Dietary alteration of particle breakdown and passage from the rumen in lactating dairy cattle.

To evaluate the effect of dietary alteration on the rates of ruminal small particle passage or large particle breakdown, six ruminally cannulated Holstein cows 90 d postpartum were used in a 3 X 3 Latin square design. Experimental diets were 1) control: 60% concentrate, 40% corn silage; 2) bicarbonate: 60% concentrate (containing 2% sodium bicarbonate), 40% corn silage; and 3) hay: 60% concentrate, 30% corn silage, and 10% long alfalfa hay on a DM basis. Dry matter intake (kg/d), milk production (kg/d), percentage milk fat, rumination (min/d), ruminal fluid outflow (L/d), small particle (150 to 850 micron) passage rate (%h), and large particle (greater than 4.25 mm) breakdown rate for diets control, bicarbonate, and hay were 20.2, 20.9, 22.4; 26.6, 27.7, 26.8; 3.5, 3.6, 3.2; 405, 350, 370; 167, 184, 185; 6.5, 7.6, 7.4; and 7.2, 6.8, 7.4, respectively. Rumen DM content and digesta particle size were reduced at the end of a 24-h feeding period. Data suggested that small particle passage may be more closely related to ruminal fluid outflow and DM intake than large particle breakdown rate.