In vitro unfermented fiber is a good predictor of the digestible and metabolizable energy content of corn distillers dried grains with solubles in growing pigs

Abstract

Characterizing fiber into fermentable and unfermentable fractions may enhance the accuracy of estimating digestible (DE) and metabolizable (ME) energy content in fiber-rich ingredients. Therefore, the objective of the study was to analyze the concentrations of neutral detergent fiber (NDF), representing both the fermentable (fNDFom) and unfermentable (uNDFom) portions among sources of corn distillers dried grains with solubles (DDGS), and determine their relative contributions to DE and ME content. The concentrations of DE and ME, as well as apparent total tract digestibility (ATTD) of gross energy (GE), were measured in a previous experiment. Samples of DDGS (0.5 g) were mixed with fecal inoculum and incubated for 8, 12, and 72 h. The ash corrected NDF (NDFom) content of DDGS residues at each time point was determined. The fNDFom increased with fermentation time of 8 h (21.6%), 12 h (29.0%) and 72 h (68.6%). The ATTD of GE increased as the uNDFom decreased at 8 h (uNDFom8; R2 = 0.83; P < 0.01) and 72 h (uNDFom72; R2 = 0.83; P < 0.01). Likewise, ME content of DDGS increased as uNDFom72 decreased (R2 = 0.59; P < 0.01). The best fit DE equation was DE (kcal/kg DM) = 2,175 – 3.07 × uNDFom8 (g/kg, DM) – 1.50 × uNDFom72 (g/kg, DM) + 0.55 × GE (kcal/kg DM) [R2 = 0.94, SE = 36.21]. The best fit ME equation was ME (kcal/kg DM) = 1,643 – 2.31 × uNDFom8 (g/kg, DM) – 2.54 × uNDFom72 (g/kg, DM) + 0.65 × GE (kcal/kg DM) – 1.42 × crude protein (g/kg DM) [R2 = 0.94, SE = 39.21]. These results indicate that in vitro unfermented fiber is negatively associated with GE and NDF digestibility, and therefore, is a good predictor of DE and ME content in corn-DDGS.

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1
1Financial support was provided by National Pork Board
(#17–036). Mention of trade names or commercial prod-
ucts in this publication is solely for the purpose of providing
specic information and does not imply recommendation or
endorsement by the University of Minnesota or the USDA.
The USDA is an equal opportunity provider and employer.
2Corresponding author: urrio001@umn.edu
Received October 14, 2018.
Accepted June 28, 2019.
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In vitro unfermented ber is a good predictor of the digestible and metabolizable
energy content of corn distillers dried grains with solubles in growing pigs1
Zhikai Zeng,* Jae Cheol Jang,* BrianJ. Kerr,†,2 GeraldC. Shurson,* and PedroE. Urriola*,
*Department of Animal Science, University of Minnesota, Saint Paul, MN 55108; and †USDA-ARS National
Laboratory for Agriculture and the Environment, Ames, IA 50011
ABSTRACT: Characterizing ber into ferment-
able and unfermentable fractions may enhance
the accuracy of estimating DE and ME energy
content in ber-rich ingredients. Therefore, the
objective of this study was to analyze the concen-
trations of NDF, representing both the ferment-
able (fNDFom) and unfermentable (uNDFom)
portions among sources of corn distillers dried
grains with solubles (DDGS), and determine
their relative contributions to DE and ME con-
tent. The concentrations of DE and ME, as well
as apparent total tract digestibility (ATTD) of
GE, were measured in a previous experiment.
Samples of DDGS (0.5g) were mixed with fecal
inoculum and incubated for 8, 12, and 72h. The
ash corrected NDF (NDFom) content of DDGS
residues at each time point was determined. The
fNDFom increased with fermentation time of
8 h (21.6%), 12 h (29.0%), and 72 h (68.6%).
The ATTD of GE increased as the uNDFom de-
creased at 8h (uNDFom8; R2=0.83; P<0.01) and
72h (uNDFom72; R2=0.83; P<0.01). Likewise,
ME content of DDGS increased as uNDFom72
decreased (R2=0.59; P<0.01). The best-t DE
equation was DE (kcal/kg DM)=2,175– 3.07×
uNDFom8 (g/kg, DM) – 1.50 × uNDFom72
(g/kg, DM) + 0.55× GE (kcal/kg DM) (R2=0.94,
SE = 36.21). The best-t ME equation was
ME (kcal/kg DM)=1,643– 2.31× uNDFom8
(g/kg, DM) – 2.54× uNDFom72 (g/kg, DM) +
0.65× GE (kcal/kg DM) – 1.42× crude protein
(g/kg DM) (R2 =0.94, SE = 39.21). These re-
sults indicate that in vitro unfermented ber is
negatively associated with GE and NDF digest-
ibility, and therefore, is a good predictor of DE
and ME content in corn-DDGS.
Key words: corn distillers dried grains with solubles, energy prediction, fermentable ber, in
vitro ber fermentation, growing-nishing pigs, metabolizable energy
© The Author(s) 2019. Published by Oxford University Press on behalf of the American Society of
Animal Science. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.
J. Anim. Sci. 2019.XX:XX-XX
doi: 10.1093/jas/skz221
INTRODUCTION
Corn distillers dried grains with solubles
(DDGS) has been widely used in swine diets
because it is a cost-competitive source of ME
and digestible amino acids but the ME content
in DDGS is quite variable (Stein and Shurson,
2009). Because energy is the most expensive nu-
tritional component in animal feeds, DE and ME
prediction equations have been developed using
chemical composition data, to dynamically es-
timate the DE and ME content among various
DDGS sources (Anderson etal., 2012; Kerr etal.,
2013; Li et al., 2015). Dietary ber from total
dietary ber (TDF) or NDF have been shown to
be necessary components to predict DE and ME
content among sources of DDGS (Kerr et al.,
2013). However, these equations may not provide
AADate
AAMonth
AAYear
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2Zeng etal.
the most precise inputs in energy prediction equa-
tions because they depend on the measurement of
TDF, NDF, or ADF as a homogenous portion of
thediet.
In fact, dietary ber in corn DDGS is composed
of multiple carbohydrates with distinct properties
(Jaworski et al. 2015) and rate of disappearance
along the gastrointestinal tract (Urriola et al.,
2010). Therefore, using more specic character-
istics of ber may improve the prediction of DE
and ME content. For example, ber fermentation
produces short-chain fatty acids, which contribute
to DE and ME, while unfermented ber reduces
apparent total tract digestibility of lipids (Kim
et al., 2013) and CP (Urriola and Stein, 2010),
which subsequently decrease digestibility of en-
ergy (Gutierrez et al., 2013; Iyayi and Adeola,
2015). Therefore, analyzing the unfermented ber
fraction of corn DDGS may enhance the accuracy
and consistency of DE and ME estimates derived
from energy prediction equations. We hypothe-
sized that uNDF is a useful predictor of the DE,
ME, and nutritive value of DDGS. The objective
of this study was to determine the in vitro fNDF
and uNDF content of various DDGS sources
using fecal inoculum to determine their relative
contributions for predicting DE and ME content
of DDGS.
MATERIALS AND METHODS
Sample Collection
Fifteen sources of corn DDGS were obtained
from different ethanol plants and diverse geo-
graphical locations of U.S. corn production, as
well as different co-product processing technolo-
gies, to represent the variability in chemical com-
position among sources in the U.S.market. These
15 corn DDGS samples were used in a previous
study (Kerr etal., 2013) to determine the nitrogen
(CP), ether extract (EE), and dietary ber (TDF,
NDF, or ADF) content and were kept at −20°C.
We used nutrient composition from the previous
manuscript, calculated other nutrients such as in-
soluble hemicellulose (NDF minus ADF), and
used the published values for in vivo apparent total
tract digestibility (ATTD) of nutrients, DE, and
ME content for growing pigs. The digestible nu-
trient content of DDGS was calculated by multi-
plying total concentrations by the corresponding
ATTD values, and the indigestible portion was
subsequently calculated by the difference between
total and digestible nutrient content.
In Vitro Fermentation
Feces were obtained from 3 nishing pigs (90kg
BW) from Cargill Animal Nutrition (Elk River,
MN), which had been fed a corn-wheat-soybean
meal diet with no antibiotics. Fecal samples were
collected directly from the rectum, immediately
placed in zipper plastic bags without air, and kept
in a water bath at 39°C until used as inoculum for
incubation. The time from fecal collection until in-
cubation was less than 1h.
For the in vitro fermentation assay, 500 mg
of DDGS samples were added to 125 mL serum
bottles with rubber-stoppers containing 40 mL
buffer solution containing macro- and micro-
minerals (Menke and Steingass, 1988). The in-
oculum was prepared by diluting blended feces in
an inoculation solution composed of distilled water
(474 mL/L), trace mineral solution (0.12 mL/L
containing CaCl2 132 g/L, MnCl3·4H2O 100 g/L,
CoCl2•6H2O 10 g/L, and FeCl3•6H2O 80 g/L),
in vitro buffer solution (237 mL/L containing
NH4HCO3 4.0 g/L and NaHCO3 35 g/L), macro-
mineral solution (237mL/L composed of Na2HPO4
5.7g/L, KH2PO4 6.2g/L, MgSO4·7H2O 0.583g/L,
and NaCl 2.22g/L), and resazurin (blue dye, 0.1%
w/v solution; 1.22mL/L), and was ltered through
4 layers of cheesecloth. The nal inoculum con-
centration was adjusted to 0.094 g feces per mL
of buffer, which represented the same feces to sub-
strate ratio used in previous studies (Jha et al.,
2011; Huang etal., 2017a, 2017b). Forty milliliters
of inocula were transferred into bottles containing
DDGS samples, and bottles were sealed with
rubber stoppers before placing in a 39 °C water
bath for incubation. Anaerobiosis was maintained
in the inoculation solution by the addition of a re-
ducing solution (distilled water 47.5 mL/L, 1 M
NaOH 2mL/L, Na2S 335 mg/L) and CO2. Bottles
were sealed with a rubber stopper and placed in the
water bath for incubation. The fermentation was
terminated after 8, 12, or 72h of incubation by pla-
cing bottles in ice, and gas production was recorded
at 2, 5, 8 12, 16, 20, 24, 30, 36, 48, and 72h.
After the termination of fermentation, the
inoculum (40mL) was directly mixed with NDF
washing detergents (60mL) and loaded on a re-
ux apparatus for NDF analyses as described by
Mertens (2002). After reux solubilization, res-
idues were ltered with the use of a glass micro-
ber lter (934-AH by Whatman, Whatman
Limited-GE Healthcare, Maidstone, UK) with
a porosity of 1.5 μm in Pyrex Gooch crucibles
(40 to 60 μm; Corning, Inc., Corning, NY).
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3
Unfermentable ber for energy predictions
Subsequently, residues were ashed at 550°C to de-
termine ash-free NDF (NDFom). Blanks were cre-
ated by inoculating the bottles with buffer, fecal
inoculum, and were subjected to the same process
as the test feed ingredients to adjust for any par-
ticles introduced into the in vitro fermentation
system. The NDF content of DDGS samples was
analyzed in duplicate, and all unfermented NDF
residues after 8, 12, and 72 h fecal incubation
(uNDFom8, uNDFom12, and uNDFom72, re-
spectively) were analyzed in triplicate. The in vitro
fermented NDFom (fNDFom8, fNDFom12, and
fNDFom72) was calculated by subtracting total
NDFom and uNDFom after 8, 12, and 72h of in-
cubation. The digestibility coefcients of NDFom
(DigNDFom8, DigNDFom12, and DigNDFom72)
were calculated by the ratio of digestible NDFom
and total NDFom content.
Gas accumulation curves were recorded during
the 72 h of fermentation and were modied ac-
cording to France etal. (1993):
G(mL g/DM)=0, if 0 <t<L,
G(mL g/DM)=Gf(1−exp(−[b(t−L)
+c(
√
t−
√
L)])), if t ≥L,
where G denotes the gas accumulation at a spe-
cic time (t), Gf (ml g−1 DM) was the maximum gas
volume for t=∞, and L (h) represents the lag time
before the fermentation began. Gas accumulation
rapidly reached one-fourth of the maximum ac-
cumulation in 2h, and the parameter L was very
close to 0, which resulted in the model failing to
converge. Therefore, L(h) data were removed from
the nal model. The constants b (h−1) and c (h−1/2)
were used to determine the fractional rate of deg-
radation of the substrate µ (h−1), which is postu-
lated to vary with time as follows:
µ
=
b
+
c
/(
2√t
)
,t
=
T
/
2
, representing the time
to half-asymptote when G=Gf/2.
Statistical Analyses
The PROC CORR of SAS (Version 9.3; SAS
Inst. Inc., Cary, NC) was used to determine if there
was a correlation among DE, ME, fNDFom, and
uNDFom fractions, and the chemical composition
of corn DDGS samples. Correlations with a value
of P<0.05 were considered signicant. The PROC
REG STEPWISE of SAS was used to select input
variables for the equations to predict DE and ME
content from chemical composition, and in vitro
fNDFom and uNDFom of the corn DDGS sam-
ples. Variance Ination Factor (VIF) was used to
determine multicollinearity, and variables with VIF
> 10 were considered as multicollinear and were re-
moved from the prediction equations. The P value,
R2, and the root of SEM were used as parameters to
determine the accuracy of the prediction equations.
RESULTS AND DISCUSSION
Variability of Energy and Fiber-Related
Composition
The sources of DDGS obtained in this study
represent a wide range in GE, DE, and ME con-
tent, with differences between maximum and
minimum values of were 387 kcal/kg for GE, 396
kcal/kg for DE, and 430 kcal/kg for ME (Table 1).
The ber-related composition was more variable
(CV > 5.6) compared to ME and DE (CV < 4.1).
The average NDFom value was slightly greater
(388g/kg NDF) than the average value reported
by Kerr etal. (2013), with a difference of 122 g/
kg NDF between the minimum and maximum
values of the 15 DDGS sources. In our study, the
NDFom analyses involved using glass microber
lters with pore sizes that were in the submicron
range, resulting in retention of very ne particles
(Barbosa et al., 2015), which may partially ex-
plain the slightly greater average NDF values in
our study compared with those reported by Kerr
etal. (2013).
A 21.6% of the NDFom (average value of 15
sources DDGS) disappeared after 8h of incuba-
tion (fNDFom8), which was similar to the average
apparent ileal digestibility (AID, 21.5 %) value of
TDF for 10 sources of corn DDGS reported by
Urriola and Stein (2010). This portion of NDF
had the greatest CV among the 3-time tested sug-
gesting that it may provide great differentiation
among sources of DDGS, the disappearance of
NDF in this rapid portion (fNDFom8) was quite
variable 43 to 121 g/kg. Furthermore, the per-
centage of NDFom disappearing after 8 h fecal
incubation (DigNDFom8) ranged from 11.0 to
30.4% among DDGS sources. The fNDFom8 (as-
sumed to be readily degradable ber in the small
intestine of pigs) of corn DDGS was highly vari-
able with covariation coefcients greater than
20%. The use types and amounts of enzymes
in the ethanol and co-product production pro-
cess varies among ethanol plants and may have
contributed to high variance of readily degrad-
able ber in the sources of corn DDGS evalu-
ated in this study. Cellulolytic enzymes have been
shown to improve ethanol yield and oil recovery
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4Zeng etal.
during bioethanol production (Luangthongkam
etal., 2015). Therefore, it is possible that greater
amounts of readily degradable ber in corn
DDGS were removed in some sources due to the
type of enzyme pretreatment during the ethanol
production process (Pedersen etal., 2015).
A 68.6% of the NDFom disappeared after 72h
of fecal incubation (fNDFom72), which was greater
than the ATTD of NDF (59.3%) in corn DDGS re-
ported by Urriola etal. (2010). This difference may
have been a result of using the 72-h in vitro incu-
bation time compared with a typical 30 to 51h in
vivo digesta transit time (Latymer etal., 1990). In
addition, nondietary materials with ber-like prop-
erties in the gastrointestinal tract may have contrib-
uted to the measurement of TDF or NDF, which
are likely to be detected as endogenous ber loss
and consequently result in a relatively low apparent
ber digestibility (Montoya etal., 2015; Montoya
etal., 2016).
Unfermented Fiber and Apparent Total Tract
Digestibility
The ATTD of GE, DM, EE, and carbon lin-
early decreased (P<0.05) in response to increased
uNDFom8, uNDFom72, and NDFom of corn
DDGS (Fig. 1a–c, respectively). The ATTD of
CP decreased (P < 0.01) with increasing NDFom
and tended to decrease linearly (P = 0.067) with
increasing uNDFom72, but ATTD of P was not
affected by any ber fraction (NDFom, fNDFom
or uNDFom) of corn DDGS (data not shown). It
has been well documented that there is a linear de-
crease in ileal and total tract apparent digestibility
of GE, DM, and CP with increased levels of dietary
ber (Nortey etal., 2007; Gutierrez etal., 2013). In
a meta-analysis review, Zeng et al. (2018) also re-
ported that standardized ileal digestibility of CP
and amino acids linear decreased with increasing
NDF or ADF content of different sources of
Table 1. Fitted kinetics parameters of gas accumulation and fermented and unfermented NDF of corn
distillers dried grains with solubles, DM basis1
Item Mean Minimum Maximum Interval SD CV
Chemical composition (Kerr etal., 2013)
GE, kcal/kg 4,996 4,780 5,167 387 111 2.2
DE, kcal/kg 3,650 3,474 3,870 396 130 3.6
ME, kcal/kg 3,435 3,266 3,696 430 140 4.1
Crude protein, g/kg 305 277 329 52 14 4.5
Ether extract, g/kg 97 49 132 84 23 23.3
TDF, g/kg 342 308 378 69 19 5.6
NDF, g/kg 354 288 440 152 40 11.3
ADF, g/kg 117 90 140 50 18 15.3
Fiber fractions characterized by fecal incubation
NDFom, g/kg 388 335 457 122 28 7.2
uNDFom8, g/kg 304 247 350 103 30 9.8
uNDFom12, g/kg 276 234 329 95 28 10.3
uNDFom72, g/kg 123 84 165 81 28 22.6
fNDFom8, g/kg 84 43 121 78 19 23.2
fNDFom12, g/kg 112 73 146 73 18 16.4
fNDFom72, g/kg 265 232 297 65 21 7.7
DigNDFom8, % 21.6 11.0 30.4 19.4 4.8 22.4
DigNDFom12, % 29.0 18.7 36.6 17.9 4.6 15.8
DigNDFom72, % 68.6 59.1 75.9 16.8 5.7 8.4
Gas accumulation kinetics
Gas8, mL/g 97 81 117 36 8.5 8.8
Gas12, mL/g 120 105 139 33 9.2 7.7
Gas72, mL/g 199 174 214 39 13 6.5
Gf, mL/g 240 211 271 60 22.4 9.3
T/2, hour 17.2 10.9 27.1 16.2 4.8 27.6
μ
T/2, h−1 0.036 0.024 0.048 0.024 0.007 18.8
1SD=standard deviation; CV=coefcient of variation; GE=gross energy; DE=digestible energy; ME=metabolizable energy; TDF=total
dietary ber; ADF=acid detergent ber; NDFom=ash-free NDF; DigNDFom=percentage of fermented NDFom after 8, 12, or 72h of fecal
incubation; fNDFom=fermented NDF after 8, 12, or 72h of fecal incubation; uNDFom=unfermented NDF after 8, 12, or 72 h of fecal in-
cubation; Gas=cumulative gas production after 8, 12, or 72h of fecal incubation; T/2=half-time to asymptote (hour); μ
T/2=fractional rate of
degradation (h−1) at t=T/2; Gf=maximal gas production.
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5
Unfermentable ber for energy predictions
DDGS. The ATTD of EE has been reported to be
greater in extracted corn oil compared with intact
oil from corn germ meal (Kil etal., 2010). This im-
plies that intact oil is encased within the ber matrix
and is more resistant to the formation of emulsions
and enzymatic digestion than extracted corn oil
(Knudsen etal., 1993; Grundy etal., 2016). In add-
ition, slow fermentable ber (the difference between
uNDFom8 and uNDFom72) can ultimately reach
the hindgut for microbial digestion and synthesis of
microbial lipids (Drackley, 2000). Therefore, dietary
components that increase microbial activity and
microbial synthesis of lipids in the hindgut will in-
crease endogenous EE loss and reduce the ATTD
of EE. This is consistent with our results where
DDGS sources that had greater unfermented ber
(uNDFom8 or uNDFom12), had less ATTD of EE,
but uNDFom72 had no effect on ATTD ofEE.
ATTD_GE = -0.0885x + 99.978
R² = 0.8807
ATTD_DM = -0.0804x + 95.77
R² = 0.7327
ATTD_EE = -0.1717x + 114.79
R² = 0.3822
ATTD_C = -0.0798x + 96.433
R² = 0.7663
50
60
70
80
90
250270 290310 330350 370
% ,ytilibitsegid tcart latot tnerappA
uNDFom8, g/kg DM basis
ATTD_GEATTD_DMATTD_EEATTD_N ATTD_C
A
ATTD_GE = -0.0886x + 83.95
R² = 0.7572
ATTD_N = -0.0405x + 86
R² = 0.2356
ATTD_C = -0.0871x + 82.852
R² = 0.7819
50
55
60
65
70
75
80
85
90
70 90 110130 150170
% ,ytilibitsegid tcart latot tnerappA
NDFom72, g/kg DM basis
ATTD_GEATTD_DM
ATTD_EEATTD_N
ATTD_C Linear (ATTD_GE)
B
ATTD of GE = -0.0803x + 104.21
R² = 0.6291 P <0.01
ATTD of DM = -0.0701x + 98.5
R² = 0.4829 P <0.01
ATTD of EE = -0.1581x + 123.9
R² = 0.2812 P =0.042
ATTD of N = -0.0537x + 101.85
R² = 0.4196 P <0.01
ATTD of C = -0.0745x + 101.05
R² = 0.5793 P <0.01
50
60
70
80
90
300350 400450
% ,ytilibitsegid tcart latot tnerappA
NDFom, g/kg DM basis
ATTD_GEATTD_DMATTD_EEATTD_N ATTD_C
C
Figure 1. Association between apparent total tract digestibility of gross energy (GE), carbon (C), dry matter (DM), and ether extract (EE) and
unfermented NDFom8 (A), unfermented NDFom72 (B) and NDFom (C) in 15 sources of corn distillers dried grains with solubles. uNDFom=un-
fermented NDF after 8, 12, or 72h of fecal incubation; NDFom=ash-free neutral detergent ber.
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6Zeng etal.
Correlation Among Energy Components, Gas
Production, and Chemical Composition
Gross energy was positively (P < 0.01) as-
sociated with total EE and digestible EE con-
tent in corn DDGS (Fig. 2). The DE content was
negatively correlated with uNDFom8 (r = −0.86,
P < 0.01), uNDFom12 (r = −0.86, P < 0.01),
uNDFom72 (r = −0.86, P < 0.01), and NDFom
(r = −0.84, P < 0.01), and positively correlated
with digestible DM (r = 0.73, P < 0.01; Fig. 3).
Similarly, ME was negatively (P<0.05) associated
with ber-related components (NDF, fNDFom,
and uNDFom), and tended to be positively correl-
ated (r= 0.48, P= 0.07) with digestible EE (Fig.
4). However, there were no signicant correlations
between the digestible content of CP, ADF, NDF,
and NDFom with DE or ME content (data not
shown). In contrast, Noblet and Perez (1993) ob-
served good correlations between DE and digest-
ible CP, EE, and NDF content, and developed
DE, ME, and NE prediction equations using these
measures. Apossible explanation for these incon-
sistent results can be attributed to differences in ap-
proach for determining the digestibility of nutrients
(i.e., protein, starch, and lipid) that contribute to
DE. Noblet and Perez (1993) directly measured the
ATTD of diets and used these values for developing
DE and ME prediction equations. Low molecular
weight sugars were recently proposed as a necessary
component of carbohydrates that contribute to en-
ergy (Navarro et al., 2018). However, the overall
relevance of low molecular sugars for development
of prediction equations depend on the overall con-
tribution of this component to energy and the vari-
ability among sources and needs to be investigated.
In the current study, the ATTD values of CP
and EE for corn DDGS (Kerr et al., 2013) were
obtained by calculating the difference between the
corn-basal diet and the corn + 30% DDGS diets.
The assumption of using the difference method
is that there is linear additivity of ATTD in corn
+ 30% DDGS diets. However, the low dietary CP
and EE in the corn-basal diet increase the relative
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
r,stneiciffeocnoittalerroC
**
*
**#
#
Figure 2. Correlation coefcients (r) between chemical compos-
ition (digestible and unfermented) and GE of 15 corn distillers dried
grains with solubles samples. ADF=acid detergent ber; EE=ether
extract; NDF = neutral detergent ber; NDFom = ash-free NDF;
TDF=total dietary ber; dDM, dEE, dADF, and dNDF=digestible
EE, ADF, and NDF on a dry matter basis calculated by multiplying
total concentration by the corresponding apparent total tract digest-
ibility coefcients; dNDFom=fermented NDFom after 8, 12, or 72h
of fecal incubation; Gas12=gas production after 12h of fecal incuba-
tion; uNDF and uCP= undigested NDF and CP (total – digestible);
uNDFom=unfermented NDFom after 8, 12, or 72h of fecal incuba-
tion. #Means are different (P<0.10), *Means are different (P<0.05).
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
r,stneiciffeocnoitalerroC
*
*
*
**
#
*
**
*
Figure 3. Correlation coefcients (r) between chemical composition, digestible and unfermented content, and DE of sources of 15 corn distil-
lers dried grains with solubles. NDFom=ash-free NDF; TDF=total dietary ber; dDM, dEE, dADF, and dNDF=digestible ether extract (EE),
ADF, and NDF by multiplying total concentrations by the corresponding apparent total tract digestibility coefcients); dNDFom= fermented
NDFom after 8, 12, or 72h of fecal incubation; Gas12=gas production after 12h of fecal incubation; uNDF and uCP=undigested NDF and
crude protein (total – digestible); uNDFom = unfermented NDFom after 8, 12, or 72h of fecal incubation. #Means are different (P<0.10),
*Means are different (P<0.05).
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7
Unfermentable ber for energy predictions
contribution of endogenous loss of CP and EE to
total CP and EE in the fecal outow compared with
the corn + 30% DDGS diets. As a result, ATTD of
CP and EE of DDGS may be overestimated due
to the lack of additivity of ATTD values in mixed
diets (Stein etal., 2005; Stein etal., 2007).
The ATTD of NDF and ADF digestibility
may also vary with increasing dietary inclusion
rates. Gutierrez etal. (2013) fed diets containing
increasing levels (10, 20, 30, and 40%) of corn
bran in corn-soybean meal diets to growing pigs
and showed that the ATTD of NDF slightly de-
creased from 42.6% to 41.9% as corn bran was
added at 10% or 20% of the diet, respectively, and
was sharply reduced to 29.3% and 30.5 % when
corn bran was added at 30% and 40% of the diet,
respectively. This implies that the digestibility of
corn ber may be quite different when pigs are
fed a corn basal diet or a corn + 30% DDGS diet.
Therefore, it may be less accurate to estimate the
ATTD of NDF and ADF in corn DDGS by sub-
tracting the differences in digestibility between a
corn basal diet and corn + 30% DDGS diets, as
reported by Kerr etal. (2013). The in vitro system
allows evaluating the nutritive value of single in-
gredients without the confounding effects of diet
inclusion rate of the ingredient in in vivo deter-
minations. Therefore, the in vitro unfermented
ber (uNDFom8, 12 or 72) had greater Pearson
correlation coefcients with DE and ME com-
pared with the in vivo unfermented NDF or ADF
correlations.
Gas production at 8, 12, and 72h were posi-
tively (P<0.05) associated with fermented NDFom
at the corresponding time points (Table 2). The dis-
appearance of NDFom at 8 and 12h was negatively
associated (P<0.05) with a time of achieving half
maximal gas production (T/2). These results are
consistent with previous data reported by our re-
search group, where an increase in ATTD of TDF
was observed as the maximal gas production in-
creased among sources of DDGS, wheat straw, and
soybean hulls (Huang etal. 2017a). In vitro gas ac-
cumulation measurements can be used to estimate
substrate degradation and yield valuable informa-
tion about feed ingredient fermentation kinetics of
feed ingredients (France etal. 1993). However, the
disappearance of NDF may not precisely match
with gas accumulation from fermentation because
gas is generated from fermenting a wide range of
substrates, including both soluble and insoluble
ber components (Schoeld etal. 1994).
Prediction Equations for DE andME
Stepwise regression analysis of ber-related
measurements was used to generate a series of pre-
diction equations for DE (Table 3). The initial re-
gression equation (Eq. 1) included uNDFom8 as
the most important component to predict DE fol-
lowed by Eq. 2, which included both uNDFom8 and
uNDFom72, and ultimately resulted in the best-t
equation (Eq. 3), where DE (kcal/kg DM)=2,175–
3.07× uNDFom8 (g/kg, DM) – 1.50× uNDFom72
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
r,tneiciffeocnoitalerroC
*
*
*
*
#
*
**
*
#
#
#
#
Figure 4. Correlation coefcients (r) between chemical composition (digestible and unfermented) and ME of 15 corn distillers dried grains
with solubles samples. NDFom=ash-free NDF; TDF=total dietary ber; dDM, dEE, dADF, and dNDF=digestible ether extract (EE), ADF,
and NDF on a dry matter basis by multiplying by the total concentration by the corresponding apparent total tract digestibility coefcients);
dNDFom= fermented NDFom after 8, 12, or 72h of fecal incubation; Gas12 = gas production after 12 h of fecal incubation; uNDF and
uCP=undigested NDF and crude protein (total – digestible); uNDFom=unfermented NDFom after 8, 12, or 72h of fecal incubation. #Means
are different (P<0.10), *Means are different (P<0.05).
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8Zeng etal.
Table 2. Correlation coefcients (r) between energy values, total dietary ber, and fermented neutral detergent ber among 15 sources of corn distillers
dried grains with solubles1
Item TDF
NDF
om uNDF om8 uNDF om12
uNDF
om72 fNDF om8 fNDF om12
fNDF
om72 DigNDF om8
DigNDF
om12
DigNDF
om72 Gas8 Gas12 Gas72 Gf T/2 μ
T/2
TDF 1.00
NDFom 0.30 1.00
uNDFom8 0.43 0.78** 1.00
uNDFom12 0.51*0.79** 0.94** 1.00
uNDFom72 0.60*0.73** 0.81** 0.88** 1.00
fNDFom8 −0.24 0.24 −0.43 −0.31 −0.21 1.00
fNDFom12 −0.33 0.30 −0.27 −0.35 −0.25 0.84** 1.00
fNDFom72 −0.40 0.38 −0.04 −0.12 −0.36 0.60*0.75** 1.00
DigNDFom8 −0.34 −0.11 −0.71** −0.60*−0.47 0.94** 0.76** 0.49 1.00
DigNDFom12 −0.47 −0.14 −0.63*−0.72** −0.59*0.76** 0.90** 0.61*0.83** 1.00
DigNDFom72 −0.60 −0.52*−0.72** −0.79** −0.96** 0.36 0.43 0.60*0.56*0.68** 1.00
Gas8 −0.62*−0.15 −0.51 −0.58*−0.45 0.57*0.67** 0.40 0.62*0.75** 0.48 1.00
Gas12g −0.18 −0.07 −0.41 −0.53*−0.38 0.54*0.72** 0.43 0.57*0.77** 0.44 0.77** 1.00
Gas72 −0.49 −0.06 −0.42 −0.43 −0.64*0.56*0.58*0.78** 0.59*0.62*0.78** 0.62*0.59*1.00
Gf −0.25 0.06 −0.02 −0.07 −0.46 0.12 0.20 0.70** 0.11 0.18 0.60*0.13 0.18 0.73** 1.00
T/2 0.27 0.10 0.42 0.41 0.12 −0.51 −0.49 −0.03 −0.54*−0.55*−0.09 −0.58*−0.48 −0.15 0.54*1.00
μ
T/2 0.10 −0.44 −0.40 −0.44 −0.12 −0.03 0.01 −0.44 0.13 0.22 −0.04 0.21 0.25 −0.30 −0.70** −0.66** 1.00
1GE=gross energy; DE=digestible energy; ME=metabolizable energy; TDF=total dietary ber; NDFom=ash-free neutral detergent ber; DigNDFom=percentage of fermented NDF after 8, 12, or
72h of fecal incubation; fNDFom=fermented NDFom after 8, 12, or 72h of fecal incubation; uNDFom=unfermented NDFom after 8, 12, or 72h of fecal incubation. Gas= cumulative gas production
(mL/g) after 8, 12, or 72h of fecal incubation; T/2=half-time to asymptote (hour); μ
T/2=fractional rate of degradation (h−1) at t=T/2; Gf=maximal gas production (mL/g).
*Means are different (P<0.05).
**Means are different (P<0.01).
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9
Unfermentable ber for energy predictions
(g/kg, DM) + 0.55× GE (kcal/kg DM) (R2=0.94,
SE=36.21).
A series of prediction equations were also de-
veloped for ME content of corn DDGS (Table 4).
The initial regression (Eq. 4)included uNDFom72
as the most important component to predict ME,
followed by Eq. 5, which included both uNDFom12
and GE, and Eq. 6, which included uNDFom8 (Eq.
6), and ultimately resulted in the best-t equation
(Eq. 7), where ME (kcal/kg DM)=1,643– 2.31×
uNDFom8 (g/kg, DM) – 2.54× uNDFom72 (g/kg,
DM) + 0.65× GE (kcal/kg DM) – 1.42× CP (g/kg
DM) (R2=0.94, SE=39.21).
Kerr etal. (2013) suggested that TDF is a better
variable to use in DE and ME prediction models
than NDF because TDF provides a more com-
plete estimate of the ber content in corn-DDGS.
However, it is more expensive and time consuming
to analyze TDF than NDF and ADF, even though
most of the ber (95 to 100%) in corn DDGS is in-
soluble ber (Urriola etal., 2010). Therefore, NDF
or ADF content (insoluble ber) may serve as al-
ternative variables to predict DE and ME in corn
DDGS when TDF content is difcult to obtain
in practice. Li etal. (2015) reported that NDF or
ADF are useful predictors of DE and ME in both
conventional high-oil and reduced-oilDDGS.
In the current study, the in vitro uNDFom8 and
uNDFom72 were selected as the initial variables in
the DE and ME regression model, which indicates
Table 3. Stepwise regression equation for estimating the DE content among 15 sources of corn distillers
dried grains with solubles
Item
Regression coefcient1Statistics2
Intercept uNDFom8 uNDFom72 GE SE R2Adjust R2
Eq. 1 4,783 −37.28 69.96 0.73 0.71
SE3191 6.24
P-value3<0.01 <0.01
Eq. 2 2,388 −4.19 0.51 42.89 0.91 0.89
SE3517 0.395 0.11
P-value3<0.01 <0.01 <0.01
Eq. 3 2,175 −1.5 −23.59 0.55 35.41 0.94 0.93
SE3435 0.54 0.58 0.09
P-value3<0.01 <0.01 0.026 <0.01
1Equations were based on analyzed nutrient content expressed on a DM basis. GE=gross energy. Units are kcal/kg DM for GE and DE and g/
kg DM for unfermented NDFom after 8 and 72h fecal incubation (uNDFom8 and uNDFom72).
2SE=SE of the regression estimate dened as the root of the mean square error.
3SE and P-values of the corresponding regression coefcient.
Table 4. Stepwise regression equation for estimating the ME content among 15 sources of corn distillers
dried grains with solubles
Item
Regression coefcient1
Statistics2
Intercept uNDFom8 uNDFom72 GE CP SE R2Adjust R2
Eq. 4 3,911 −38.78 93.53 0.59 0.55
SE3113 9.02
P-value3<0.01 <0.01
Eq. 5 648.7 −4.49 0.67 58.49 0.85 0.83
SE3711 0.58 0.14
P-value30.380 <0.01 <0.01
Eq. 6 899 −2.23 −2.57 0.71 42.64 0.93 0.91
SE3524 0.65 0.71 0.11
P-value30.114 <0.01 <0.01 <0.01
Eq. 7 1,643 −2.31 −2.54 0.65 −1.42 39.21 0.94 0.92
SE3645 0.60 0.65 0.10 0.82
P-value30.029 <0.01 <0.01 <0.01 0.013
1Equations were based on analyzed nutrient content expressed on a DM basis. CP=crude protein; GE=gross energy. Units are kcal/kg DM for
GE and ME and g/kg DM for CP and unfermented NDFom after 8 and 72h fecal incubation (uNDFom8 and uNDFom72).
2SE=SE of the regression estimate dened as the root of the mean square error.
3SE and P-values of the corresponding regression coefcient.
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10 Zeng etal.
that unfermented ber is a better predictor of en-
ergy utilization of DDGS in pigs than analyt-
ical ber measures (e.g., TDF, NDF, and ADF).
Although these analytical ber measures represent
both fermentable and unfermentable ber portions,
they have been reported to be negatively associated
with DE or ME of brous ingredients in pigs in sev-
eral studies (Anderson etal., 2012; Kerr etal., 2013;
Li etal., 2014, 2015). However, the fermentable por-
tion of ber can result in the production of a con-
siderable amount of short-chain fatty acids in the
hindgut and contribute 10–20% to DE in the diet
(Anguita etal., 2006; Iyayi and Adeola, 2015). The
energy obtained from the fermentable ber may de-
crease the negative correlations between analytical
ber measures (e.g., TDF, NDF, and ADF) and
DE or ME, especially because the fermentability
of ber is highly variable among high ber ingredi-
ents. Therefore, it may be more appropriate to use
in vitro unfermented ber (as measured by TDF)
as a predictor for energy utilization of DDGS.
Urriola etal. (2010) reported a strong positive as-
sociation between the ATTD of TDF and NDF in
diets that contained 30% DDGS as the exclusive
ber source. Therefore, uNDFom may show good
correlations with unfermented TDF and thus, serve
as a good alternative predictor for DE and ME in
corn DDGS when in vitro disappearance of TDF is
not available.
It is worth mentioning that the procedure as
proposed in this manuscript has some limitations.
The ingredients in the fermentation process were
incubated with feces prior simulation of small in-
testine digestion, which would remove amino acids,
lipids, and starch. There is biological relevance in
adding a predigestion step with pepsin and pan-
creatin to the model, but the objective is to pre-
dict with the simplest process possible (Bohn etal.,
2018). We decided that simplicity of the model was
necessary for the intended use because the pepsin
and pancreatin steps would add complexity to the
model. This research also provided equations that
were not validated as has been done in previous
work using DDGS from another data set (Urriola
etal., 2014). Therefore, the proposed equations re-
quire a new experiment for validation.
In conclusion, the use of the in vitro fermen-
tation assay is an effective method to estimate
fermented and unfermented ber content in corn
DDGS. The in vitro unfermented ber measures
of uNDFom8 and nNDFom72 are the best pre-
dictors for DE and ME content of corn DDGS fed
to growing pigs, compared with in vivo apparent
total tract digestible nutrients, which were not
good predictors for DE and ME content in corn
DDGS. Further investigations are encouraged
to develop energy prediction equations based on
in vitro digestible nutrients (CP, EE, starch, and
carbohydrates) and unfermented residues (ash and
TDF).
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Unfermentable ber for energy predictions
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