Characteristics of Stratified Bedded Pack Dairy Manure.

“Compost” dairy barns are a relatively new housing system that generates a deep (0.9 to 1.5m), stratified bedded pack (SBP) manure source. Bedding composed of sawdust, wood chips, or crop residues accumulates as additions are made to maintain a dry surface. Surface drying is promoted by a combination of forced air ventilation and twice daily stirring to incorporate manure into the pack. The result is a manure pack with two layers: 1) a loose surface about 20 to 30 cm thick that gradually increases in manure content, moisture, and density until new bedding is added; and 2) the compact layer, which increases in thickness as old surface layers are buried under new bedding. The compact layer is only partially composted, presumably due to oxygen limitations. This manure has neither been characterized in detail nor defined in terms of its N supply. We measured physical characteristics, nutrient concentration, N mineralization, and N supply to corn (Zea mays L.) of SBP dairy manure from eight Minnesota farms. Concentrations of N, P, and K were generally higher than in typical solid dairy manure, and were highly variable within and among barns. Average bulk density of SBP dairy manure was 934 kg/m³. All SBP dairy manures released a net addition of nitrate to soil during a 4-month-long incubation at 35 C, but the four with highest C:N ratios (19 to 21) immobilized N for 30 to 60d. In-field fertilizer N equivalents to corn ranged from 0.7 to 6.0 kg N/Mg for quickly incorporated SBP dairy manure, but only -0.1 to 2.6 kg N/Mg for fall-applied manure that was not incorporated until spring. Guidelines for solid dairy manure were not reliable for predicting N availability from SBP dairy manure.