Manure Storage Volume

Manure storage sizing is a three-term multiplication — head count, production per head per day, and the number of days the storage has to hold — and the days are usually not a choice. Nutrient management regulations across most of Canada and the northern United States require 180 to 270 days of storage, and the reason is agronomic rather than bureaucratic: manure spread on frozen or saturated ground runs off instead of soaking in, so it has to be held until the land can accept it.

Production per head varies with species, size, ration and how much bedding and water go in with it. A mature dairy cow produces roughly 55 L a day of manure and urine, a finishing pig about 5 L, a beef animal on a bedded pack somewhat less as liquid but considerably more as total volume once the straw is counted. Book values are a starting point; a farm's own records, or a measured storage over a known period, are worth more.

The classic mistake is sizing to the manure alone. A real storage also receives bedding, parlour and equipment wash water, spilled drinking water, silage leachate, and every drop of rain and snowmelt that falls on the uncovered surface and on any yard that drains into it. In a wet climate the precipitation term alone can add a quarter to the required volume, and wash water on a dairy is frequently as large as the urine. On top of all that a storage needs freeboard — the unfilled margin that keeps a heavy rain from putting the contents over the wall — typically half a metre or more, and required by regulation in most jurisdictions.

Under-sizing does not announce itself until the worst possible moment. A storage that reaches capacity in February leaves exactly two options, both bad: spread on frozen ground, which is illegal in many places and environmentally indefensible everywhere, or an emergency haul at whatever price a contractor names in the middle of winter. Build the margin in at the design stage, where it costs concrete, rather than discovering it later, where it costs a river.