Improving N and P Estimates for Swine Manure Lagoon Irrigation Water

Author(s): McLaughlin, M.; Brooks, J.; Adeli, A.; Jenkins, J.

Nutrient management plans (NMPs) for confined animal feeding operations (CAFOs) require a record of N and P loads from manure land-applications, including irrigation with lagoon water. Mississippi regulations require nutrient records for lagoon irrigation water be based on at least one annual analysis. Research on swine CAFOs in Mississippi has shown that N and P levels in lagoon water, and the N:P ratio, follow predictable annual cycles, but vary significantly through the April to October irrigation season. Nutrient estimates based on a single annual analysis may not account for this variability and may over- or underestimate nutrient loads and yield inaccurate NMP records. The present study reports an improved method to more accurately estimate N and P loads in irrigation water from swine lagoons. Derived by analyses of data from Mississippi lagoons and other lagoon studies, the method used predicable annual cycles of N and P in lagoon water to fit lagoon-specific models and produce date-based data on nutrient levels. Data were converted to tables displaying estimated N and P levels in the lagoon water for each day of the irrigation season. The farm manager uses the calendar table to find estimated nutrient levels for the date of a respective irrigation event, multiplies those values by the volume of water applied per irrigated area, and enters the results in the NMP record. Similarity of curves from analyses of lagoons in Mississippi and other states suggests that the method can be applied using data from a single nutrient analysis for each lagoon. Although annual cycles followed polynomial models, the irrigation season could be reduced to simpler linear models. An interesting mathematical result from the seasonal linear model showed that lagoon water samples from early July, midway in the irrigation season, represented the average N and P levels for the season, therefore, making it possible to estimate N and P loads using the single analysis data without fitting the data to a seasonal model and calendar table. The accuracy of the single early July analysis approach without curve fitting and a calendar table was, however, dependent on lagoon water volumes being uniformly distributed throughout the irrigation season. Fitting a curve and producing a lagoon-specific calendar table was more accurate for estimating nutrient loads when irrigation events and volumes were not evenly distributed through the season. Both methods were more accurate than using a single analysis from early or late season nutrient concentrations.

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