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Milk urea nitrogen (MUN) and milk protein (MP) are increasingly used as indicators of the protein-energy balance and the actual faun feeding practice, and also are related to nutritional and reproductive disorders. The purpose of this study was to investigate the MUN and MP concentrations by month and geographical distribution in individual and bulk milk, and to evaluate the protein-energy balance for feeding according to the level of MP and MUN by stage of lactation in Holstein cows. MP and MUN contents were determined using automated infrared procedures. Mean MUN and MP concentrations in the bulk milk samples obtained from Jan. 1999 until Dec. 1999 in the 26,467 cows of 1,077 herd were 16.9±4.2㎖/㎗ and 3.18±0.18%, respectively. The highest values were found during spring and lowest values during winter in MUN. But, the average contents of MP were the highest values during winter and the lowest ones during summer. Out of the total herd surveyed, 10.7% had MUN values lower than 12.0㎖/㎗ and 39.7% had values higher than 18.0㎖/㎗, and 53.0% had MP values outside the standard interval (3.0-3.2%). In order to evaluate protein-energy balance for feeding, we set the level of standard range of MP of 2.90-3.29% in early lactation considering dairy cows to experience a negative energy balance and the level of MP in mid-lactation and late lactation were 3.10-3.49%, and 3.30-3.69%, respectively. Standard MUN of 12-18㎖/㎗ was applied through the whole lactation because it is relatively constant without being affected by days in milk. Milk yield, body condition score, and milk composition were analyzed according to the 9 types based on the levels of MP and MUN. The lower body condition score than the recommended level by stage of lactation was associated with low MP, high MUN content and decreased milk yield. Out of a total, 26.8%, 25.8%, and 22.2% had protein-energy balance for feeding by stage of lactation, respectively. Therefore, when MUN and MP analysis is used for gross diagnosis of protein-energy balance and feeding management practice at the farm level.
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