Short communication
In¯uence of non®ber carbohydrate concentration
on forage ®ber digestion in vitro
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S.G. Haddad
a, R.J. Grant
b,*aDepartment of Animal Production, Jordan University of Science and Technology,
P.O. Box 3030, Amman, Jordan
bDepartment of Animal Science, University of Nebraska, Lincoln, NE 68583-0908, USA
Received 22 November 1999; received in revised form 9 May 2000; accepted 25 May 2000
Abstract
We quanti®ed the effect of dietary non®ber carbohydrate (NFC) concentration (30, 35, 40, or 45% of DM) on in vitro digestion kinetics of neutral detergent ®ber (NDF) from alfalfa and corn silages at pH 5.8 or 6.8. The objective was to simulate the effect of diets differing in effective ®ber content on the optimal NFC-to-NDF ratio that resulted in maximal ®ber digestion. Ash-free NDF was determined at 0, 3, 6, 9, 12, 24, 30, 36, 48, 72, and 96 h of fermentation. Kinetic parameters were estimated using logarithmic transformation and linear regression. The optimal NFC-to-NDF ratio for maximal NDF digestion differed between the two forages. For alfalfa fermented at pH 6.8, apparent extent of ruminal NDF digestion was greatest between 30 and 40% NFC, but at pH 5.8 NDF digestion was greatest at 35% NFC. For corn silage fermented at either pH 6.8 or 5.8, apparent extent of NDF digestion was greatest at 30% NFC. An NFC-to-NDF ratio of 0.70±1.20 maximized NDF digestion for alfalfa only when fermentation pH was maintained at 6.8. These in vitro results demonstrate that the optimal dietary NFC content for maximum NDF digestion in the rumen for a particular forage will be a function of fermentation pH that re¯ects the physically effective NDF content of the diet.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Non®ber carbohydrate; Fiber; Ruminal pH; Digestion kinetics
1. Introduction
Carbohydrates comprise the largest single dietary component of rations fed to dairy cows (up to 75% of DM). The commonly used carbohydrate fractions, namely neutral detergent ®ber (NDF) and non®ber carbohydrates (NFC), comprised primarily of starch,
86 (2000) 107±115
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Published with the approval of the director as Paper Number12831, Journal Series, Nebraska Agricultural Research Division.
*Corresponding author. Tel.:1-402-472-6442; fax:1-402-472-6362.
E-mail address: rgrant1@unl.edu (R.J. Grant)
sugars, pectin, and b-glucans (Van Soest et al., 1991). The NFC content of a diet or feedstuff is estimated commonly by subtracting crude protein (CP), NDF, and ether extract from organic matter with a correction for CP included in NDF (Van Soest et al., 1991). Although the optimum dietary concentration of NFC for lactating dairy cows is uncertain, recent research with alfalfa-based diets fed to high-producing dairy cows suggested that diets should contain >30% NFC (DM basis), with a negligible bene®t of feeding 42 versus 36% NFC (Batajoo and Shaver, 1994).
Milk production and ruminal ®ber digestion responses to varying dietary concentra-tions of NFC appear to be a function of ruminal degradability of NFC, ruminal pH, and ®ber source. Previous research indicates that lower dietary NFC was associated with higher ruminal NDF digestibility when pH increased; however, ruminal NDF digestion was unaffected by reduced NFC content when ruminal pH was unchanged (Sievert and Shaver, 1993a, b; Batajoo and Shaver, 1994). During the ®rst 8 h post-feeding, Sievert and Shaver (1993a, b) measured ruminal pH to be consistently below 6.0 for dairy cows fed diets containing 42 and 35% NFC in dietary DM. Additionally, the diurnal range in ruminal pH observed in lactating dairy cows can fall between 6.8 and 5.5, with ruminal pH below 6.2 for 70±80% of the day for diets containing 50±60% concentrates (Robinson et al., 1986). Consequently, the effect of NFC content on ruminal pH is crucial for determining the optimal NFC content in lactation diets.
Physically effective NDF has been de®ned as the proportion of NDF that stimulates rumination, and has been proposed as a major determinant of ruminal pH (Mertens, 1997). The importance of the long particle fraction (measured as physically effective NDF) at determining ruminal pH is illustrated by diets for which NFC content remains the same, but forage is either coarse or ®nely chopped. At the same dietary NFC content, ®nely chopped forage has resulted in low ruminal pH and ®ber digestion, while the same forage, when coarsely chopped, resulted in pH above 6.0 and higher ®ber digestion (Grant et al., 1990). Consequently, dietary NFC content and fermentability is important in determining ruminal pH, but forage physical form (physically effective NDF) exerts the dominant control over pH.
Grant and Mertens (1992a) developed and described a buffer system that can be used with a batch in vitro fermentation system to maintain constant pH in the range of 5.5 to 6.8 in order to facilitate measurement of the effect of pH on NDF digestion. Using a buffer system that maintains stable pH over 96 h of in vitro fermentation allows the effects of NFC content and fermentation pH on forage NDF digestion to be partitioned. The objective of this experiment was to determine the effect of dietary NFC concentration on the kinetics of NDF digestion for two common forages at high and low pH, representative of adequate or de®cient physically effective NDF.
2. Materials and methods
2.1. Substrate preparation
Forage and concentrate substrates were dried at 558C for 48 h and ground through a 1-mm screen using a Wiley mill (Arthur H. Thomas Co., Philadelphia, PA). Substrates were analyzed for CP (Association of Of®cial Analytical Chemists, 1990), ADF and NDF
(Van Soest et al., 1991), ether extract (Association of Of®cial Analytical Chemists, 1990), and ash (Association of Of®cial Analytical Chemists, 1990). Chemical compositions of the late-bud stage alfalfa silage, physiologically mature corn silage, soybean hulls, corn, and soybean meal are listed in Table 1. The individual ingredients were combined in the proportions shown in Table 2 to create substrates representing isonitrogenous diets containing either 30, 35, 40, or 45% NFC with either alfalfa silage or corn silage as the forage. To achieve these calculated NFC concentrations in the samples, soybean hulls were substituted incrementally for the corn grain.
Table 1
Chemical composition (g per 100 g DM) of substrate ingredients
Ingredient CPa ADFb NDFc Ash EEd NFCe
Alfalfa silage 19.0 35.1 45.1 10.8 3.8 21.3
Corn silage 8.5 25.0 46.0 3.9 3.0 34.6
Corn grain 10.0 3.8 9.0 2.1 4.3 74.6
Soybean hulls 12.1 50.0 67.0 5.1 2.1 13.7
Soybean meal 47.6 4.9 10.6 6.9 1.5 33.4
aCrude protein. bAcid detergent ®ber. cNeutral detergent ®ber. dEther extract.
eNon®ber carbohydrates (100
ÿCPÿNDFÿAshÿEE).
Table 2
Ingredient and chemical composition (g per 100 g DM) of substrates used for in vitro experiment
Item Diet
Alfalfa Corn
30a 35 40 45 30 35 40 45
Ingredients
alfalfa silage 50.0 50.0 50.0 50.0 ± ± ± ±
corn silage ± ± ± ± 50.0 50.0 50.0 50.0
corn grain 14.7 22.9 31.3 39.6 ± 6.1 14.6 24.8
Soybean hulls 25.0 16.7 8.3 ± 26.5 19.5 11.0 ±
soybean meal 10.3 10.4 10.4 10.4 23.5 24.4 24.4 25.2
Compositionb
CP 18.5 18.4 18.3 18.4 18.5 18.5 18.6 18.5
ADF 30.7 27.4 23.5 19.7 27.2 24.0 19.9 15.1
NDF 41.0 36.9 32.0 26.2 42.3 38.4 33.1 27.1
EE 3.2 3.4 3.6 3.8 2.4 2.5 2.7 2.9
Ash 7.6 7.4 7.2 7.0 4.9 4.7 4.5 4.2
NFC 29.7 33.9 39.2 44.5 31.5 35.5 40.9 47.1
NFC-to-NDF 0.72 0.92 1.23 1.69 0.74 0.92 1.24 1.74
aPercentage of non®ber carbohydrate contained in the substrate.
bCP, crude protein; ADF, acid detergent ®ber; NDF, neutral detergent ®ber; EE, ether extract; NFC, non®ber
A 300-mg sample of each of the eight substrates was weighed into 50-ml polypropylene tubes for measurement of in vitro NDF digestion kinetics. The entire experiment was replicated three times.
2.2. In vitro procedure
The in vitro procedure utilized was that described by Grant and Weidner (1992). The buffer solution was that of Goering and Van Soest (1970) adjusted to pH 5.8 with 1 M citric acid or formulated for pH 6.8 as described by Grant and Mertens (1992a). Fermentation times (at 398C) were 0, 3, 6, 9, 12, 24, 30, 36, 48, 72, and 96 h. Tubes were swirled gently at inoculation and at each remaining time thereafter.
Ash-free NDF was measured at each time (Van Soest et al., 1991). To ensure complete starch solubilization with the starch-containing substrates, a heat-stable amylase (ANKOM Tech. Corp., Fairport, NY) was used during sample re¯ux and at ®ltering. The ruminal ¯uid inoculum was obtained from a steer-fed medium-quality alfalfa hay. At collection, the mean pH of the ruminal ¯uid was 6.20 (SE0.11) for the three replicates of the experiment. Although inoculum source can affect ®ber digestion kinetics measured in vitro, donor animals were not available for every substrate studied, so alfalfa was fed to the donor steer because it has been shown to promote rapid NDF digestion rates (Van Soest, 1994). Jung and Varel (1988) demonstrated that ruminal inoculum from cattle fed alfalfa hay degrades ®ber fractions of forages more rapidly than inoculum from a wide range of forage diets including bromegrass, switchgrass, or corn silage.
As tubes were removed from the water bath, the pH of the contents for each substrate was measured to monitor stability of the buffer pH, which did not decrease more than 0.18 pH unit for any substrate during 96 h of fermentation. Therefore, actual pH for each treatment remained suf®ciently separated for the results to be biologically meaningful.
2.3. Statistical analysis
The model for the kinetics of NDF digestion was that described by Mertens and Loften (1980),
YD0eÿkd tÿLINDF;
whereYis the NDF residue at timetof in vitro fermentation,D0the potentially digestible
NDF fraction (proportion of initial DM),kdthe fractional rate constant of NDF digestion
(hÿ1),Lthe discrete lag time (h),tthe time of fermentation (h), and INDF the indigestible NDF residue at 96 h (proportion of initial DM).
The remaining potentially digestible fraction at each time was transformed logarithmically, and the NDF digestion parameters were estimated by linear regression. Discrete lag time was calculated using the equation
L lnD0ÿlnD
00
ÿkd ;
whereD00is intercept of the equation of ln (Yÿ(96-h) NDF residue) over time att0.
Potential extent of NDF digestion (PED) was determined using the formula
PED100 D0
D0INDF :
Predicted apparent extent of ruminal NDF digestion (AED) was calculated using the equation described by Miller and Muntifering (1985),
AEDPEDeÿkpL kd
kdkp ;
wherekpis the fractional rate of ®ber particle passage from the rumen, which was set at
either 0.050/h for particles of NDF or at 0.030/h (Shaver et al., 1986) to simulate faster and slower particle passage from the rumen as a result of high or low feed intake.
Parameter estimates of the NDF digestion model were analyzed using the general linear models procedure of SAS (1985), with a factorial arrangement of forage (alfalfa or corn silage), NFC concentration (30, 35, 40, or 45%), and buffer pH (5.8 or 6.8). The model included factors for replicate, forage, NFC concentration, pH, and interactions. Differences among treatment means for signi®cant main effects were detected using Student±Newman±Keul's multiple range test (SAS, 1985). Signi®cance was declared at
p<0.05 unless otherwise stated.
3. Results
3.1. Substrate composition
All in vitro test diets contained8.5% CP and similar amounts of lipid. Dietary NDF content ranged from 26.2 to 41.0% for alfalfa diets, and from 27.2 to 42.3% NDF for corn silage diets. Likewise, NFC ranged from 29.7 to 44.5% for alfalfa diets and from 31.5 to 47.1% for corn silage diets. The NDF and NFC contents of these diets bracketed the range typically observed in practice.
3.2. Digestion kinetics of NDF
When the sole dietary source of forage was alfalfa, in vitro fermentation at pH 6.8 resulted in the greatestKdfor NDF at 40% NFC (Table 3). For alfalfa, as NFC content
decreased, the lag time increased between 45 and 40% NFC and potential extent of digestion for NDF increased between 35 and 40% NFC. In contrast, at pH 5.8 theKdof
NDF increased with NFC concentration of 30 and 35% of DM versus 40 and 45% NFC. As with higher fermentation pH, when NFC content decreased, the potential extent of digestion of dietary NDF increased.
For corn silage-based diets fermented at pH 6.8, as NFC content decreased, theKdand
lag time for dietary NDF increased between 40 and 45% NFC and again between 30 and 35% NFC. The potential extent of digestion was greatest at 30% NFC for corn silage diets at pH 6.8. With fermentation pH of 5.8, the Kd for NDF increased as NFC content
NFC. The potential extent of digestion was lower for 30% NFC versus 40 and 45% NFC, with 35% NFC being intermediate. If a Kpof 0.050 or 0.030 hÿ
1
is assumed, then the apparent extent of ruminal NDF digestion in Table 4 can be calculated. Apparent extent of NDF digestion at pH 6.8 for diets containing alfalfa silage was greatest between 30 and
Table 3
Effect of forage, non®ber carbohydrate (NFC), and pH on neutral detergent ®ber digestion kinetics
Forage NFC (g per
aMeans within a column and forage combination with unlike letters differ (p<0.05).
Table 4
Effect of forage, non®ber carbohydrate (NFC), and pH on apparent extent of ruminal neutral detergent ®ber digestion (%)a
aCalculated from data in Table 3 and assumed fractional passage rates of 0.050 and 0.030 hÿ1. bMeans within a row with unlike letters differ (p<0.05).
40% NFC. At pH 5.8, the apparent extent of NDF digestion was greatest at 35% NFC. For corn silage-based diets, apparent extent of NDF digestion was maximal at 30% NFC regardless of fermentation pH.
4. Discussion
The limited research conducted to-date suggests that forage source may in¯uence the optimal dietary NFC concentration regarding ruminal NDF digestion. Nocek and Russell (1988) suggested that 40% NFC was optimal for diets containing alfalfa silage, corn silage, and 50:50 mixtures of each silage. Other research with alfalfa-based diets suggests that the optimal NFC content may be closer to 35% (Sievert and Shaver, 1993a, b; Battajoo and Shaver, 1994). Previous research has shown differences among forage sources in their susceptibility to the negative effect of starch on NDF digestion (Grant and Mertens, 1992a, b; Grant, 1994). For example, Grant and Mertens (1992b) found that raw corn starch decreased fractional rate of NDF digestion for alfalfa hay, but observed little effect of starch for bromegrass.
Nocek and Russell (1988) proposed that the interaction between NFC and NDF was an important consideration when formulating diets for lactating dairy cows and suggested an optimal ratio of NFC-to-NDF of 0.9 to 1.2. Carbohydrates in the NFC fraction generally are rapidly degraded in the rumen, resulting in low ruminal pH, reduced ®brolytic bacterial activity, and have been related to reduced milk fat synthesis. Therefore, this ratio may indicate whether ruminal conditions are optimal for ®ber fermentation. To-date, very few studies have measured the impact of NFC-to-NDF ratio on ruminal pH and ®ber fermentation.
For diets that contain low content of NDF from forages, reducing NFC from 39 to 29% increased milk yield, microbial yield, and ruminal turnover rate (Feng et al., 1993). In practice, dietary NFC is often adjusted by altering the ratio of forage to concentrate which confounds the separate effects of forage and NFC on ruminal NDF fermentation. In the few studies that have adjusted NFC-to-NDF ratio using ®brous byproduct feeds, the effects on milk yield and composition have been inconsistent (Beauchemin et al., 1997). It is possible that the differing cow responses to dietary NFC content are, at least in part, a function of ruminal pH which may re¯ect the physically effective NDF content of the diet. Source and particle length of dietary forage affects NFC degradability due to differing buffering capacities, chewing time responses, and fractional passage rates from the rumen (Beauchemin et al., 1997). In our study, we used high (pH 6.8) and low (pH 5.8) fermentation pH conditions to simulate diets with either adequate or inadequate physically effective NDF. This interpretation is based on the importance of forage particle length in determining ruminal pH for diets of similar NFC content.
both forage sources. However, for NFC-to-NDF ratios of 1.25 or 1.50, agreement was poor when potential extent of NDF digestion from fermentation at pH 6.8 was used (59.8%) versus extents of digestion measured at low pH (49.3%) for which agreement with the in situ data set was good. These in vitro digestion results indicate that pH will have a major impact on NDF digestion, for any NFC-to-NDF ratio, and that factors affecting ruminal pH, most notably physically effective NDF as well as NFC digestion rate, must be taken into account when attempting to predict ®ber digestion and animal performance as dietary NFC content is varied. Any discussion of NFC-to-NDF ratio recommendations needs to include a measurement of the dietary effective NDF content. The optimal dietary NFC concentration for maximal NDF digestion for a particular forage will be a function of the mean ruminal pH that is characteristic of a given diet's physical and chemical composition. So, high or low effective NDF, for any ratio of NFC to NDF, will potentially have a large impact on the optimal dietary NFC content to maximize ruminal ®ber fermentation. It is possible that diets of smaller particle size (lower effective NDF) which would result in low ruminal pH require less NFC for maximal NDF digestion than diets of higher effective NDF.
5. Conclusions
The effect of NFC content and pH on NDF digestion differed between alfalfa and corn silage. At pH 6.8, extent of NDF digestion was unaffected between 30 and 40% NFC for alfalfa-based diets. In contrast, extent of NDF digestion was maximized at 30% NFC for corn silage-based diets for both high and low pH. The optimal NFC-to-NDF ratio ranged between 0.70 and 1.20 for alfalfa and close to 0.70 for corn silage-based diets when pH was 6.8. Fermentation at pH 5.8 reduced NDF digestion when NFC-to-NDF ratio was >0.92 for alfalfa and 0.74 for corn silage. Understanding the interactions among forage source, ruminal pH, and NFC content will ultimately aid in formulating dairy lactation diets for maximal ruminal NDF digestion and optimal milk and milk component production.
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