Potential Reduction of Forage and Rumen Digesta Particle Size by Microbial Action

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Abstract
The direct effects of microbial action on particle size of feed and rumen digesta samples were examined. The 1st experiment examined reduction of particle size during 48-h in vitro fermentation of rumen digesta from steers fed alfalfa hay or hay-concentrate diets. Distributions of particle size of material recovered on sieves were not altered significantly. A 2nd in vitro experiment studied fermentation of a size-defined fraction of coarsely ground alfalfa hay. Over 50% of substrate dry matter and 37% of insoluble dry matter were digested after 48 h. Logarithm base 10 of mean particle size was not changed, but log10 standard deviation increased linearly with incubation time. This was interpreted to indicate that enough size reduction occurred to skew the distribution of particles without changing the mean. In a 3rd experiment, the same hay fraction was incubated for 96 h in situ. Over 60% of substrate dry matter and 51% of insoluble dry matter and 51% of insoluble dry matter were digested in 48 h. Logarithm base 10 of mean particle size decreased with time of incubation, but log10, standard deviation was not affected. The 19% reduction of mean particle size in situ made only a minor direct contribution to total reduction of size necessary before passage from the rumen would be likely.