Milk Urea Nitrogen as a Tool to Assess Effi ciency of Nitrogen Utilization in Dairy Cows

Insuffi cient intake of nitrogen (N) penalizes milk produc on and milk protein produc on of lacta ng dairy cows, but the excessive use of protein supplements, especially commercial sources of rumen undegraded protein (RUP), may translate into unnecessary addi onal costs and losses of N to the environment in the form of urinary urea-N. Thus the ques on of how low in crude protein can a diet be formulated to avoid losses of produc on, unnecessary expenses, and unnecessary losses of urinary urea-N? This ar cle focuses on milk urea-N (MUN) as one of the tools available to assess N use effi ciency (NUE) defi ned as the conversion of dietary N to milk N, and more broadly the adequacy of a diet to deliver nutrients (in proper amounts and propor on) to maximize milk protein produc on. Extensive literature of experiments conducted in North America indicated that on a daily basis, the modern lacta ng dairy cow converts 26% of the N consumed into milk N, and she excretes as much urinary urea-N (168 g/d) than she secretes N as milk protein (166 g/d). In general increasing the percentage of dietary crude protein increases N intake and urinary urea-N excre on and decreases NUE, but does not alter milk protein produc on. For its part, MUN refl ects closely the percentage of crude protein in the diet. It is a reliable predictor of urinary urea-N and NUE, but (unfortunately) not a good predictor of milk protein produc on. For example, analysis of data collected on farm by a dairy herd improvement (DHI) associa on from the Midwest of the United States has indicated that milk protein produc on can be near or at maximum for any MUN value ranging from 10 to 16 mg/dL. As opposed to common assump ons, MUN does not refl ect only an excess rumen degradable protein (RDP) in the diet. Milk urea-N should rather be interpreted as an indicator of the overall adequacy of the diet to provide amino acids and energy yielding nutrients (in par cular glucose) that support the most effi cient use of the N consumed by a cow. Data from DHI have demonstrated that test-day MUN is infl uenced substan ally by numerous cow factors (e.g., breed, body weight, parity and gene cs) and many complex interac ons among nutri onal, animal, and managerial factors. As a result, a within-herd baseline may be more appropriate than an industry standard. Nevertheless, our analysis of studies in which Holstein cows were fed diets typical of the Midwest of the United States (corn silage, alfalfa silage, corn grain and protein supplement based on soybean by-products primarily) revealed that maximum protein yield (1.20 kg/d) occurred at MUN of 11.3 mg/dL, with diets of 16.2% crude protein (diet dry ma er basis). Under this dietary situa on, expected urinary urea-N excre on would be approximately 134 g/d, which would be a 25% reduc on compared with the expected 178 g/d in urinary urea-N excre on with diets of 18% crude protein with essen ally no change in milk protein yield. Calcula on of income over feed cost (IOFC) with prices prevailing in the Midwest of the United States between 2007-2015 indicated that maximum IOFC occurred at MUN of 10.9 mg/dL. Due to possible gene c and permanent management eff ects on MUN, producers and consultants are encouraged to determine a reference MUN for a herd when fed a ra on balanced according to NRC standards and experiment with reducing RUP and (or) RDP in order to determine the most appropriate herd-specifi c MUN target.