Effect of sodium bicarbonate on rate of passage and degradation of soybean meal in postpartum dairy cows.

Effects of sodium bicarbonate on rate of passage and disappearance of soybean meal from the rumen were determined in a change-over experiment with eight cows. Experimental diets containing 50 or 60% roughage were fed over five 21-day periods with four cows per diet. Sodium bicarbonate at 1.0 and 2.5% (diet dry matter) were changed over in periods 2 and 4, whereas periods 1, 3, and 5 served as control. Rate of passage of soybean meal was measured with chromium-mordanted soybean meal and rate of disappearance by nylon bag technique. Effects of diet were similar for all responses. Response to the two percents of buffer was similar for dry matter intake, milk yield, milk fat, and protein. Percent buffer fed was associated positively with ruminal pH and with disappearance of nitrogen from nylon bags. The 0, 1, and 2.5% of buffer resulted in turnover rates of mordanted soybean meal of 8.22, 9.80, and 10.52%/h, but degradation of protein remained relatively constant at 36.0, 38.4, and 38.2%. The influence of rate of passage on ruminal degradability is discussed.