Characteristics and Fertilizer Value of Compost Dairy Barn Manure

This report summarizes some of the nutrient characteristics of compost dairy barn manure and suggests a sampling protocol. Eight cooperating farms were identified at locations ranging throughout Minnesota. We sampled and collected data from the farmers in September 2006. Area per cow ranged from 60 to 97 ft². All bedded packs were based on wood chips or sawdust, but the sources varied from ground particleboard to fine hardwood sawdust. Some farmers had added other bedding on a trial basis, including sunflower hulls, wheat and barley middlings, small grain straw, and waste hay. All stirred the pack twice daily with a skid steer or small tractor and cultivator. Results showed two distinct layers:

1. the loose surface layer, ranging from about 3 to 8 inches thick that gradually increases in manure content and density; and
2. the compacted layer, which is partially composted .

Manure characteristics

Total nitrogen (N) concentration averaged 1.09% and did not differ among areas in the barn or depth. The range of carbon (C) to N (C:N) ratio for the entire pack depth in each barn was 11.2 to 20.9. At C:N ratios higher than 30.1, the soil uses more nitrogen to breakdown the excess carbon than is released for plant growth, resulting in less available nitrogen for the crop. Many producers with compost barns had this concern, but this research shows that this should not be a problem.

Phosphate (P₂O₅) concentration averaged 0.28% and did not vary with depth or sampling location. Potash (K₂O) concentration averaged 0.74% at the surface and 0.67% in the compacted area.

Recommendations

Because ammonium-N comprised 41 to 65% of the available N, sample collection and storage must provide good estimates of this component. The pH of CDB manure typically is above 7.5 and in the two other studies averaged about 8.5. High pH will result in high ammonia volatilization when the pack is exposed to air, especially when the barn is emptied and during and after land application. Farmers are advised to incorporate this manure resource quickly to minimize ammonia losses.

Based on this study, we recommend that farmers take samples through the entire depth of the pack in at least 6 sites in the barn. One of these sites should be within 6 ft of the outside wall; the others should be taken randomly in the rest of the barn. We did not sample the ramps, and recommend that they be avoided, because they likely have higher nutrient status than the majority of the pack.