The effect of monensin supplementation on ruminal ammonia accumulation in vivo and the numbers of amino acid-fermenting bacteria.

When nonlactating Holstein cows (685 +/- 59 kg) were fed chopped timothy hay (9% CP, 7.0 kg/d) 12 times daily, the steady-state ruminal ammonia concentration was 2.6 mM, and the specific activity of ammonia production by mixed ruminal bacteria was 27.4 nmol/mg of protein-1.min-1. The addition of soybean meal (53% CP, 1 or 2 kg/d) to the basal diet caused a linear increase in ruminal ammonia (7.0 and 12.4 mM, respectively; P < .001), but there was only a small increase in the specific activity of ammonia production (30.7 and 33.8 nmol/mg of protein-1.min-1, respectively; P < .05). The addition of monensin (350 mg/d) to the diets caused more than a 30% decrease (P < .01) in ruminal ammonia at all levels of soybean supplementation, and there was a similar decrease (P < .001) in the specific activity of ammonia production. Before monensin addition, the most probable number of bacteria that could utilize peptides and amino acids, but not carbohydrates, as an energy source for growth was 5.8 to 7.0 x 10(6)/mL. When monensin was added to the diets, these bacteria decreased (P < .001) nearly 10-fold. Based on these results, it seemed that monensin inhibited highly active amino acid-fermenting ruminal bacteria, and this inhibition, in turn, decreased ruminal amino acid deamination and ammonia production. Because monensin did not increase soluble protein, peptides, or amino acids in ruminal fluid, it did not seem that the decrease in ammonia increased flow of dietary amino N to the lower gut.(ABSTRACT TRUNCATED AT 250 WORDS)