Effects of Particle Size and Forage Composition on Functional Specific Gravity

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Abstract
Functional specific gravity of forage particles influences the rate at which they pass from the rumen. Effects of forage constituents (neutral detergent fiber, acid detergent fiber, lignin, protein, fat and ash) and particle size on change of functional specific gravity in vitro were investigated. Rate of change of functional specific gravity was higher in legumes than in grasses. Hay samples were ground through 1-, 2-, 4- and 6-mm screens in a Wiley mill and immersed in water. Specific gravity measurements were at 1, 2, 4, 6, 8, 10, 12, 24 and 48 h. Specific gravities of samples with small particles were higher at all times than those of samples with large particles. Particle size influenced rate of change of specific gravity. Specific gravities of 4 hays that had been placed in the rumen in nylon bags for 1, 2 and 4 h were measured by flotation technique. Prior to incubation in the rumen, approximately 80% of the particles were lighter than 0.9 specific gravity in 3 of the forages. After 1 h in the rumen, no more than 28% of any of the samples floated in the 0.9 specific gravity solution. Specific gravity of ground forage samples changed rapidly in vivo.