Feed Preservation with Propionic Acid: Dosage by Moisture and Crop is governed by the antimicrobial activity of propionic acid (CAS 79-09-4, EC 201-176-3), a short-chain fatty acid with pKa 4.88 at 25 °C and atmospheric boiling point 141 °C. In feed preservation, the active agent is the undissociated molecule, which penetrates fungal and bacterial cell membranes; the dissociated anion contributes comparatively little to antifungal action. Consequently, the required dosage is not fixed for all feeds but is selected from the moisture level, crop matrix, storage period, and ambient temperature of the specific preservation system.

Propionic acid is registered as a technological feed additive in major regulatory frameworks. Under FDA 21 CFR 184.1081, propionic acid is affirmed as generally recognized as safe for use as an antimicrobial agent under current good manufacturing practice. Under Regulation (EC) No 1831/2003, propionic acid is classified in the category of technological additives and functional group of preservatives for feed materials and compound feed. These authorizations do not replace dosage adjustment based on moisture; they assume application rates consistent with good preservation practice for the stated matrix.

At field level, dosage recommendations are conventionally expressed on a weight-by-weight basis as active propionic acid relative to fresh feed. A separate expression is product basis, which depends on the concentration of commercial acid. A product containing 65 % active propionic acid requires a product dose equal to the active dose divided by 0.65; a 99 % product requires division by 0.98–0.99. Failure to distinguish between active and product basis is a recurrent source of underdosing in high-moisture maize and whole-crop cereal preservation.

What limits propionic acid efficacy in wet feed matrices?

The moisture content of the feed influences preservation through water activity and fungal ecology. Cereal grains at 14 % moisture generally maintain water activity below the threshold for active storage mould proliferation; at 16–18 % moisture, water activity approaches 0.80–0.85, where species of Aspergillus and Penicillium become competitive. Propionic acid demand therefore rises non-linearly once moisture exceeds 15 % in whole cereal grains. The dose is further influenced by pH, because the undissociated acid fraction declines as pH increases. At pH 4.5, approximately 70 % of propionic acid is undissociated; at pH 5.0, the undissociated fraction falls to approximately 43 %; at pH 6.0, it is less than 7 %. Feedstuffs with higher buffering capacity, such as legume-containing by-products or rations containing limestone, require higher acid input to achieve equivalent antimycotic activity at the same moisture level.

In wet cereal systems, dose escalation also depends on contact distribution. Application of propionic acid to ground or rolled grain creates localised acid concentration gradients; untreated pockets remain susceptible to heating and mycotoxin formation. Industrial treatment therefore requires not only adequate average dosage but also sufficient mixing residence time and droplet size to achieve a coefficient of variation below 15 % of the target concentration across the mass. This requirement is especially strict for whole moist sorghum and oats, where glumes and hulls can shield surface area from acid contact.

Maize stored at 24–26 % moisture for 6 months typically receives 0.50–1.00 % active propionic acid by fresh weight when ambient temperature remains below 25 °C. Above 28 % moisture, the inclusion is generally extended to 1.00–1.50 % w/w and expected storage duration is limited to 3–4 months. At this moisture range, application through a 316L stainless steel positive-displacement pump at the auger discharge is normal practice, with nozzle placement selected to wet the moving grain stream before the next mixing point. These values are compiled from extension guidance and preservation literature; they are not a substitute for silo-specific trials because batch-to-batch moisture variance of ±2 % can shift acid demand by several tenths of a percentage point.

Dosage thresholds for maize, sorghum, and small grains at harvest moisture levels

The following dosage matrix summarises typical active propionic acid ranges reported in preservation literature and feed-additive technical bulletins for common crop matrices. Values apply to clean concentrates at the stated moisture; foreign material, broken kernels, and fine particulates can increase measured acid demand because they contribute surface area and nutrient availability for spoilage organisms.

The table values refer to active propionic acid; product-based rates are derived by dividing by the active fraction of the commercial solution. For a 65 % product, a required active dose of 0.50 % w/w corresponds to approximately 0.77 % w/w product. For a 99 % product, the same active dose corresponds to approximately 0.51 % w/w product. Metering systems should be named with both active and product delivery rates to reduce calibration errors during batch preparation.

Whole-crop cereal and maize silage require a different preservation rationale from dry grain. The ensiled mass is already acidified by fermentation, but the feedout phase is destabilised by oxygen ingress and yeast metabolism. Propionic acid is therefore applied at the silage face, in the TMR mixer, or during transfer to suppress the rapid heating that occurs after oxygen exposure. The effective dose is lower than that required for preserving dry grain at the same water activity because the target organisms are acid-tolerant aerobic yeasts rather than storage moulds. Application at 0.20–0.50 % of wet weight is a common range in total mixed rations for dairy and beef systems, with the higher end required when face management is poor or when ambient temperature exceeds 30 °C.

Cereal Grain, Whole-Crop, and By-product Preservation Matrices

Crop matrix effects are not limited to moisture. Ground grains expose starch granules and fracture planes that increase surface area and acid-neutralising components; whole grains have much lower exposed surface area but also lower uniformity of acid coverage. Legume-containing by-products and oilseed meals can exhibit buffering from protein and mineral residues, increasing the apparent acid demand relative to cereal grain at identical moisture. In moist by-products such as citrus pulp, distillers grains, and wheat middlings, published data for specific propionic acid dosage configurations are limited; field trials generally indicate that the acid requirement may approach or exceed the high-moisture maize range at equivalent moisture. Treatment of these matrices requires pilot-scale mixing evaluation because viscosity, particle size distribution, and high moisture combine to decrease the efficiency of low-pressure spraying.

For moist by-products handled through a mixer wagon, a common production strategy is to treat at the point of receiving with a spray lance and then mix for a defined number of auger revolutions. The target is to achieve acid distribution before fungal activity begins. Batch records should capture incoming moisture, ambient temperature, product concentration, pump calibration, and mixing time. These records are essential for diagnosing preservation failures because a single dosage value cannot be interpreted without the associated moisture and mixing conditions.

At production scale, metering equipment generally uses acid-resistant positive displacement diaphragm or peristaltic pumps with wetted components constructed from 316L stainless steel, HDPE, PVDF, or PTFE. Carbon steel, copper, and brass are not acceptable for contact with concentrated propionic acid. Flow rates are calibrated daily against a graduated cylinder or mass flow meter because acid viscosity changes with temperature, and batch-to-batch acid concentration can vary between 65 % and 99 %. The application nozzle is placed in a grain transfer screw, belt conveyor, or mixer entry point. The target is a fine spray that wets moving feed without pooling; large droplets reduce coverage uniformity, while an excessively fine mist increases vapour load and operator exposure. The recommended mixing residence time after acid injection is generally 30–120 seconds at the design throughput of the wagon or conveyor, depending on moisture and particle size.

When Oxygen Ingress and Bunker Face Heating Destabilize Treated High-Moisture Feed

Treatment of grain at harvest does not eliminate the risk of yeast recontamination during feedout. High-moisture maize stored in a bunker or pile and subsequently moved once daily can require supplemental propionic acid at the TMR mixer if face heating exceeds 10 °C above ambient within 24 hours of oxygen exposure. The supplemental dose is commonly in the range of 0.20–0.30 % w/w of the wet ration. This secondary application is not a substitute for good face management; it is a control measure when feedout rate is slower than the aerobic stability window of the silage. Oxygen ingress through damaged film, loose surface seals, or wide face exposure consumes residual acid and permits lactate-assimilating yeasts to metabolise organic acids, raising pH and reducing the undissociated acid reserve.

Operational boundaries are explicit. Propionic acid treatment of ground grain above 35 % moisture is generally not recommended as a standalone preservation method because free water, leachate, and acid pooling create pumping, nozzle blockage, and coverage failures. In that moisture range, drying, blending with dry carrier, or ensiling is preferred. Basic feed ingredients such as limestone, sodium bicarbonate, and urea reduce free acid efficacy when combined in a dry premix before adequate acid contact has occurred. These materials should not be added at the same injection point as propionic acid. Treated grain should not be held in contact with carbon steel bins or unprotected galvanised surfaces; 316L stainless steel or HDPE-lined storage is used to avoid corrosion and metal ion contamination.

Regulatory compliance is anchored to specific standard designations and chemical identification. The following matrix records the principal frameworks governing feed preservation use.

Incompatibilities are equally important as authorisations. Propionic acid should not be blended with strong alkalis, oxidising agents, or high-buffering mineral premises without process evaluation, because neutralisation consumes the active undissociated acid before it can interact with spoilage fungi. The application of propionic acid to feed is a preservation technology with defined operational limits; its reliability depends less on the absolute molecular activity of the acid than on matching dosage to harvest moisture, crop matrix, mixing uniformity, storage geometry, and feedout management.