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Nutrition of grass carbohydrates

Authors:

Abstract

As livestock production continues to improve through breeding, the need for high quality forage has also increased. The producers control the quality of selecting fodder harvest or grazing date. This should increase the emphasis on a better understanding of the effect of environmental factors on forage quality. Unfortunately, the mechanisms by which environmental factors influence the quality of forage are not well understood, particularly at the molecular level and our understanding is not enough to predict the influence of environmental factors. Usually, the temperature has great influence on forage quality, more than other environmental factors, and is, in this particular area, where more information is needed. The increase in temperature normally causes maturity, however the primary effects on digestibility may be through the effect of the relationship between leaves and stems. High temperatures promote the growth of the stem over the leaf growth. The digestibility of stems and leaves is low in forages warm weather due to the high concentrations of cell wall and low content of non-structural carbohydrates. An increase in temperature can have a positive effect on forage quality by raising the concentration of CP. Soil nutrients have only small effects on forage quality. N fertilization, usually increase CP levels of some non-leguminous fodder. Forage species with low concentrations of N, such as winter pastures can improve digestibility, because N fertilization can stimulate microbial activity in the rumen. Additionally, the application of sulfur in deficient soils, this often stimulates digestibility. Foliar diseases probably have the most adverse effects on the quality of the forage plant. Pesticides in plants may reduce digestibility.
Book chapter by:
Roque Ramirez Lozano. (2015). Nutrition of grass carbohydrates. In: Grass
Nutrition. Edited by R. Ramirez Lozano. pp. 29-56. Palibrio Press,
Bloomington, Indiana USA. ISBN: 978-1-5065-0808-5
Introduction
As livestock production continues to improve through breeding, the need for high
quality forage has also increased. The producers control the quality of selecting
fodder harvest or grazing date. This should increase the emphasis on a better
understanding of the effect of environmental factors on forage quality.
Unfortunately, the mechanisms by which environmental factors influence the
quality of forage are not well understood, particularly at the molecular level and our
understanding is not enough to predict the influence of environmental factors.
Usually, the temperature has great influence on forage quality, more than other
environmental factors, and is, in this particular area, where more information is
needed. The increase in temperature normally causes maturity, however the
primary effects on digestibility may be through the effect of the relationship
between leaves and stems. High temperatures promote the growth of the stem
over the leaf growth. The digestibility of stems and leaves is low in forages warm
weather due to the high concentrations of cell wall and low content of non-
structural carbohydrates. An increase in temperature can have a positive effect on
forage quality by raising the concentration of CP. Soil nutrients have only small
effects on forage quality. N fertilization, usually increase CP levels of some non-
leguminous fodder. Forage species with low concentrations of N, such as winter
pastures can improve digestibility, because N fertilization can stimulate microbial
activity in the rumen. Additionally, the application of sulfur in deficient soils, this
often stimulates digestibility. Foliar diseases probably have the most adverse
effects on the quality of the forage plant. Pesticides in plants may reduce
digestibility.
2
Forage
In general, forages are the vegetative parts of grasses containing a high proportion
of fiber (more than 30% of neutral detergent fiber). They are required in the diet in
a coarse physical form (particles larger than 1 or 2 mm in length). Forages usually
occur: 1) at the farm, 2) directly grazed, and 3) harvested and preserved as silage
or hay. Depending on the stage of growth, forages can contribute from almost
100% (in non-lactating animals) and not less than 30% (in cows in early lactation)
of the dry matter in the diet. The general, characteristics of forages are:
1. The volume limits consumption of the ruminant. Too much forage in the diet
may limit energy intake and milk production. However, bulky feeds are
essential to stimulate rumination and maintain the health of the ruminant.
2. They can contain 30-90% of neutral detergent fiber (NDF). In general, the
higher the fiber content, the lower the energy content of the forage.
3. Depending on the maturity, legumes may contain 15-23% crude protein
(CP), grasses; however, contain 8-18% CP (according to the level of
nitrogen fertilization) and crop residues (straw or stubble) may have only 3-
4% of CP.
From a nutritional standpoint, forages may range from very good feeds (lush young
grass, legumes at a vegetative stage) to very poor (straw and roughage).
Grasses and Legumes
High quality forage can make up two-thirds of the dry matter in the diet of
ruminants, that consumes 2.5 to 3 % of their body weight (a cow of 600 kg can eat
15-18 kg of dry matter of a good quality forage). Cows can eat more than one
legume than grasses at the same stage of maturity. However, good quality forages
3
in good balanced diets, can provide much of the protein and energy needed for
milk production.
The soil and climate conditions typically determine the most common types of
forages in a region. Both grasses and legumes are widely known around the world.
Grasses need nitrogen fertilizers and moisture conditions to grow well. However,
legumes are more resistant to drought and require less N in the soil because they
live associated with bacteria that can convert air N to soil N for fertilization.
The nutritive value of forages is highly influenced by the stage of growth when are
harvested or grazed. Growth can be divided in three successive stages:
1. Vegetative stage
2. Flowering stage and
3. Seed formation stage
Usually, the feeding value of a forage is highest during vegetative growth stage and
the lowest during seed formation stage. As maturity progresses, the concentration
of CP, energy, calcium, phosphorus and digestible dry matter in the plant is
reduced; while, increasing the concentration of NDF. When NDF is increasing,
lignin content increases, making less available the carbohydrates to the rumen
microbes. Thus, the energy value of the forage decreases. Therefore, when
forages are produced for feeding cattle, they have to be harvested or grazed at an
early stage. Corn and sorghum harvested for silage are exceptions because,
despite the nutritive value of the vegetative parts of the plant (stem and leaves),
during the seed formation, a high amount of digestible starch accumulates in the
grains. The maximum yield of digestible dry matter of a forage crop is obtained:
1. In grasses, during the first part of maturity
2. In legumes, at the stage of mature button medium
3. In corn and sorghum, before the grains are fully completed
4
The nutritional value of a forage is reduced with advanced of maturity. The delay of
the harvest after the optimum maturity may reduce the potential animal production
of cattle consuming forage. However, several strategies are available to maintain
the availability of forage that has good nutritive value:
1. Develop a grazing strategy that matches the number of animals in a pasture
and the rate of grass growth.
2. Plant a mixture of grasses and legumes that have different rates of growth
and maturity throughout the season.
3. Harvest at an early stage of maturity and preserve as hay or silage.
4. Feed lower quality forage to dry cows or the cows in late lactation, and the
good quality forage to the cows in early lactation.
Crop residues and byproducts
The residues are the parts of plants that remain in the field after harvesting the
main crop (e.g. corn roughage, cereal straw, sugar cane bagasse, peanut hay).
The residues may be grazed, processed as dry feed, or made into silage. General
characteristics of most residues are:
1. They are cheap and bulky foods
2. High in indigestible fiber because of its high lignin content. Although
chemical treatments can improve its nutritional value
3. Low crude protein
4. Require adequate supplementation especially with protein and minerals
5. Require be chopped when harvested or before feeding
6. Can be included in the diets of lactating cows that have lower energy
demands.
5
Concentrates
There is no clear definition of the concentrates, but can be described by their
characteristics as food and its effects on rumen function. Usually, the concentrates
can be referred as:
1. They are low in fiber and high in energy.
2. They can be high or low in protein. Cereal grains contain <12% CP, but
oilseed meals (soybean, cotton, peanut) called protein foods can contain
>50% of CP.
3. They have high palatability and are usually eaten rapidly. In contrast to
forage, concentrates have low volume per unit of weight (high specific
gravity).
4. Do not stimulate rumination.
5. They usually ferment faster than forages in the rumen. Thereby, increasing
acidity (lower pH) in the rumen may interfere with normal fiber fermentation.
6. When they comprise more than 60-70% of the diet may lead to health
problems.
Lactating ruminants have high requirements for energy and protein. Because cows
can eat only a certain amount each day, forage alone may not supply the required
amount of energy and protein. The purpose of adding concentrates to the diet of
lactating cattle is to provide a source of energy and CP to supplement the forage
and meet the requirements of the animal. Thus, concentrates are important feeds
that allow for formulating diets that will maximize milk production. Generally, the
maximum amount of concentrates that a cow can receive per day should not
exceed 12 to 14 kg.
6
Types of carbohydrates
Carbohydrates are the most important source of energy and are the main
precursors of fat and sugar (lactose) in milk. Microorganisms in the rumen allow
the ruminant to obtain energy from fibrous carbohydrates (cellulose and
hemicellulose) that are bound to the lignin in cell walls of the plant (Table 3.1). The
fiber is bulky and is retained in the rumen because the cellulose and hemicellulose
are fermented slowly. As the plants mature, the lignin content of the fiber increases
and the degree of fermentation of cellulose and hemicellulose in the rumen is
reduced. The presence of fiber in the diet is necessary to stimulate rumination.
Rumination increases the breakdown and fermentation of fiber, stimulates
contractions of the rumen and increases the flow of saliva to the rumen. Saliva
contains sodium phosphates that help maintain the acidity (pH) of the rumen
contents to a nearly neutral pH. Diets low in fiber and high in concentrates result in
a low percentage of fat in the milk and contribute to digestive disorders, such as
displaced abomasum and rumen acidosis.
Table 3.1 Carbohydrates contained in plants
Component Function Fruits Seeds Legumes Grasses Trees and shrubs
Soluble sugars Nonstructural 17-77 0-1 2-16 5-15 5-15
Starch Nonstructural 0-3 80 1-7 1-5 --
Pectin Structural 5-17 0-1 5-10 1-2 6-12
Hemicellulose Structural 2-7 7-15 3-10 15-40 8-12
Cellulose Structural 3-17 2-5 7-35 20-40 12-30
Obtained from: Robbins (2001).
7
Nonstructural carbohydrates (starch and sugars) are fermented rapidly and
completely in the rumen. The content of nonstructural carbohydrates increases the
energy density of the diet by improving energy supply and increasing microbial
protein produced in the rumen. However, the nonstructural carbohydrates not
stimulate rumination fermentation or saliva production and when are excess may
inhibit fermentation of fiber. Therefore, the balance between structural and
nonstructural carbohydrates is important in ruminant feed for efficient production.
Glucose synthesis in liver
All propionic acid produced in the rumen is converted to glucose in the liver. The
liver uses amino acids for glucose synthesis. This is an important process because
usually the glucose can be absorbed from the digestive tract and the liver produces
all the sugars found in milk. The exception is when the cow is being fed large
amounts of concentrates rich in starch or a source of resistant starch ruminal
fermentation. The glucose formed by the digestion in the intestine is absorbed and
transported to the liver where it contributes to glucose supply of the cow. Lactose is
an alternative source of glucose for the liver. Lactose is found in well-preserved
silage, but lactose production in the rumen occurs when there is excess starch in
the diet. This is undesirable because the acid in rumen environment, fiber
fermentation stops and, in extreme cases, the animal stops eating.
Lactose and fat synthesis in the liver
During lactation, the mammary gland has high priority for the use of glucose.
Glucose is used primarily for the formation of lactose (milk sugar). The amount of
lactose synthesized in the mammary gland is closely related with the amount of
milk produced every day. The concentration of lactose in milk is relatively constant
and water is added to the amount of lactose produced by the secretory cells to a
8
concentration of approximately 4.5% lactose. Therefore, milk production in dairy
cows is highly influenced by the amount of glucose derived from the propionic acid
produced in the rumen. Furthermore, glucose is converted to glycerol that is used
for the synthesis of milk fat. Volatile fatty acids (VFA) acetic acid and β-
hydroxybutyric acid are used for the formation of fatty acids of milk fat. The
mammary gland synthesizes saturated fatty acids containing from 4 to 16 carbon
atoms (short chain fatty acids). Almost half of milk fat is synthesized in the
mammary gland. The other half that is rich in unsaturated fatty acids containing 16
to 22 carbon atoms (long chain fatty acids) are derived from dietary lipids. The
energy required for the synthesis of fat and lactose is obtained from the
combustion of ketone bodies; however, acetic acid and glucose can also be used
as energy sources for the cells of many tissues.
Carbohydrates and grass quality
Carbohydrates are the main reservoir of photosynthetic energy from plants. The
nutritional characteristics of carbohydrates for animal feeds are variable and
depend on their sugar components and their bounds. However, the variety of
sugars and bounds in plants is much wide than animal tissues. Carbohydrates of
plants contain many sugars and uncommon links than animal systems. The
nutritional availability depends on the capacity to break the glycosidic linkages in
the carbohydrates of plants and between carbohydrates and other substances.
They form the bulk of the food supply for animals, and is the most abundant class
of compounds found in plants. Play important roles such as 1) intermediary
metabolism, 2) energy transfer, 3) storage and 4) plant structure. Photosynthetic
energy is set to carbohydrates via the Calvin cycle and serve as initial substrates
for intermediate pathways in almost all plants. The energy is transported within the
plants as the disaccharide sucrose, and stored in polymers such as starch and
fructans.
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Carbohydrates also constitute most of the cell wall of plants and as such play an
important role in the structural integrity of individual cells, tissues and organs. They
are extremely important from a nutritional perspective, and are the main source of
energy in the diet of a ruminant. In ruminants most of the carbohydrate digestion
occurs within the rumen (over 90%), although under certain circumstances such as
high passage rates, a significant amount of carbohydrate digestion can occur in
small intestine and large intestine. Sugars are rapidly fermented in the rumen to
give VFA that are absorbed into the blood through the rumen wall. The
polysaccharides must be degraded into simple sugars before being used.
Nonstructural polysaccharides such as starch and fructans are rapidly and
completely degraded in the rumen, while the degradability of structural
polysaccharides (cellulose and hemicellulose) varies considerably.
In general, the degradability of cellulose in forages ranges from 25 to 90%, while
the digestibility of hemicellulose varies from 45 to 90%. The degradation of β-
glucans is intermediate to cellulose. The ability to degrade and utilize structural
carbohydrates gives to ruminants a unique ecological niche. Besides being an
important source of energy in the diet of ruminants, carbohydrates have other
nutritional roles as components of dietary fiber. Structural carbohydrates are
important for normal rumen function. Fiber stimulates rumination and salivation as
previously mentioned, and promotes the exchange of cations that are important in
ruminal buffering capacity. The fiber is also involved in the regulation of voluntary
intake.
Chemistry of carbohydrates of the forage
The terms fiber and cell wall of the plant are often misleading. These terms,
however, are not synonymous and reflect different functional perspectives. Plants
are unique among higher organisms, although they have rigid cell walls. The cell
walls of plants can be considered a compound consisting of cellulose fibrils
10
embedded within a matrix of lignin and hemicellulose polysaccharides (Figure 3.1).
In addition, intact cell wall contains components such as water, organic solvents
and phenolic that give unique properties to the structure (Table 3.2). The
macromolecular composition of the cell walls of the cells varies considerably
between organs, tissue and subcellular level. The primary cell wall is formed
adjacent to the plasmalemma during cell elongation and consists almost entirely of
polysaccharides. The secondary wall is formed during cellular differentiation inner
wall and the primary composition varies greatly depending on the cell type.
Individual cells stick to the middle lamella, which consists mainly of pectic
substances that serve as an intercellular cementing agent.
FORAGE
Digest with neutral detergent fiber
Cell contents Hemicellulose, Cellulose and Lignin
Protein, starch,
(NDF)
sugars, organic
acids and pectin Digest with acid detergent fiber
Hemicellulose Cellulose and lignin
(ADF)
Digest with 72%
sulfuric acid
Cellulose Lignin
Figure 3.1. Schematic of detergent system of forage analysis
Table 3.2. Classification of forage fraction using the
Van Soest Method
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The most obvious function of the cell wall (Figure 3.2) is its role in morphogenesis.
The cell walls form the structural design of the architecture of the plant and provide
mechanical and structural support to the plant organs. Also, the walls play
important roles in water balance, ion exchange, cell recognition and protection of
biotic stress. In contrast, the fiber is a nutritional entity which is defined by its
biological properties and chemical composition. The concept of fiber, particularly
fodder, refers to complex dietary nutrients, which are relatively resistant to
digestion and are slowly and partially degraded by ruminants. In this definition, the
main components of the fiber are cellulose, hemicellulose and lignin. This definition
also includes pectins and β-glucans. The holocelulosa and lignocellulose terms are
often used in relation to forage quality. The holocelulosa collectively refers to
cellulose and hemicellulose. While lignocellulose includes lignin, in addition to the
structural polysaccharides. The term lignocellulose is often misleading with the
fiber, especially in areas that are not related to nutrition, such as biofuels.
CELL WALL
Middle lamella
Primary cell wall
Secundary cell wall
CELL WALL COMPOSITION
Primary cell wall
Cellulose ADF
Lignin (LDA) NDF
Hemicellulose
Secondary cell wall
CELL CONTENTS
Protein , Pectin,
Sugars, Starch, etc.
Figure 3.2. Schematic representation of plant cell wall composition (ADF = acid detergent fiber;
NDF = neutral detergent fiber)
Biosynthesis of carbohydrates
Carbohydrates are produced by photosynthetic carbon fixation process (Figure
3.3). The formation of individual types of sugars typically occurs through the action
12
of the enzymes epimerases, isomerases, oxidoreductases and/or decarboxylase,
of activated monosaccharides leaving Calvin cycle or from the breakdown of
storage carbohydrates. The biosynthesis of oligosaccharides and polysaccharides
requires activated sugars, in the form of nucleoside diphosphate monosaccharides.
The predominant pattern of interconversions of glucose is derived directly from
photosynthetic activity or starch degradation. There are a few alternate routes such
as the conversion of inositol or glucuronic acid degradation pathways that can
claim the galactose and galacturonic acid through direct phosphorylation.
Figure 3.3. Reactions of carbon fixation for the biosynthesis of carbohydrates
Variation in composition of structural carbohydrates
There is considerable variation between species of plants with respect to the
concentration and composition of structural carbohydrates. The cellulose
concentration is typically higher in the walls of legumes than in grasses. This
reflects a much lower concentration of hemicellulose in legumes compared to
grasses. The concentration of cellulose often appears similar between grasses and
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legumes. The warm weather perennial grasses contain structural carbohydrates in
greater proportion than temperate grasses.
The hydrolysis of forage hemicellulose produces the neutral monosaccharides:
glucose, xylose, arabinose, mannose, galactose, rhamnose, fructose, and the
acids uronic, galacturonic, glucuronic and 4-0-metilglucorónic. The relative
proportions of each monosaccharide vary between species, reflecting differences
in the structure of polysaccharides. Xylose and arabinose are produced from most
neutral sugars isolated of hemicelluloses from grasses and legumes. Comparative
studies were conducted in neutral structural carbohydrates isolated from stems of
legumes and grasses. Glucose (predominantly from native cellulose) and xylose
comprised 67% and 20% from the cell wall of legumes and 63% and 30% in
grasses, respectively. It was recognized that the degradation of cell wall
polysaccharides is much more affected by interactions between cell wall polymers,
than for the individual properties of the polymers. Cellulose is degraded in the
rumen by a complex of anaerobic microorganisms including bacteria, protozoa and
fungi.
The cellulolytic bacteria from which Ruminococcus flavefaciers, Fibrobacter
succinogenes and R. albus are the most important, and are responsible for
cellulose digestion that occurs in the rumen (Figure 3.5). Although ciliated protozoa
and fungi have been identified in the rumen that have cellulolytic activity, their
contribution is relatively minor to the degradation of cellulose. Cellulolytic bacteria
adhere to the surface of the cell wall, placing enzymes in close proximity to the
substrate. The cellulolysis is accomplished by the action of several extracellular
enzymes that bind to the surface of the body or are secreted into the surrounding
medium. However, three basic enzyme activities are involved: 1) endo-β-1, 4-
glucanase that breaks the polysaccharide random into oligosaccharides, 2) exo-β-
1, 4-glucanase which attacks the no reducing end of oligosaccharides, giving
cellobiose, and 3) cellobiose β-1, 4-glucosidase that hydrolyses cellobiose to
glucose. The amount by which the native cellulose is used by rumen
14
microorganisms is limited because it is association with lignin and other cell wall
constituents.
There are, however, inherent factors that can limit the rate at which the cellulose is
digested. The crystallinity of the cellulose has been suggested as a factor in
reducing the accessibility of the cellulose to enzymatic attack. Cellulose
degradation has been shown to be inversely proportional to the degree of
crystallinity for purified substrates. However, actually, there is little evidence that
the crystallinity is a limiting factor in the rate of degradation of native cellulose by
rumen microbes.
CELLULOSE
HEMICELLULOSE
gas Microbial action
RUMEN
METANE VOLATILE FATTY
ACIDS
ACETIC PROPIONIC BUTYRIC
BLOOD
TISSUES
Catabolism Acetic
Butyric Propionic
ENERGY
FATS GLUCOSE
Figure 3.5. Ruminal digestion and absorption of carbohydrates
Hemicellulose degradation in the rumen occurs in a manner analogous to that of
cellulose, but involves a broader arrangement of enzyme activities. The same
cellulolytic bacteria listed above are responsible for most cellulose degradation in
the rumen and are for the major hemicellulosic bacteria. In addition, Butirivibrio
fibrisolvens that has a relatively minor role in the degradation of cellulose has a
proportionally greater role in the degradation of xylans.
15
Some fungi and rumen protozoa have also hemicellulolytic activity, but its activity in
the degradation of hemicellulose is relatively minor compared to ruminal bacteria.
Isolated hemicelluloses are general, completely digested by rumen
microorganisms; degradation of hemicellulose occurs through activities endo and
exo glycanases that depolymerize and solubilize the major polysaccharide chains.
Substitutes groups and side chains are removed from the hemicellulose and
subsequently degraded by several glucosidases.
Cell wall (NDF) content in cultivated grasses
Seasonal NDF, cellulose and hemicellulose content of in cultivated grasses grown
in different counties of the state of Nuevo Leon, Mexico are listed in Table 3.3. The
NDF average in grasses was 76% with values ranging from 74 to 78%. Cellulose
was slightly higher (34% annual mean) than hemicellulose (31%) in most grasses.
However, in Cynodon dactylon the hemicellulose (34%) was higher than cellulose
(32%) content. Due to small differences between grasses on the cellulose and
hemicellulose contents, it may indicate that are digested by bacteria in the rumen
in similar proportions. Differences in digestion might be due to genetic variations
between grasses.
The NDF content of the six genotypes of the grass Cenchrus ciliaris was similar
between collections (Table 3.4). However, cellulose doubled, in all cuts, the
hemicellulose content. The same tendency as the NDF and its constituents
(cellulose and hemicellulose), was observed in the 84 new genotypes of C. ciliaris
(Table 3.5). Cellulose is degraded in the rumen by a complex anaerobic
microorganisms among which are included bacteria, protozoa and fungi.
Ruminococcus flavefaciens cellulolytic bacteria, R. albus and Fibrobacter
succinogens are most important and are responsible for most of the digestion of
cellulose in the rumen. Digestion of hemicellulose in the rumen occurs in the same
manner as cellulose. The same bacteria mentioned above are responsible for
digestion of hemicellulose, although it also includes Butyrivibrio fibrisolvens, but
has much less effect than the other ones. Some fungi and protozoa also digest the
16
cellulose and hemicellulose in the rumen, but to a much lesser extent than
bacteria.
Cultivated grasses such as C. ciliaris, C. dactylon, D. annulatum and P. coloratum
(Table 3.6) growing in the county of Linares, Nuevo Leon, Mexico, contained more
NDF in stems to the leaves; additionally, cellulose and hemicellulose were largely
higher in the stems than in the leaves. During wet seasons (spring and fall)
grasses had less NDF content compared to the dry seasons (winter and summer).
Table 3.3. Seasonal content of neutral detergent fiber (NDF), cellulose y hemicellulose (Hemicel, % dry matter) in cultivated grasses collected
in different counties of the state of Nuevo Leon, Mexico
Grasses Place and date of collection Concept Seasons Annual
Winter Spring Summer Autumn mean
Cenchrus ciliaris Marin, N.L., México (1994) NDF 78 74 76 74 76
Cellulose 36 35 35 34 35
Hemicel 32 30 31 33 32
Cenchrus ciliaris Teran, N.L., México (2001-02) NDF 81 73 86 68 77
Cellulose 35 31 36 32 35
Hemicel 32 30 30 30 32
Cenchrus ciliaris Linares, N.L., México (1998-99) NDF 72 72 70 71 71
Cellulose 35 32 33 32 33
Hemicel 33 30 36 26 31
Cynodon dactylon Linares, N.L., México (1998-99) NDF 81 72 83 78 79
Cellulose 33 33 30 34 32
Hemicel 32 27 44 32 34
Cynodon dactylon Marin, N.L., México (1994) NDF 69 77 82 76 76
Cellulose 40 37 39 36 38
Hemicel 25 26 21 27 25
Cynodon dactylon II Marin, N.L., México (1994) NDF 69 81 81 76 77
Celulose 29 28 30 25 28
Hemicel 29 37 40 36 35
Dichanthium annulatum Marin, N.L., México (1994) NDF 78 77 78 79 78
Cellulose 29 34 38 33 33
Hemicel 33 37 34 34 35
Dichanthium annulatum Linares, N.L., México (1998-99) NDF 78 77 83 77 79
Cellulose 38 38 37 39 38
Hemicel 22 26 32 24 26
Panicum coloratum Linares, N.L., México (1998-99) NDF 80 77 82 76 79
Cellulose 33 34 33 35 34
Hemicel 35 33 40 32 35
Rhynchelytrum repens Teran, N.L., México (2001-02) NDF 73 69 74 73 72
Cellulose 28 27 29 28 28
17
Hemicel 24 29 28 26 27
Mean NDF 75 76 78 76 76
Cellulose 34 33 35 33 34
Hemicel 29 32 32 30 31
Obtained from: Ramírez et al. (2002); Ramírez et al. (2003); Ramírez (2003); Ramírez et al. (2004); Ramírez et al. (2005);
Table 3.4. Neutral detergent fiber (NDF), cellulose y hemicellulose (% dry matter) of the hybrid buffel Nueces
and five new genotypes of Cenchrus ciliaris collected in different dates in the county of Teran of the state of
Nuevo Leon, México
Genotypes Concept Dates of collections
Aug
1999
Nov
1999
Nov
2000
Jun 2000
(Fertilized)
Cenchrus ciliaris Nueces NDF 74 71 72 72
Cellulose 38 35 38 40
Hemicellulose 25 27 24 24
Cenchrus ciliaris 307622 NDF 71 71 70 71
Cellulose 38 35 33 43
Hemicellulose 23 24 18 19
Cenchrus ciliaris 409252 NDF 73 69 70 70
Cellulose 40 33 40 40
Hemicellulose 24 26 21 18
Cenchrus ciliaris 409375 NDF 73 69 70 73
Cellulose 39 35 38 40
Hemicellulose 23 24 19 23
Cenchrus ciliaris 409460 NDF 72 69 70 74
Cellulose 40 33 36 37
Hemicellulose 23 22 22 24
Cenchrus ciliaris 443 NDF 71 66 66 69
Cellulose 38 36 40 40
Hemicellulose 24 20 15 20
Means NDF 72 69 70 71
Cellulose 39 35 38 40
Hemicellulose 24 24 20 21
Obtained from: Garcia-Dessommes et al. (2003ab)
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Cell wall (NDF) content in native grasses
Native grasses that grow in Marin county of the state of Nuevo Leon, Mexico
contained about 10% more NDF (Table 3.7) than native grasses that grow in Teran
county of the state of Nuevo Leon, Mexico (Table 3.8). The differences in NDF
content may be due to differences in sites and dates of harvested. During the year
of the study, in Marin county, the rainfall was 516 mm and in Linares was 613 mm.
Thus, lower cell wall content in the grasses growing in Linares was because there
was a greater precipitation. The cellulose content was similar to hemicellulose
content in all native grasses (Tables 3.7 and 3.8). However, the grass Cenchrus
incertus and Chloris ciliata had an intermediate NDF content when compared with
other native grasses that grow in northeastern Mexico.
Table 3.5. Neutral detergent fiber content (NDF), cellulose (Cel) y hemicellulose (Hemi; % dry matter) in 84 new genotypes of
Cenchrus ciliaris collected in the county of Teran of the state of Nuevo Leon, México in November 2000.
Genotype
s
NDF Cel Hemi
Genotype
s
NDF Cel Hemi
Genotype
s
NDF Cel Hemi
Genotype
s
NDF Cel Hemi
202513 73 38 29 409185 72 40 24 409258 67 39 21 409449 73 40 25
253261 72 38 27 409197 70 43 25 409263 70 40 27 409459 67 41 15
307622 69 33 18 409200 79 39 29 409264 73 41 27 409460 72 41 24
364428 69 38 22 409219 68 39 24 409266 74 41 31 409465 75 42 27
364439 76 42 23 409220 71 42 26 409270 73 41 29 409466 76 41 28
364445 73 40 22 409222 76 41 31 409278 71 43 25 409472 72 41 28
365654 72 41 23 409223 75 41 27 409280 70 42 26 409480 76 41 26
365702 72 41 27 409225 77 42 29 409300 75 41 23 409529 76 42 30
365704 71 42 21 409227 75 39 29 409342 70 40 25 409691 74 39 29
365713 75 40 27 409228 71 41 26 409359 73 43 23 409711 76 39 29
365728 76 41 25 409229 68 38 27 409363 68 40 25 414447 76 42 27
365731 77 41 27 409230 71 40 23 409369 70 41 23 414451 75 41 26
409142 75 40 27 409232 70 41 26 409373 69 37 19 414454 75 39 28
409151 72 42 26 409234 70 40 24 409375 72 41 24 414460 75 41 23
409154 70 41 19 409235 70 41 27 409377 71 40 24 414467 75 42 25
409155 71 42 26 409238 68 38 26 409381 71 40 27 414499 78 42 26
409157 71 42 23 409240 70 40 24 409391 75 40 24 414511 76 42 26
409162 69 41 22 409242 76 44 28 409400 75 40 24 414512 74 42 25
409164 74 40 22 409242 70 40 21 409410 74 40 25 414520 76 41 26
409165 69 39 22 409252 75 41 28 409424 70 41 21 414532 71 36 22
19
409168 70 40 24 409254 73 41 24 409448 76 41 30 443 72 38 24
Obtained from: Rodríguez-Morales (2003)
The environment and plant quality
There is no factor that affects the quality of the forage as the maturity of the plant;
however, environmental changes the impact of maturity. The environment includes
biotic and abiotic factors that influence the growth and development of plants.
Cumulative effects are integrated through physiological processes and reflected in:
1) the growth rate of passage, 2) growth rate, 3) production and 4) quality of
forage. Variations between seasons and changes in the environment related to the
geographical location affect the quality of the forage, even when forages are
harvested at similar morphological stages. This makes it difficult to predict the
quality of forage, which affects the behavior of animals consuming the forage.
Table 3.6. Neutral detergent fiber content (NDF), cellulose y hemicellulose (Hemi; % dry matter) in leaves and stems in cultivated grasses and
collected at different counties and dates of the state of Nuevo Leon, México
Grasses Places and dates of collection Parts Concept Seasons Annual
Winter Summer Summer Fall mean
Cenchrus ciliaris Linares, N.L., Mexico (1998-99) Leaves NDF 78 62 83 63 71
Cellulose 35 28 26 29 30
Hemi 29 25 40 25 30
Stems NDF 88 75 88 74 81
Cellulose 40 35 36 35 36
Hemi 33 28 36 27 31
Cynodon dactylon Linares, N.L., Mexico (1998-99) Leaves NDF 77 80 82 76 79
Cellulose 32 31 27 31 30
Hemi 34 38 47 34 38
Stems NDF 82 73 84 80 80
Cellulose 44 35 31 36 36
Hemi 31 26 44 32 33
Dichanthium annulatum Linares, N.L., Mexico (1998-99) Leaves NDF 76 73 80 74 76
Cellulose 36 37 34 38 36
Hemi 23 24 35 25 27
Stems NDF 84 82 84 83 83
Cellulose 43 43 37 43 41
Hemi 22 24 35 24 26
Panicum coloratum Linares, N.L., Mexico (1998-99) Leaves NDF 80 72 78 68 75
Cellulose 31 26 28 27 28
Hemi 38 38 41 33 38
20
Stems NDF 85 81 86 83 84
Cellulose 37 37 40 38 38
Hemi 35 32 36 33 34
Obtained from: Ramirez et al. (2001ab); Foroughbackhch et al. (2001); Ramirez et al. (2003); Ramirez (2003); Ramirez et al. (2005);
Plants rarely grow in their ideal environment, instead experience fluctuations in the
environment and stress that: 1) changes its morphology and growth rate, 2) limits
its production and 3) alter its quality. Stress is caused when some environmental
factor is not ideal for plant growth and development. This can be caused by many
factors but those that must be considered are 1) temperature, 2) water deficit, 3)
solar radiation, 4) nutrient deficiencies and 5) pests. The cell wall of plants provides
the first line of defense against most stresses. Cells develop secondary wall
lignification, which is an important aspect of protection. Lignification also restricts
the availability of nutrients in the cell wall for animals that consume them. The cell
walls vary in digestibility, they are available only in part; however, the cell contents
are completely digestible.
Table 3.7. Seasonal content of neutral detergent fiber (NDF), cellulose y hemicellulose (% dry matter) in native grasses
collected in Teran county of the state of Nuevo Leon, México in 2001 and 2002
Grasses Concept Seasons and years of collection Annual
Winter 2002 Spring 2002 Summer 2002 Fall 2001 mean
Bouteloua curtipendula NDF 72 72 73 79 74
Cellulose 28 28 29 31 29
Hemicellulose 27 27 29 29 28
Bouteloua trifida NDF 75 70 76 76 74
Cellulose 30 28 30 30 30
Hemicellulose 27 26 30 29 28
Brachiaria fasciculata NDF 72 63 62 60 65
Cellulose 27 23 23 22 24
Hemicellulose 25 22 25 22 24
Digitaria insulares NDF 70 67 65 71 68
Cellulose 33 32 31 33 32
Hemicellulose 34 31 29 31 31
Chloris ciliata NDF 70 70 74 75 72
Cellulose 26 26 27 28 27
Hemicellulose 26 27 26 26 26
Leptochloa filiformis NDF 75 67 73 67 70
Cellulose 29 25 28 25 27
Hemicellulose 31 21 29 25 26
Panicum hallii NDF 74 67 68 71 70
Cellulose 31 28 28 30 30
Hemicellulose 31 26 29 31 29
Panicum obtusum NDF 74 65 66 66 65
Cellulose 28 25 25 25 26
Hemicellulose 28 23 24 24 24
Panicum unispicatum NDF 70 64 69 67 68
Cellulose 29 27 29 28 28
21
Hemicellulose 32 26 28 26 28
Setaria grisebachii NDF 73 71 61 73 69
Cellulose 28 28 23 28 26
Hemicellulose 26 27 21 26 26
Setaria macrostachya NDF 72 68 63 73 69
Cellulose 28 27 25 28 27
Hemicellulose 28 27 23 25 26
Tridens eragrostoides NDF 71 74 72 76 73
Cellulose 30 31 30 32 30
Hemicellulose 29 31 31 31 30
Tridens muticus NDF 76 72 73 78 75
Cellulose 28 27 27 29 28
Hemicellulose 31 28 28 28 27
Means NDF 73 68 69 72 70
Cellulose 31 29 29 30 30
Hemicellulose 30 28 29 28 29
Obtained from: Cobio-Nagao (2004)
The stress caused by the environment has a greater effect on the production of
forage than in the digestibility or other factors related to quality. The environment of
the plant often demonstrates its great influence not only in forage quality by altering
the relations between the stems and leaves, but also cause other morphological
changes in chemical composition of plant parts. Changes in the morphology of
plants can alter the availability of forage, especially influencing consumption
affecting grazing animals potential bite size. The height cover of the vegetation is
the most important variable affecting grass bite size and altering the ratio stem:leaf.
Environmental influence maturation rates and the amount of dead material. The
animals generally selected young green tissues rather than the stems and dead
leaves plant tissues. Many stresses reduce plant growth and development, the
result is that the quantity of forage remains at very low levels.
Table 3.8. Neutral detergent fiber content (NDF), cellulose y hemicellulose (% dry matter) in native grasses
collected in Marin county f the state of Nuevo Leon, México in 1994
Native grasses Concept Seasons Annual
Winter Spring Summer Fall mean
Aristida longiseta NDF 87 85 87 88 87
Cellulose 37 37 37 37 37
Hemicellulos
e
37 33 37 37 37
Bouteloua gracilis NDF 90 81 82 77 82
Cellulose 33 38 32 33 34
Hemicellulos
e
43 30 32 29 34
Cenchrus incertus NDF 80 74 80 75 77
Cellulose 29 28 35 30 31
Hemicellulos
e
36 26 34 33 32
Hilaria belangeri NDF 83 75 82 75 79
22
Cellulose 39 28 38 32 32
Hemicellulos
e
32 31 31 30 31
Panicum hallii NDF 76 69 73 68 72
Cellulose 32 30 29 31 31
Hemicellulos
e
31 29 32 26 29
Setaria
macrostachya
NDF 80 79 86 74 80
Cellulose 37 35 42 33 36
Hemicellulos
e
32 35 35 31 33
Means NDF 83 77 82 76 80
Cellulose 35 33 36 33 34
Hemicellulos
e
35 31 34 31 33
Obtained from: Ramirez et al. (2004)
Effect of temperature on nutritional quality of grasses
Because the nutritional value of a forage is regulated by the amount and availability
of metabolic and anabolic products, including cell wall and cell contents, thus any
factor that influences these products also affects the quality of forage. The
temperature usually has a great influence on forage quality than other
environmental factors found in plants. The temperature of the plant is the result of
complex interactions between the plant and its environment and is influenced by
the flux density of radiation, heat conduction, convection heat, latent heat and
anatomical and morphological characteristics. Moreover, due to variations in
coverage, particular aspects of plant parts and the result of differences in the
radiation charge, the tissue temperature can vary widely at any time.
Temperature effects on plant development
In general, temperature and soil affect the quality of forage in certain species
growing in certain regions. The temperature is the major determinant of the
geographical adaptation of plant species. This is particularly manifested in the
extreme temperatures encountered by ontogeny of plants. These extremes can
cause plant death or severe weakness. Under field conditions, the high
23
temperature stress often occurs along with the stress of water making it difficult to
separate the two effects.
Within a region or area, the primary effects of temperature on the quality of grasses
determine the rate of plant growth and influence relative proportion of leaves and
stems. A side effect of the temperature difference is found in the tissue morphology
of the leaves and stems. The temperature has a greater effect on the digestibility
more than other environmental variables, the economic implications of changes in
temperature should not be ignored.
Effect of temperature on chemical composition and digestibility
The negative effects of elevated temperature on the digestibility of the foliage of
grasses have been subject to numerous studies over the last 30 years. In most of
the plants that flower, environmental controls such as length of the day and
temperature modulate the rate of development. The temperature cannot only
increase the concentration of cell wall, but can also reduce cell wall digestibility. A
decrease of 80 g kg-1 in vitro digestibility (IVDMD) in Festuca arnudinacea occurred
when the temperature was increased from 15/10 °C to 25/20 °C. Results of several
related experiments related with temperature and digestibility concluded the leaves
of temperate grasses, during growth, showed an average decrease of 6.6 g kg-1 of
IVDMD per each increase of one-Celsius degree in temperature. The decrease in
IVDMD associated with high temperatures is most often attributed to high
concentrations of the constituents of the cell wall, but little research has been
conducted to test the causes involved in this phenomenon.
Winter and summer grasses
24
It has been reported that the geographical distribution of species C3 and C4 are
determined by regional and seasonal temperatures. The C4 types are more
numerous in the warmer and warmer seasons climates, C4 grasses have been
recognized for their relatively low digestibility and high concentrations of structural
polysaccharides compared with C3 grasses. In fact, in the grass, the difference in
digestibility and composition of the cell wall are caused by the temperature. Within
the Festucoidae and the tropical subfamily Panicoidae, the first exhibited the
photosynthetic C3 pathway and the last had C4. Warm grasses also were high in
cellulose and hemicellulose, rather than temperate grasses, this aspect has not
been sufficient clarify whether the differences are truly taxonomic or are caused by
different environmental conditions.
Using growth chambers with controlled temperature growth, it has been shown that
an increase in temperature during growth of warm season grasses (Brachiaria
ruziziensis) increased production of dry matter (DM), size, but also on the number
of new leaves in leaf/stem ratio, and the concentration of organic nitrogen (N) in
the DM. The authors also noted a positive relationship between temperature and
concentration of crude fiber in both leaf and stem tissues and postulated that the
temperature itself is the main factor contributing to the relatively poor quality grass
in warm climates.
Effects of water on grass quality
Water is a crucial component of plant cells and is needed for all metabolic
processes that depend on its presence. A suitable amount of water is required for
maintenance of turgor pressure, the protection function and diffusion of the solutes
into cells. Water provides oxygen during photosynthesis and the hydrogen used for
the reduction of carbon dioxide. The amount of water varies with the cell type and
the physiological status. The newly formed cells are necessarily composed of
water, while fibrous consistency cells contain almost no water. On average, the
concentration of water in grasses can be about 750 g kg -1, depending on the
25
species and environmental conditions, and decline as the maturity of the plant
progresses.
Most of the water of grasses comes from the soil through the roots. The plants
function as water pump, moving soil water into the atmosphere in response to
differences in the potential of soil and air. About 1% of the water entering the
growing plants is retained and most of it is lost through perspiration. Excess of
water, for metabolic needs, usually is used for important functions in the movement
of solutes from the roots to the leaves and stems and evaporative cooling of plant.
Strong resistance to water movement through the plant normally occurs in the air
spaces within the leaves. Stomata occupy about 1% of the leaf surfaces, but most
water lost by living leaves, passes through these open organs. Some water is also
lost through the cuticle.
General effects of water on grasses
The excess or deficiency of water can produce stress in grasses. Too much water
that can result from waterlogged soils imposes stress because in waterlogged
soils. The oxygen is lost by microorganisms and the root respiration, leaving the
roots of grasses in an environment without oxygen. Even that anoxia can greatly
reduce forage production; there is little information on how this affects the quality of
grasses.
It is more common that most grasses are growing areas with dry soil than watery
soils. In fact, the biggest concerns in the future will related to climate change and
droughts. The water deficit stress is usually the greater physical limitation to
grasses. When transpiration exceeds water absorption by the roots, the water
deficit in the plant increases and stress can occur, which adversely affects many
enzymatic reactions of most physiological processes. The water deficit causes
26
stomatal closure reduces transpiration rates, and increases the temperature of the
grass. Cell enlargement is particularly sensitive to water deficit.
Cell division appears to be less sensitive than cell enlargement. Turgor pressure
plays an important role in cell enlargement, providing the necessary pressure to
the cell wall expand. As the cell walls expand, decreased turgor pressure, which
causes the water potential within the cell, decrease. This creates a difference
between the inside and outside of the cell, which moves more water into the cells.
Solutes should continue settling in growing cells.
The ability of pasture to maintain a constant positive or turgor under water potential
decreases is an important adaptation process to water deficit. The most important
physiological mechanisms allow plants maintain their turgor under water stress
conditions in osmoregulation, which is the osmotic potential and can result from the
condensation of the cells for water loss and increased solute in cells under
conditions of water stress. Solutes that are in concentration include soluble sugars,
organic amino acids. Under moderate to severe stress concentration of the amino
acid proline, it increases more than other amino acids. Proline may serve as a
store of N and an aid in drought tolerance acting as a solute in the osmoregulation.
The photosynthetic rates are usually less affected by the drought that respiration
rates and growth, resulting in an overall increase in concentration of nonstructural
carbohydrates. The translocation of fotocinatos, however, is relatively sensitive to
water deficit. The effect varies depending on the species level of stress and the
state of development of the plant. Accumulation of nonstructural carbohydrates and
stores of N can facilitate rapid regrowth after water stress is released.
Solar radiation
The first step in the use of solar energy is the conversion to chemical energy
through photosynthesis. During this process, the energy flow starts in the
27
ecosystem of the earth biosphere. The carbon used is fixed from atmospheric CO2,
representing around 0.03% of the total gas composition. Photosynthesis occurs
when the green sheets are exposed to radiation in the visible part of the field
(radiation is in a wavelength of 400 to 700 nm). The energy in these wavelengths
represents about half the total solar radiation. Under ideal conditions up to 7% of
the solar energy can be stored in photosynthetic products in fast-growing crops.
However, in grasses throughout the growing season, the average is much lower
(less than 1%).
Shading typically has a small effect on grass quality, compared with the
morphology or production. It has been found that by imposing a 63% shade of 5
perennial grasses reduced by 43% the production and leaf 24% but, only reduced
the concentration of NDF in 3%, the concentration of lignin in the cell wall in 4%
and increased digestibility of forage in 5%. The N concentration is much more
sensitive to shading than other quality characteristics. It has been found that 63%
of the shadow the N concentration in grasses increased by 26%. The response
was generally higher in the leaves than in the stems.
The cell wall components are deposited in the following order: hemicellulose,
cellulose and lignin, although there are many overlap between these activities. The
reduction in cell wall composition by shading is reflected in an increase in DM
digestibility in some studies. It has been reported that forage digestibility was
improved by 5% with heavy shading. Moreover, they found that DM digestibility of
grasses developed under shade was higher than the grasses that grow under the
influence of sunlight.
Interaction of environmental factors and plants
Among the climatic variables, light and temperature are the most important and
then follows the water supply. This sequence becomes more noticeable in
28
temperate climates. The growing season begins in spring, and then starts the slow
growth of temperature and light faster, then the temperature reaching a maximum
in the summer when the length of day begins to decline. Light, temperature and
plant maturity have different effects on the composition of the plant, and these
effects vary and interact differently in relation to the season of the year. The effect
of irrigation, fertilization and predators should not be ignored. A summary of the
main factors that influence and interact on the composition and nutritional value of
forages is shown in Table 3.9.
Table 3.9. Environmental factors that influence and interact in composition and
nutritional quality of forages
Parameter Temperature Ligh
tNitrogen Water Defoliation
Production + + + + -
Soluble carbohydrates - + - - +
Nitrates - - + NA NA
Cell wall + - ± + -
Lignin + - + + -
Digestion - + ± - +
Note:
+ = positive effect; - = negative association; ± = inconstant association; NA = not available data
Obtained from: Van Soest et al. (1978).
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