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EFFECT OF IN VITRO STARCH DIGESTIBILITY,
PROCESSING METHOD, AND NITROGEN
SUPPLEMENTATION ON SITE AND EXTENT OF NUTRIENT
DIGESTION IN HOLSTEIN STEERS FED A HIGH GRAIN
DIET

presented by

Charles Andrew McPeake

has been accepted towards fulﬁllment
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Doctoral degree in Animal Science
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5/08 KtIProj/AcoSPres/ClRC/DaleDue.indd

 

 

EFFECT OF IN VITRO STARCH DIGESTIBILITY, PROCESSING METHOD, AND
NITROGEN SUPPLEMENTATION ON SITE AND EXTENT OF NUTRIENT
DIGESTION IN HOLSTEIN STEERS FED A HIGH GRAIN DIET
By

Charles Andrew McPeake

A DISSERTATION
Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Animal Science

2009

ABSTRACT

EFFECT OF IN VITRO STARCH DIGESTIBILITY, PROCESSING METHOD,
AND NITROGEN SUPPLEMENTATION ON SITE AND EXTENT OF
NUTRIENT DIGESTION IN HOLSTEIN STEERS FED A HIGH GRAIN DIET
By
Charles Andrew McPeake
Corn hybrids with different genetics result in corn that has different kernel

characteristics. The physical structure and chemical composition of cereal starches,
along with the interactions between starch and protein concentration can alter the
digestibility of grain for livestock. Recent research (Philippeau et al., 1999; Macken et
al., 2003) suggests that grain source (i.e., wheat VS. corn) and corn endosperm type (i.e.,
ﬂour vs. ﬂint) can increase both ruminal starch degradability and postrurninal starch
digestion. Degradable intake protein requirements for an animal increase with greater
ruminal starch degradability (NRC, 1996) and could be altered based on the extent of
ruminal fermentation. Collectively, two experiments were conducted to determine the
interaction between corn grain endosperm type/ in vitro starch digestibility (IVSD) and
processing method on site and extent of nutrient digestibility and subsequent effects of N
concentration on nutrient digestibility. In the ﬁrst experiment, eight ruminally,
duodenally, and ileally cannulated Holstein steers were used in a replicated 4 x 4 Latin
square design experiment with a 2 x 2 factorial arrangement of treatments. Treatments
were com grain with high or low IVSD (HIGH vs. LOW) as either dry-rolled or high
moisture corn (DRC vs. HMC). When expressed as a percentage of duodenal ﬂow,

21.5% less starch was digested in the small intestine of steers consuming diets containing

DRC. Likewise, an 8% decline in apparent total tract starch digestibility was

documented when feeding DRC. Total molar concentration of volatile fatty acid tended
to be greater for HIGH diets. Diets containing HMC resulted in greater amounts of
microbial nitrogen ﬂow to the duodenum, greater apparent post-ruminal nitrogen
digestibility, and greater microbial efﬁciency. Both processing method and IVSD of corn
hybrids appear to impact site and extent of starch digestibility and nitrogen metabolism.
In the second experiment, the interaction between IVSD of two corn hybrids and
nitrogen concentration on site of nutrient digestion and nitrogen metabolism in Holstein
steers consuming a high-grain diet was investigated. Four Holstein steers (initial BW =
500 i 21 kg) were used in separate 4 x 4 Latin square design experiments with a 2 x 2
factorial arrangement of treatments. Treatments were IVSD of corn grain (HIGH and
LOW) and nitrogen concentration (HN IT and LNIT). A signiﬁcant interaction of
treatments was present for total tract starch digestibility. Both HIGH and LNIT
treatments resulted in greater molar concentration of propionate. Corn grain with high
IVSD resulted in numerically greater microbial efﬁciency when compared to LOW corn

grain.

ACKNOWLEDGEMENTS

First, I would like to thank the most important person involved in my dissertation
program, Dr. Steven Rust. Several times throughout my program the road got tough, but
we persevered as a team. Dr. Rust showed me the value of teamwork and for that I will
be forever grateful.

Secondly, I would be remiss if I didn’t extend a deep debt of gratitude toward the
BCRC staff and Dr. Mike Allen and his faithful lab. Speciﬁcally, Dave Main, Dewey
Longuski, Steve Mooney, Barry Bradford, and Yun Ying (aka Jackie) for their unselﬁsh
ability to help me with whatever I asked for. Without their insight and willingness to
help a “feedlot guy” this research would Simply not have been possible.

I would also like to thank the remaining members of my graduate committee, Drs.
Dan Buskirk, David Hawkins, Clint Krehbiel, and Dale Romsos. They have all offered
their wisdom and guidance and helped me complete the most difﬁcult educational
endeavor of my life.

I would also like to thank Julie. I think she will tell you that she is a strong
motivational factor in me completing my dissertation. Life isn’t fair, but it sure is easier
when you’re side-by-side with the person that makes you the happiest. I consider myself

very lucky.

iv

TABLE OF CONTENTS

 

 

 

LIST OF TABLES - _ -- - _ - vii

LIST OF ABBREVIATIONS AND SYMBOLS - ix
CHAPTER 1

REVIEW OF LITERATURE -- - - - 1

1.1 INTRODUCTION ..................................................................................................... 1

1.2 PHYSICAL CHARACTERISTICS OF CORN HYBRIDS ...................................... 4

TABLE .................................................................................................................... 5

1.2.1 DIAGRAM .................................................................................................... 7

1.2.2 Corn Starch Characteristics ............................................................................ 7

1.2.3 Grain Processing ............................................................................................ 9

1.3 SITE OF STARCH DIGESTION ........................................................................... 11

1.3.1 Ruminal Starch Degradability ..................................................................... 12

1.3.2 Intestinal Starch Digestibility ..................................................................... 13

1.4 METABOLIZABLE PROTEIN SUPPLY ............................................................. 15

1.5 ENVIRONMENTAL NITROGEN AND PHOSPHORUS ................................. 18
CHAPTER 2

EFFECT OF IN VITRO STARCH DIGESTIBILITY AND PROCESSING
METHOD OF CORN ON SITE AND EXTENT OF NUTRIENT DIGESTION
AND RUMINAL METABOLISM IN HOLSTEIN STEERS FED A HIGH-

 

 

 

GRAIN DIET __ - - - - 22
ABSTRACT - -- - - - - - - - - - - 22
2.1 INTRODUCTION ................................................................................................... 23
2.2 MATERIALS AND METHODS ............................................................................. 24
2.2.1 Corn Grain Planting, Harvest, and Processing ............................................. 24
2.2.2 Experimental Design and Data Collection .................................................. 25
2.2.3 Sample Analyses ......................................................................................... 29
2.2.4 Statistical Analysis ...................................................................................... 32
2.3 RESULTS AND DISCUSSION ............................................................................. 33
2.3.1 Ruminal Fermentation ................................................................................ 33
2.3.2 Dry Matter and Organic Matter Digestibility ............................................. 34
2.3.3 Site of Starch Digestion .............................................................................. 35
2.3.4 Site of Nitrogen Digestibility ...................................................................... 37
2.4 CONCLUSIONS .................................................................................................... 39
TABLES- - - - -- - . - 40
CHAPTER 3

EFFECT OF IN VITRO STARCH DIGESTIBILITY OF CORN AND
SUPPLEMENTAL NITROGEN CONCENTRATION ON SITE AND EXTENT

OF NUTRIENT DIGESTION AND PROTEIN METABOLISM IN HOLSTEIN

 

 

 

 

 

STEERS FED A HIGH-GRAIN DIET - -- - -- - ..... 49

ABSTRACT - - - -- - 49

3.1 INTRODUCTION .................................................................................................. 50

3.2 MATERIALS AND METHODS ............................................................................ 51

3.2.1 Treatment Determination ............................................................................ 51

3.2.2 Experimental Design and Data Collection ................................................. 52

3.2.3 Sample Analyses ......................................................................................... 54

3.2.4 Statistical Analysis ...................................................................................... 57

3.3 RESULTS AND DISCUSSION ............................................................................. 58

3.3.1 Ruminal Fermentation ................................................................................ 58

3.3.2 Digestibility Marker .................................................................................... 59

3.3.3 Dry Matter and Organic Matter Digestibility ............................................. 59

3.3.4 Site of Starch Digestion .............................................................................. 61

3.3.5 Site of Nitrogen Digestion .......................................................................... 62

3.4 CONCLUSIONS .................................................................................................... 64

TABLES--- - - -- - -- - - -- - - - - -- 66
CHAPTER 4

FINAL DISCUSSION -- -- - 75

FUTURE WORK - - - 80

REFERENCES -- - - -- - 81

 

vi

Table 1.1.

Table 2.1.

Table 2.2.

Table 2.3.

Table 2.4.

Table 2.5.

Table 2.6.

Table 2.7.

Table 2.8.

Table 3.1.

Table 3.2.

Table 3.3.

Table 3.4.

Table 3.5.

Table 3.6.

Table 3.7.

LIST OF TABLES

Chemical composition of cracked yellow dent corn grain ....................... 5
Physical and chemical characteristics of corn grain ................................ 41
Composition of experimental diets (DM basis) ....................................... 42
Nutrient composition of experimental diets ............................................. 43

Effects of corn grain in vitro starch digestibility and processing
method on ruminal VFA, pH, and ammonia concentration ..................... 44

Effects of corn grain in vitro starch digestibility and processing method
on site of DM and OM digestion ............................................................. 45

Effects of corn grain in vitro starch digestibility and processing method
on site of neutral detergent ﬁber digestion .............................................. 46

Effects of corn grain in vitro starch digestibility and processing method
on site of starch digestion ........................................................................ 47

Effects of corn grain in vitro starch digestibility and processing method
on site of nitrogen digestibility ................................................................ 48

Physical and chemical characteristics of corn grain ................................ 67

Composition of experimental protein/vitamin-mineral pellet
(DM basis) ............................................................................................... 68

Composition of experimental diets (DM basis) ....................................... 69
Effects of corn grain in vitro starch digestibility and nitrogen
supplementation concentration on ruminal VF A, pH, and ammonia

concentration ............................................................................................ 70

Effects of corn grain in vitro starch digestibility and nitrogen
supplementation concentration on site of DM and OM digestion ........... 71

Effects of corn grain in vitro starch digestibility and nitrogen
supplementation concentration on site of NDF digestion ........................ 72

Effects of com grain in vitro starch digestibility and nitrogen
supplementation concentration on site of starch digestion ...................... 73

vii

Table 3.8. Effects of corn grain in vitro starch digestibility and nitrogen
supplementation concentration on site of nitrogen digestibility .............. 74

viii

A:P

ADF

ADG

BCP

BW

CP

DIP

DM

DMI

DRC

HMC

MCP

MNE

MP

NDF

OM

TRDOM

UIP

VFA

SFC

LIST OF ABBREVIATIONS AND SYMBOLS
acetate to propionate ratio
acid detergent ﬁber
average daily gain
bacterial crude protein
body weight
crude protein
degradable intake protein
dry matter
dry matter intake
dry-rolled corn
high moisture corn
microbial protein
microbial nitrogen efﬁciency
metabolizable protein
neutral detergent ﬁber
organic matter
truly ruminally degraded organic matter
undegradable intake protein
volatile fatty acid

steam ﬂaked corn

ix

CHAPTER 1

REVIEW OF LITERATURE

1.1 INTRODUCTION

Corn is the world’s most widely utilized feed concentrate, and in the United
States, comprises roughly 80% of the feed grain consumed by livestock (USDA, 1992).
Grain quality is best deﬁned as a measure of the suitability of a grain for its intended use.
The highest quality corn for ﬁnishing cattle would be the corn that results in the greatest
amount of weight gain relative to the amount of corn consumed. Corn hybrids with
different genotypes, or grown under diverse environmental conditions, result in corn that
has different kernel characteristics. These unique kernel characteristics result in corn that
can be designed for speciﬁc end uses.

High-grain diets consumed by feedlot cattle contain high concentrations of starch.
Typically, processed corn supplies much of the starch found in feedlot rations. Starch in
the corn kernel is fermented to volatile fatty acids (VFA; primarily propionate, acetate,
and butyrate) in the rumen, then absorbed serving as the main source of energy for the
ruminant animal. Propionate is the primary precursor for synthesis of glucose by the
liver. Theoretically, the conversion of dietary starch energy into net energy utilized by
various body tissues (i.e., portal-drained viscera, etc.) is greater if assimilation occurs via
intestinal glucose absorption rather than ruminal fermentation and subsequent volatile

fatty acid absorption (Harmon and McCloud, 2001). Ruminal fermentation results in

energetic losses due to methane formation, heat of fermentation, and lower efﬁciency for
use of absorbed substrates. However, limits to small intestinal starch digestion by cattle
have been reported (Krehbiel et al., 1996) and Huntington (1997) estimated the threshold
to be less than 1.5 kg starch/d in concentrate—adapted animals. Therefore, maximal
energetic efﬁciency of growing-ﬁnishing cattle necessitates a high extent of ruminal
starch fermentation (Owens et al., 1997), while maximizing starch digestibility in the
small intestine for that which escapes ruminal fermentation. For the beneﬁts of the
increased efﬁciency of intestinal starch digestion to be realized, carbohydrate
assimilation must occur in the small intestine because large intestinal fermentation is less
efﬁcient due to energetic losses attributed to fermentation of substrate in the large
intestine (Harmon and McLeod, 2001).

Nitrogen (N) and phosphorus (P) excretion is receiving increased attention from
both the beef industry and the Environmental Protection Agency (EPA). Excess
environmental N can cause soil build-up, water contamination via leaching, or increased
ammonia emissions (Sutton and Beede, 2003). Ammonia is a major concern because it
can be dispersed via acid rain, contaminating the ground and water where it falls.
Generally, diets that are excessive in protein or have a greater degree of hind gut
fermentation result in increased protein excretion and thereby, increased amounts of
environmental N.

Recent research (Philippeau et al., 1999; Macken et al., 2003) suggests that both
grain source (i.e., wheat vs. corn), as well as corn vitreousness can inﬂuence ruminal
starch degradability. Little research is available that examines the effects of kernel

vitreousness and grain moisture on extent of ruminal, duodenal, and ileal starch

digestibility. With more available information regarding interactions between site of
starch digestion and metabolizable N intake, nutritionists could more accurately
formulate diets to maximize feed conversion efﬁciency while simultaneously limiting
environmental N pollution.

Improving the feed conversion efﬁciency of feedlot cattle is of utmost importance
when documenting enterprise proﬁtability. Likewise, one of the more expensive
nutrients to add to feedlot rations is N. Corn grain with greater vitreousness has been
shown to undergo a Shift in site of starch digestion from the rumen to the lower
gastrointestinal tract (Philippeau etal., 1997). Site of starch digestion and the
fermentation proﬁles associated with such can also have profound effects on an animal’s
N requirement. Therefore, the interaction between corn grain endosperm type and N
supply could affect site of nutrient digestion, microbial protein production, and
environmental N losses.

The objectives of the current experiments were to determine the effect of corn
vitreousness and processing method on site and extent of starch digestion, digestion
kinetics, and nutrient utilization. Based on site of starch digestion and ruminal
fermentation proﬁles, investigation of differing degradable intake protein concentrations
were also evaluated. Collectively, these results should yield a clearer understanding of
the interaction between site of starch digestion and animal N needs, while also providing

important data pertaining to the effect of site of digestion on bovine N and P balance.

1.2 PHYSICAL CHARACTERISTICS OF CORN HY BRIDS

The corn (Zea mays) kernel is classiﬁed botanically as a caryopsis, which means
it has a dry, indehiscent, single-seeded fruit of which the mature ovary wall (pericarp)
does not separate naturally from the seed. The corn kernel contains four major structural
components: germ, endosperm, pericarp, and the tip cap. These basic structural
components of the corn kernel are depicted in Diagram 1.1. The germ is comprised of
the embryo (precursor tissues for leaves, stern, and root), and scutellum (absorbs nutrients
from the endosperm during germination). The germ contains precise genetic information,
enzymes, a large proportion of the kemel’s essential amino acids, and assorted vitamins
and minerals (most notably phosphorus). The endosperm is made up of elongated cells
packed with starch granules embedded into a continuous protein matrix within the
individual cells (Wolf et al., 1952). Generally, the endosperm constitutes about 82 to
84% of the dry weight of the kernel and is 86 to 89% starch by weight (Earle et al.,
1946). The endosperm contains about 98% of the starch of the kernel, as well as about
74% of the total protein (Earle et al., 1946). The starchy endosperm can be of two types,
ﬂoury (soft, less vitreous) and ﬂinty (hard, more vitreous), where vitreousness represents
the proportion of horny endosperm of the kernel. Many different corn hybrids are
commercially available and differ largely in their chemical composition and structure.
From a cattle feeding perspective ﬂinty, ﬂoury, dent, waxy, and high-oil corns have been
evaluated. Flinty corn has a rounded crown and the hardest kernels due to the presence
of a large and continuous volume of vitreous endosperm. In contrast, ﬂoury corn has a
rounded or ﬂat crown, but contains less vitreous endosperm that is more readily digested.

This type of corn is also more extensively damaged during handling and artiﬁcial drying.

Fine pieces of corn that are produced (ﬁnes) after such handling, combined with the
formation of stress cracks result in an increased susceptibility to insect and mold damage
due to reduced airﬂow during storage.

Dent corn is the most commercially used corn type in the United States. Dent
corn has a depressed crown that forms as the maturing kernel dehydrates. This “dent” is
formed as the rigidity of the cylinder of vitreous endosperm prevents the central core of
ﬂoury endosperm from shrinking uniformly during drying causing an indentation on the
crown (Watson, 1987). Dent corns of the United States were developed in precolonial
North America from natural hybridization between northern ﬂint and southern ﬂour corns
(Watson, 1987). Because corn was developed as a hybrid cross, it can exhibit vast
differences in vitreousness induced by environmental and heritable inﬂuences (Hamilton
et al., 1951). Speciﬁcally, environmental factors such as moisture differences between
ﬁelds, temperature, and soil N supply and uptake effect physical characteristics between
corn hybrids. The average chemical composition of yellow dent corn is illustrated in

Table 1.1.

Table 1-1. Chemical composition of cracked yellow dent corn grain "

 

Protein, % Fat, % NDF, % ADF, % Calcium, % Phosphorus, %

 

 

9.8 i 1.08 4.06 :I: 0.64 10.8 i 3.57 3.3 :t 1.83 0.03 :t 0.07 0.32 :l: 0.04

 

“NRC (1996)

F loury endosperm is characterized by large starch granules loosely associated

within a protein matrix (Dado, 1999), resembling basketballs held in a large mesh bag

 

(Owens and Zinn, 2004). Floury endosperm is Opaque to transmitted light. Duvick
(1961) reported this was due to minute air pockets between starch granules. Air pocket
formation between starch granules in ﬂoury endosperm is a result of the protein matrix
tearing during kernel drying. After processing, the cells of ﬂoury endosperm are
completely disrupted, releasing free starch granules due to incomplete closure by the
protein matrix (Watson, 1987). Consequently, starch availability in ﬂoury endosperm is
more susceptible to processing techniques (Huntington, 1997).

Alternatively, the cell walls of more vitreous endosperm are broken after drying,
but little release of free starch granules occurs due to the strength of the surrounding
protein matrix implying a tightly compacted endosperm where starch granules are
embedded in a thick protein matrix (Watson, 1987). Corn hybrids with this type of
physical characteristic tend to be more resistant to disease and also produce heavier ﬁeld
test weights relative to less vitreous corn hybrids (Owens, personal communication). "Not
surprisingly, these two attributes have been important selection criteria for a vast majority
of seed corn producers.

The pericarp is the outermost membrane structure of the corn kernel. The
pericarp is resistant to water, while also protecting the kernel from insect susceptibility
and ﬁtngal infection. The pericarp makes up 3 to 6% of the corn kernel dry weight, while
containing nearly half of the NDF of the kernel (Owens and Zinn, 2004). The tip cap is
the only kernel component not covered by the pericarp, and is the attachment site to the

cob.

Diagram 1.1. Structural components of corn kernel

Scutellum
Pericarp

Endosperm

 

 

Tip cap

1.2.1 Corn Starch Characteristics

Starch granules are 5 to 30 pm in diameter and are embedded in the protein matrix
of the kernel. Starch granules are made up of two glucan polymers, amylose and
amylopectin. Amylose is an essentially linear molecule of glucose units linked by 0t-1,4
bonds while amylopectin is a branched molecule composed of linear regions of 0t-1 ,4-
linked glucose units with 0t-1,6-linked branch chains every 20 to 25 glucose residues
(Marshall and Whelan, 1974). The ratio of amylose to amylopectin is often utilized as an
indicator of grain digestibility. While it is generally accepted that amylose makes up

about 25 to 30% of the starch in corn while amylopectin constitutes 70 to 75% (Boyer

and Shannon, 1987), Allen (1991) documented that corn starch can range from nearly
100% amylose to nearly 100% amylopectin due to genetic and environmental differences.

As previously mentioned, the physical properties of starch granules are important
in determining their biological and economic value relative to the corn kernel. Some of
these important features include starch granule morphology, amylose content,
crystallinity, gelatinization temperature, and digestibility. These factors are all altered by
different endosperm types (Boyer and Shannon, 1987).

Corn maturity and moisture concentration can have a profound affect on
endosperm starch and protein fractions. During kernel development, the cells in the
central crown region of the endosperm begin starch accumulation ﬁrst, with the lower
endosperm cells initiating starch synthesis and accumulation much later (Boyer et al.,
1977). Creech (1965) documented that with maturity, the hard starch layer and milk line
move toward the cob and kernel sugar content declines while starch increases. As corn
matures, sugars are translocated from the stover to the ear and these sugars are converted
to starch. The accumulated starch is hard above the milk line, and soft below the line.
Thus, it is more difﬁcult to fracture a mature kernel because of hard starch accumulation.
Likewise, as kernel maturity increases, the amylose content increases resulting in
decreased digestibility (Owens and Zinn, 2004). Similarly, Philippeau and Michalet-
Doreau (1997) documented that while vitreousness differed between dent and ﬂint
genotypes by 11.8%, immature grain had 28.2% less vitreousness when compared to

more mature grains.

1.2.2 Grain Processing

Many different dry- and wet-milling techniques are used to amplify starch
utilization. Physical processing increases the rate of ruminal starch digestion by breaking
the outer coat of the kernel to increase access sites for ruminal microorganisms and
enzymes. The application of heat, moisture, and pressure Giigh moisture or steam-ﬂaked
grains) also alters ruminal fermentation; increasing starch digestion by disrupting the
protein matrix surrounding starch granules and gelatinizing starch (destroying its
crystalline structure; Kotarski et al., 1992). Thus, the main function of grain processing
is to improve the efﬁciency of feedlot cattle production by enhancing starch availability
(Owens et al., 1997). Galyean (1996), in a survey of six feedlot consultants responsible
for the nutritional programs of 3.6 million cattle, reported that all corn was processed
before feeding, most commonly by steam ﬂaking, followed by dry-rolling and high-
moisture storage. Huntington (1997) summarized data from 14 trials published on the
inﬂuence of corn processing on starch digestibility. Ruminal starch digestibility for dry-
rolled, high moisture, and steam-ﬂaked corn was 76.2, 89.9, and 84.8% of intake,
respectively. Postruminally, 68.9, 67.8, and 92.6% of starch entering the duodenum was
digested, while total tract starch digestibilities were 92.2, 95.3, and 98.9% of intake,
respectively.

In a more recent study, Macken et a1. (2003) reported that starch digestibility was
signiﬁcantly greater for less vitreous corn grains harvested as either dry rolled (18%
moisture) or high moisture (29% moisture) corn when compared to corn grain with
greater vitreousness. When fed as dry-rolled corn, steers consuming less vitreous corn

grain had improved ADG and feed efﬁciency. These results suggest that feeding less

vitreous corn grain, regardless of grain processing technique, results in greater ruminal
starch fermentation and pre-ileal starch digestibility.

Wet processing methods are used to increase surface area of starch. Corn grain
that is to be steam ﬂaked is harvested as dry corn. However, the whole corn kernels are
placed into a steam chamber and moisture content is increased to approximately 18%.
Afterwards, the kernels are passed through rollers to produce a “ﬂake”, which results in a
completely disrupted endosperm structure of the corn kernel. Use of corn grain with less
vitreousness in the ﬂaking process is not generally practiced as a higher proportion of
ﬁnes, more fragile ﬂakes, and a reduced ﬂaking rate is typically witnessed (Owens and
Zinn, 2004). Greater amounts of ﬁnes post-ﬂaking can hasten the development of
metabolic disorders.

High-moisture grains vary widely in moisture content and can be processed in
many different ways before storage and feeding (Owens et al., 1997). In order to
maximize ruminal starch degradability and feed efﬁciency, high moisture corn grain must
have adequate moisture content (26 to 31% moisture) and a sufﬁcient duration of
fermentation (Owens and Zinn, 2004). For storage in bunker silos, or other oxygen-
limiting structures, grain is typically harvested at higher moisture content (i.e., 26%
moisture or above) and rolled or ground to permit thorough packing. Regressions of
daily gain and body weight-adj usted metabolizable energy (ME) against the percentage
of moisture of high-moisture grain fed in all forms revealed that both ADG and ME
should be optimized between 30 and 31% moisture (Owens et al., 1997).

Length of the ensilment and particle size can also affect feeding value. In three

separate experiments, Stock et al. (1991) documented that yearling steers consuming

coarse ground HMC gained faster and were more efﬁcient than steers fed whole HMC.
Further, as length of storage increased, in vitro rate of starch digestion and soluble N
content increased, while grain pH decreased (Stock et al., 1991). Benton et a1. (2004)
also reported that in situ starch disappearance corresponded with increased N solubility as
the length of storage increased. Knowlton et a1. (1998) documented that little starch
digestion occurred in the small intestine of lactating dairy cows fed diets consisting of
coarsely rolled corn, while a moderate increase was witnessed when the same corn was
ground (13% of abomasal ﬂow prior to the ileum). Interestingly, when the same corn
hybrid was included in the diet as either rolled or ground high moisture com, 64% and
59% disappeared in the small intestine, respectively. Particle size has been shown to
limit starch digestion in the small intestine, as larger particle sizes are more likely to pass
into the large intestine (Rust, 1983).
1.3 SITE 0F STARCH DIGESTION

Research has clearly documented that starch digestion can be shifted to various
portions of the gastrointestinal tract based on physical characteristics of the corn kernels.
The major end product from starch digestion at each site along the gastrointestinal tract
varies. Ruminal degradation of carbohydrates produces VF A, while increasing energy
loss due to the formation of methane and heat losses associated with fermentation.
Digestion in the small intestine is thought to be more energetically efﬁcient (Harmon and
McCloud, 2001). Digestion that occurs in the large intestine results in similar energetic
losses as ruminal degradation, plus energy and N that is contained in bacteria is excreted
in the feces. Understanding how the physical and chemical characteristics of corn kernels

affect digestibility is discussed below.

11

1.3.1 Ruminal Starch Degradability

Structure of the kernel endosperm can range from 0% to 100% vitreous due to
genetics. A study involving steers ﬁtted with both ruminal and duodenal ﬁstulas
indicated a shift in the site of starch digestion from the rumen to the small intestine when
com with mostly vitreous endosperm replaced corn with little vitreous endosperm
(Philippeau et al., 1999). Likewise, corn with greater vitreousness had a higher
percentage of starch intake digested in the hindgut, relative to steers consuming corn
grain with less vitreousness (Philippeau et al., 1999). Waxy varieties of corn or sorghum
contain only amylopectin, and have a faster rate of digestion than nonwaxy varieties
(Huntington, 1997). Philippeau and Michalet-Doreau (1997) observed that amount of
vitreous endosperm in corn grain explained 86% of the variation in ruminal starch
degradability; as vitreousness increased, starch degradability linearly decreased. Less
vitreous corn varieties were observed to have higher ruminal and small intestinal starch
digestibility than more vitreous corn varieties (Philippeau and Michalet-Doreau, 1997).

Starch in cereal grain can be completely digested during passage through the total
digestive tract. However, the rate and extent of ruminal starch digestibility, rate and
extent of VF A production, and post-ruminal starch ﬂow vary in response to grain type
(Owens et al., 1986). Digestibility of corn grain in vivo can range from 51 to 93% and is
dependent on a variety of factors (Nocek and Tamminga, 1991). Likewise, different
processing (i.e. rolling, grinding, or steam ﬂaking) and conservation methods (i.e. dry or
high-moisture) can inﬂuence both total tract digestibility of grains as well as site of

digestion (Firkins et al., 2001).

Although ruminal starch digestion can be manipulated, its effect on feed intake
varies. Site of starch digestion can change the availability of metabolites to the animal.
Ruminally degraded starch is fermented to VFA (primarily propionate and acetate) and
contributes to production of microbial cells. Increased degradation of starch in the rumen
generally increases ME yield from starch because of limited potential for starch
digestibility in the small intestine (Kotarski et al., 1992; Huntington, 1997). However,
rapid fermentation of starch to VFA in the rumen may overwhelm the buffering and
absorptive capacity of the rumen, leading to reductions in rumen pH that may decrease
dry matter intake (DMI; McCarthy et al., 1989). Owens et a1. (1997) reported that more
extensive grain processing reduced average daily gain (ADG) slightly. This reduction
was attributed largely to reduced DMI. Reduced DMI of rapidly fermented grain sources
and extensively processed grain has been attributed to excessive rates of acid production
in the rumen. This accumulation of acids in the rumen leads to subclinical acidosis,
which increases day-to-day variation in DMI (Stock et al., 1995). Alternatively, Allen
(2000) proposed that propionate decreases DMI by stimulating oxidative metabolism in
the liver. Other studies (Herrera-Saldena and Huber, 1989; Oliveira et al., 1995) have
shown no changes in DMI as a result of increased ruminal degradation of starch.

1.3.2 Intestinal Starch Digestibility

While it is clear that increased total tract starch digestion improves performance
(N ocek and Tamminga, 1991), the optimal amount of postruminal starch digestion is
unclear. Much of the available research compiled measures the site of starch digestion
relative to different grain sources (i.e., corn, grain sorghum, wheat, etc.), which is

confounded by protein content of the diet and ﬁber concentration.

Starch can bypass the rumen and be enzymatically digested and absorbed as
glucose. Intestinal absorption of glucose is theoretically more efﬁcient than ruminal
fermentation to VP A (Harmon and McLeod, 2001). Digestibility of starch that passes the
rumen intact varies signiﬁcantly. Owens et a1. (1986) calculated that starch digestibility
in the small intestine accounts for 80% of the post-ruminal starch digestion, and starch
digestion in the small intestine increases as starch ﬂow increases. Site of post-ruminal
starch digestion is important because end products of small intestinal and large intestinal
starch digestion differ.

Starch is digested in the small intestine by (Jr—amylase secreted by the pancreas,
which hydrolyzes amylose and amy10pectin into limit dextrins and oligosaccharides (2-3
glucose units; Harmon, 1993). The process is completed by oligosaccharidases that are
located on the brush border membrane of intestinal microvilli (Huntington, 1997).
Ruminants rely heavily on maltase and isomaltase activity for the production of glucose
(Huntington, 1997); however, the capacity of the small intestine for starch digestion
appears to be limited by supply of pancreatic amylase, rather than capacity for glucose
absorpton (Kreikemeier et al., 1991). Pancreatic amylase secretion is a function of
energy intake (Harmon and Taylor, 2005).

The structure of starch entering the small intestine may be more important than
the amount of starch when determining starch digestibility. Kreikemeier et al. (1991)
infused graded levels of three different carbohydrates (i.e., glucose, raw corn starch, or
corn dextrin) into the abomasum of steers and measured starch ﬂow at the ileum. Within
any infusion concentration, the disappearance of infused dextrin anterior to the ileum was

always greater than the disappearance of raw corn starch (81% vs. 58%). These

14

differences may be attributed to the fact that com dextrin is a heat and acid treated
derivative of corn starch. Small intestinal enzymes may have limited ability to digest
starch that is still surrounded by the protein matrix and still maintains a crystalline
structure.

Research has also shown that glucose absorption from the small intestine
decreases from the duodenum to the ileum (Krehbiel et al., 1996). Regardless, glucose
that is assimilated in the small intestine is either transported into and utilized by intestinal
enterocytes or transported to the portal circulation as lactate or glucose (Allen, 2000).
Earlier research completed by Rust (1983) indicated that some glucose absorbed from the
small intestine is deposited in omasal adipose tissue.

1.4 METABOLIZABLE PROTEIN SUPPLY

Metabolizable protein (MP) must be present in the diet in sufﬁcient amounts and
is accounted for by two separate fractions, degradable intake protein (DIP) and
undegradable intake protein (U IP). Degradable intake protein is ruminally degraded and
supplies peptides, amino acids, and other growth factors for microbial fermentation and
growth (Owens and Bergen, 1983). Russell et al. (1992) indicated that insufﬁcient DIP
in the diet may reduce energy yield from carbohydrate fermentation, thereby lowering
VFA production and energetic efﬁciency of the diet. According to the NRC (1996),
increasing ruminally available starch should increase the synthesis and efﬁciency of
production of bacterial crude protein (BCP) in the rumen and therefore, increase the DIP
requirement. This assumption is based primarily on results obtained from Shain et a1.
(1998) and Cooper et al. (2001), who concluded that ﬁnishing diets using high-moisture

corn (HMC) require more DIP than dry-rolled corn (DRC) diets because of increased

15

ruminal starch degradability. Undegradable intake protein is the fraction of MP that
escapes ruminal degradation and is equivalent to the MP requirement for the animal
minus the MP supplied from BCP (NRC, 1996). Undegradable intake protein is not
digested in the rumen and is partially utilized by the small intestine. Approximately one-
half of the protein digested in the small intestine is microbial protein (Owens and Bergen,
1983).

The type of protein being supplemented has been shown to greatly impact starch
digestion, MCP production, and microbial N efﬁciency (MNE) for ruminant animals.
Microbial efﬁciency is deﬁned as gram of microbial N reaching the duodenum per
kilogram of truly ruminally degraded organic matter (TRDOM). Microbial efﬁciency
can be inﬂuenced by passage rate, where a faster rate of passage is positively correlated
to microbial efﬁciency (Oba and Allen, 2000). Conversely, MNE can be reduced when
peptides or amino acids are not supplied in sufﬁcient amounts (Van Kessel and Russell,
1996). Ruminal bacteria responsible for fermentation of non-structural carbohydrates
grow faster and more efﬁciently when incorporating amino acids and peptides into
microbial protein (Russell and Sniffen, 1984). Likewise, ruminal microorganisms
fermenting non-structural carbohydrate can obtain approximately two thirds of their N
from amino acids or peptides (Russell et al., 1983). Ruminal ﬁber digesting bacteria
derive all of their N requirements from ammonia, while several species have peptide
requirements (Bryant, 1973). Soybean meal and urea are two commonly used sources of
protein in ﬁnishing diets. Urea is exclusively degraded in the rumen and is used
extensively in ﬁnishing diets because of its low cost and ease of diet incorporation. Urea

supplementation has been shown to increase dietary energy utilization, but does not

directly contribute to the MP supply. Supplemental urea is necessary for organic matter
(OM) and starch digestion to be increased, however, ﬂows of total N and microbial N to
the duodenum remain unchanged (Milton et al., 1997). Therefore, the use of urea is
limited to the amount that provides sufﬁcient ruminal ammonia to maximize MCP
production and/or OM digestion. In contrast, soybean meal contains a degradable protein
fraction that supplies ammonia, amino acids, and peptides to rumen microbes, as well as
an escape fraction that increases MP reaching the small intestine (Milton and Brandt,
1994). In a feeding study to compare urea and soybean meal, Milton and Brandt (1994)
found that steers receiving soybean meal had greater feed efﬁciency. This result was
likely due to increased MP supply and/or improvements in fermentation from the
provision of ruminally degradable amino acids. Improvements in carcass weight and
longissimus dorsi area, with little increase in carcass fat, indicate that soybean meal
increased total supply of peptides and amino acids available to the animal for protein
deposition (Milton and Brandt, 1994). In a subsequent metabolism trial by Milton et a1.
(1997), total tract starch digestion, duodenal microbial N ﬂow, and MNE were greater in
steers supplemented with soybean meal compared to urea, leading to the conclusion that
soybean meal supplementation increased MP supply and dietary energy utilization for
steers consuming dry-rolled, com-based diets compared to urea supplemented diets.
Alternatively, Gill et a1. (1979) reported that urea supplementation was superior to
soybean meal for rate of gain and feed efﬁciency with dry corn, while soybean meal was
superior to urea for HMC-based diets. Shain et a1. (1998) documented improved ADG
and feed efﬁciency when diets were supplemented with urea. If digestion is limited by

available ruminal ammonia for synthesis of MCP, higher solubility of nutrients in HMC

or N should prove beneﬁcial. Conversely, when ruminal fermentation occurs faster than
the ATP produced can be utilized by the microbial population, MNE can be reduced
when ATP is used for non-growth functions in energy spilling reactions (Russell, 1998).

Available data suggests that high-moisture and steam-ﬂaked com-based diets
require 50% more DIP due to increased potential for MCP production (Klopfenstein and
Erickson, 2002). Relative to its’ total MP content, high-moisture corn has lower inherent
UIP available as a result of increased N solubility, indicating the need for UIP
supplementation (Klopfenstein and Erickson, 2002). This conclusion is apparent when
evaluating data comparing different protein sources in high-moisture corn based diets.
Metabolizable protein requirements for growing feedlot steers are changed throughout the
feeding period due to changes in intake, body weight, and composition of gain. It is
generally assumed that an animal’s DIP requirement increases due to increased intake as
body weight increases. Theoretically, the requirement for UIP decreases as body weight
increases due to both a larger supply of MCP via increased DIP utilization and the
requirement for amino acids are reduced later in the growth curve.
1.5 EVIRONMENTAL NITROGEN AND PHOSPHORUS

Concentrated animal agriculture may affect air quality as well as water quality.
After excretion, the organic substrate in solid and liquid animal waste is subject to
microbial conversion to cellular biomass and gases, including ammonia. Nitrogen
contamination of ground and surface water, combined with air pollution from ammonia
emissions, are major environmental concerns. Nitrogen can enter the farm in feed,
fertilizer, and legume ﬁxation and may be exported in product (milk, meat, cash crop) or

may be lost to the environment (via direct deposition in surface water, leaching, runoff,

ammonia volatilization, or denitriﬁcation; Kohn et al., 1997). Nitrogen contamination of
surface water can result in algae blooms. These blooms shade aquatic vegetation, thereby
reducing photosynthetic activity. Likewise, decomposition of algae consumes dissolved
oxygen in the water (eutrophication), impairing its usefulness for recreation, drinking,
ﬁshing, and industry uses (Sims et al., 1998).

The amount of surplus N available for conversion to ammonia gas is a function of
the dietary N concentration, ration composition and feed efﬁciency (Auverrnann, 2002).
Nitrogen digestibility for beef cattle consuming a high concentrate diet is approximately
65% of the feed—borne N (Auverman, 2002). Therefore, when cattle are consuming 11.3
kg/animal/d DM, total N excreted by feedlot cattle consuming a diet containing 13.5%
CP diet is 0.24 kg N/animaI/d. While ammonia emission for domesticated animals is
estimated to be 21.95 x 106 metric tons of leear or 40% of total N emission, beef cattle
operations are responsible for an estimated 8.74 x 106 metric tons of N/year or 16% of the
total (Asman, 2002). James et a1. (1999) found that decreasing N intake of Holstein
heifers by 14% resulted in a 28.1% reduction in ammonia loss from mixed feces and
urine. Signiﬁcant decreases in concentrations of urinary urea N and total N and in
proportions of N excreted in urine were also observed (James et al., 1999). F rank and
Swensson (2002) found that manure from dairy cows fed low protein diets emitted
signiﬁcantly less ammonia than manure from cows fed high protein diets. Likewise,
Marini and Van Amburgh (2003) observed linear decreases in excretion of both urea N
and total urinary N in heifers fed diets with reduced N content.

Phosphorus (P) is also a current issue with regard to environmental implications

of conﬁnement animal feeding operations (CAFO). Phosphorus also enters the farm via

feed and/or fertilizer. Roughly 30 to 50% of feed P is captured in meat and milk, and 50
to 70% is excreted in manure and urine (Knowlton et al., 2001; Knowlton and Herbein,
2002). Unlike N, P does not volatilize from the pen surface, so whatever is excreted is
typically spread over land used for crop production via manure or lost in surface runoff.
If manure P application is greater than crop P uptake, P accumulates in the soil and can
rtm off into surface water, causing eutrophication and algae blooms.

Recent data ﬁom P feeding trials have demonstrated that P concentration in feces
is directly related to P levels in diets. Therefore, P surpluses and potential environmental
losses can be reduced through diet manipulation. Dou et a1. (2002), through data
compiled from three dairy feeding trials, documented that lower P in diets resulted in
lower P content in feces. Supplemental P in ﬁnishing beef cattle diets appears to be
unnecessary (Erickson et al., 1999; Erickson et al., 2000) as the P requirement is met by
com grain in the diet (0.32% P; DM basis; NRC, 1996). Thus the amount of P
contributed from corn grain in typical feedlot rations is sufﬁcient to meet requirements.
Phytate-P is degraded by ruminal microbes that render the P available to ruminant
animals. On average, 95% or more of the P bound to phytate is released to a useable
form during ruminal fermentation (Morse et al., 1992). Removing supplemental P from
feedlot diets fed to yearling and calf-fed animals resulted in reduced P intakes (51% and
41%, respectively; Erickson et al., 2000). Likewise, reduced dietary P caused a 59% and
38% reduction in manure P concentration for yearling and calf-fed animals.

In summary, corn hybrids with different genetics result in corn that has different
kernel characteristics. The physical structure and chemical composition of cereal

starches, along with the interactions between starch and protein content can alter the

20

digestibility of grain for livestock. Recent research (Philippeau et al., 1999; Macken et
al., 2003) suggests that grain source (i.e., wheat vs. corn) and corn endosperm type (i.e.,
ﬂoury vs. ﬂinty) can increase both ruminal starch degradability and postruminal starch
digestion. Additionally, degradable intake protein requirements for an animal increase
with greater ruminal starch degradability (NRC, 1996) and could be altered based on the
extent of ruminal fermentation. While corn processing methods have been shown to
inﬂuence protein requirement (Cooper et al., 2002), little research has been conducted to
determine the speciﬁc effects of corn grain endosperm type and degradable intake protein
(DIP) fractions on N utilization and retention in growing steers with varying rates of
ruminal starch degradation. Collectively, two experiments were conducted to determine
the interaction between corn grain endosperm type (IVSD) with processing method and
dietary N concentration on site and extent of nutrient digestibility in Holstein steers

consuming a high-grain diet.

21

CHAPTER 2

EFFECT OF IN VITRO STARCH DIGESTION AND PROCESSING METHOD
OF CORN ON SITE AND EXTENT OF NUTRIENT DIGESTION AND
RUMINAL METABOLISM IN HOLSTEIN STEERS FED A HIGH-GRAIN DIET

ABSTRACT

This experiment was conducted to evaluate the interaction between in vitro starch
digestion and processing method on site of nutrient digestion in Holstein steers
consuming a high-grain diet. Ten ruminally, duodenally, and ileally cannulated Holstein
steers (initial BW = 200 i 21 kg) were used in a replicated 4 x 4 Latin square design
experiment with a 2 x 2 factorial arrangement of treatments. Treatments were in vitro
starch digestion (high and low) and conservation method (high moisture and dry-rolled).
Interaction of treatments was not detected (P > 0.15) for any measure of digestibility or
ruminal metabolism. When expressed as a percentage of duodenal ﬂow, 21.5% less
starch was digested in the small intestine of steers consuming diets containing dry-rolled
corn, with an 8% decline in apparent total-tract starch digestibility. Total molar
proportion of volatile fatty acid tended to be greater (P = 0.10) for diets containing corn
hybrids with high in vitro starch digestion. Relative to nitrogen digestibility, diets
containing high moisture corn resulted in a greater amount of microbial nitrogen ﬂow to
the duodenum and greater microbial efﬁciency (P < 0.05). Both processing method of
corn and ruminal starch degradation of corn hybrids appear to have a profound impact on

site and extent of starch digestibility and nitrogen metabolism.

22

2.1 INTRODUCTION

Corn hybrids with different genotypes or grown under diverse environmental
conditions, result in corn that has different kernel characteristics. The physical structure
and chemical composition of cereal starches, along with the interactions between starch
and grain protein content can alter the digestibility of grain for livestock. Research
(Philippeau et al., 1999; Macken et al., 2003) suggests that grain source (i.e., wheat vs.
corn) and corn endosperm type (i.e., ﬂoury vs. ﬂinty) can increase both ruminal starch
degradability and postruminal starch digestion. Steam-ﬂaking of more vitreous corn
grain has been shown to increase total tract digestibility when compared to diets
containing dry-rolled corn (Corona et al., 2006).

Theoretically, the conversion of dietary starch to energy utilized by various body
tissues is greater if assimilation occurs via intestinal glucose absorption rather than
ruminal fermentation and subsequent volatile fatty acid absorption (Harmon and
McCloud, 2001). Fermentation in the rumen and large intestine results in energetic
losses due to methane formation, heat of fermentation, and lower efﬁciency for use of
absorbed substrates. However, ruminal degradation of carbohydrate allows for VFA
formation and subsequent absorption which is critical to meet the animal’s energetic
requirement. Therefore, maximal energetic efﬁciency of ﬁnishing cattle necessitates a
high extent of ruminal starch fermentation, while maximizing starch digestibility in the
small intestine and limiting digestion in the large intestine (Owens et al., 1997).

We hypothesized that com grain with high in vitro starch digestibility (IVSD)
would be more ruminally ferrnentable than corn grain with low IVSD. Also, corn grain

with high IVSD that bypasses ruminal degradation may have greater small intestinal

23

digestibility. Likewise, high moisture processing of corn would result in both an increase
in ruminal starch degradability and the percentage of starch digested in the small intestine
when compared to dry-rolled corn. Consequently, greater amounts of starch might be
digested in the small intestine for steers consuming diets containing dry-rolled corn due
to greater ruminal starch passage. The objective of this experiment was to evaluate the
potential interaction between high and low IVSD and conservation method (high
moisture ensiled vs. dry rolled) on site of nutrient digestion in Holstein steers.
2.2 MATERIALS AND METHODS
2.2.1 Corn Grain Planting, Harvest, and Processing

Two corn hybrids were obtained from Pioneer Hi-Bred international to represent
ﬂoury (33R77) and ﬂinty (34B97) endosperm types (Owens, personal communication).
While high moisture corn grain containing ﬂoury endosperm did not differ in
vitreousness compared to high moisture corn grain containing ﬂinty endosperm (45.6%
vs. 41.8%; P = 0.17), IVSD was signiﬁcantly greater for 33R77 than 34B97 when
evaluated as either HMC or DRC (P < 0.01). Therefore, instead of vitreousness,
treatments are represented as corn grain with low IVSD (LOW) or high IVSD (HIGH).

Two corn hybrids representing high (Pioneer 33R77; HIGH) and low (34B97;
LOW) IVSD (Pioneer Hi-Bred International, Des Moines, IA) were planted in a lS-acre
ﬁeld located on the Michigan State University farm (East Lansing, M1) on May 8, 2003.
These speciﬁc hybrids were chosen based on chemical composition differences
determined and recommended by Pioneer Hi-Bred International. The ﬁeld was divided
in half, with each half containing one experimental hybrid. Each half was monitored

extensively for DM throughout the summer. For high moisture corn conservation, half of

24

LOW was harvested on September 15, 2003 at a DM of 66.3% and half of HIGH was
harvested on September 23, 2003 at a DM of 67.3%. The remaining corn for each variety
was harvested as dry corn at 81% DM on December 12, 2003 and commercially dried at
15°C to 87% DM (Michigan Crop Improvement Association and Foundation Seed,
Okemos, MI). High moisture corn (HMC) was coarsely ground at the Michigan State
University feed mill prior to storage in two separate Ag-Bags for ensiling (Miller-St.
Nazianz, St. Nazianz, WI). Dry corn was coarsely rolled (DRC) prior to feeding. Corn
was planted at a rate to provide a population for both varieties of 28,000 plants per acre.
The average yield for LOW was 130 bushel per acre, while HIGH yielded 162 bushel per
acre. Nutrient compositions and physical characteristics of the corn grain treatments used
in the experiment are shown in Table 2.1.

2.2.2 Experimental Design and Data Collection

All surgical and animal care procedures described herein were approved by the
All-University Committee for Animal Use and Care of Michigan State University
(#11/03-145-00).

Eight Holstein steers were utilized in a replicated 4 x 4 Latin square design, with
two extra steers. The squares were balanced for carryover effects so that each dietary
treatment followed the other dietary treatments an equal number of times. Within each
square, four steers were randomly assigned to one of four experimental treatments. A 2 x
2 factorial arrangement of treatments was utilized within each square. Experimental
treatments consisted of two corn hybrids (HIGH and LOW) and two corn processing
methods (HMC and DRC). Steers were vaccinated and treated for both internal and

external parasites prior to surgical cannulation procedures.

25

Steers (four months old; BW = 267 kg) were transported to the Department of
Large Animal Clinical Science, College of Veterinary Medicine, Michigan State
University where they were ﬁtted with ruminal, duodenal, and ileal cannulas according to
procedures outlined by Streeter et al. (1992). Steers were retained overnight for
observation and then transported to the Michigan State University Purebred Beef Center.
Steers were monitored and surgical Sites cleaned twice daily over a four week period to
prevent infection. If infection was determined to be present, steers were treated with
penicillin and monitored by a consulting veterinarian. After recovery, steers were moved
to the Beef Cattle Teaching and Research Center.

Experimental diets contained either DRC or HMC treatment hybrids, corn silage,
soybean meal, vitamins and minerals, and limestone. The diets were formulated to
contain 13% crude protein, 2.1 Meal/kg NEm, and 1.4 Meal/kg NEg (NRC, 1996).
Supplemental limestone was added to balance calcium concentration for each diet due to
high levels of phosphorus associated with the experimental treatments. Ingredient and
nutrient compositions of the experimental diets are presented in Table 2.2 and 2.3. In
order to keep diets isoenergetic, 5.6% more corn silage was added to the diets of steers
consuming HMC treatments.

High moisture or dry rolled com, corn silage, soybean meal, vitamins, minerals,
and limestone were weighed and mixed daily. Steers were fed once daily at 0800 h at
110% of expected intake. Amount of feed delivered and orts were recorded daily.
Dietary feed samples (0.5 kg) and individual steer orts (10% of orts on a wet weight

basis) were obtained daily during the collection period, frozen and composited within

26

each collection period. Diet components were evaluated after each period for DM and
diets were re—balanced accordingly.

Experimental periods were 21 d in length: 12 d allotted for diet adaptation, 4 d
for total fecal and urine collection, and 5 d for ruminal, intestinal, and fecal digesta
sampling. Steers were adapted to metabolism stalls and fecal collection bags utilized
throughout the experiment by placing steers in metabolism stalls for a period of 10 (1.
After this time, DMI of steers was equivalent to previously recorded intake. Steers were
housed in covered pens bedded with wood chips to preserve intestinal cannula integrity
and prevent lameness. Two steers, one from each square consuming the same treatment,
were housed in a pen for use in the experiment if needed. During adaptation, steers were
fed 110% of previously recorded intake. After treatment adaptation, steers were placed
into metabolism stalls for the remainder of the period for digesta sampling. Steers were
weighed at the beginning of the trial and at the end of each period. Data from two steers,
receiving identical treatments, were removed prior to the second period due to duodenal
and ileal cannula failure. These steers were replaced with two alternate Holstein steers
that had been previously adapted to similar diets and to metabolism stalls for the
remainder of the trial.

Total urine and feces collection was conducted during the ﬁrst 4 d of each
sampling period. Steers were ﬁtted with a harness and fecal collection bag. Fecal bags
were removed at 0700 h, weighed, and a 500 g subsample obtained daily. Samples were
immediately frozen at -20°C until nutrient analyses were conducted. Urine collection
bags were placed at the end of each metabolism stall to collect all urine excreted by each

steer. Urine collection bags were acidiﬁed with 50 mL concentrated 6N sulﬁtric acid as a

27

preservative for ammonia and emptied into individual plastic containers at 0730 h,
weighed, volume determined, and a 100 mL subsample obtained daily.

Chromic oxide (0203) was used as a marker of passage to estimate nutrient
digestibility in the rumen, small intestine, large intestine and total tract. Gelatin capsules
(1.5 oz; Torpac Inc., Philadelphia, PA) were ﬁlled with 5 g 0203 and dosed through the
rumen cannula at 0730, 1530, and 2330 h (total of 15 g 0203 per d) from d 14 to 20 of
experimental period. A priming dose of Cr203 was administered on d 14 at three times
the regular dose.

Determination of ruminal, small intestinal, and total tract digestibility was
accomplished via collection of duodenal and ileal digesta, and feces collected every 9 h
starting at 1200 h on d 16 and continuing until 0300 h on d 18. This sampling time
reﬂects every 3 h time point across a 24 h time interval to account for diurnal variation.
One 500 mL mixed rumen sample was obtained during sampling for later purine analysis.
Ruminal contents from six different sites were combined and strained to obtain ruminal
ﬂuid. One 100 mL liquid subsample was obtained for VFA analysis. Ruminal pH was
determined using a calibrated pH probe (model 230A, ATI Orion, Boston, MA) at each
sampling time. After collection, each sample was immediately frozen at -20°C until
further analysis. Effect of experimental treatments on relative rate of VFA absorption
and rate of liquid passage was measured on d 20 using a pulse dose of valeric acid and
cobalt EDTA (Allen et al., 2000). Solutions (634 ml, pH 6.0) containing valeric acid
(1.84 moles) and Co-EDTA (5 g) were pulse-dosed 2 h post-feeding. Ruminal ﬂuid
samples were collected every 30 min during the initial 12 h, every 1 h from 12 to 16 h

time points, every 2 h from 16 to 20 h time points, and once at 24 h. Samples were

28

frozen immediately after collection until later analysis for cobalt and valerate
concentration.
2.2.3 Sample analyses

In vitro starch digestibility of composited corn samples obtained at harvest was
completed after a 7 h incubation (Goering and Van Soest, 1970). Prior to trial initiation,
corn samples obtained at harvest were subsampled, dried in a 55°C forced-air oven for 72
h and ground through a Wiley mill (l-mm screen; Arthur H. Thomas, Philadelphia, PA).
Starch was analyzed by a one-stage enzymatic method of glucoamylase (Diazyme L-200,
Miles, Inc., Elkhart, IN) with a NaOH gelatinization step (Karkalas, 1985); glucose
concentration was measured using a glucose oxidase method (glucose kit #510; Sigma
Chemical Co., St. Louis, MO), and absorbance was determined with a microplate reader
(SpectraMax 190, Molecular Devices Corp., Sunnyvale, CA). Ruminal ﬂuid utilized for
in vitro incubations was obtained from a lactating cow consuming a 50% concentrate,
50% forage diet. All assays were performed in duplicate.

Corn kernel particle size was determined for each experimental treatment by dry
sieving through 8 sieves (Sieve aperatures: 4750, 2360, 1180, 600, 300, 150, 75 um and
bottom pan) using a sieve shaker apparatus (Model RX-86, W.S. Tyler Inc., Gastonia,
NC) for 15 min until the bottom pan weight remained constant and mean particle size
was calculated (ASAE, 1968). Corn grain vitreousness for both hybrids was determined
according to the procedure of Dombrink—Kurtzman & Bietz (1993). Brieﬂy, one hundred
grains of each hybrid prior to HMC conservation were randomly selected. Among those
100 grains, 10 groups of 10 grains visually homogeneous in Size and shape were formed.

From each of those 10 groups, one grain was randomly sampled. A composite sample

29

was then formed with 10 grains from each hybrid, at each stage. The sample was
immersed in distilled H20 for 5 m, and after that the grains were dried with a paper towel
and the pericarp and germ were separated from the endosperm by scalpel dissection.
Next, the vitreous endosperm was separated from the farinaceous endosperm with a
rotary tool. The weight of vitreous endosperm was determined and expressed as
percentage of the total endosperm weight.

Absolute density for experimental corn hybrids was completed using a
pycnometer (Ultrapycnometer 1000c, Quantachrome Instruments, Boynton Beach, F L).
In general, the typical volume measurement cycle of the pycnometer consists of: opening
the venting valves and measuring the ambient pressure, opening the gas input valve to
purge the sample cell, followed by closing the vent valves and allowing the pressure to
build in the sample cell until the desired target pressure is reached. When the pressure
stabilized, the pressure value was saved (Pressure A), the valve to the added volume was
opened and once the pressure stabilized the pressure was recorded (Pressure B). The
kernel volume was calculated from these two pressure readings. The kernel weight was
used in conjunction with the kernel volume to determine density.

Composited diet ingredients and orts were analyzed for DM, ash, NDF, ADF,
starch, CP, and P. Samples were dried in a 55°C forced-air oven for 72 h and ground
through a Wiley mill (l—mm screen; Arthur H. Thomas, Philadelphia, PA). Ash was
determined following sample ignition at 500°C for 5 h. Composited samples were
analyzed for NDF (Van Soest et al., 1991), ADF (Goering and Van Soest, 1970), CP
(Leco, 1988), and starch (Karkalas, 1985), with glucose concentration determined

according to the above procedures. All analyses were performed in duplicate and

30

 

 

concentrations of all nutrients, disregarding DM, were expressed as percentages of DM
determined after drying at 105°C through a forced-air oven.

Frozen duodenal, ileal, and fecal samples obtained during digesta collection were
lyophilized and ground to pass through a l-mm screen. All digesta samples were
analyzed for DM, ash, NDF, ADF, CP, starch and purine concentration. Concentrations
of all nutrients, except DM, were expressed as percentages of DM determined by drying
at 105°C in a forced-air oven for more than 8 h.

Chromium concentration of digesta samples was evaluated using atomic
absorption spectrometry according to the manufacturers’ recommendations (Smith-
Hieftje 4000, Thermo Jarrell Ash Co., Franklin, MA) following digestion with a
phosphoric acid-manganese sulfate solution (Williams et al., 1962). Duodenal digesta
ﬂow was calculated according to procedures described in Armentano and Russell (1985).

Rumen samples for purine analysis were prepared following the procedure of
Overton et a1. (1995). Purines were measured, using a modiﬁed procedure of Zinn and
Owens, 1986 (Overton et al., 1995), as a bacterial marker. Purine to N ratio was
determined and microbial protein production was calculated by dividing purine to N ratio
of ruminal microorganisms by the purine percentage of duodenal DM per day (Zinn and
Owens, 1986).

Ruminal ﬂuid samples obtained at sampling were thawed and composited by
taking two 10 mL aliquots from each time point for each animal yielding two 80-mL
subsarnples per animal. The ﬁrst composited ruminal ﬂuid subsample was analyzed for
concentrations of VFA, lactate, and NH3 concentration. The aliquot to be analyzed for

ruminal NH3 concentration was acidiﬁed with 2 mL of concentrated sulfuric acid. For

31

VF A analysis, ruminal samples were centrifuged at 26,000 x G for 30 min.
Concentrations of VFA and lactate of supernatant were determined by HPLC (Waters
Corp., Milford, MA). Ammonia was measured using the procedure of Broderick and
Kang (1980), absorbance was determined using a microplate reader (SpectraMax 190,
Molecular Devices Corp, Sunnyvale, CA).

The second ruminal ﬂuid subsample was used to measure valerate concentration
by HPLC (Waters Corp, Milford, MA) and measurement of cobalt concentration by
ﬂame atomic absorption spectrophotometry according to manufacturer’s
recommendations (SpectrAA 220/F S, Varian Australia Pty. Ltd., Mulgrave, Victoria,
Australia).

Nutrient intake was calculated using the amounts and composition of feed offered
and refused. Duodenal digesta was analyzed for purines and duodenal ﬂow of microbial
N, OM, and starch ratios were estimated by analysis of microbial pellets. Truly
ruminally degraded OM was calculated by subtracting duodenal ﬂow of nonmicrobial
OM from OM intake. Truly ruminally degraded starch was calculated by subtracting
duodenal ﬂow of nonmicrobial starch from starch intake.

2.2.4 Statistical Analysis

Data were analyzed using the linear mixed model procedure of SAS (Version 8e,

SAS Institute Inc., Cary, NC) as duplicated (n = 2) 4 x 4 Latin squares using the

following model:

Yijkl = II + Al + Pj + Qk + R1 + QRkI + PQRjkl + eijkl
where u = overall mean, A, = random effect of steer (i = 1 to 10), PJ- = ﬁxed effect of

period (j = l to 4), Qk = ﬁxed effect of corn grain IVSD (k =1 to 2), Rl= ﬁxed effect of

32

processing method (1 = 1 to 2), QR“ = interaction of corn grain IVSD and processing
method, PQRjk] = interaction of period, corn grain IVSD and processing method, and cum

= residual, assumed to be normally distributed. Orthogonal contrasts were utilized to
determine the effect of corn grain IVSD, corn grain processing method, and the
interaction of corn grain IVSD and conservation method. Period by treatment interaction
was included in the model as two steers consuming the same treatment were removed
after the ﬁrst period and replaced by two alternate steers. Treatment main effects were
considered signiﬁcant at P < 0.05, and a tendency declared when P < 0.10. Period by
treatment interaction was considered signiﬁcant when P < 0.05 and acknowledged in
tables with an asterisk rather than probability values. Period by treatment interaction was
removed from the model if signiﬁcance was not detected (P > 0.05).

Rates of valerate and cobalt disappearance were determined by nonlinear
regression (JMP Version 4, SAS Institute, Inc., Cary, NC) of their decline in
concentration in ruminal ﬂuid over time post-dosing using a one-pool, ﬁrst-order model
accounting for background concentrations of each. Rate of concentration decline of
valerate and cobalt from the rumen over time were estimated by nonlinear regression.
Rate of absorption of valerate was estimated by subtracting rate of concentration decline
over time determined for cobalt from that determined for valerate (Allen et al., 2000).
2.3 RESULTS AND DISCUSSION
2.3.1 Ruminal Fermentation

No interactions of main treatment effects were signiﬁcant for any observed
measure of ruminal VF A concentration, pH, or ammonia (Table 2.4). HIGH tended to

have greater molar concentrations of total VFA (P = 0.10), with numerically greater

33

proportion of propionate (36.2% vs. 32.7%) and less proportion of butyrate (7.2% vs.
9.0%) than LOW. Previous research has shown that as fermentable grain starch is
increased there is greater total VFA concentration, increased production of microbial
protein, reduced ﬁber digestion, decreased ruminal ammonia concentration and decreased
acetatezpropionate ratios (Huntington et al., 2006). These results demonstrate that HIGH
is fermented differently than. LOW. Rate of valerate absorption was utilized in this trial
as an estimate of VFA absorption across the ruminal epithelium. Voelker and Allen
(2003) reported a positive relationship between mean ruminal pH and rate of valerate
absorption in dairy cows. Although mean ruminal pH was similar between treatments
during this trial, DRC HIGH had a 9% numeric advantage in rate of valerate absorption
compared to HMC HIGH. Rate of VFA absorption can be affected by ruminal
consistency, blood ﬂow, and rumen motility.
2.3.2 Dry Matter, Organic Matter, and Neutral Detergent Fiber Digestibility
Interaction of treatments was not detected for any measure of DM or OM
digestibility (Table 2.5). Likewise, digestion of DM and OM were not affected by com
grain IVSD in this study. However, steers consuming DRC had signiﬁcantly greater DM
and OM intake compared to steers consuming HMC (P < 0.05). Dry-rolled corn
treatments resulted in greater apparent ruminal degradation of OM (P < 0.10) when
compared to HMC diets, while no differences between treatments existed when evaluated
as a percentage of intake. Steers consuming HMC had signiﬁcantly greater apparent
small intestinal OM digestibility (P < 0.05) compared to steers consuming DRC when
evaluated as a percentage of intake, with a similar tendency detected for HMC when

evaluated as a percentage of duodenal passage (P < 0.10). Decreased OMI, coupled with

34

greater small intestinal OM digestibility resulted in signiﬁcantly greater (P < 0.05) total
tract OM digestibility for HMC treatments compared to DRC. These results are similar
with those reported by Cooper et al. (2001) who showed a 4% advantage in total tract
OM digestibility for steers consuming HMC or steam ﬂaked corn (SF C) relative to DRC.

Treatment effects on neutral detergent ﬁber (N DF) digestion are presented in
Table 2.6. Interaction of treatments was not detected for NDF digestibility. Likewise, no
differences were detected for NDF digestibility based on corn grain IVSD. However,
treatments containing DRC resulted in greater NDF intake (P < 0.05), and signiﬁcantly
greater ruminal, post—ruminal, and total tract NDF digestibility (P < 0.05) compared to
steers consuming HMC. This result is somewhat surprising as HMC treatments
contained greater concentrations of NDF which could promote a better ruminal
environment for ﬁber digestion (Gorocica—Buenﬁl and Loerch, 2005).
2.3.3 Site of Starch Digestion

Our initial hypothesis was for a potential interaction between corn grain IVSD
and processing method to occur. Treatment effects on site of starch digestion are shown
in Table 2.7. Corn grain vitreousness reﬂects the association between starch and protein
in the endosperm. In ﬂinty, more vitreous endosperm, the starch granules are surrounded
by a more compact and dense protein matrix, thereby limiting accessibility to enzymatic
digestion (Kotarski et al., 1992)). In contrast, ﬂoury, less vitreous endosperm is
comprised of starch granules that are more accessible to rumen bacteria because the
granules are less compact and the protein matrix is discontinuous and more soluble
(Kotarski et al., 1992). Converse to our initial hypothesis, there were no treatment

interactions for any measure of starch digestibility (P > 0.10). Likewise, corn grain

35

IVSD did not affect (P > 0.10) site or extent of starch digestion during digesta sampling.
Our data obtained during digesta sampling did not corroborate ﬁndings from Philippeau
et a1. (1999) who reported that apparent ruminal starch digestibility increased from 35.0
to 57.0% when a more vitreous corn grain was replaced by com grain with less
vitreousness, nor reported results from Corona et al. (2006) who documented a reduction
in post-ruminal and total-tract starch digestibility when vitreousness increased in diets
consisting of 73.2% DRC.

A tendency for reduced starch intake was documented for steers consuming HMC
compared to DRC (P < 0.10). Compared to steers consuming HMC, starch apparently
degraded in the rumen was reduced by 10.2% (P < 0.05) when evaluated as a percent of
intake for steers consuming DRC. Steers consuming DRC tended to have 8.6% less truly
ruminally degraded starch than steers consuming HMC treatments (P < 0.10).
Subsequently, feeding DRC resulted in 10.1% greater starch passage to the duodenum (P
< 0.01). Even so, no differences between processing methods were documented for
apparent small intestinal starch digestibility when expressed as a percent of intake.
Contrary to this ﬁnding, when small intestinal starch disappearance was expressed as a
fraction of duodenal ﬂow, 21.5% more starch was digested in the small intestine of steers
consuming HMC (P < 0.05) compared to diets containing DRC. The protein matrix
associated with HMC is more soluble, allowing more damage to occur from potential
ruminal microbial attack, thereby enabling the partially degraded endosperm that passes
to the small intestine to undergo heightened enzymatic hydrolysis. Alternatively, factors
related to limited time and surface exposure in the small intestine for DRC and the

potential limited capacity of gut tissue to produce starch-hydrolyzing enzymes (Owens et

36

al., 1986; Harmon, 1993) may have also contributed to this result. Steers consuming
HMC had 8.3% greater apparent total tract starch digestion when compared to steers
consuming DRC (96.7% vs. 88.2%; P < 0.01). A primary goal in corn processing is to
increase starch availability. These results agree with previous research Showing that
processing method has a profound effect on site and extent of starch digestion.
Huntington (1997) summarized data from 14 research trials and reported average ruminal
starch digestibility values of 76% and 90% for DRC and HMC, respectively. Galyean et
al. (1976), while not ﬁnding signiﬁcant differences in postruminal starch digestion across
processing treatments, documented that HMC was numerically 7% greater than DRC.
Cooper et al. (2002) documented a 3% advantage in total tract starch digestibility for
HMC and SFC relative to DRC. Likewise, in a review pertaining to the inﬂuence of corn
grain processing on site and extent of digestion, Owens and Zinn (2005) reported that
ﬁne grinding of less vitreous grain will help to maximize starch digestion. Theurer et al.
(1999), in a trial involving sorghum grain, reported that steam ﬂaked compared to dry-
rolled and reduced steam ﬂaking density resulted in greater ruminal starch degradation.
Likewise, Ladely et al. (1995) documented that processing of three different corn hybrids
selected on the basis of in-vitro rate of starch digestion (IVRSD) had a more profound
impact on IVRSD and cattle performance than did differences among grain hybrids.
Diets containing HMC contained 5.6% more corn silage (DM basis) in an effort to
maintain treatment diets isoenergetic. Likewise, a tendency for greater starch intake was
shown for DRC diets compared to HMC diets (P < 0.10). Increased concentrations of
corn silage and decreased starch intake can result in greater total tract digestibility of

starch (Turgeon et al., 1983).

37

2.3.4 Site of Nitrogen Digestion

No differences were detected for N intake between treatments (P > 0.10; Table
2.8). Even so, steers consuming HIGH treatments had numerically greater amounts of N
truly degraded in the rumen compared to steers consuming corn grain with low IVSD.
Russell (1998) reported that when ruminal fermentation is increased, ruminal bacteria can
more effectively utilize amino N. Ruminant N requirement is increased due to
heightened ruminal fermentation of carbohydrate (NRC, 1996). No differences between
treatments for duodenal N ﬂow were detected. However, greater amounts of N reached
the ileum of steers consuming DRC (P < 0.05), while steers consuming HMC tended to
have a 6% advantage in N digested and absorbed in the small intestine compared to steers
consuming DRC (P < 0.10). Steers consuming HMC had a greater fraction of duodenal
N from microbial origin when compared to the DRC treatments (P < 0.05; 143.5 vs.
114.6, respectively). Coincidentally, HMC treatments resulted in signiﬁcantly greater
microbial N efﬁciency (MNE; g microbial N / kg truly fermented OM) compared to DRC
(P < 0.05). These results document the difference in protein solubility for diets
containing HMC; soluble compounds in the rumen are attacked more rapidly and
digested more completely than are insoluble compounds due in part to differences in
microbial access (Owens and Zinn, 1988). Greater MNE is usually witnessed when
greater ruminal fermentation exists, which is corroborated by increased apparent and true
ruminal starch digestion in steers consuming HMC in the present study. However,
Owens and Goetsche (1988) reported that MNE is greater with less extensive digestion in

the rumen (i.e., whole corn vs. DRC) and almost always with forage vs. concentrate diets.

38

In our study HMC treatments resulted in numerically less truly ruminally fermented OM,
but did result in greater microbial nitrogen yield.

Steers consuming HIGH corn hybrids as DRC had the highest concentration of N
apparently digested in the large intestine either on a g/d or percentage of duodenal ﬂow
basis. These results, combined with N truly degraded in the rumen, document that N
escaping ruminal utilization was less digested and absorbed in the small intestine. Even
so, no differences were detected between treatments for apparent total tract N
digestibility.

2.4 CONCLUSIONS

In the present study, IVSD and processing method of corn grain appear to affect
site and extent of nutrient digestion. Feeding corn hybrids with high IVSD resulted in
greater ruminal starch degradability. Steers consuming HMC had greater MNE
compared to DRC, presumably due to heightened ruminal fermentation of starch. While
differences in digestibility due to corn grain IVSD were not evident throughout this trial,
diets containing corn grain with high IVSD resulted in greater total VF A concentration
and numerically greater proportions of propionate, providing evidence that IVSD does
impact ruminal fermentation. Experiments focusing on corn grain with greater
differences in vitreousness merit further research attention. However, high moisture
conservation of corn grain or rolling corn grain prior to feeding may alleviate potential
differences in digestibility of corn grain in high-concentrate feedlot rations due to

reduced particle size.

39

TABLES

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Table 2.2. Composition of experimental diets (DM basis)

 

 

 

Processing Method
Item Dry rolled High moisture
Ingredient, % (DM Basis)
Corn 78.3 72.0
Corn silage 12.6 18.2
Soybean meal 3.8 3.6
Limestone 0.4 1 .0
Premixa 5.0 5.4
Calculated composition
Crude protein, % 12.9 12.8
NEm, Meal/kg 2.1 2.1
NEg, Meal/kg 1.4 1.5

 

aContained (DM Basis): 47.7% soybean meal, 22.1% calcium carbonate, 10.0% trace
mineral salt, 7.5% urea, 5.6% potassium chloride, 3.3% ground corn, 2.2% dicalcium
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42

Table 2.3. Nutrient composition of experimental diet (DM basis)

 

HMC1 DRCl

 

HIGH2 Low2 HIGH2 Low2 SEM

 

0M 948° 95.0’1 95.2b 95.3b 0-1
Starch 58.8 58.6 56.3 57.9 0.9
CF 13.0 13.1 12.9 13.0 0.1
NDF 6.8a 6.861 8.5b 7.8b 0-4
ADF 3.4 3.3 3.3 3.3 0.1
P 0.5 0.4 0.4 0.5 0.03

 

lHigh moisture and dry-rolled corn
2High and low in vitro starch digestibility

a’bMeans in the same row followed by unlike superscript letters
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48

CHAPTER 3

EFFECT OF IN VITRO STARCH DIGESTIBILITY OF CORN AND
SUPPLEMENTAL NITROGEN CONCENTRATION ON SITE AND EXTENT OF
NUTRIENT DIGESTION AND PROTEIN METABOLISM IN HOLSTEIN
STEERS FED A HIGH-GRAIN DIET

ABSTRACT

This experiment was conducted to evaluate the interaction between corn grain in
vitro starch digestibility (IVSD) and nitrogen concentration on site of nutrient digestion
and nitrogen metabolism in steers consuming a high—grain, dry-rolled corn based diet.
Eight Holstein steers (initial BW = 500 :l: 21 kg; four ruminally and duodenally
cannulated) were used in separate 4 x 4 Latin square design experiments with a 2 x 2
factorial arrangement of treatments. Treatments were dry-rolled corn having either high
(HIGH) or low (LOW) IVSD and nitrogen (N) concentration (HNIT and LNIT), where
HNIT diets contained a greater degradable intake protein fraction as a percentage of
crude protein (60.1% vs 48.6%, respectively). A tendency for treatment interaction was
documented for ruminal starch degradation (P < 0.10). Steers consuming LNIT diets had
0.4 kg/d more starch apparently degraded in the rumen compared to HNIT diets, with
LNIT LOW resulting in 0.6 kg/d more ruminal starch degraded than all other treatments.
However, no differences were present between treatments when evaluated as a
percentage of starch intake. A signiﬁcant interaction of treatments was present for total
tract starch digestion (P < 0.05), with LNIT LOW having greater total tract starch
degradation than HNIT LOW (P < 0.05). Low N concentrations or high IVSD had a

5.6% and 4.2% increase in molar propionate concentration when compared to HNIT or

49

low IVSD (P = 0.01), respectively. Steers consuming LNIT and HIGH had the lowest
ruminal pH (P < 0.05) indicating greater ruminal fermentation of carbohydrate. High
IVSD diets had numerically greater microbial efﬁciency when compared to LOW.
Lower N concentrations may negatively impact total tract N digestibility, while high
IVSD can result in improved microbial nitrogen efﬁciency.

3.1 INTRODUCTION

Diets consumed by feedlot cattle often contain high concentrations of corn and
thereby high concentrations of starch. Starch is fermented in the rumen to volatile fatty
acids (V FA) that are then absorbed and serve as the main source of energy for the
ruminant animal. Maximal energetic efﬁciency of ﬁnishing cattle necessitates a high
extent of ruminal starch fermentation (Owens et al., 1997). Research (Philippeau et al.,
1999b; Macken et al., 2003) suggests that corn vitreousness (i.e., ﬂoury vs. ﬂinty) can
increase ruminal starch degradability and vitreousness of corn grain can be used to
predict ruminal starch disappearance (Philippeau et al., 1999a).

Degradable intake protein (DIP) is the fraction of dietary protein used by the
ruminal microbial population for synthesis of microbial N and is derived from preformed
amino acids and non-protein N (N PN). Metabolizable protein (MP) is the combination
of microbial protein, rumen bypass protein, and endogenous protein which can be utilized
by the animal. Metabolizable protein requirements increase with greater ruminal starch
fermentation (NRC, 1996) and could be altered based on the extent of ruminal
fermentation. Increased diet degradability can have a profound impact on the efﬁciency
with which microbes use energy from ruminal fermentation of substrate for growth

[deﬁned as grams of duodenal microbial N per kilogram OM truly ruminally degraded

50

(TRDOM)]. Microbial N efﬁciency (MN E) is difﬁcult to predict due to differences in
ruminal retention time, pH, and substrate availability. Likewise, MNE can be reduced
when peptides or amino acids are not supplied in sufﬁcient amounts (Van Kessel and
Russell, 1996). While corn processing methods have been shown to inﬂuence protein
requirement (Cooper et al., 2002), little research has been conducted to determine the
speciﬁc effects of corn grain starch availability and DIP fractions on N utilization and
retention in growing steers experiencing varying rates of ruminal starch degradation.

We hypothesized that ruminal starch degradability would be increased in diets
containing corn grain with ﬂoury endosperm and high IVSD. Likewise, higher
concentrations of supplemental N would be beneﬁcial to diets containing corn grain with
ﬂinty endosperm and low IVSD by causing an increase in microbial N production, MNE,
and retained N. Furthermore, low IVSD diets might beneﬁt from less ruminally available
N due to less ruminally degradable starch and less ruminal N demand. The objectives of
this experiment were to evaluate the effects of corn grain IVSD and supplemental
nitrogen concentration on site of nutrient digestion in growing Holstein steers consuming
a high grain diet.

3.2 MATERIALS AND METHODS
3.2.1 Treatment Determination

Two corn hybrids were obtained from Pioneer Hi-Bred international to represent
ﬂoury (33R77) and ﬂinty (34B97) endosperm type (Owens, personal communication)
and were utilized in a previous experiment (McPeake et al., 2008). No differences were
documented for hybrids processed as high moisture corn (HMC; P < 0.17). However,

IVSD was 14.1% greater for 33R77 than 34B97 when evaluated as dry rolled corn (DRC;

51

P < 0.01). Therefore, hybrids are represented as corn grain with low IVSD (LOW) or
high IVSD (HIGH).

Agronomic and harvesting conditions were reported in McPeake (2008). Brieﬂy,
ﬂoury (Pioneer 33R77; HIGH) and ﬂinty (Pioneer 34B97; LOW) corn hybrids were
planted in a 15-acre ﬁeld located on the Michigan State University farm (East Lansing,
M1) on May 8, 2003. The ﬁeld was divided in half, with each half containing one
experimental hybrid and harvested as HMC or DRC. Dry corn was coarsely rolled prior
to feeding during the trial. Nutrient compositions and physical characteristics of the corn
grain treatments used in the experiment are shown in Table 1. Nitrogen treatments were
chosen based on crude protein concentration and amount of DIP supplied to the rumen
(i.e., HNIT vs. LNIT), where HNIT supplied 13.3% CF with a DIP balance of -109.7 g/d
and LNIT supplied 10.3% CF with a DIP balance of -392.8 g/d. Degradable intake
protein balance was estimated using Level 1 of 1996 NRC (NRC, 1996). Additionally,
HNIT treatments resulted in daily urea intake of 1.0% DM, whereas LNIT treatments
contained no supplemental N derived from urea.

3.2.2 Experimental Design and Data Collection

All animal care procedures described herein were approved by the All-University
Committee for Animal Use and Care of Michigan State University (#11/03-145-00).

Four ruminally and duodenally cannulated Holstein steers from a previous
metabolism study were used in a 4 x 4 Latin square design. The square was balanced for
carryover effects so that each dietary treatment followed the other dietary treatments an
equal number of times. Initially, the four steers were randomly assigned to one of four

experimental treatments. A 2 x 2 factorial arrangement of treatments was utilized.

52

Experimental treatments consisted of two corn hybrids with different IVSD and two
nitrogen concentrations. Physical and chemical characteristics of corn grain hybrids are
presented in Table 3.1, while composition of supplemental protein pellets is displayed in
Table 3.2.

Experimental diets contained DRC, cottonseed hulls, and pellets containing both

the protein source and vitamin/mineral mix. The diets were formulated to be isocaloric
and contained 2.3 meal/kg NEm, and 1.4 meal/kg NEg (NRC, 1996). Ingredient and

nutrient compositions of the experimental diets are presented in Table 3.3.

Dry rolled corn, cottonseed hulls, and pellets were weighed and mixed daily.
Steers were fed once daily at 0800 h at 110% of expected intake. Amount of feed
delivered and orts were recorded daily. Dietary feed samples (0.5 kg) and individual
steer orts (10% of orts on a wet weight basis) were obtained daily during the collection
period, frozen and composited within each collection period. Diet components were
evaluated aﬁer each period for DM and diets were re-balanced accordingly.

Experimental periods were 18 d in length: 14 d allotted for diet adaptation and 4
d for digestion sampling. All steers were housed in a barn equipped with Calan gates and
bedded on wood chips to promote cannula integrity. Prior to trial initiation, steers were
allowed appropriate time for adaptation to the Calan gate system and for dietary
treatments speciﬁc to each steer. During treatment adaptation, steers were fed 110% of
previously recorded intake.

During digesta sampling, chromic oxide (0203) was used as a marker to estimate
nutrient digestibility in the rumen and total tract. Gelatin capsules (1.5 oz; Torpac Inc.,

Philadelphia, PA) were ﬁlled with 5 g Cr203 and dosed through the rumen cannula at

53

0730, 1530, and 2330 h (total of 15 g Cr203 per (1) from d 14 to 20. A priming dose of
CrzO3 was administered on d 12 at three times the regular dose.

Determination of ruminal and total tract digestibility was accomplished via
collection of duodenal digesta and feces every 9 h starting at 1200 h on d 16 and
continuing until 0300 h on d 18. This sampling time reﬂects every 3 h time point across
a 24 h time interval to account for diurnal variation. One 500 mL mixed rumen sample
was obtained during sampling for later purine analysis. Ruminal contents from six
different sites were combined and strained to obtain rumen ﬂuid. Two 100 mL liquid
subsarnples were obtained for VFA and ruminal NH3 analyses. Ruminal liquid samples
to be analyzed for ruminal NH3 were acidiﬁed at collection with 2 mL concentrated
sulfuric acid. Ruminal pH was determined using a calibrated pH probe (model 230A,
ATI Orion, Boston, MA) at each sampling time. Each sample after collection was
immediately frozen at -20°C until further analysis.

3.2.3 Sample Analyses

Corn kernel particle size for each experimental treatment was dry sieved through
8 sieves (Sieve aperatures: 4750, 2360, 1180, 600, 300, 150, 75 um and bottom pan)
using a sieve shaker apparatus (Model RX-86, W.S. Tyler Inc., Gastonia, NC) for 15 min
until the bottom pan weight remained constant and mean particle size was calculated
(ASAE, 1968). Corn grain vitreousness for both hybrids was determined according to the
procedure of Dombrink-Kurtzman & Bietz (1993). Brieﬂy, one hundred grains of each
hybrid prior to HMC conservation were randomly selected. Among those 100 grains, 10
groups of 10 grains visually homogeneous in size and shape were formed. From each of

those 10 groups, one grain was randomly sampled. A composite sample was then formed

54

with 10 grains from each hybrid, at each stage. The sample was immersed in distilled
H20 for 5 m, and after that the grains were dried with a paper towel and the pericarp and
germ were separated from the endosperm by scalpel dissection. Next, the vitreous
endosperm was separated from the farinaceous endosperm with a rotary tool. The weight
of vitreous endosperm was determined and expressed as percentage of the total
endosperm weight.

Absolute density for experimental corn hybrids was completed using a
pycnometer (Ultrapycnometer 1000e, Quantachrome Instruments, Boynton Beach, FL).
In general, the typical volume measurement cycle of the pycnometer consists of: opening
the venting valves and measuring the ambient pressure, opening the gas input valve to
purge the sample cell, followed by closing the vent valves and allowing the pressure to
build in the sample cell until the desired target pressure is reached. When the pressure
stabilized the pressure value was saved (Pressure A), the valve to the added volume was
opened and once the pressure stabilized the pressure was recorded (Pressure B). The
kernel volume was calculated from these two pressure readings. The kernel weight was
used in conjunction with the kernel volume to determine density

Composited diet ingredients and orts samples were analyzed for DM, ash, NDF,
iNDF, ADF, starch, N, and P. Samples were dried in a 55°C forced-air oven for 72 h and
ground through a Wiley mill (1 -mm screen; Arthur H. Thomas, Philadelphia, PA). Ash
was determined following sample ignition at 500°C for 5 h. Composited samples were
analyzed for NDF (Van Soest et al., 1991), ADF (Goering and Van Soest, 1970), N
(Leco, 1988), and starch (Karkalas, 1985). Starch was analyzed by a one-stage

enzymatic method of glucoamylase (Diazyme L-200, Miles, Inc., Elkhart, IN) with a

55

NaOH gelatinization step (Karkalas, 1985); glucose concentration was measured using a
glucose oxidase method (glucose kit #510; Sigma Chemical Co., St. Louis, MO), and
absorbance was determined with a microplate reader (SpectraMax 190, Molecular
Devices Corp., Sunnyvale, CA). Indigestible NDF was estimated as NDF residue after a
240-h in vitro fermentation (Goering and Van Soest, 1970). All analyses were performed
in duplicate and concentrations of all nutrients, disregarding DM, were expressed as
percentages of DM determined after drying at 105°C through a forced-air oven.

Frozen duodenal and fecal samples obtained during digesta collection were
lyophilized and ground with a coffee grinder due to limited dry sample availability. All
digesta samples were analyzed for DM, ash, NDF, ADF, N, starch, purine concentration,
and iNDF. Concentrations of all nutrients, except DM, were expressed as percentages of
DM determined by drying at 105°C in a forced-air oven for more than 8 h.

Chromium concentration of digesta samples was evaluated using atomic
absorption spectrometry according to the manufacturers’ recommendations (Smith-
Hieftje 4000, Thermo Jarrell Ash Co., Franklin, MA) following digestion with a
phosphoric acid-manganese sulfate solution (Williams et al., 1962). Duodenal digesta
ﬂow was calculated according to procedures described in Armentano and Russell (1985).
Indigestible NDF was also quantiﬁed in digesta samples as an alternative marker of
digestibility.

Ruminal samples for purine analysis were prepared following the procedure of
Overton et a1. (1995). Purines were measured by the procedures deﬁned by Zinn and
Owens (1986) with the modiﬁcations of Overton et a1. (1995) as a bacterial marker.

Microbial protein production was calculated by dividing purine to N ratio of ruminal

56

microorganisms by the purine percentage of duodenal DM per day (Zinn and Owens,
1986).

Ruminal ﬂuid samples obtained at sampling were thawed and composited by
taking two 10 mL aliquots from each time point for each animal yielding two 80 mL
subsarnples per animal. Composited ruminal ﬂuid was analyzed for concentrations of
VP A and lactate, and the alternate subsample for NH3 concentration. The aliquot to be
analyzed for ruminal NH; concentration was acidiﬁed with 2 mL of concentrated sulfuric
acid. For VFA analysis, rumen ﬂuid samples were centrifuged at 26,000 x g for 30 min.
Concentrations of VFA and lactate of supernatant were determined by HPLC (Waters
Corp., Milford, MA) using a modiﬁed procedure developed by Dawson (1994).
Ammonia was measured using the procedure of Broderick and Kang (1980), with
absorbance being determined using a microplate reader (SpectraMax 190, Molecular
Devices Corp., Sunnyvale, CA).
3.2.4 Statistical Analysis

Data were analyzed using the linear mixed model procedure of SAS (Version 8e,

SAS Institute Inc., Cary, NC) as a 4 x 4 Latin square using the following model:

Yijkl = H + Ai + Pj + Qk + R1 + QRkl + PQRjk1+ eijkl
where p = overall mean, A, = random effect of steer (i = 1 to 4), PJ- = ﬁxed effect of
period (j = 1 to 4), Qk = ﬁxed effect of IVSD (k =1 to 2), R1= ﬁxed effect of nitrogen
concentration (I = 1 to 2), QRk1= interaction of IVSD and nitrogen concentration, PQRJ-kl

= interaction of period, IVSD and nitrogen concentration, and eiJ-k. = residual, assumed to

be normally distributed. Orthogonal contrasts were utilized to determine the effect of

57

IVSD, nitrogen concentration, and the interaction of IVSD and nitrogen concentration.
Main effects of treatment were considered signiﬁcant at P < 0.05, and a tendency
declared when P < 0.10. Treatment means were separated using the pdiff option of SAS
when a tendency for interaction was detected (P < 0.15). Period by treatment interaction
was removed from the model when signiﬁcance was not detected (P > 0.05).
3.3 RESULTS AND DISCUSSION
3.3.1 Ruminal Fermentation

No interactions (P > 0.10) of main treatment effects were observed for ruminal
VF A concentration or NH3 (Table 3.4). However, a tendency for an IVSD by nitrogen
concentration interaction was present for ruminal pH. While no differences existed
between HIGH and LOW IVSD within the HNIT treatment, steers consuming LNIT

HIGH had lower ruminal pH when compared to the LNIT LOW treatment (5.46 vs. 5.72;
P < 0.05). Ruminal NH3 was predictably lower (P < 0.05) for LNIT compared to HNIT.

Satter and Slyter (1974) observed that 5.0 mg/ 100 mL concentration is necessary to
support microbial protein production. The LNIT HIGH treatment had the lowest numeric
ruminal ammonia concentration and tended to have the lowest ruminal pH. This may
imply a signiﬁcant role of NH3 as a buffer in high grain diets. Ammonia has been shown
to stimulate Na+ transport across the ruminal epithelium of concentrate-fed cattle and
urea-fed sheep by creating an adaptive metabolism by the rumen mucosa to metabolize
greater quantities of NH3 and/or an increased permeability of the rumen epithelium to the
charged ammonium ion (NH4+; Abdoun et al., 2003).

Steers consuming HIGH had decreased proportion of VF A as acetate compared to

LOW (P < 0.05) with a tendency for greater proportion of VF A as propionate. Steers

58

consuming LNIT treatments had a greater proportion of total VF A as propionate, with
less acetate (P < 0.05) compared to HNIT treatments. In a previously conducted trial by
McPeake (2008), Pioneer 33R77 was more ruminally degradable which can lead to
greater ruminal concentration of propionate (McPeake, 2008). In the present study,
HIGH IVSD or LNIT concentrations resulted in reduced proportions of branched—chain
VF A concentration (P < 0.05) compared to LOW and HN IT treatments. One would

expect greater BC VFA with higher nitrogen concentrations as they are metabolites of

microbial protein degradation. As expected, ruminal NH3 concentration was greater for

HNIT (P < 0.01) compared to LNIT. Higher ruminal NH3 concentrations can be

attributed to increased ruminally available N in the present study.
3.3.2 Digestibility Marker
Comparisons between iNDF and chromium for digestibility of ruminal and total
tract OM were completed according to the previously documented procedures.
Indigestible NDF provided a more realistic estimate of both ruminal and total tract OM
and starch digestibility, as estimates obtained using chromium were unrealistically low.
We believe that chromium disassociated from ruminal contents due to high amounts of
cottonseed bulls in the diet. Therefore, estimates of digestion were completed using
iNDF.
3.3.3 Dry Matter, Organic Matter, and Neutral Detergent Fiber Digestibility
Treatment effects on DM and OM digestibility are presented in Table 3.5. A
tendency for treatment interaction was witnessed for DM intake (P = 0.06) even though
no signiﬁcant differences existed between treatments. Across all treatments, steers

consumed 2% of BW. Steers receiving HNIT HIGH and LNIT LOW had numerically

59

greater DMI relative to both HN IT LOW and LNIT HIGH treatments (11.0 and 10.8 vs.
9.7 and 9.9 kg/d, respectively). A signiﬁcant interaction of treatments was documented
for apparent total tract DM digestibility (P < 0.05). When expressed on a kg/d basis, a
greater amount of DM tended to be digested in the total tract for HNIT HIGH and LNIT
LOW treatments. However, when expressed as a percentage of intake, no signiﬁcant
difference between IVSD or N concentration was detected in the present trial.

A tendency for interaction between main effects was detected with regards to
OMI and followed the same documented trend of DMI. Interestingly, a tendency for
interaction between main effects was present for truly ruminally degraded OM. Upon
evaluation of simple main effects, steers that consumed the HN IT HIGH treatment tended
to experience greater truly ruminally degraded OM (TRDOM) than did steers that
consumed the HNIT LOW treatment (8.3 vs. 6.5; P = 0.12). Other measures of OM
digestibility were similar among treatments in this study.

Treatment effects on site and extent of NDF digestibility are presented in Table
3.6. No differences in NDF intake were realized between treatments. Likewise, no
treatment differences were detected for ruminal NDF degradability. However, HIGH
HNIT was the lowest numerically for apparent NDF degradability and had signiﬁcantly
more duodenal NDF passage when compared to either N treatment consuming corn grain
with low IVSD (P < 0.05). A tendency for interaction of treatments was detected for
postruminal digestion of NDF. Steers consuming HIGH HNIT diets had signiﬁcantly
greater postruminal digestion of NDF compared to all other treatments (P < 0.05) when
expressed as either a percentage of DMI or duodenal passage. However, no differences

were documented for total tract NDF digestion among treatments.

60

3.3.4 Site of Starch Digestion

Treatment effects on site of starch digestion are shown in Table 3.7. A tendency
for an interaction between treatments was (P = 0.11) present for starch intake. A similar
tendency for interaction was detected among treatments for starch apparently degraded in
the rumen on a kg/d basis, with steers consuming LNIT LOW having signiﬁcantly greater
ruminal starch degradation (kg/d) compared to all other treatments (P < 0.05). This result
was understandable considering LNIT LOW had the greatest starch intake, but when
ruminal starch degradability was evaluated as a percentage of intake no differences
between treatments were detected. We hypothesized that LOW would require less
ruminal N to maximize ruminal starch degradability compared to HIGH, as we
documented a 14% difference for in vitro starch digestibility when comparing these two
corn hybrids in a previous study (McPeake, 2008). A tendency for an interaction (P <
0.10) was also detected between treatment main effects for starch truly degraded in the
men with both HIGH HN IT and LOW LNIT being numerically higher than either
LOW HNIT or HIGH LNIT. However, no differences were detected when evaluated as a
percentage of intake.

No differences were detected between treatments for duodenal starch passage (P
> 0.05). However, steers consuming HN IT treatments had 17% more starch digested
postruminally compared to LNIT (P < 0.05) when expressed as a percentage of duodenal
starch ﬂow.

Signiﬁcant interaction of treatments was discovered for apparent total tract starch
digestion (P = 0.03). Steers consuming LNIT LOW had greater total tract starch

digestion compared to HNIT LOW (P < 0.05), but no differences were detected. for the

61

HIGH treatments. Vitreousness of corn grain reﬂects the association between starch and
protein in the endosperm. In corn grain containing ﬂinty endosperm the starch granules
are surrounded by a more compact and dense protein matrix, thereby limiting
accessibility to microbial and enzymatic digestion (Kotarski et al., 1992). In contrast,
ﬂoury endosperm is comprised of starch granules that are more accessible to rumen
bacteria because the granules are less compact and the protein matrix is discontinuous
(Kotarski et al., 1992). While most research has documented that com grain with ﬂoury
endosperm is more ruminally degradable than more vitreous/ﬂinty hybrids (Philippeau et
al., 1999b), Szasz et al. (2002) documented greater ruminal starch degradability in ﬂinty
vs. ﬂoury HMC treatments. The magnitude of difference in vivo between the two corn
hybrids utilized in this trial for ruminal starch degradability does not appear to be great
enough to illicit a large enough response to test potential differences in supplemental N
level on nutrient digestion. Results from a previous trial in our lab utilizing these same
corn hybrids documented that as high moisture corn (HMC), ruminal starch degradability
was 10% higher compared to dry-rolled corn (DRC; McPeake, 2008). Total tract starch
digestibilities for the current study were within the reported ranges for DRC (Owens,
1997). Therefore, in order to study the importance of N synchrony based on ruminal
fermentation characteristics, grain processing (steam-ﬂaked vs. DRC), grain type
(sorghum vs. wheat), or corn hybrids with greater vitreousness differences than used in
this study may prove to be more appropriate models.

3.3.5 Site of Nitrogen Digestibility

62

We hypothesized that N concentration would interact with degree of IVSD
because supplemental N would be beneﬁcial to diets containing corn hybrids with high
IVSD as a result of increased ruminal starch degradability.

A tendency for interaction between treatments was detected for N intake (P <
0.10; Table 3.8). By design, N intake was signiﬁcantly higher for the HNIT treatment
during the present trial (202.2 vs. 154.2 g/d; P < 0.01), while N intake for all treatments
followed the same pattern as did OM intake. Steers consuming HNIT HIGH had
signiﬁcantly greater N intake than did LNIT HIGH or LNIT LOW (P < 0.05). Likewise,
HNIT LOW resulted in greater N intake than did LNIT HIGH (P < 0.05).

While no differences were observed for the amount of total or microbial N ﬂow to
the duodenum among treatments (P > 0.05), a numeric advantage was present for HNIT
HIGH with regards to microbial N ﬂow to the duodenum compared to other treatments.
We hypothesized that com hybrids with low IVSD may undergo less ruminal
fermentation of starch, thereby requiring less ruminally degradable N to satisfy the
microbial N requirement. This result may illustrate greater ruminal hydrolysis of starch
and thereby more extensive utilization of DIP in the HIGH HNIT diet, a premise the most
current NRC (1996) is based on.

No differences between treatments were observed for N truly degraded in the
rumen. Interestingly, LNIT LOW had numerically the largest value for true ruminal N
degradability when evaluated as a percent of N intake. This result is somewhat surprising
as we believed that com hybrids with low IVSD would result in decreased ruminal
fermentation of starch. In the present trial, the LNIT treatment supplied 4.9% dietary

DIP, while the HNIT treatment supplied 7.9% dietary DIP. Interestingly, it appears as

63

though the LNIT treatment was able to meet the ruminal N requirement as evidenced by
no difference in MNE when compared to the HN IT treatment. Cooper et a1. (2002)
documented through a 90 animal individually-fed feeding trial that 6.3% DIP was
necessary for DRC-based diets, which agrees with current NRC (1996)
recommendations. Surprisingly, the LNIT treatment supplied well below the 5.0 mg/ 100
mL ruminal ammonia level believed necessary to sustain microbial protein production
(Satter and Slyter, 1974), but with no detrimental effects on MNE when compared to the
HNIT treatment.

A signiﬁcant interaction of treatments was observed for total tract digestibility of
N (P = 0.03). While steers consuming the HNIT HIGH treatment had the highest total
tract N digestibility on a g/d basis (P < 0.05), HN IT LOW was intermediate, and both
were higher than either LNIT treatments (P < 0.05). When expressed on a percentage
basis, both HNIT treatments resulted in an almost 20% advantage in total tract N
digestibility when compared to LNIT HIGH.
3.4 CONCLUSIONS

The present study was conducted to determine if site of nutrient digestion would
be affected in growing steers based on potential differences in ruminal starch
degradability and varying concentrations of supplemental N. No differences between
treatments for any measure of starch digestibility were detected when expressed on a
percentage basis. Dietary N concentration impacted the amount and extent of N
digestibility during this trial. Differences in ruminal starch fermentation that may exist
between corn hybrids due to differences in IVSD in this trial were not great enough to

warrant differences in supplemental N concentrations. Further research is necessary to

64

document the need for synchrony of N with ruminal starch fermentation based on other
processing methods or grain types. However, corn grain with high IVSD fed in
conjunction with greater dietary N concentrations resulted in numerically greater

TRDOM, MCP production, and MNE.

65

 

TABLES

66

Table 3.1. Physical and chemical characteristics of corn grain hybrids

 

 

Item HIGH 1 LOW 1 SEM P
DM 90.4 90.3 0.01 0.41
------ % of DM-----
Starch 67.6 65.8 1.6 0.53
CP 9.7 9.5 0.1 0.69
NDF 6.6 6.2 0.7 0.82
ADF 1.9 2.3 0.1 0.20
IVSD?" 57.4 43.3 0.7 < 0.01
Absolute density (g/cc) 1.285 1.306 0.004 0.01
Vitreousness, %3 45.6 41.8 1.8 0.17
Sieve aperture (um) ----% retained -----
4750 17.0 21.5 4.7 0.53
2369 69.0 67.4 5.0 0.43
1180 9.3 7.0 1.2 0.75
600 2.3 2.0 0.5 0.60
300 1.0 1.1 0.3 0.82
150 0.9 0.6 0.1 0.20
75 0.32 0.10 0.04 < 0.01
Pan 0.04 0.01 0.02 0.26
Mean particle size (um) 3844.1 4064.7 171.8 0.65
Standard deviation 1949.0 1843.8 84.2 < 0.01

 

lCorn hybrids having either high or low in vitro starch digestibility
2IVSD = In vitro starch digestion determined after 7 h incubation

3 . .
V1treousness determmed on corn samples preserved as HMC

67

Table 3.2. Composition of experimental protein/vitamin-mineral pellet (DM basis)

 

Supplemental Nitrogen Concentration
Item Low High

Ingredient, % (DM Basis)

Ground corn 39.8 25.1
Soybean meal, 48% CP 31.8 36.5
Urea 0.0 10.0
Limestone 1 0.6 10.6
Vitamin-Minerala 1 7-9 17'9

 

aContained (DM Basis): 12.0% potassium chloride, 3.0% sodium chloride, 1.8%
magnesium oxide, 0.02% selenium, 0.07% vitamin A (30,000 IU/g), 0.001% vitamin E,
0.04% copper sulfate, 0.17% ferrous sulfate, 0.03% manganese sulfate, 0.09% zinc
sulfate, 0.0005% iodine, 0.50% Ameribond, and 0.20% Bovatec 91

68

Table 3.3. Composition of experimental diets (DM basis)

 

Supplemental Nitrggen Concentration

Item Low High

 

Ingredient, % (DM Basis)

Corn 78.0 78.0

Cottonseed hull 12.0 12.0
1 1,2 10 0 10 0

Protern pellet - -

Calculated Composition

Crude protein, % 10.3 13.4
DIP, % CF 48.6 60.1
NEm, Mcal/kg 2.3 2.2
NEg, Meal/kg 1.4 1.3

 

1Low pellet contained: 39.8% ground corn, 31.8% soybean meal, 10.6% limestone, and
17.9% vitamin/mineral premix

2High pellet contained: 25.1% ground corn, 36.5% soybean meal, 10.0% urea, 10.6%
limestone, and 17.9% vitamin/mineral premix

69

 

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74

CHAPTER 4
FINAL DISCUSSION AND CONCLUSIONS

The current experiments were conducted to determine the potential interactions
between in vitro starch degradability (IVSD) and either processing method or
supplemental N on site of nutrient digestion. In the ﬁrst experiment, the interaction
between IVSD of two corn hybrids (HIGH vs. LOW) and processing method (HMC vs.
DRC) on site of nutrient digestion in Holstein steers consuming a high-grain diet was
investigated. Di gestibility and ruminal metabolism was not inﬂuenced by an interaction
between main effect treatments. Feeding HIGH resulted in a tendency for greater molar
VF A concentrations. Even so, 21.5% less starch was degraded in the small intestine of
steers consuming diets containing DRC with a subsequent decline in apparent total-tract
starch digestibility. Likewise, a tendency for greater ruminal ammonia levels was present
for diets consisting of LOW. Collectively, feeding LOW corn grain results in reduced
ruminal fermentation of starch. Diets containing HMC resulted in greater amounts of
microbial nitrogen ﬂow to the duodenum, greater apparent post-ruminal nitrogen
digestibility, and greater microbial efﬁciency.

Previous research has documented that endosperm type can have a profound
impact on site and extent of starch digestibility (Philippeau et al., 1999). In research
trials utilizing dairy cows, Taylor et a1. (2005) documented that com grain containing
ﬂoury, less vitreous endosperm resulted in greater ruminal and total tract starch

digestibility. The corn hybrids utilized in the present trial were provided by Pioneer

75

 

based on agronomic data and research ﬁndings conducted across a variety of locations.
Ideally, these two hybrids would have been grown in conjunction with other hybrids
selected for differences in vitreousness and tested thoroughly prior to trial initiation.
However, current experimental treatments represent Pioneer corn hybrids that would be
commercially planted based on agronomic value and that have historical differences in
vitreousness. Results indicate that vitreousness was not different between the two corn
hybrids planted, while IVSD was signiﬁcantly higher for HIGH vs. LOW as DRC
(14.1%; P < 0.01). Even so, the effect of corn hybrid did not interact with processing
method for any measure of digestibility. Therefore, we believe that the effect of
endosperm type may not be as pronounced in feedlot diets that typically contain 70 to
80% com grain. However, processing method of corn grain hybrids has a profound
impact on site and extent of starch digestibility and nitrogen metabolism.

Between trials, in vitro rate of starch digestibility after 7 h was completed for the
corn hybrids deﬁned in the ﬁrst trial. This was completed in order to establish if DRC or
HMC would be a more appropriate model for the second trial which was based on
potential differences in ruminal starch fermentation. A highly signiﬁcant interaction of
treatments was detected (P < 0.01; Figure 4-1). The magnitude of difference (14%) was
greater between HIGH and LOW when evaluated as DRC as compared to HMC (4.7%,
respectively). Therefore, DRC containing HIGH or LOW corn grain was utilized for the
second trial. Dietary ﬁber was provided in the ﬁrst trial via corn silage, whereas

cottonseed hulls were used exclusively during the second trial.

76

80—

Starch degradability after 7 h

 

FLRY HMC FLTY HMC FLRY DRC FLTY DRC

Figure 4-1 In vitro starch degradability for corn grain containing ﬂoury
and ﬂinty endosperm type processed as HMC or DRC after 7 h.

77

 

Evaluation of the interaction between IVSD and nitrogen concentration on site of
nutrient digestion and nitrogen metabolism in Holstein steers consuming a high-grain diet
was investigated in the second experiment. Treatments were IVSD of corn grain (HIGH
and LOW) and N concentration (HNIT and LNIT). Cottonseed hulls were incorporated
at a rate of 10% of DMI due to reduced quality of corn silage available and a low,
unavailable N content. Noticeable visual differences were apparent when collecting
ruminal digesta samples compared to the ﬁrst trial which utilized corn silage. Ruminal
digesta was very watery and loose and may explain why the chromium oxide apparently
disassociated from the other ruminal contents, leading to the decision to utilize iNDF as a
marker in this trial.

Steers consuming LOW LNIT diets had signiﬁcantly greater ruminal starch
degradability compared to all other treatments and increased total tract starch digestibility
compared to steers consuming LOW HNIT (P < 0.05). Steers consuming HIGH LNIT
had the least total tract N digestibility when evaluated on either a g/d or percentage basis
(P < 0.05). We hypothesized that steers consuming HIGH corn grain would have greater
ruminal starch fermentation, and thereby greater ruminal utilization of dietary N. While
diets containing HIGH corn grain did result in greater MNE when compared to LOW, no
differences in absorption or retention of N and phosphorus were found between IVSD
treatments. Greater microbial efﬁciency is generally related to increased ruminal
fermentation. These results show that com grain hybrids utilized in the second trial do
not require different concentrations of DIP. Although the in vitro data suggested that
ruminal fermentation of starch would be much higher for ﬂoury endosperm, data

collected in vivo did not correspond.

78

Understanding nutrient synchrony is a very promising concept that may allow
beef cattle operations to be more efﬁcient. However, nutrient synchrony requires the
ability to quantitatively predict the fate of ruminal protein and carbohydrate. Successful
nutrient synchrony requires accounting for the complex interaction between the animal,
its environment, the ruminal microbial population, and the diet (Hall and Huntington,
2008)

The research described herein is a small piece of the puzzle that pertains to
enhancement of beef cattle efﬁciency. The current research trials document that IVSD of
com grain could be considered during diet formulation in order to maximize nutritional
inputs. Further research is needed to evaluate the effects of grain vitreousness and

processing methods on growth performance and end-product merit in feedlot cattle.

79

 

FUTURE WORK

Further research is necessary to ascertain whether or not performance differences
can be realized in feedlots based on fermentation differences between corn hybrids with
different vitreousness. A major obstacle for feedlots will be their ability to source
speciﬁc corn grain hybrids based on potential differences in ruminal fermentation. While
nutrient synchrony is theoretically a sound strategy to reduce excess environmental N and
P, nutrient synchrony in general is very hard to predict as available research does not
precisely quantify fates of various feedstuffs. This is due in part to our current
understanding of ruminant digestion and metabolism. Additional research is needed to
elucidate these responses. With current input prices at an all-time high, enhancement of

beef cattle efﬁciency should be of paramount importance within the research community.

80

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