**3. Evaluating of fiber digestibility in ruminants**

Understanding the mechanism of fiber digestion is very important to accurately estimate the digestible energy of fiber and to improve animal performance. Fiber is digested primarily in the rumen as the result of the dynamic operation that is affected by the chemical nature of the fiber and by the passage and digestion rate of fiber within the digestive tract of the animal. The potentially digestible NDF (pdNDF) and the digestion rate (kd) vary greatly between and within different silage types [14, 17]. The passage rate of fiber (kp) is in the first place influenced by the animal, where the digestion of fiber increases along with increased retention time of feed in the rumen [15, 18]. Several models have been developed to describe the process of digestion in the rumen; some models are simple or complex. Most of these models have been developed by fractional schemes to correlate the disappearance or gas production curves with rumen digestibility of feed components, which assume that the feed component includes at least two portions: a potentially degradable fraction and an undegradable fraction. The potentially degradable portion will be degraded at a fractional rate (per hour), after a discrete lag time (h). The undegradable fraction is calculated from the longer time of incubation as proposed by Waldo et al. [19] (**Figure 1**). By using this model, Allen and Mertens [21] educed mathematical equations to define fiber digestibility and rumen fill. For fiber digestibility, the following equations were educed:

$$\mathbf{D} = \left( \text{pdNDF} \left( \text{dFINTAKE/dt} \right) \right) / \left( \text{kd} + \text{kp} \right) \tag{1}$$

$$\mathbf{I} = \left( \text{fi} \left( \text{dFINTAKE} \,/\, \text{dt} \right) \right) / \left( \text{kp} \right) \tag{2}$$

Finally, the rumen fill would be estimated as the sum of the digestible (D) and indigestible (I) fiber pools in the rumen

$$\text{Fill} = \text{D} + \text{I} \tag{3}$$

and indigestible (fi), as well as digestion rates (kd) and passage (kp). Jung and Allen ranked the factors that influence ruminal fill, and the most important element was the fiber content, followed by kp, the fraction that is indigestible, and the lowest factor was the kd [20]. The digestion kinetics of fiber can be measured in vivo using rumen evacuation technique, where cannulated animals are used for measuring the digestible and indigestible fiber pools that flow from the rumen [23]. In spite of the high precision for rumen evacuation technique to estimate rumen digestion kinetics, this technique is unwieldy for routine forage analysis. It has been proved that the use of other biological methods, that is, in vitro or in situ techniques, could give better characterization to degradation kinetics of fibrous fraction of forages. Over the last 50 years, the in vitro system has not been widely used in farm to implement analysis on forages because of its difficulty to perform in farm. This situation has changed in recent years with the use of a shorter digestion time (30 or 48 h) along with the enhancements that occurred in spectral analysis using near-infrared spectroscopies, where the laboratories were facilitated to assess the digestion of forages without the need to obtain rumen fluid. Some mathematical equations have been developed, which can use single time points like 24 or 30 h in vitro NDFD

Maximizing Fiber Utilization of Silage in Ruminants

http://dx.doi.org/10.5772/64471

127

along with fixed lag time and lignin in the forages to calculate the kd rates [24].

(%) = −3.62 + 1.11 × ivttNDFD (%) with *R*<sup>2</sup>

In recent times, the feeding studies have found the indigestible neutral detergent fiber (iNDF) after longer incubation time (240 h in vitro or 288 h in situ) was highly correlated with dry matter intake (DMI) and would be used to predict pdNDF [25]. Furthermore, there were sufficient data being created by commercial laboratories. Thus, the iNDF was applied as a new approach rather than using lignin × 2.4 to calculate pdNDF (CB3) and indigestible NDF (CC) using the updated CNCPS 6.5 [25]. It has been found that the model, which could accurately predict NDF digestibility, should partition NDF into iNDF and pdNDF, fractionate feed particles by their retention and passage in the rumen, using a predicted kd by an in vitro system [26]. Based on this approach, Combs developed a new method for predicting fiber digestibility; he used shorter incubation time (24, 30, and 48 h) along with iNDF (240 h) to predict kd (kdCB3) of pdNDF [27]. The CB3 kd rates derived from in vitro analysis were entered in the updated CNCPS model to calculate the ruminal fiber digestibility according to this equation; rumen degradability for pdNDF = CB3 × (kdCB3/(kdCB3 + kp). Finally, they calculated the in vitro total-tract NDFD (ivttNDFD) assuming that the intestinal digestibility of available NDF (CB3) amount escaping rumen digestion was 5%. Lopes et al. have found that in vivo total-tract NDF digestibility was highly correlated with the ivttNDFD. The regression equation to describe the relationship was described as follows: in vivo total-tract NFDF

differences between two methods (ivttNDFD and in vivo total-tract NDFD) were not significant, and mean values varied by only 1% unit, showing promise for this approach [28].

The use of high-resolution spectroscopic techniques (e.g., high-field nuclear magnetic resonance, mid-infrared, Raman spectroscopy, and pyrolysis mass spectrometry) is finding increased usage in forage assessment. These advanced technologies would provide more broad information about a primary nature [29]. A spectroscopic method such as Fourier transform infrared (FT/IR) spectroscopy has been developed as rapid, direct, nondestructive and noninvasive bioanalytical technique [29–37]. Thereby, this technique paves the way to better

= 0.70, RMS = 4.27, *P*-value < 0.01; *n* = 21 diets. The

Eq. (1) shows that digestibility is directly related to (pdNDF) and (kd), and inversely proportional (kd + kp; the rate of total fiber digestibility). Thus, as the ruminal retention time increases (1/kp), the extent of ruminal digestibility increases [22]. The fiber weight in the rumen is dependent on fiber intake per unit of time (dFINTAKE/dt), and parts that are digestible (fd),

**Figure 1.** Schematic model of total-tract fiber digestibility. Redrawn from Waldo et al. and Jung and Allen [19, 20].

and indigestible (fi), as well as digestion rates (kd) and passage (kp). Jung and Allen ranked the factors that influence ruminal fill, and the most important element was the fiber content, followed by kp, the fraction that is indigestible, and the lowest factor was the kd [20]. The digestion kinetics of fiber can be measured in vivo using rumen evacuation technique, where cannulated animals are used for measuring the digestible and indigestible fiber pools that flow from the rumen [23]. In spite of the high precision for rumen evacuation technique to estimate rumen digestion kinetics, this technique is unwieldy for routine forage analysis. It has been proved that the use of other biological methods, that is, in vitro or in situ techniques, could give better characterization to degradation kinetics of fibrous fraction of forages. Over the last 50 years, the in vitro system has not been widely used in farm to implement analysis on forages because of its difficulty to perform in farm. This situation has changed in recent years with the use of a shorter digestion time (30 or 48 h) along with the enhancements that occurred in spectral analysis using near-infrared spectroscopies, where the laboratories were facilitated to assess the digestion of forages without the need to obtain rumen fluid. Some mathematical equations have been developed, which can use single time points like 24 or 30 h in vitro NDFD along with fixed lag time and lignin in the forages to calculate the kd rates [24].

lag time (h). The undegradable fraction is calculated from the longer time of incubation as proposed by Waldo et al. [19] (**Figure 1**). By using this model, Allen and Mertens [21] educed mathematical equations to define fiber digestibility and rumen fill. For fiber digestibility, the

Finally, the rumen fill would be estimated as the sum of the digestible (D) and indigestible (I)

Eq. (1) shows that digestibility is directly related to (pdNDF) and (kd), and inversely proportional (kd + kp; the rate of total fiber digestibility). Thus, as the ruminal retention time increases (1/kp), the extent of ruminal digestibility increases [22]. The fiber weight in the rumen is dependent on fiber intake per unit of time (dFINTAKE/dt), and parts that are digestible (fd),

**Figure 1.** Schematic model of total-tract fiber digestibility. Redrawn from Waldo et al. and Jung and Allen [19, 20].

D= pdNDF dFINTAKE/dt /(kd+kp) ( ( )) (1)

I fi dFINTAKE / dt / kp = ( ( )) ( ) (2)

Fill D I = + (3)

following equations were educed:

126 Advances in Silage Production and Utilization

fiber pools in the rumen

In recent times, the feeding studies have found the indigestible neutral detergent fiber (iNDF) after longer incubation time (240 h in vitro or 288 h in situ) was highly correlated with dry matter intake (DMI) and would be used to predict pdNDF [25]. Furthermore, there were sufficient data being created by commercial laboratories. Thus, the iNDF was applied as a new approach rather than using lignin × 2.4 to calculate pdNDF (CB3) and indigestible NDF (CC) using the updated CNCPS 6.5 [25]. It has been found that the model, which could accurately predict NDF digestibility, should partition NDF into iNDF and pdNDF, fractionate feed particles by their retention and passage in the rumen, using a predicted kd by an in vitro system [26]. Based on this approach, Combs developed a new method for predicting fiber digestibility; he used shorter incubation time (24, 30, and 48 h) along with iNDF (240 h) to predict kd (kdCB3) of pdNDF [27]. The CB3 kd rates derived from in vitro analysis were entered in the updated CNCPS model to calculate the ruminal fiber digestibility according to this equation; rumen degradability for pdNDF = CB3 × (kdCB3/(kdCB3 + kp). Finally, they calculated the in vitro total-tract NDFD (ivttNDFD) assuming that the intestinal digestibility of available NDF (CB3) amount escaping rumen digestion was 5%. Lopes et al. have found that in vivo total-tract NDF digestibility was highly correlated with the ivttNDFD. The regression equation to describe the relationship was described as follows: in vivo total-tract NFDF (%) = −3.62 + 1.11 × ivttNDFD (%) with *R*<sup>2</sup> = 0.70, RMS = 4.27, *P*-value < 0.01; *n* = 21 diets. The differences between two methods (ivttNDFD and in vivo total-tract NDFD) were not significant, and mean values varied by only 1% unit, showing promise for this approach [28].

The use of high-resolution spectroscopic techniques (e.g., high-field nuclear magnetic resonance, mid-infrared, Raman spectroscopy, and pyrolysis mass spectrometry) is finding increased usage in forage assessment. These advanced technologies would provide more broad information about a primary nature [29]. A spectroscopic method such as Fourier transform infrared (FT/IR) spectroscopy has been developed as rapid, direct, nondestructive and noninvasive bioanalytical technique [29–37]. Thereby, this technique paves the way to better understand the quantity, composition, structure, and distribution of chemical constituents and functional groups in a tissue (feed and ingredients) [38–42]. Intrinsic chemical structures were found to effect on nutritive value, degradation characteristics, utilization, and availability of feed [43, 44]. Many studies have reported that AT/IR would accurately predict rumen degradability of DM, NDF, concentrations of lignin, ferulic, and coumaric acids in forage samples [45– 47].

component of hemicellulose and compromises about 30–35% of the cell-wall material of annual plants. The main chain of xylan is composed of 1,4-β-linked d-xylopyranose units [50, 51].

Maximizing Fiber Utilization of Silage in Ruminants

http://dx.doi.org/10.5772/64471

129

The collaborative activity of the cellulolytic and noncellulolytic microorganisms in the rumen is critical in fiber digestion [52]. Rumen cell-wall degradation initiated by the attachment of rumen microbes to fiber and the bacterial species specialized to start this attachment/colonization process are the cellulolytic species *Ruminococcus albus*, *R. flavefaciens*, and *Fibrobacter succinogenes*. Rumen fungi and protozoa also colonize and degrade plant fragments to differing degrees [48]. The fermentation of structural carbohydrates by cellulolytic consortium results in the progressive process where volatile fatty acids (VFAs) are liberated at a lower rate than starch fermentation. The fermentation of structural carbohydrates is associated with an increase in the proportion of acetic and butyric acid [53]. Following absorption, the large proportion of acetate is not changed by hepatic metabolism and may be augmented by endogenous acetate production in the liver. The posthepatic supply of acetate to peripheral tissues constitutes a major part of the total energy available to the animal and may be either oxidized to produce adenosine triphosphate (ATP) or used as a substrate in the production of long-chain fatty acids [54]. While ruminally derived butyrate is quantitatively metabolized to b-OH-butyrate during absorption through the rumen epithelium, in posthepatic tissues it has

Lignin is an indigestible polymer in plants that plays an important role in the structural integrity of plant tissue. Although lignin comprises little of the total structural carbohydrate system in plants, it has been recognized to exert the negative effect on cell-wall polysaccharide digestibility by coating the plant cell-wall polysaccharides from enzymatic hydrolysis [55]. Lignin arises from an enzyme-initiated dehydrogenative polymerization of three originators: p-coumaryl alcohols, coniferyl, and sinapyl. The phenylpropanoid metabolism and shikimic acid pathway lead to the synthesis of lignin intermediates like p-coumaric acid, ferulic acid, and diferulic acid [56], which are converted into coniferyl, sinapyl, and p-coumaryl alcohols

With the maturation of forage cell walls, the guaiacyl-type lignin changes to lignin-rich syringyl units, and the digestibility of mature cell walls decreased. Taboada et al. found that guaiacyl and syringyl have negative correlation with organic matter or dry matter digestibility in ruminants fed on silages. They concluded that guaiacyl and syringyl could be used as

The brown midrib (BMR) mutation in annual C4 grasses such as corn and sorghum results in both a reduction in lignin concentration and a shift in lignin composition to a more guaiacylrich polymer [20]. Jung and Deetz have suggested that the improved digestibility of cell walls in BMR mutants is a result of both the reduced lignin concentration and the reduction in

Cross-linking of lignin to cell-wall polysaccharides has been reported as additional mechanisms limiting fiber digestibility [20]. In grasses, ferulate and p-coumarate molecules are

and ultimately to guaiacyl, syringyl, or p-hydroxyphenyl lignin, respectively [55].

predictors of digestibility than total lignin content in silage [57].

a similar metabolic fate to that of acetate [54].

**4.2. Lignin and phenolic acids**

syringyl lignin content [58].
