Seven double-muscled Belgian Blue bulls (initial BW: 341 ± 21 kg) with cannulas in the rumen and proximal duodenum were used in an incomplete replicated Latin square. The study examined the effect of imbalance between energy and N in the rumen on microbial protein synthesis and N metabolism by giving the same diet according to 3 different feeding patterns. The feed ingredients of the diet were separated into 2 groups supplying the same amount of fermentable OM (FOM) but characterized by different levels of ruminally degradable N (RDN). The first group primarily provided energy for the ruminal microbes (12.5 g of RDN/kg of FOM), whereas the second provided greater N (33.3 g of RDN/kg of FOM). These 2 groups were fed to the bulls in different combinations with the aim of creating 3 levels of imbalance (0, 20, and 40 g/ kg of DM) between energy and N supplies in the rumen. Imbalance was measured by the variation of the degradable protein balance (OEB value in the Dutch system) of the diet between the 2 meals each a day. Diurnal variations in ruminal NH 3 -N concentrations and plasma urea concentrations were greatly influenced by the feeding patterns of the diet. Introduction of imbal-

In a previous study, Valkeners et al. (2004) reported that feeding a concentrate-based diet with an imbalance between energy and N release in the rumen did not greatly influence the N balance of double-muscled Belgian Blue (dm-BB) bulls if daily ruminal degradable N (RDN) and fermentable OM (FOM) ratio did not exceed 6.2 g RDN/kg FOM. The objective of the present study was to examine the effects of a higher level of imbalance between energy and N release in the rumen on microbial protein synthesis and N metabolism in dm-BB bulls fed maize silage based diet.