Optimizing accuracy of measurement protocols for nitrogen application in dilute dairy manure

Variability of dairy process wastewater (PWW) nitrogen (PWW-N) content makes precise fertilizing of forage challenging. Our objective was to improve measurement accuracy of PWW-N. We characterized PWW-N and quantified its variability using over 1,000 PWW samples from 91 lagoons on commercial California dairies. We used statistical modeling and stochastic simulations to compare the accuracy of various protocols for measuring PWW-N applied to forage crops. Electrical conductivity (EC) was positively correlated with N concentration within a lagoon (conditional R²  = .69, mixed model). Simulations compared the accuracy of N application rates when lagoon samples for N analyses were collected more frequently and from more homogeneous lagoons. When sampled quarterly, the N application measurement error was ±50%. Homogenizing PWW and sampling weekly reduced measurement error to ±42 and ±30%, respectively. Simulations using EC as an N concentration proxy within an automated irrigation system reduced N application measurement error to ±5%. More accurate automated N concentration measurements allow for precision N management that meets yield goals and reduces environmental effects.