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The objective of this study was to investigate the effect of NDF content in highly digestible grass/clover silage on particle size reduction, ruminal stratification, and selective retention in dairy heifers. The reduction in particle size from feed to feces was evaluated and related to feed intake, chewing activity, and apparent digestibility. Four grass/clover harvests (Mixtures of Lolium perenne, Trifolium pratense, and Trifolium repens) were performed from early May to late August at different maturities, at different regrowth stages, and with different clover proportions, resulting in silages with NDF contents of 312, 360, 371, and 446 g/kg DM, respectively, and decreasing NDF digestibility with greater NDF content. Four rumen-fistulated dairy heifers were fed silage at 90% of ad libitum level as the only feed source in a 4 × 4 Latin square design. Silage, ingested feed boluses, medial and ventral ruminal digesta, and feces samples were washed with neutral detergent in nylon bags of 10-μm pore size, freeze dried, and divided into small (0.212 mm), medium (0.212 to 1 mm), and large : LP;1 mm) particles by dry-sieving. Chewing activity, rumen pool size, and apparent digestibility were measured. Intake of NDF increased linearly from 2.3 to 2.8 kg/d with greater NDF content of forages (P = 0.01), but silages were exposed to similar eating time (P = 0.55) and rumination time per kg NDF (P = 0.35). No linear effect of NDF content was found on proportion of LP in ingested feed boluses (P = 0.31), medial rumen digesta (P = 0.95), ventral rumen digesta (P = 0.84), and feces (P = 0.09). Greater proportions of DM (P0.001) and particulate DM (P = 0.008) were found in medial ruminal digesta compared with ventral rumen, and differences in DM proportion increased with greater NDF content (P = 0.02). Particle size distributions were similar for digesta from the medial and ventral rumen regardless of NDF content of the silages (P0.13). The LP proportion was30% of particles in the ventral and medial rumen, whereas in the feces, the LP proportion was2%. Particle size stratification of the rumen was undetectable regardless of NDF content of the silages, stressing that the retention mechanism of large undigested particles lies elsewhere than with particle entrapment in the rumen mat. In this study, forage particle breakdown, ruminal stratification, and retention of particles in the rumen were not affected by NDF content of highly digestible grass/clover silages.

This study was conducted with the objective to validate a rumination monitoring system (RMS). The RMS was developed by SCR Engineers Ltd. (Netanya, Israel) to record individual rumination times in commercial dairy herds. Four heifers were fed 4 forage types at 3 feeding levels in 4 periods in a Latin square design. The forage types were spring growth of grass/clover harvested early (May 9) and late (May 25) conserved as silage and hay, creating 4 forage types: early harvest silage (ES), early harvest hay (EH), late harvest silage (LS), and late harvest hay (LH). Feeding levels were ad libitum, 90% of ad libitum, and 60% of ad libitum. Data were collected for 24 h at ad libitum feeding level, for 96 h at 90% of ad libitum, and for 24 h at 60% of ad libitum. Rumination time was recorded by the RMS in minutes per 2-h interval (RT(RMS2_i)) and per 24-h interval (RT(RMS24)). As a reference method, rumination time was recorded by chewing halters identifying rumination time by jaw movements (JM) and was accumulated into minutes per 2-h interval (RT(JM2_i)) and per 24-h interval (RT(JM24)) over the same time intervals as RT(RMS2_i) and RT(RMS24). Differences between RT(RMS2_i) and RT(JM2_i) were observed for EH, LS, and LH, with differences of 3.4 (P0.001), 1.1 (P = 0.03), and 3.3 (P0.001) min per 2-h interval. In addition, feeding restrictively at 90% and 60% of ad libitum resulted in greater rumination time recorded by the RMS compared to JM, with differences of 2.6 min per 2-h interval (P0.001) for 90% of ad libitum and 1.6 min per 2-h interval (P = 0.01) for 60% of ad libitum. Further, the differences between rumination times recorded by the RMS compared to JM were different among the 4 heifers. Despite these effects, rumination time recorded by the RMS correlated well with rumination time recorded by JM with correlation between RT(RMS2_i) and RT(JM2_i) of r = 0.91 (P0.001) and between RT(RMS24) and RT(JM24) of r = 0.79 (P0.001).