Quantitation of microorganisms associated with the particulate phase of ruminal ingesta

Microbial organic matter associated with rumen particulate and fluid phases was quantified using 15N as the microbial marker in two rumen fistulated cows fed a 65% alfalfa haylage diet. During two collection periods, feed was removed 1 h after initiation of feeding, and cows were dosed with (15NH4)2SO4. Whole rumen contents were sampled before feeding and at various times up to 12 h after feeding. Fluid microorganisms were those that passed through eight layers of cheesecloth. Particle-associated microorganisms were obtained by chilling squeezed particles prior to seven successive extractions with saline solution. The amount of microorganisms removed from particles ranged from 32.1 to 59.9% as measured by 15N. Organic matter (mg/mL strained rumen fluid equivalent) of fluid and particle-associated microorganisms was respectively: 10.7 and 47.5; 12.5 and 35.5; 12.2 and 30.1; 10.7 and 26.1; 10.9 and 26.7; and 8.9 and 20.6, at 2, 3, 4, 7, 9 and 11 h after initiation of feeding. These ratios indicated that 70-80% of microbial organic matter in whole rumen contents was associated with the particulate phase and that particle-associated microbial organic matter was greatest soon after feeding. Analysis of 15N in extracted rumen particles indicated that 50-65% of particle nitrogen and 17-27% of particle dry matter was of microbial origin. These results provide evidence that particle-associated microorganisms make up a major proportion of the total microorganisms in ruminal ingesta and that a large proportion of ingesta particle N can be of microbial origin.