Composition and digestive tract retention time of ruminal particles with functional specific gravity greater or less than 1.02

The objective of this study was to determine composition, particle size distribution, and in vivo kinetics of ruminal particles having functional specific gravity (FSG) greater or less than FSG of particles found in the omasum and reticulum of lactating dairy cows. Particles from the reticulum and the omasal had FSG of 1.03 and 1.02, respectively. Particles from ruminal contents with FSG higher (HP) or lower (LP) than 1.02 were isolated and labeled with Er or Dy, respectively. Four ruminally cannulated, lactating Ayrshire dairy cows were fed all-grass silage (AS) or 54% grass silage:46% concentrate (SC) diets in a cross-over design trial and used to study chemical composition and ruminal and total tract kinetics of HP and LP. Labeled particles were pulse dosed into the rumen of the cows and disappearance of the markers from ruminal HP and LP pools and excretion in feces was monitored for 72 and 120 h, respectively. Fecal marker excretion data were fitted using two-compartment mathematical age-dependent/age-independent (Gn-->G1) models. Inclusion of concentrate in the diet (SC) increased (P < 0.05) apparent total tract digestibility of dietary DM, OM and N. Digestibility of fiber fractions, NDF and ADF, was lower (P < 0.01 and P < 0.05, respectively) for SC compared with AS. The heavy particles had higher (P < 0.01) indigestible NDF and lower (P < 0.01) N concentration than LP. Particles from the HP pool passed from the rumen more rapidly (P < 0.01) than particles from LP (0.044 and 0.019 h(-1), respectively). Diet had no effect on particle rate of disappearance or pool size in the rumen. Across diets, pool size of LP was consistently larger (P < 0.05) than that of HP. Diet had no effect on total tract mean retention time (MRT) of LP or HP. Total tract MRT of LP was greater (P < 0.05) than MRT of HP (59.6 vs. 49.0 h, respectively). Results from this study support the hypothesis that functional specific gravity is an important factor determining the rate of outflow and residence time of feed particles within the reticulo-rumen and total digestive tract. Our data indicate that digesta particles with functional specific gravity greater or less than 1.02 have different composition and flow characteristics. Heavier particles contain more indigestible fiber and less N and are likely depleted of substrate available for microbial fermentation, are smaller in size, and have a higher passage rate/shorter retention time in the digestive tract than lighter particles.