Species and variety of herbage affects release of cell contents during ingestive mastication in dairy cows fed indoors

To achieve a better understanding of the nutritional value of grazed forages to ruminants, it is necessary to improve our knowledge of the accessibility and nature of nutrients available to micro-organisms in the rumen. The objectives of this experiment were to compare release of cell contents and comminution of particles during ingestive mastication in dairy cows fed four temperate species or varieties of fresh forages, and to analyse the respective effects of chewing behaviour and mechanical properties of plant tissue on release of cell contents. Six Holstein dairy cows were fed indoors with four species/varieties of fresh forages being white clover (WC) (Trifolium repens L., cv Alice), tetraploid perennial ryegrass (tPR) (Lolium perenne L., cv Chéops), diploid perennial ryegrass (dPR) (Lolium perenne L., cv Ohio) and tall fescue (TF) (Festuca arundinacea Schreb., cv Barcel). The design was two 4 × 4 Latin squares with 4 periods of 1 day each and two replications of two treatments on two cows each period. One Latin square was completed in May and the other in June. Ingestive boli were collected directly at the cardia of the cows during the morning meal after manually emptying the rumen. The intake rate averaged 62.5 g DM/min, and chewing behaviour parameters were unaffected by herbage. The average proportions of intracellular constituents (ICs) released during ingestive mastication were 265, 203 and 364 g/kg of ICs ingested for neutral detergent soluble (NDS), intracellular N and water-soluble carbohydrates (WSC), respectively. For most ICs considered, the proportions released during mastication were higher for white clover and fescue compared with ryegrasses. The proportions of intracellular N released were higher for tetraploid compared with diploid ryegrass. The median size of the bolus particles was unaffected by the herbage species or variety. In contrast, the median size of the particles in the boli was strongly affected by individual cow (P<0.01), whereas the proportions of ICs released showed no such effect. It appeared that those cows with the lowest reduction in particle sizes by ingestive chewing were those with a smaller molar surface area. None of the parameters measured (i.e., release of ICs, median size of particles in the boli and chewing behaviour) could be related to the shear properties of the herbage. Results illustrate the complexity of the interaction between the structure of the plant tissue and the physical damage of the particles during chewing.