Calf Nutrition Issues and Factors Affecting
Rumen Development
Dr. Howard Tyler
Department of Animal Science
Iowa State University
Meeting The Nutrient Requirements
of the Calf
A nutritionist’s nightmare
 Entire pre-weaning period is transition period from
non-ruminant to ruminant

– proportions of nutrients provided by liquid vs. solid feeds
in constant flux
– physical capacity of rumen increases
• potential for dry matter intake also increases
– fermentative capacity of rumen increases
– absorptive capacity of rumen increases
– therefore, digestibility of solid feeds also changes as
rumen develops
Energy Requirements
Approximately 20 kcal/pound of body weight in a
thermoneutral environment for maintenance
 Eighty pound calf requires 1500 kcal maintenance
and 850 kcal for 0.5 lb of gain (2350 kcal/day total)
 Requirements increase 1% for every degree below
500 F

– High surface area:body mass ratio
– Require 450 kcal more at 200 F (2800 kcal/day total)
– Difference between ruminating calves and nonruminating calves
– Difference between small and large calves
Body Size and Growth Rate Affects
Protein Requirements

The size of the calf affects the protein requirements
at any given level of intake
– For 95 lb calf receiving 10 ounces of milk replacer twice
daily, a 20:20 milk replacer meets the protein needs
– An 85 lb calf receiving the same amount of milk replacer
needs 22.5% protein, and a 75 lb calf needs 25% protein
• Less energy needed from maintenance in lighter calves, so more
energy available for growth
• Therefore, more protein required to support lean growth

Required protein:energy ratio needed changes with
rate of gain targeted and size of animal being fed
Factors Affecting Protein Requirements

Rate of gain determines amount of protein required
– If facilities, genetics, and calf source/management permit rapid
gain, then high levels of protein and energy required in diet
– If facilities, genetics or calf source/management LIMIT rate of
gain, then protein and energy must be reduced accordingly

The size of the animal affects the protein requirements at
any given level of intake
– Less energy needed from maintenance in lighter animals, so more
energy available for growth
• Therefore, more protein required to support lean growth

Frequency of feeding affects digestive efficiency
– Natural suckling frequency allows greater intake than most “handfeeding” systems can achieve
– Required protein:energy ratio changes accordingly
Imbalanced Protein:Energy Ratios

If energy diverted to immune function (poor
facilities), then it is NOT available for growth
– Excess protein must be de-aminated and ammonia
detoxified and excreted
• Energy required (3 ATP per molecule of urea) and therefore even
LESS energy available for growth and immunity

Overfeeding protein (in this case by underfeeding
energy) impairs growth and may impair immunity
Is Mortality Rate an Issue? (Summary)
Under excellent management conditions, there is no
evidence that calves fed higher levels of protein
(appropriate for maximal growth rates) are at more
risk or less risk for dying
 Under marginal conditions, calves fed higher levels
of protein may be at higher risk
 Under highly challenging conditions, calves fed
higher protein levels are at higher risk of dying than
calves fed conventionally
 This begs the question – do calves on
a higher plan of nutrition have a stronger
immune system or a weaker immune system?

Does Higher Plane of Nutrition = Higher
Functioning Immune System?
110
No Growth
100
Low Growth
High Growth
Weight, kg
90
80
70
60
50
40
0
1
2
3
4
Time, wk
5
6
7
Does Higher Plane of Nutrition = Higher
Functioning Immune System?
20
20
CD4+ cells
CD8+ cells
Percent CD45RO+
No Growth
16
16
Low Growth
High Growth
12
12
8
8
4
4
0
0
0
1
2
3
4
Time, wk
5
6
7
0
1
2
3
4
Time, wk
5
6
7
Does Higher Plane of Nutrition = Higher
Functioning Immune System?
Absorbance @410/490
0.5
OVA-specific IgG 1
No Growth
0.4
OVA-specific IgG 2
0.5
0.4
Low Growth
High Growth
0.3
0.3
0.2
0.2
0.1
0.1
0
0
0
1
2
3
4
Time, Wk
5
6
7
0
1
2
3
4
Time, Wk
5
6
7
100 PPD-NS, 72 hr
Low Growth, P<0.01
80
High Growth, P<0.01
70
IFN-g, ng/mL
Does Higher Plane of
Nutrition = Higher
Functioning Immune
System?
No Growth, P<0.0001
90
60
50
40
30
20
10
0
-10
0
1
2
3
4
Time, wk
5
6
7
Challenges with Liquid Feeding
Strategies

High energy intake from liquid feed delays
initiation of starter intake and suppresses
appetite for starter
– high fat inclusions during cold weather
– higher solids in the same volume
of milk replacer
– higher volume intakes

Must balance accelerated growth with
accelerated rumen development
Establish Microbial Populations
After birth aerobic bacteria colonize
 Anaerobic bacteria soon dominate

– cellulolytic and methanogenic first
– lactate-fermenting exceed adult values then
decline
– protozoa introduction requires contact with
mature ruminants

Influenced by access to solid feed and the
composition of the ration
Water

Water availability may limit starter intake
– free water necessary for development of rumen
fermentative capacity
• water in milk replacer bypasses rumen
The Rumen at Birth
At birth, the rumen and
reticulum are under
developed, sterile, and
non-functional
 Milk bypasses the
reticulorumen by the
esophageal groove
 The reticulorumen
comprises less than 1/3 of
the entire stomach volume

The Rumen at Weaning
The rumen is the primary
compartment (over ½ of
stomach volume)
 Size, metabolic activity,
and blood flow have
increased markedly
 The rumen modifies both
energy and protein prior
to digestion by the calf

Absorptive Ability of Rumen Tissue
The rumen wall
 Muscle is develops in
consists of two distinct response to material in
layers:
the rumen
– musculature
 Development of the
– absorptive epithelium
mucosa depends on the
(mucosa)
production of VFA
 The mucosa is
 Roughage does NOT
responsible for
stimulate mucosal
absorbing VFA
growth

Rumens of 4-Week-Old Calves
Milk and Hay
Milk, Grain, and Hay
What is the Right Balance?

Concentrates
– VFA production
• butyrate
– High in energy
• fermentable
carbohydrates
– Palatable

Forages
– Low in energy
• structural
carbohydrates
– Ruminal abrasion
value (RAV)
– Bulk
– Rumination
Starter Without Hay
Textured
Commercial textured starter
CP - 23.58% ADF
- 6.39%
Ground
CP - 25.44% ADF
- 6.44%
Importance of Particle Size

Particle size of the diet influences
– Palatability
– Speed of digestion
• Rate of acid production
– Rumen retention time for digesta
– Rumen pH via saliva production
– Integrity of rumen papillae (RAV)
Starter With Grass Hay
Coarse + 7.5% Hay
Hay of consistent particle size
CP - 23.08% ADF
- 6.47%
Coarse + 15% Hay
Hay of consistent particle size
CP - 22.60% ADF
- 7.43%
Calf Starter with 7.5% Grass Hay
Increased body weight
 Increased feed efficiency
 Alters rumen VFA production

– increased acetate:propionate
Creates a more stable rumen environment
 Decreased starter costs

– significantly improved feeding profitability
– 4 lbs chopped hay per 50 lb bag of starter
Changes in Nutrients Utilized by
Calves After Rumen Development



VFA are produced in the
rumen and become an
important source of energy
Blood glucose declines
Ruminal bacteria increase in
population and become a
primary source of protein
– 50% protein
– 80% digestible
– High biological value
Weaning Strategies
Level of calf starter intake correlates with
maturity of rumen fermentative function as
well as physical development of rumen
 Weaning should be intake-based
 Weaning reduces labor costs by 50% and
costs of gain by 3 to 5-fold

– earlier weaning of healthy calves is most
profitable
Can A Ruminant Survive Without A
Rumen???

Rumenectomies (early removal of the rumen) or
prolonged milk feeding used to answer this
question
– Young ruminants will survive for a time without rumen
fermentation
– Animal viability decreases and sudden death occurs
between 6 and 8 months of age
• Can be reversed almost immediately by providing food to the
rumen!!!

Ruminant animals “hard-wired” metabolically to
function as ruminants
– Must utilize the end-products of microbial fermentation
On raising calves


The first four weeks of its life the calf must
receive the whole of its mother’s milk, because in
this period the nutrition contained in the milk in
so small volume can be replaced by no other
equally nutritious and as easily digestible means
of food.
After four weeks the milk may be replaced by that
means of fodder which nutritious substance next
to it in equal weight of dry volume, in the greatest
possible amount.
USDA, 1847