Livestock & Disinfection

The ammonia measured inside a barn or a poultry house is really the last output of a water problem. Urea excreted in urine does not turn into ammonia by itself; the conversion is done by urease, an enzyme produced by ureolytic bacteria in the manure, and that enzyme only works when it finds enough water activity. Below 20% litter moisture urea hydrolysis all but stops; above 35% the enzyme accelerates while footpad dermatitis, breast lesions and the survival time of coccidial oocysts rise together. Looking for where the water comes from is therefore a more productive starting point than looking for where the smell comes from.

Lime enters this chain at three separate points, and the three are not covered by one product. Quicklime (CaO) binds water chemically and evaporates part of it with the heat of hydration; hydrated lime (Ca(OH)₂) takes the saturated surface solution to pH 12.4 and collapses both vegetative pathogens and the urease-producing flora; ground carbonate conditions the litter physically while keeping pH below the critical threshold. What goes wrong in the field is usually not the choice of product but the use of the right product in the wrong window.

The most widespread belief is that lime “binds” ammonia. It does not; by raising pH it converts the ammonium ion dissolved in the litter into volatile ammonia and pushes ambient concentration up in the first hours. The gain from lime shows not in those hours but in the weeks that follow, once urease activity has been cut at its source. This page sets out which product goes where, at what dose and contact time, which measurement misleads and when, and the general logic of moisture management in an enclosed volume — alongside the same logic on the industrial moisture and humidity control side.

Urea hydrolysis is the reaction CO(NH₂)₂ + H₂O → 2NH₃ + CO₂, and because it is enzymatic its rate is set by three variables: water activity, temperature and the size of the microbial population producing the enzyme. Between 25 and 35 °C and above 35% litter moisture, much of the urea in fresh urine is hydrolysed within hours. Bring the same litter down to 20% moisture and the rate falls by a large multiple. The first intervention in ammonia control is therefore physical, not chemical: until drinker leakage, ventilation rate, dew point and litter depth are corrected, no dose produces a lasting result.

The second variable is pH. Nitrogen in litter sits on the equilibrium NH₄⁺ ⇌ NH₃ + H⁺, whose pKa is 9.25 at 25 °C. At pH 7 only about six parts per thousand of that nitrogen is free volatile ammonia; at pH 9.25 half of it, at pH 11 roughly 98%, at pH 12 almost all. Spreading hydrated lime on the surface pushes the equilibrium right in one step and releases the ammonium pool accumulated up to that moment within a few hours. The ppm spike measured after application is not a failure but an expected transition — it does mean the animals should not be in the building while it happens.

The measurement itself can also mislead. Litter moisture cannot be represented by a single sample: a zone above 50% under the drinker line and an 18% zone behind the feeder sit side by side in the same house, and their average describes neither. The caked layer usually looks dry while the layer beneath it is saturated. A reliable reading comes from zone samples dried at 105 °C for 24 hours and weighed; handheld capacitive probes drift by several points with cake density. Height matters in ammonia too: a reading taken at animal head height, 20–30 cm off the litter, can differ by a factor of two from one taken at 1.5 m in the passage.

The fourth and most frequently skipped point is that “disinfection” does not describe a single target. Two hours of contact at pH 12 gives a 4–5 log reduction in vegetative bacteria such as Salmonella, E. coli and Campylobacter; Clostridium spores, Cryptosporidium parvum oocysts and Mycobacterium avium subsp. paratuberculosis largely survive the same conditions. Against oocysts, the heat released by quicklime is more decisive than pH. On top of that, organic matter consumes alkali: lime applied over a floor still carrying manure and litter residue cannot hold the target pH for more than a few minutes. A dose calculated without pre-cleaning is correct on paper and void in the house.

The most productive window in the biosecurity calendar is the empty period after the flock leaves and the litter is removed. Onto a floor still damp from mechanical cleaning and washing, hydrated lime classified CL 90-S to EN 459-1, with Ca(OH)₂ above 90% and finely ground, is spread at 300–600 g/m² or sprayed as a 15–20% milk of lime. A saturated calcium hydroxide solution reads pH 12.4 at 25 °C, enough to denature vegetative cell membranes and enzyme proteins. Hydrated lime is preferred in this window for a simple reason: it is already hydrated and produces no sudden heat or steam on contact with a wet floor.

Quicklime is reserved for heavier work: manure pit and lagoon stabilisation, burial after culling, treatment aimed at Cryptosporidium, and drying out saturated patches in an empty building. The stoichiometry is plain — 56 g of CaO binds 18 g of water, so 1 kg of CaO chemically holds about 0.32 kg. The practical effect is larger: hydration releases roughly 1140 kJ per kilogram, and with a latent heat of vaporisation of 2260 kJ/kg that energy can evaporate a further 0.4 kg or so. Total effect is of the order of 0.6–0.7 kg of water per kg of CaO. The same heat, for the same reason, produces serious burns on contact with wet skin; CaO is never used with animals present.

With the flock inside, no product that takes pH above 9.25 is the right choice. The material for this window is ground calcium carbonate: its low solubility keeps litter pH from rising past the 8.3–9.0 band, leaves the ammonium equilibrium below the pKa, and takes up moisture not chemically but by capillary retention across its surface area. It also improves slip resistance on the floor. Part of what is sold as “bedding lime” is in fact carbonate; what to read on the label is not the trade name but the Ca(OH)₂ and CaCO₃ percentages. In small enclosed volumes — feed stores, hatchery equipment cabinets, medicine cupboards — a sachet of moisture and gas absorbent is a cleaner answer than spreading anything.

In manure stabilisation the aim is to take pH above 12 and hold it there for at least two hours — the same logic accepted for alkaline treatment of animal by-products. In slurry the CaO dose is typically 5–15% of dry solids, moving to the upper end for highly buffered liquids rich in volatile fatty acids. A deliberate trade-off is made here: high pH strips ammonia and lowers the nitrogen value of the manure, but it holds sulphide species in solution as HS⁻ (pKa₁ ≈ 7.0 for H₂S) and markedly reduces the H₂S release that can be lethal during agitation. The stripped ammonia need not be lost; in a covered store it can be captured in an acid scrubber and recovered as ammonium sulphate.

Stabilised manure and limed litter end up on the field, and they carry there not only nitrogen and phosphorus but a substantial neutralising value. If that contribution is not counted into the lime requirement calculation on the agriculture and soil improvement side, a few seasons later you meet unwanted over-liming and the zinc–manganese availability problem that follows it. Lagoon supernatant, wash water and parlour effluent are a separate line; the suspended solids, phosphorus and pathogen load of those streams are handled with water and wastewater treatment logic.