The aim of this study was to validate a mass balance method based on N to ash or N to specific mineral (P, K, Ca, Mg, or Na) concentrations in manure to estimate NH3 losses. Six multiparous Holstein cows were used in replicated 3 x 3 Latin squares with three 14 d periods. Two diets were balanced to contain 15.5% CP but in one the forage (58% of diet DM) was mainly corn silage (CS) and alfalfa silage in the other (AS). A third diet (CSU) was the same as the CS diet except that 0.5% urea was added (17.3% CP). During total collection trials, urine and feces samples were collected to prepare three types of slurry mixtures: 1) as excreted (AsEx), 2) as excreted + sand (Sand) and 3) low urine (Low urine). Both AsEx and Sand slurries were mixed in the same proportion as excreted by the cows (1200 g), but sand (240 g) was added to Sand slurries. Low urine slurries had an averaged 33.4% reduction in the proportion of manure that was urine with a concomitant increase in feces. Slurries were incubated at ambient temperature for 3 d. Initial and final concentrations of N, ash, and specific minerals and slurry masses were used to determine change in N, ash, and specific minerals mass. Nitrogen losses were estimated using the following equation: (N intake – N milk) – N/Ash*(Ash intake – Ash milk), where N and Ash intake and N and Ash milk are g/d and N and ash concentrations in slurry samples at 3 d constitute the N/Ash ratio. For N to specific minerals, the same equation was used except that specific minerals replaced the ash term.
Average daily DM intake and milk yield were 22.4 kg and 32 kg, respectively. Average manure output was 65.4 kg/d across diets. Manure N excretion was highest for cows fed CSU diet followed by CS and AS diets (422, 343 and 392 g/d, P<0.01). Urinary N as a percent of excreted N was 34, 41, 53 % for AS, CS, CSU diets, respectively (P<0.01). Ash excretion in manure was highest for cows fed AS diet (1134 g/d); CS and CSU diets had similar ash excretion (936 g/d) (P<0.01). Urine contributed 46, 40, and 42% of excreted ash for AS, CS and CSU diets.
Measured NH3-N losses (across diets) from the trays were highest for Sand, followed by AsEx and lowest for Low urine slurries. Urine N but not total manure N was positively correlated with NH3-N losses. Ash and specific minerals remained constant in the slurries. The method underestimates NH3-N losses in cows with negative N balance and overestimates it for cows in positive N balance. For Sand and Low urine slurries, the accuracy to estimate N losses using N to ash or specific minerals was generally inaccurate. For N to ash or N to specific mineral ratios to estimate with acceptable accuracy both N and ash or specific minerals need to be in similar balance (% basis).