Use of manure concentrations of ash or specific minerals and nitrogen to estimate loss of volatile nitrogen from manure incubated under laboratory conditions

Abstract

Changes in the ratio of volatile N to non-volatile specific minerals or total ash in manure can potentially be used to estimate losses of N from manure. The aim of this study was to evaluate the accuracy of using specific minerals (P, K, Ca, Mg, and Na) or total ash as markers to estimate volatile N losses from incubated manure slurries. Holstein cows were fed diets with the forage as predominantly corn silage (CS) or alfalfa silage (AS) or a diet identical to the CS diet except 5g/kg urea was added (CSU). Total output of urine and feces were measured and used to prepare two types of slurry mixtures (all slurry mixtures contained 1200g of manure per tray): (1) as excreted (AsEx) and (2) AsEx+sand (SAND). The SAND slurry was designed to mimic manure from barns that used sand as a bedding material. Feces and urine were mixed in the same proportion as excreted by the cows for both slurry types and 240g of sand were added to the SAND slurries. Initial and final (3d incubation period) slurry weights and concentrations of N, ash, and specific minerals were measured to calculate loss of volatile N. Losses of volatile N from the slurries were estimated as: (N intake−N milk)−[N/ash×(ash intake−ash milk)], where N and ash intake=g/d consumed by the cow; N and ash milk=g/d secreted in milk; and N/ash=ratio of N and ash concentrations in slurry samples at 3d. To evaluate specific minerals as markers, the same equation was used except that Ca, K, Mg, Na or P replaced the ash term. Measured N losses from the AsEx slurries were 0.44, 0.62, and 1.03g/3d for the AS, CS, and CSU treatments and estimated N losses (using N/ash) were 0.29, 0.49, and 1.14g/3d. Estimated losses were less than measured losses for cows with negative N balance and greater than measured losses for cows in positive N balance. For AsEx slurries, the use of N to specific mineral ratios was generally less accurate than using the N/ash ratio. Estimated N losses from SAND slurries were extremely inaccurate for all markers. The use of the N/ash ratio to estimate N volatilization from manure shows promise but markers that are in appreciable concentrations in bedding material will not be accurate.