Release of cell contents and comminution of particles of perennial ryegrass herbage during ingestion by dairy cows fed indoors or grazing A. Boudon*, A. Acosta†, R. Delagarde* and J.-L. Peyraud* *Unite ´ Mixte de Recherches INRA/Agrocampus Production du Lait, Institut National de la Recherche Agronomique, St-Gilles, France, and †Facultad de Agronomı ´a, Universidad de Buenos Aires, Ciudad de Buenos Aires, Argentina Abstract The effect of feeding indoors fresh perennial ryegrass vs. grazing on ingestive behaviour, release of cell contents and comminution of particles during ingestion, as well as on gas production of ingested boli fermented in vitro, was studied. Indoor feeding and grazing were compared using four dairy cows according to a triple reversal design with six periods. Chemical and morphological composition of the ingested herbage was similar for both indoor feeding and grazing treatments. The intake rate was markedly higher indoors compared with grazing [52Æ1 vs. 22Æ9 g dry matter (DM) min )1 ] with heavier boli and less saliva added per gram of DM intake. The proportions of intracellular nitrogen and chlorophyll released during mastication after ingestion of herbage fed indoors were lower, and the median size of the particles in the boli was larger (5Æ97 vs. 4Æ44 mm) compared with grazing. As a result, the rate of gas production in vitro was also lower for herbage fed indoors compared with grazing (0Æ423 vs. 0Æ469 mL min )1 g )1 incubated DM). Indoor feeding or grazing may have limited consequences in vivo on the kinetics of availability of nutrients for micro-organisms in the rumen, because the consequences of the more extensive physical damage suffered by herbage ingested at grazing could be compensated by a lower intake rate. Keywords: dairy cows, ingestive behaviour, grazing, rumen fermentation, mastication Introduction In ruminants fed forages, ingestive mastication is the first step leading to physical damage to the plant structure, which then determines the availability of forage nutrients for micro-organisms in the rumen. This step could be particularly important for grazed forages that have around half of their dry matter (DM) in the form of cell contents, mostly nitrogenous compounds and soluble carbohydrates. These intracellular constit- uents (ICs) need to be released from living plant cells by mastication before becoming available for the micro- organisms in the rumen. Moreover, the cell walls are partly enclosed in physical barriers, such as the cuticle, that needs to be altered by mastication to facilitate the colonization of the plant tissue by micro-organisms (Pond et al., 1984). Mastication during ingestion of herbage releases more than 0Æ50 of the soluble carbo- hydrates and 0Æ30 of the intracellular nitrogen (Mangan et al., 1976; Boudon and Peyraud, 2001). It would also appear to be responsible for 0Æ25 of the breakdown of long particles in forages, i.e. the particles collected on a 1Æ18-mm sieve, in cattle fed dried herbage (McLeod and Minson, 1988). It remains difficult to explain between-forage variab- ility in the proportions of ICs released during ingestion and the average particle size in the ingestive bolus. For instance, the proportions of ICs released increased with increasing stage of maturity of fresh ryegrass offered to cows in the experiment of Boudon et al. (2002b). However, the proportions of ICs released were not affected by the stage of maturity in the experiment of Boudon and Peyraud (2001) where the difference in the stage of maturity was larger. In the same way, the differences in particle size in the bolus observed between forages are often difficult to relate to the supposed shear tenacity of the forage (Lee and Pearce, 1984; McLeod and Minson, 1988). This may be because cattle adapt their chewing behaviour to compensate Correspondence to: A. Boudon, Unite ´ Mixte de Recherches INRA/Agrocampus Production du Lait, Institut National de la Recherche Agronomique, 35590 St-Gilles, France. E-mail: anne.boudon@rennes.inra.fr Received 17 October 2005; revised 27 January 2006 Ó 2006 The Authors Journal Compilation Ó 2006 Blackwell Publishing Ltd. Grass and Forage Science, 61, 205–217 205