Aim for at least 6 months of storage capacity for any commercial dairy; 9–12 months is the modern best practice and is required by regulators in many regions.
Why 9 months? You only spread manure when soils are warm, dry, and crops can absorb the nutrients. In temperate climates that's a 3–4 month window. Anything less than 6 months of storage forces emergency spreading on frozen or saturated ground — that's where runoff and complaints come from.
Sizing A milking cow produces roughly 70 L/day of slurry (manure + urine + parlor wash water). For 300 cows × 270 days = 56,700 m³. Add 1 m of freeboard for rain. Build covered to reduce odor and rainwater intake.
Separation A screw-press or roller separator removes 25–30% of dry matter as solids. Solids can be composted and sold, or dried and reused as bedding. The liquid fraction stores in a lagoon and pumps easily through irrigation.
Digesters Anaerobic digesters become attractive at 1,500+ cows or as part of a co-op cluster. They convert manure methane to electricity or pipeline gas, reduce odor sharply, and produce a digestate that's lower-pathogen than raw slurry.