To investigate the factors affecting the milk urea nitrogen (MUN) in Chinese Holstein cows, a large commercial dairy farm participated in a 30-mo study. 103 593 test-day records were used for the analysis. In this study, the mean MUN concentration was 11.75 mg/dL (Table 1). The MUN reached the maximum value on 90 d of lactation for the first parity and the third or greater parities (P < 0.05), but it peaked at the end of lactation during the second parity. The MUN of the first parity was lower than other parities. The MUN showed its minimum level in January (P < 0.05), and reached its maximum in July (P < 0.05). The MUN of cow calving in summer was higher than other seasons at the first month of lactation, while the MUN of cow calving in autumn was significantly lower than other seasons at the fourth month of lactation (P < 0.05). The positive correlations were observed between daily milk yield, net energy for lactation, crude protein and MUN for the first and third parities, but negative correlations were observed in the second parity. The MUN showed significantly positive correlations with fat content, total solid content, and dry matter intake for all parities (P < 0.01). The negative correlation was observed between MUN and protein content, with the exception of the second parity. For all data, as MUN concentration increased, milk protein content decreased. It has been recommended that MUN concentration should be evaluated in association with parity, days in milk, season (or month), day matter intake and dietary nutritional components, to improve the management and economic benefits of dairy farm. Table 1. The mean and SD of DHI variable for Chinese Holstein cows

Variables	N	Minimum	Maximum	Mean	SD
DIM	103,384	1.00	999.00	181.57	129.56
Parity	103,593	1.00	5.00	1.59	0.81
Daily milk yield, kg/d	103,593	8.00	65.00	29.63	9.46
Fat, %	103,593	2.00	7.00	4.27	0.81
Protein, %	103,593	2.00	6.00	3.37	0.41
SCS	103,593	0.00	9.00	2.13	1.60
Lactose, %	103,593	1.96	5.65	4.94	0.24
Total solid, %	103,593	7.86	20.38	13.76	1.32
MUN, mg/dL	103,593	5.00	30.00	11.75	2.81
DMI (kg)	103,384	13.13	23.83	20.35	2.25
NEL (Mcal/d)	103,384	1.32	60.13	33.52	5.89
NEL (Mcal/kg)	103,384	1.40	2.91	1.64	0.14
CP (%)	103,384	13.20	18.40	16.66	1.18