Quantitation of Microorganisms Associated with the Particulate Phase of Ruminal Ingesta

Abstract

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.