Animal Feed Science and Technology 123–124 (2005) 139–154 Effects of particle size of a total mixed ration on in vivo ruminal fermentation patterns and inocula characteristics used for in vitro gas production M. Tafaj, Q. Zebeli, B. Junck, H. Steingass, W. Drochner ∗ University of Hohenheim, Institute of Animal Nutrition (450), Emil-Wolff-Str. 10, 70599 Stuttgart, Germany Abstract The objective was to investigate effects of particle size (PL) of a total mixed ration (TMR) on in vivo ruminal fermentation patterns and characteristics of the inocula used for in vitro gas production. A 3 × 3 randomized block design was used with three ruminally cannulated Holstein cows (640 kg BW, 63 DIM, 36 kg milk/d) fed ad libitum with TMR diets consisting of 450 g/kg grass silage (GS) with three PL levels (25, 11 and 5.5 mm), 500 g/kg cereal-based concentrate mixture and 50 g/kg of long hay. Replicates of samples of solid digesta from the dorsal (SRF-T) and ventral (SRF-B) rumen and free fluid (FRL) samples were collected 1 h before and 2, 8 and 12 h after the morning feeding. Samples were analyzed for pH and short chain fatty acids (SCFA), thiamine and protozoa concentrations. The same digesta collected 1 h before the morning feeding was used as inoculum for in vitro incubation in a gas test, and the gas production (GP) profile of all TMR were determined. The SCFA concentration of SRF-T increased with reduction of PL of TMR (P < 0.05), but that of FLR decreased (P < 0.05). There was no effect of PL of TMR on SCFA concentration in SRF-B. Reducing the PL of TMR to 5.5mm decreased ruminal pH and the acetate: propionate ratio 1 h prior to the morning feeding (P < 0.05), while 11 mm did not affect ruminal pH. Ruminal protozoa numbers increased by reducing the PL to 11 mm. These effects of PL on the ruminal milieu are also reflected in the digestive characteristics of inocula used for in vitro incubation. The inocula of SRF-T collected from cows fed TMR of 11 and 5.5 mm PL increased in vitro GP of blank and substrate (TMR) in comparison to the 25 mm PL Abbreviations: TMR, total mixed ration; SCFA, short chain fatty acid; GP, gas production; SRF-T, squeezed digesta fluid from dorsal rumen; SRF-B, squeezed digesta fluid from ventral rumen; FRL, free rumen liquid; PL, theoretical particle size ∗ Corresponding author. Tel.: +49 711 459 2420/3520/3064; fax: +49 711 459 2421. E-mail address: drochner@uni-hohenheim.de (W. Drochner). 0377-8401/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2005.04.032