Size and structure of leaf and stalk components of digesta regurgitated for rumination in sheep offered five forage diets

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Abstract
A microscopic examination was undertaken of particulate dry matter (DM) in digesta collected from oesophageal fistulae of sheep during rumination. The digesta was separated onto sieves with 4, 2, 1, 0.5, and 0.25 mm apertures by wet-sieving before examination. The principal parameters measured were: the proportions of leaf, stalk, sheath, and cuticle; the length and width of particles; number of sclerenchyma and vascular bundles; and the number and length of fractures in leaf and stalk particles across all sieves for the five feeds studied. There were no differences in any of these parameters between regurgitated (pre-chewed) and ruminated (chewed) digesta, although particle size reduction did occur as a consequence of rumination. Leaf particles accounted for less than 5% of digesta with fresh lucerne (Medicago sativa L.) (FL) and lucerne chaff (LC) despite an initial content in the feed of 65 and 43% respectively. In contrast, perennial ryegrass (Lolium perenne L.) (PR) leaf and leaf from meadow hay (MH) accounted for 35-50% of digesta particles, only slightly less than the leaf content of the feeds themselves. Red clover (Trifolium pratense L.) (RC) was intermediate between PR and FL in leaf loss. Perennial ryegrass leaf had the highest concentration of vascular and sclerenchyma bundles, especially in small particles. Fractures in articles of PR leaf were parallel to the bundles, in contrast to PR stalk and the leaf and stalk of the other feeds where fractures occurred in all directions. These structural aspects of PR leaf suggest a high resistance to particle size reduction which could account for the slow rate of rumen clearance, characteristic of ryegrass diets. Microscopic measurement of particle length and width showed that particles passing through sieves had a length 1.4-2.0 times the sieve aperture size (side of square hole), so that the true upper limit to the length of particles leaving the rumen of sheep is about 1.8-2.0 mm.