Digesta Passage Measured by Markers in Dairy Cows Fed Two Ratios of Corn Silage:Grain with 0 or 1.2% Sodium Bicarbonate

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Abstract
Four multiparous Holstein cows in midlactation fitted with rumen cannulae were in a 4 .times. 4 Latin square experiment to examine effects of 2 ratios of corn silage:grain (50:50 or 75:25) and sodium bicarbonate (0 or 1.2% of ration dry matter) on rate of passage of digesta. Marker techniques and mathematical equations were compared. Markers for liquid phase were polyethylene glycol and cobalt-EDTA and for particulate phase were La, Sm, Ce and Cr-mordanted fiber. A single compartment model was used to measure rate of passage of liquid from concentration of the 2 markers in ruminal fluid samples. Two 2-compartment mathematical models were used to calculate rates of passage of liquid, grain and fiber from fecal marker concentration. Ruminal liquid volume, rate of flow and fractional rate of passage were not affected by sodium bicarbonate but increased with greater silage feeding. Ruminal grain and fiber passage were increased when more corn silage was fed. Effects of sodium bicarbonate on ruminal grain and fiber passage were variable, although fiber passage tended to be slowed. Overall, liquid passage exceeded grain passage, which exceeded fiber passage from the rumen. In the total tract, retention time of liquid, grain and fiber was less when diets contained 75% corn silage. The effect of sodium bicarbonate on retention time of liquid, grain and fiber in the total tract was variable. Markers and mathematical calculations caused differences in estimates of rate of passage and interpretation of results.