In spring, dairy cows are often gradually transitioned from a TMR to a pasture-based ration. This study aims to investigate how the populations of the liquid (LAB), particle (PAB) and epithelium (EAB) associated bacteria in the rumen are influenced by this nutritional change. A 10-wk trial (w1–10) was performed in spring 2014, including 10 rumen-fistulated dairy cows of the German Holstein breed (182 ± 24 d in milk, 23.5 ± 3.5 kg milk/day, means ± SD). The cows were divided into a pasture- and a confinement group (PG and CG, n = 5). The CG stayed on a TMR-based ration (35% corn silage, 35% grass silage, 30% concentrate; DM basis), while the PG was gradually transitioned from a TMR- to a pasture-based ration (w1: TMR-only, w2: 3 h/day on pasture, w3 and 4: 12 h/day on pasture, w5–10: pasture-only). In w1, w5 and w10 of the trial samples of solid and liquid rumen contents were taken and after emptying the rumen, papillae biopsies were taken from the ventral rumen sac. The bacterial DNA was isolated from each sample and a SSCP-analysis was performed. Similarity and cluster analysis was done using GelComparII. Data were analyzed as repeated measures using PROC MIXED of SAS Enterprise Guide 6.1. Cluster analysis displayed a clustering of LAB, PAB and EAB in both ration types and all three points in time. LAB and PAB had a similarity of 82 ± 5% (means ± SD) and differ strongly from the EAB with a similarity of 39 ± 8% EAB compared with LAB, and 37 ± 8% EAB compared with PAB (P < 0.001). When comparing the samples of the different bacteria populations in w5 and w10 to their reference sample in w1, a significant greater decrease in similarity over time in all three bacteria populations was observed for the PG compared with the CG (P < 0.01; illustrated in Table 1). To further identify the different bacterial populations 16S DNA sequencing of all samples is currently being performed. Table 1. Change in bacterial populations of the LAB, PAB, and EAB over time expressed in average similarity (in %) of samples compared with their reference sample in week 1 (means ± SD)

	LAB CG	PAB CG	EAB CG	LAB PG	PAB PG	EAB PG
Week 5	93 ± 6%	97 ± 2%	91 ± 4%	76 ± 10%	86 ± 8%	82 ± 8%
Week 10	91 ± 3%	94 ± 2%	88 ± 4%	61 ± 13%	85 ± 8%	67 ± 8%