Exploring Milk Urea-N Excretion as a Nutritional and Environmental Management Tool for the Dairy Industry

Abstract Text: In controlled experiments, concentration of urea-N in milk, commonly referred to as milk urea-N (MUN), has been highly correlated with dietary CP level, N use efficiency (NUE, milk-N/intake-N) and urinary urea-N excretion (UUNE). However, under field conditions, variations due to non-nutritional factors (e.g., sampling type, frequency of milking, milk production) and lack of information about N intake the day of DHI testing have lessened the value of MUN as a management tool. This study explored the use of milk urea-N excretion (MUNE) as a UUNE predictor, and the use of MUNE per unit of milk protein yield (PY) as an indicator of NUE. Using DHI (AgSource-CRI) records from 2005 to 2013, we determined the association between PY and MUN or MUNE (calculated as MUN x milk production, regardless of sampling type). Then, we studied the relationship between MUNE/PY and NUE, and between MUNE and UUNE with data of a 128-cow nutritional study. Finally, we compared the MUNE/PY of DHI herds to the values obtained from the nutritional study. Regression analysis of approximately 1.5 million test-day MUN records of 529 DHI herds indicated no linear (P=0.08) relationship between PY and herd-level MUN, but the relationship between PY (g/d) and MUNE (mg/d) was described as: PY = 0.173 + 4.19xMUNE (r²=0.995, P<0.001) for PY in the range of 600 to 1300 g/d. In the nutritional study, MUNE was calculated as average morning and evening MUN weighted by milk production at each milking. The relationship between UUNE and MUNE was described as: UUNE (g/d) = 26.8xMUNE (r²=0.991, P<0.001) for UUNE in the range of 40 to 125 g/d. Furthermore, MUNE/PY increased from (mean±SE) 1.1±0.3, to 2.2±0.3, 3.6±0.5 and 4.3±0.3 mg/g as NUE decreased linearly (P<0.01) when mid-to-late lactation cows were fed diets of 11.8, 13.1, 14.6 and 16.2% CP (DM basis), respectively. In the DHI database, however, MUNE/PY averaged 4.6±0.01 mg/g, but ranged from 2.5 to 6.7 mg/g for the 47 herds for which MUNE could be calculated as in the nutritional study. In conclusion, MUNE may be used as a reliable predictor of UUNE and indicator of NUE. Comparing MUNE/PY of the nutritional study to MUNE/PY of selected Wisconsin dairy herds suggested that producers could feed cows to increase NUE and lower UUNE simultaneously across a wide range of PY. Additional research is needed to ascertain the usefulness of MUNE and MUNE/PY as management tools.

Keywords: Protein Nutrition, Nitrogen, environment.