Swine Lagoon Effluent Irrigation on Bermudagrass Pasture in North Carolina

Scenario

Project Type: Waste-to-resource irrigation system upgrade Location Context: Coastal Plain, NC — sandy loam, high water table, humid subtropical climate; 2022 summer application on rotational bermudagrass pasture Constraints: State requires ≤150 kg/ha annual N loading; lagoon effluent must be applied within 48 hrs of pumping to limit NH₃ loss; pasture grazed within 14 days, so rapid N availability is critical for forage quality.

Given Data

• Total Nitrogen in Manure (wet basis): 187 kg/ha (measured via on-farm lagoon sensor array + lab composite)
• Organic Nitrogen in Manure (wet basis): 134 kg/ha
• Decay Constant for Ammonia Volatilization: 0.082 day⁻¹ (high temp, low pH lagoon effluent, surface-applied via center-pivot)
• Mineralization Rate Constant: 0.0083 day⁻¹ (soil temp avg. 28°C during application window)
• Time Since Application: 7 days (applied June 12; first grazing June 19)

Calculation

PAN = (Total_N − Organic_N) × e^(−k_decay × t) + Organic_N × (1 − e^(−k_min × t))

• Inorganic N = 187 − 134 = 53 kg/ha
• e^(−0.082 × 7) = e^(−0.574) ≈ 0.563 → Inorganic N remaining = 53 × 0.563 ≈ 29.84 kg/ha
• Organic N mineralized = 134 × (1 − e^(−0.0083 × 7)) = 134 × (1 − e^(−0.0581)) ≈ 134 × (1 − 0.9436) ≈ 134 × 0.0564 ≈ 7.56 kg/ha
• PAN = 29.84 + 7.56 = 37.40 kg/ha (tool output: 37.40)

Result and Decision

The tool calculated 37.40 kg/ha PAN at day 7 — sufficient to support early regrowth but insufficient for peak forage yield (>60 kg/ha required). The engineering team redesigned the irrigation schedule: instead of one 187 kg/ha pulse, they split applications into two 95 kg/ha doses spaced 10 days apart. This reduced per-event volatilization loss and increased cumulative PAN at first grazing to 52.1 kg/ha (validated by tissue N analysis).

Lesson

For warm-climate, surface-applied liquid manures, single large doses suffer disproportionate ammonia loss; splitting applications leverages higher short-term PAN while staying within regulatory N limits — a cost-neutral operational improvement requiring only scheduler reprogramming.