Technical factors affecting estimation of nitrogen balance of dairy cattle in climate respiration chambers.

Abstract

Nitrogen retention is often overestimated when calculating N balance of dairy cattle. This study determined if different N analysis methods [Kjeldahl (KJEL) or elemental analysis (EA)] and sample collection and storage protocols when using climate respiration chambers (CRC) affected the estimation of N intake, milk N secretion, manure N excretion, volatile N appearance, and N retention. Twenty-eight Holstein-Friesian cows were housed in CRC for a 4-d measurement period. Nitrogen intake was estimated using individual forages (corn silage and grass silage) and mixtures of forages sampled before or after storage at 4°C. Total N content of forages was the sum of NH₃-N content in wet samples and N content in dry samples analyzed by KJEL and EA. Nitrogen content in dry concentrate samples was analyzed by KJEL and EA. Secretion of N in milk was based on N content in wet samples analyzed by KJEL and EA. Manure N excretion was based on the N content of wet manure samples with and without acid preservative analyzed by KJEL and EA. Volatile N consisted of N in condensed water from the CRC heat exchanger analyzed by KJEL, and aerial N. Aerial N was determined using an NH₃ sensor or acid trap analyzed by KJEL. Volatile N was quantified during the 4-d measurement period (cow phase), CRC cleaning (cleaning phase), and between CRC cleaning and the next 4-d measurement period (post-cleaning phase). Nitrogen content, with and without accounting for NH₃-N, was higher in dry silage samples when analyzed using EA compared with KJEL. Nitrogen intake was higher when forages were analyzed by EA but was not affected by forage source (individual forages or forage mixtures pre- or post-storage) or interaction between forage source and N analysis method. Nitrogen analysis method did not affect the N content of milk or manure. Acid addition to manure samples did not affect N content on a wet basis but tended to increase the DM content of manure such that N content on a DM basis was lower in acid-preserved samples. There was no effect of aerial N analysis method (acid trap or NH₃ analyzer) and no interaction between volatile N measurement phase and N analysis method on volatile N appearance. Nitrogen retention was lower when samples were analyzed using KJEL compared with EA (19 and 36 g/d, respectively), attributed to the lower N content of forages when analyzed with KJEL. The measurement phase used for estimating volatile N production did not affect N retention. Overall, N analysis method of forages impacts the estimation of N intake but may be less important for milk and manure in terms of the impact on N retention. When using CRC to quantify volatile N, an acid trap or NH₃ analyzer for aerial N can be used with similar outcomes. Although N analysis by KJEL resulted in lower N retention relative to EA, this may be artificial if KJEL underestimates N content of forages. Overestimation of N retention was not mitigated through the sampling or analytical approaches evaluated in this study.