ARS | Publication request: Comparison of raw dairy manure slurry and anaerobically digested slurry as N sources for grass forage production

Submitted to: International Journal of Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 23, 2012
Publication Date: May 1, 2012
Citation: Saunders, O., Fortuna, A., Harrison, J., Whitefield, E., Cogger, C., Kennedy, A.C., Bary, A.I. 2012. Comparison of raw dairy manure slurry and anaerobically digested slurry as N sources for grass forage production. International Journal of Agronomy. Volume 2012, Article ID 101074, 10 pages.

Interpretive Summary: Best management practices are needed that address how to utilize the growing quantity of nitrogen produced by livestock operations. Animal agriculture in the United States has become more specialized with farms consolidating and growing in size. The number of dairy farms has decreased by 94% since 1960, but the number of animals has remained constant. Animal consolidation has created challenges with respect to on-farm N surplus, waste management and nutrient loading in the environment. One approach to ameliorate negative environmental impacts associated with animal manures is through adoption of anaerobic digestion technologies to treat farm generated manures and food processing wastes. Digestion of wastes can provide a stable and consistent source of nutrients comparable to inorganic fertilizers such as urea. Anaerobically digested dairy slurry was shown to provide adequate soil fertility and N availability for crop uptake and forage production over the three field seasons. In the short run anaerobically digested slurry did not significantly increase yield or N uptake compared with similar rates of raw slurry. We found that soil nitrates were fairly consistent across slurry treatments and application methods during each of the field seasons. Although urea treatments had the highest apparent N recovery value, the potential for nitrate leaching was also greatest under this management. Anaerobically digested slurry did not increase soil NO3- concentrations or alter the microbial composition and provided equal forage production and similar N use efficiency when compared to undigested dairy slurry. These data will assist in developing best management practices for anaerobically digested materials in dairy systems.

Technical Abstract: Our study was conducted to determine how raw dairy slurry and anaerobically digested slurry (dairy slurry and food waste) applied via broadcast and subsurface deposition to reed canary grass (Phalaris arundinacea) affected forage biomass, N uptake, apparent nitrogen recovery (ANR), and soil nitrate (NO3-) concentrations relative to urea. Slurry application rates were estimated to be equivalent on a total N basis. Nitrogen fertilizer requirements were adjusted each year to account for post harvest soil NO3- concentrations taken to a 30-cm depth. Total yearly N applications were approximately 600 kg N ha-1 in 2009, 500 kg N ha-1 in 2010 and 300 kg N ha-1 in 2011. The total phosphorus (P) content of the raw dairy slurry was higher than that of the digested) in 2009, 2010 and 2011, respectively. Forage biomass yield and N uptake were not agronomically different among plots receiving slurries over three growing seasons. Soil nitrates were greatest late summer and early fall at the beginning of the leaching season. Soil NO3- concentrations did not differ among slurry treatments or application methods. Urea-fertilized plots had the highest soil NO3- concentrations but did not consistently have greater forage biomass, N-uptake, and ANR. Slurry treatments had higher or equivalent ANRs relative to the urea treatment when calculations were based on the concentration of NH4+-N in each slurry. However, ANRs calculated on a total N basis were lower for slurry treatments (15 to 34%) due to the unavailability of portions of the organic N contained in these amendments. No consistent differences in soil microbial biomass or other biological indicators were observed among the treatments. Anaerobically digested slurry supported equal forage production and similar N use efficiency when compared to raw dairy slurry.