In Situ Particle Size Reduction of Alfalfa and Timothy Hay as Influenced by Form and Particle Size

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Abstract
Our objectives were to characterize particle size reduction of alfalfa and timothy hay by in situ incubation. Alfalfa and timothy hay were chopped at .64-cm theoretical length of cut. Subsamples were dry sieved through a 3.2-mm screen, yielding particles > 3.2 and < 3.2 mm, and another subsample ground (5.0 mm). Each forage treatment was placed in polyester bags and ruminally incubated for up to 100 h. Original and residues were wet sieved. Change in geometric mean diameter (initial to lowest) with time for both forages was less as particle size decreased from chopped to ground; however, percent change was similar between chopped and ground and between sieved > 3.2 and < 3.2 mm. Regression of geometric mean diameter on time for alfalfa exhibited quadratic relationships for all particle categories. For timothy, ground and sieved > 3.2 mm exhibited linear, whereas chopped and sieved < 3.2 mm exhibited quadratic relationships with time. Large particle fractions for both forages were reduced proportionately more than particle fractions that were small initially. Particle size distribution showed particles > 1.18 mm accounted for 63.0 to 98.4% of the initial DM, which was reduced to 14.9 and 24.1% by 100 h. Microbial fermentation and detrition associated with ruminal movements appear to contribute substantially to particle breakdown.