Short communication: Effects of drying and analytical methods on nitrogen concentrations of feeds, feces, milk, and urine of dairy cows

摘要

Nitrogen concentrations in feeds, feces, milk, andurine samples were measured using 2 analytical methodsfollowing different drying procedures. Ten samplesof corn silage, alfalfa silage, and concentrates collectedfrom 2017 to 2018 at Krauss Dairy Research Center,The Ohio State University (Wooster), were used. A 4-dtotal collection digestion trial provided fecal samplesfrom 10 cows (1 sample/cow), and another 10 cowswere used to collect milk samples (1 sample/cow) andspot urine samples (1 sample/cow). Spot urine sampleswere acidified immediately to pH ＜3.0 when collected.Feed samples were oven dried (55°C) or lyophilized andanalyzed using the Kjeldahl (KJ; copper sulfate as acatalyst) method and a combustion method (elementalanalyzer; EA). Feces, urine, and milk samples wereanalyzed for N using the following methods: (1) freshsamples by KJ (referred to as wet KJ), (2) lyophilization(urine and milk for 8 h; feces for 120 h) followedby EA (LYO-EA), and (3) oven drying (milk and urinefor 1 h; feces for 72 h at 55°C) followed by EA (ODEA).Additionally, changes in N content of acidifiedurine at −20° over 180 d of storage were examined.Nitrogen concentrations in corn silage, alfalfa silage,and concentrates were greater for EA by 6.1, 4.8, and8.3%, respectively, compared with KJ. Analysis ofdried samples via EA compared with wet KJ resultedin lower fecal N content (27.8 vs. 29.3 g/kg of DM).Nitrogen concentration in fecal samples via KJ afterlyophilization was lower by 5% compared with wet KJbut did not differ from LYO-EA, suggesting that Nlosses occurred during drying. Nitrogen determinationwith EA after drying of samples resulted in greatermilk N (5.70 vs. 5.50 g/kg) and urinary N (9.16 vs.9.06 g/kg) content compared with wet KJ. However,drying method (i.e., lyophilization vs. oven drying) didnot affect N content of milk, urine, or feces. The use ofEA resulted in lower percentage deviation of N contentfrom duplicate sample assays for most samples (no differencewas found for concentrate and fecal N), suggestingthat EA was more precise than KJ. In conclusion,drying of feces caused N losses regardless of dryingmethods. For urine and milk samples, if drying is necessary(i.e., EA), oven drying at 55°C can be used ratherthan lyophilization. The N content was greater in feeds,milk, and urine when determined with EA versus KJ.In addition, N content in acidified and undiluted urineat −20° changed and should be analyzed within 90 d ofstorage. The results in the current study, however, didnot account for laboratory-to-laboratory variation.