Daily variations in milk urea, protein and lactose concentrations and their interrelationships were investigated in 36 and 40 moderate and high milk producer cows, respectively. Milk samples were collected from the morning and afternoon milking yield for up to one week from Holstein dairy cows in Urmia, Iran. The overall mean milk urea nitrogen (MUN), milk protein and lactose concentrations were 19.7 mg/dL, 3.6 g/dL and 4.13 g/dL, respectively. Daily mean concentrations for the milk parameters revealed the highest concentrations for MUN and milk protein in the afternoon and morning milking, respectively. Milk lactose was constant for both milking yields. Significant differences (p<0.01) were found for milk indicators for the moderate and high producers, morning and afternoon milking (p<0.01). The values for each cow within sampling times were also different (p<0.01). Positive correlations (p<0.01) were found between MUN/protein, MUN/lactose and protein/lactose in the morning, afternoon and daily milking except for protein/lactose in the afternoon milking. It was concluded that the concentrations in moderate producer cows were greater than in high producer cows. The values for MUN and milk protein were different between the afternoon and morning milking. MUN showed the highest relationships among milk parameters. Thus, MUN might be considered as a useful index in studies related to milk production and reproduction performance in cows

Monthly and seasonal variations among dietary, blood and milk parameters and their interrelationships were investigated during the lactation period in Holstein dairy cows in order to determine reliable dietary indices that could be useful in the studies related to milk yield improvement. Samples were prepared monthly, up to 11 months of lactation and were analyzed by the current laboratory methods using appropriate kits. Mean monthly and seasonal comparison of the dietary, blood and milk values were different among sampling times (p<0.05). The overall mean for dietary energy (ME), crude fiber (CF), moisture, ash, phosphorus (DP), calcium (DCa), ether extract (EE) and crude protein (CP) were 1.38 Mcal/KgDM, 20.4%, 42.8%, 5.5%, 0.34%, 0.76%, 2.39% and 10.2%, respectively. Correlations were found between dietary DP/Ca (r=-0.73), DP/EE (r=0.67), DP/ash (r=0.79), DP/moisture (r=0.84), DP/ME (r=0.60), Ca/ash (r=0.95), Ca/moisture (r=-0.64), and DCa/moisture (r=-0.93). Correlations among dietary, blood, and milk parameters revealed different relationships with the most important correlations between DP and blood parameters, and CP with blood and milk parameters. Among blood parameters glucose, BHB, SK, AST, GGT, neutrophils and lymphocytes. Milk parameters: Mg, Ca, and lactose showed the strongest relationships with dietary parameters. In conclusion, the dietary, blood and milk values were different during the lactation months and seasons. Dietary CP, Ca and DP could be applied as relevant indices in studies related to milk production improvement in dairy herds