The assessment of handling time in ruminants: the effect of plant chemical and physical structure on the rate of breakdown of plant particles in the rumen of mule deer and elk

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Abstract
The rate of breakdown of plant particles in the rumen of mule deer (Odocoileus hemionus) and elk (Cervus elaphus) was investigated to determine if plant chemical or structural composition affects processing or handling time in ruminants. Two hypotheses initially tested were that particle breakdown rates in the rumen were (i) directly related to their lignin concentration and (or) (ii) inversely related to their neutral detergent fiber concentration. The relative breakdown rate of a variety of dicot and monocot leaves and stems was an inverse function of lignin concentration, not a direct function as has been hypothesized. Neutral detergent fiber concentration was inversely related to breakdown rate. However, inconsistencies in the relation suggested a third hypothesis: the breakdown rate of the particles is directly related to mean cell wall thickness. Mean cell wall thickness was more closely related to differences in breakdown rate than any of the chemical variables measured and the relations between chemical variables and breakdown rate were a result of their covariation with mean cell wall thickness. We suggest that ruminants should select forages based in part on mean cell wall thickness and that handling time may have a significant effect on forage selection and intake.