Abstract: The aim of the study was to determine the dietary nitrogen conversion efficiency of dietary nitrogen, and analyze its influential factors and provide the basis for improving the dietary nitrogen conversion efficiency in lactating cows. Dietary nitrogen conversion efficiencies were measured in 18 lactating Holstein herds distributed in 7 farms. The factors influencing the dietary nitrogen conversion efficiencies were identified by principal component analysis (PCA). The results showed that the average daily milk yield of the 18 herds ranged from 6.00 to 28.40 kg·d^-1 and dietary CP (crude protein) ranged from 11.47% to 18.34%. The nitrogen balance analysis results indicated that CP nitrogen intake ranged from 185.72 to 653.00 g per cow per day, CP in feces comprised 25.58%-42.73% of dietary CP, that in urine comprised 25.04%-74.65% of dietary CP, that in the milk comprised 12.26%-27.99% of dietary CP. The true protein intake ranged from 160.06 to 582.20 g·d^-1 per cow, and 24.10%-43.18% were secreted in feces, 29.17%-75.54% were excreted in urine, and 10.77%-30.98% were used for milk production. The percentage of nitrogen in feces and urine was negatively correlated with milk yield, and that in milk was positively correlated with milk yield. Dietary CP level, DMI and milk yield were the major factors affecting dietary CP nitrogen conversion efficiency for milk protein synthesis. The multiple regression equation was MNE_CP(%) = 36.96-1.45 CP (%)-0.83 DMI （kg·d^-1）+ 1.11milk yield（kg·d^-1），R²=0.87. The equation was verified using 21 sets of data collected from published paper, and the difference between the measured and predicted efficiencies was non-significant (P>0.05). The results indicated that the crude protein conversion efficiency was 12.26%-27.99% in lactating dairy cows, true protein conversion efficiency was 10.77%-30.98%. Dietary protein conversion efficiency was mainly affected by lactation milk yield, dietary CP content and DMI, which was positively correlated with milk yield, but negatively correlated with DMI and CP content. The results will provide the basis for the application of technologies to improve the dietary nitrogen conversion efficiency for milk protein synthesis in lactating cows.

Key words: lactating cow, dietary nitrogen, nitrogen conversion efficiency, milk protein