Quantitative aspects of ruminant digestion and metabolism - Phần 2

3 Digesta Flow G.J Faichney School of Biological Sciences A08, University of Sydney, NSW 2006, Australia Introduction The structural carbohydrates that constitute plant fibre represent a major feed resource Herbivorous animals, unable to produce fibre-degrading enzyme systems of their own, have evolved a range of strategies (Hume and Sakaguchi, 1991) to make use of a consortium of microbes, including bacteria, protozoa and anaerobic fungi, for this purpose The strategy adopted by the ruminants involves the development of a compound stomach in which the feed eaten can be fermented by the microbes before being subjected to attack by the animal’s own enzymes and, finally, to a second fermentation in the hindgut before the undigested residues are voided in the faeces This strategy suits the domestic ruminants to the utilization of diets of moderate fibre content for the production of food and fibre and the provision of motive power They are not so well adapted to poor quality diets of high fibre content because the extended time required to break down the fibre for passage out of the stomach severely limits the amount of such diets that can be eaten Thus a knowledge of digesta flow through the ruminant gastrointestinal (GI) tract, and of the factors that affect it, is important because of its role both in the processes of digestion and absorption and in the expression of voluntary feed consumption The Nature of Digesta The ruminant GI tract consists of a succession of mixing compartments – the reticulorumen, abomasum and caecum/proximal colon, in which residues from successive meals can mix – and connecting sections in which flow is directional and axial mixing is minimal Of these latter, the small intestine and the distal colon (consisting of the spiral colon, terminal colon and rectum) are tubular in nature However, the omasum is a bulbous organ whose lumen is largely ß CAB International 2005 Quantitative Aspects of Ruminant Digestion and Metabolism, 2nd edition (eds J Dijkstra, J.M Forbes and J France) 49 50 G.J Faichney occupied by leaves of tissue (the laminae) so that, although particulate matter may be retained between them, little mixing can occur The digesta in the GI tract consist of particulate matter, including microorganisms, and water, in which is dissolved a range of organic and inorganic solutes of both dietary and endogenous origin The relative proportions of these digesta components are different in the different sections of the tract The particles exist in a continuous range of sizes from the very small to pieces of plant material up to several centimetres long that can be found in the rumen when a diet of long hay is given In order to study the characteristics of these particles, various sieving procedures have been devised which divide the continuum of sizes into fractions of defined size range Both dryand wet-sieving procedures have been used but it is now generally accepted that a wet-sieving procedure is preferable for digesta particles (Kennedy, 1984; Ulyatt et al., 1986) However, plant particles are generally elongated, often having a length/width ratio in excess of six (Evans et al., 1973), and there remains uncertainty regarding the relative importance of length and diameter in the separations achieved during sieving McLeod et al (1984) concluded that discrimination in their wet-sieving procedure was mainly on the basis of diameter However, examination of their data indicates that for three of five fractions, particle diameter was less than the mesh size of the sieve which retained them, and particle length was less than the theoretical maximum (Vaage et al., 1984) for particles passing through the particular sieve Thus it seems more likely that, with their technique, discrimination between particles was mainly on the basis of length The technique used by Evans et al (1973) also appeared to discriminate on the basis of length (Faichney, 1986) Particles that pass a sieve of mesh 150 mm are sufficiently fine to behave like solutes (Hungate, 1966; Weston and Hogan, 1967; Kennedy, 1984) but, in the rumen, only a proportion of them flow in the fluid phase (FP) because many are trapped in the ‘filter-bed’ of the reticulorumen digesta mass (Faichney, 1986; Bernard et al., 2000) On the other hand, particles above a certain size are retained in the reticulorumen, few if any being found in digesta distal to the reticulorumen (Ulyatt et al., 1986) This has led to the concept of a critical size above which particles have a low probability of passage from the rumen (large particles) Poppi et al (1980) presented evidence to support the use of a sieve of mesh 1.18 mm to define the critical size for both sheep and cattle Subsequently, Kennedy and Poppi (1984) suggested that different sieve sizes could be used for cattle and sheep on the basis that sieves of, respectively, 1.18 and 0.89 mm mesh would retain 5% of the faecal particulate dry matter (DM) Values of 1.41 mm for grazing cattle and 0.91–1.08 mm for sheep given lucerne hay can be obtained from the data illustrated in Fig 3.1, and a value of 1.2 mm can be obtained for grazing cattle from the data of Pond et al (1984), supporting the suggestion of a real, albeit small, difference in critical size between cattle and sheep It has been claimed that the critical size is not constant but increases when hay is ground and when the level of intake increases (Van Soest, 1982) However, this claim has been challenged (Faichney, 1986) because it was based on an observed increase in faecal mean particle size, a measure that Digesta Flow 51 Particles retained (% particle DM) 100 80 60 40 20 (a) 3 Sieve mesh (mm) (b) (c) Fig 3.1 Cumulative particle size distribution in: (a) faeces from grazing cattle; (b) faeces from sheep given chopped (*- - - -*) or ground (*—*) lucerne hay (Van Soest, 1982); and (c) digesta leaving the stomach of sheep given chopped (*- - - -*) or ground and pelleted (*—*) lucerne hay gives no information on critical size The data of Van Soest (1982) for faecal particle size in sheep given chopped or pelleted lucerne hay are plotted in Fig 3.1b; sieves of, respectively, 0.98 and 0.91 mm mesh would have retained 5% of the particles For comparison, Fig 3.1c shows data from the author’s laboratory for particles in digesta leaving the abomasum of sheep given kg/day of lucerne hay either chopped or ground and pelleted; sieves of, respectively, 1.08 and 1.06 mm mesh would have retained 5% of the particles Faichney and Brown (1991) found no significant effect of grinding lucerne hay on critical mesh size and could find no evidence of an increase in critical mesh size as the intake by sheep increased from 20% to 90% of voluntary consumption In fact, the critical mesh size at the lowest intake (1.12 mm) was higher (P > > > 0.8> > = Manawa ryegrass 0.5 0.8> > > > White clover 0.5> > ; 0.8 42 TC 0. 42 0.50 2. 66 2. 88 2. 04 1.89 353 22 6 28 8... 11.9 l 0.59 l 62 MA 17.6 16.0 16.1 0.79 0. 72 0.73 64 66 MA 10.8 9.8 0.48 0. 42 MA 12. 7 l 8.1 l 5.7 l 23 .2 25.6 24 .7 26 .0 MA 17.8 14.1 16.8 15.1 0.61 7.7 6.9 7.5 0.67 0.63 0.63 Ulyatt and > MacRae