Factors affecting milk urea nitrogen and protein concentrations in Quebec dairy cows

Milk urea nitrogen (MUN), together with percentage mill;protein (PROT), are increasingly used as indicators of the protein-energy balance and for moni toring nutrition and diagnosing feeding disorders. The goal of this study w as to analyze the effects of parity, milk yield, days in lactation, somatic -cell count, and herd and feeding factors on MUN and PROT. In IO dairy herd s, one milk sample was taken from each of 418 cows, within +/-2 days of the routine milk-test visit. We used a four-step multiple linear-regression mo del with backward elimination, including interactions between herd and the different factors. For both dependent variables, there were significant int eractions with herd. Herd-specific models were markedly different; however, the daily amount of protein concentrates fed remained (and had a positive coefficient) in seven of 10 herd models for MUN. This factor is easy to rec ord under field conditions and has to be considered in the evaluation of th e ration by means of MUN and PROT. Overfeeding of rumen soluble protein can be easily diagnosed and corrected using MUN analyses. The relationships be tween MUN and PROT in respect of the factors parity, daily milk yield, and days postpartum also vary considerably among herds. (C) 1999 Elsevier Scien ce B.V. All rights reserved.