Chemical And Physical Digestion Of

Chemical digestion (hydrolysis) of PDF is achieved by enzymes associated with the microbial cells of fibrolytic microbes. These enzymes physically attach to and hydro-lyze specific plant tissue surfaces of PDF.[4] Digestion of PDF is via erosion from the surface attachment site. Therefore, rate and extent of digestion of PDF are functions of 1) the microbially accessible surface area of plant tissue fragments;[5] 2) the abundance of PDF on such surfaces; 3) the rate of exposure of new surface levels of PDF by ruminative mastication;[6] and 4) an adequate ruminal flux of ruminal degraded protein (RDP) for growth of the fibrolytics.

Newly ingested fragments have intrinsic buoyancy due to the structure of large fragments of ingestive mastication. Intrinsic buoyancy of vascular tissues within the relatively large fragments of ingestive mastication is the initial force that positions younger fragments of ingestive mastication into flow paths involving ruminative mastication. With microbial colonization and residence time in the lag-rumination pool, aging fragments undergo age-dependent changes in their masticated size, rate of exposure of new surfaces of microbially accessible PDF (maPDF), and fermentation-based buoyancy. Collective changes of ruminative mastication, fermentation-based buoyancy, and mass action competition among fragments of similar buoyancy constrain ruminal escape of individual fragments until fragments' surface level maPDF is extensively digested (mean of 90±5%) (Fig. 1A) and fragments are physically masticated to relatively small fragments. We propose[7'8] that the fractional rate of digestion of maPDF provides the fermentation-based buoyancy gradients that constrain the fractional rate of escape of IDF from the rumen (Fig. 1B).

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Fig. 1 A. Digestibility of microbially accessible, potentially digestible PDF is relatively complete for forages fed to cattle (F) and to lactating dairy cows (S), but not for mixed concentrate forage diets (M). B. Variations in digestion rate of PDF are postulated as causal of a positive relationship with the escape rate of IDF a relationship that results in relatively complete digestion of microbial accessible PDF (A). Relatively low digestibility of PDF from mixed concentrate forage diets (M) is postulated due to inadequate RDP for the fibrolytic bacterial ecosystem.

FRAGMENT SIZE AND ESTIMATION OF IDF AND PDF

Being digested, it is obvious that digested NDF must define maPDF, and ruminative mastication is the process that determines the rate of exposure of maPDF. Ruminative mastication is incapable of completely exposing the total mass of fragments. Consequently, considerable micro bially inaccessible PDF remains within the mass of fragments escaping the rumen. The range in mean size of fragments entering the rumen is relatively large and highly variable among feeds (1405 to 6494 mm). In contrast, the mean size of fragments escaping the rumen is on the order of 240 to 360 mm.[9] Thus, on the order of 4.6 to 21% (300 mm/1405 mm and 300 mm/6494 mm, respectively) of the mass of ingestively masticated fragments escape the

Fig. 2 Positive relationships between ruminal flux proportions of rumen degraded protein (RDP) and potentially digestible NDF (PDF) and rate of in vivo digestion of PDF (A), and the efficiency of ruminal microbial protein efflux (RMPE) (B) suggest that RDP drives the growth rate of the fibrolytic bacterial ecosystem and the consequent rate of digestion of PDF and growth of the total ruminal bacterial ecosystem.

Fig. 2 Positive relationships between ruminal flux proportions of rumen degraded protein (RDP) and potentially digestible NDF (PDF) and rate of in vivo digestion of PDF (A), and the efficiency of ruminal microbial protein efflux (RMPE) (B) suggest that RDP drives the growth rate of the fibrolytic bacterial ecosystem and the consequent rate of digestion of PDF and growth of the total ruminal bacterial ecosystem.

rumen as fragments < 300 mm. The ratio of maPDF/miIDF presumably differs for different size distributions of fragments produced in the laboratory mill-ground feed versus fragments escaping the rumen. Thus, the 5 10% indigestibility for analytically defined PDF (Fig. 1A) could easily be accounted for by differences in mean size of ground feed versus ruminal escaping fragments. Grinding feed samples to pass 1- or 2-mm screens of the laboratory mills traditionally used may not mimic the distribution of fragments produced by ruminative mastication.

The importance of ruminative mastication is indicated by differences observed between in vivo and non-in vitro methods for estimating the rate of digestion of maPDF. Mean in vivo rate of digestion of PDF (inclusive of effects of ruminative rumination) consistently exceeds (1.3- to 4.4-fold) rate estimates obtained by in vitro or in situ methods (which exclude effects of ruminative mastication) obtained with feed samples ground to pass 1- or 2 -mm screens.

A number of analytical pitfalls are associated with the gravimetric estimation of NDF and its constituents, IDF and PDF. Problems of physical loss of matter due to the typical 30- to 50-p.m porosity of in situ bags[10] are a major problem. Rather than a gravimetric ''by-difference'' method as currently used, a more direct estimation of the specific carbohydrates of conceptual NDF is needed a specific colorimetric procedure, for example. Use of a specific, heat-stable amylase to remove starch and adequate washing to remove neutral detergent-solubilized matter are commonly unrecognized problems.

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