Initial Particle Form and Size on Change in Functional Specific Gravity of Alfalfa and Timothy Hay

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Abstract
In situ techniques were employed to characterize functional specific gravity change of alfalfa and timothy in the rumen. Alfalfa and timothy hay were chopped at .64-cm theoretical length of cut. Subsamples of each forage were dry sieved through a 3.2-mm screen, yielding particles > 3.2 and < 3.2 mm, and another subsample was grouped through a 5.0-mm screen. Each forage treatment was placed in polyester bags and ruminally incubated for up to 100 h. Functional specific gravity was determined on the original and residues. Mean functional specific gravity increased with time in the rumen for all treatments. Change in mean functional specific gravity was affected by initial particle size of the sample; however, no differences could be attributed to sieving. Mean functional specific gravity increased rapidly for timothy during the first hour of ruminal incubation, an effect not evident for alfalfa. Regardless of whether forages were ground or sieved, distribution of organic matter dramatically shifted from low to high functional specific gravity with time in the rumen. Microbial fermentation and ruminal movement caused considerable increase in organic matter functional specific gravity.