state.
• The secondary limits that move delivered value: loss on drying, ash, heavy metals, particle size.
• Compatibility inside a premix, above all whether choline can be kept away from the vitamins.
• The grade, since the same CAS is routinely supplied as feed, food and pharmaceutical material.

None of this needs a laboratory. It needs the second and third rows of the specification, which is where the price difference actually lives.