NUTRITIONAL MANAGEMENT and HOW to CALCULATE

NUTRITIONAL MANAGEMENT and HOW to CALCULATE
Ron Lemenager, Extension Beef Specialist
Purdue University, Department of Animal Sciences
Feed Requirements
Nutrient requirements of the beef herd vary according to age, gender, stage of production,
animal weight, body condition and desired level of performance. In general, the priority of
nutrient use from first to last is; maintenance, fetal development, lactation, growth, and conception.
It is not unusual, however, for late gestation and early lactation cows to lose weight and body
condition to maintain a pregnancy, or some level of lactation to support a calf, respectively. In
beef cow herds, the nutrients of greatest concern are energy, protein, calcium, phosphorus, salt,
vitamin A, water and sometimes magnesium, copper, sulfur and zinc.
In most cases, the vitamin and mineral requirements can be balanced by the use of a good
commercial vitamin/mineral supplement designed for the type of ration being fed. There are
differences, however, between vitamin/mineral supplements that should be considered before
purchase. Many different mineral sources are available for use in beef cattle diets and it is
important to check the mineral tag to be sure the ingredients being used are high in bioavailability.
For example, the oxide and carbonate forms of some minerals are not as available to the animal as
other mineral forms such as the sulfate form. Minerals are often described as being either
inorganic or organic. Inorganic mineral sources generally are associated with oxygen, chloride or
other non-carbon based compounds. Organic mineral sources are often referred to as “chelated”
minerals which are metals bound to an organic compound such as an amino acid. Because
chelated minerals are associated with organic compounds they can in some cases, but not all cases,
be more readily absorbed in the small intestine. The primary disadvantage of chelated minerals is
their cost. Table 1 shows the mineral composition and relative bioavailabilities of common
mineral forms used in mineral supplements.
Table 1. Source, empirical formulas, mineral concentrations and relative bioavailabilities of
common mineral sourcesa.
Mineral
Supplement
Mineral
Relative
Mineral
Empirical
concen-
bioavail-
avail-
formula
tration
ability
ability
(MC, %)
(RV)
(MC x RV)
Calcium
Calcium carbonate CaCO3
38
100
38.00
Bone meal
variable
24
110
26.40
Calcium chloride
(dihydrate)
CaCl2(H2O)
31
125
38.75
Dicalcium
phosphate
Ca2(PO4)
20
110
22.00
36
90
32.40
Limestone
Cobalt
Copper
Iodine
Iron
Monocalcium
phosphate
Ca(PO4)
17
130
22.10
Cobaltous sulfate
CoSO4(H2O)7
21
100
21.00
Cobaltic oxide
Co3O4
73
20
14.60
Cobaltous
carbonate
CoCO3
47
110
51.70
Cobaltous oxide
CoO
70
55
38.50
Cupric sulfate
CuSO4(H2O)5
25
100
25.00
Copper EDTA
variable
variable
95
variable
Copper lysine
variable
variable
100
variable
Cupric chloride
(tribasic)
Cu2(OH)3 Cl
58
115
66.70
Cupric oxide
CuO
75
15
11.25
Cupric sulfide
CuS
66
25
16.50
Cuprous acetate
CuC2O2H3
51
100
51.00
Potassium iodide
KI
69
100
69.00
Sodium iodide
NaI
84
100
84.00
Calcium iodate
Ca(IO)3
64
95
60.80
Diiodosalicyclic
acid
C7H4I2O3
65
15
9.75
Ethylenediamine
dihydriodine
C2H8N2(HI)2
80
105
84.00
Pentacalcium
orthoperiodate
Ca5(IO6)2
39
100
39.00
Ferrous sulfate heptahydrate
FeSO4(H2O)7
20
100
20.00
Ferric citrate
variable
variable
110
variable
Ferric EDTA
variable
variable
95
variable
Ferric phytate
variable
variable
45
variable
Ferrous carbonate
FeCO3
38
10
3.80
Magnesium Magnesium sulfate
MgSO4
20
100
20.00
Magnesium acetate
MgC2O2H4
29
110
31.90
Magnesium basic carbonate
MgCO3
31
100
31.00
Magnesium oxide
MgO
55
100
55.00
MnSO4(H2O)
30
100
30.00
Manganese carbonate
MnCO3
46
30
13.80
Manganese dioxide
MnO2
63
35
22.05
Manganese methionine
variable
variable
125
variable
Manganese monoxide
MnO
60
60
36.00
Manganese Manganese sulfate
Phosphorus Sodium phosphate
Selenium
Sodium
Zinc
NaPO4
variable
variable
Bone meal
variable
21
100
21.00
Defluorinated phosphate
variable
12
80
9.60
Dicalcium phosphate
CaHPO4
18
85
15.30
Sodium selenite
Na2SeO3
45
100
45.00
Cobalt selenite
variable
variable
105
variable
Selenomethionine
variable
variable
245
variable
Selenoyeast
variable
variable
290
variable
Sodium chloride
NaCl
40
100
40.00
Sodium bicarbonate
Na(CO3)2
27
95
25.65
Zinc sulfate
ZnSO4(H2O)
36
100
36.00
Zinc carbonate
ZnCO3
56
60
33.60
Zinc oxide
ZnO
72
100
72.00
a
Adapted from: Ammerman, C.B., D.H. Baker, and A.J. Lewis. 1995. Bioavailability of Nutrients for
Animals. New York: Academic Press; National Research Council. 1998. Nutrient Requirements of Swine,
10th revised edition. Washington, D.C.: National Academy Press; and Mineral Supplements for Beef Cattle,
University of Missouri Extension, Chad Hale and K.C. Olson.
Understanding the types and amounts of minerals provided by the base ration ingredients is
also important. For example, rations containing corn co-products (distiller’s grains and corn gluten
feed) require a mineral supplement with a high level of calcium (Ca) and low level of phosphorus
(P) to assure the correct Ca:P ratio in the diet. In addition, these corn co-products contain
relatively high levels of sulfur which can complex with copper, making it less available. Since
copper is important in reproduction and immune function, a mineral supplement that contains
higher levels of copper may be needed. Higher copper levels are also recommended for cattle
consuming endophyte-infected fescue. Help in design rations for the cow herd that are correctly
balanced for all the nutrients of concern can be obtained by consulting with a professional animal
nutritionist, your local Extension educator, or the state beef specialist at your land-grant
agricultural college.
The cow herd should be divided into management groups by nutritional requirements. In
herds where a limited breeding season is used (45-75 days), the management groups might be; 1)
replacement heifers, 2) young cows plus thin older cows, 3) mature cows in moderate and above
condition, and 4) bulls. If the breeding season is significantly longer than 75 days, groups 2 and 3
could be divided into early and late calving groups to allow delivery of feed to cows according to
their requirements (gestation vs. lactation).
Tables 2 through 6 show the nutrient requirements of beef cattle at various stages of
production. Table 2 shows the requirements of a mature (four years or older), 1200 pound, body
condition score (BCS) 5 cow during the course of a complete production cycle. The weight of a
BCS 5 cow shortly after calving is considered her base weight and the dry matter intake and
nutrient requirements are based on that weight. As the cow moves through the production cycle
she typically gains weight due to advancing pregnancy (developing fetus, gravid uterus, fluids, and
membranes). Requirements are the lowest for cows shortly after their calves are weaned and they
are in mid-gestation. Requirements increase significantly as cows advance into the last trimester
of pregnancy and into early lactation. Nutrient requirements are highest approximately two
months after calving when cows reach peak lactation and then begin to decrease until their calves
are weaned. For these reasons, the lowest quality forages should be fed to animals with the lowest
nutrient requirements (dry cows in mid-gestation) and highest quality forages should be fed to
animals with higher requirements (cows in late pregnancy and early lactation). Additionally, the
cow’s genetic potential for milk production should be matched to forage quality. When excess
nutrients are supplied to low-producing cows, surplus nutrient intake will be seen as an increase in
cow weight and BCS. Conversely, inadequate nutrients supplied to high producing cows will
cause them to; lose weight and body condition, delay return estrus, have lower fertility, and
produce calves with lighter weaning weights, unless they are provided supplements to correct
nutrient deficiencies. It is common to strategically supplement cows during some phases of the
production cycle, but the need to routinely provide significant amounts of supplement can be
expensive.
Table 2. Daily Energy and Protein Requirements for a 1200 l b., BCS 5, Mature Cow a,b
Expected Peak Milk (lbs/day)
Months
Cow Low (15 l bs)
Moderate (20 l bs)
High (25 l bs)
d
c
CP
Since
Scale Wt.
NEm
NEm
CP
NEm
CP
Calving
BCS = 5
Mcal
lbs
Mcal
lbs
Mcal
lbs
1
1200
14.5
2.4
15.8
2.7
17.2
3.0
2 (peak l actation)
1200
15.3
2.6
16.9
3.0
18.6
3.4
3
1205
14.8
2.5
16.3
2.8
17.8
3.2
4
1205
14.0
2.3
15.1
2.5
16.3
2.8
5
1205
13.1
2.1
14.0
2.3
14.9
2.5
6
1210
12.5
1.9
13.1
2.0
13.7
2.2
7 (weaning)
1215
9.0
1.5
9.0
1.5
9.0
1.5
8
1225
9.3
1.5
9.3
1.5
9.3
1.5
9
1240
9.8
1.6
9.8
1.6
9.8
1.6
10
1260
10.7
1.7
10.7
1.7
10.7
1.7
11
1290
12.0
1.9
12.0
1.9
12.0
1.9
12
1340
13.9
2.2
13.9
2.2
13.9
2.2
a
Adapted from NRC, 1 996 b
c
Does not a ccount for i ncreased e nergy needs due to cold s tress
Net e nergy for maintenance, Mcal/day
d
Crude protein, l b/day
Young cows require higher quality feeds (more nutrient dense diets) than older cows at
every stage of production because they are still growing, they do not have a complete set of mature
teeth, and they lack the volume and capacity to eat as much as a mature cow. Table 3 shows the
nutrient requirements of first- and second-calf heifers that have an expected mature weight of 1200
pounds. Nutrient requirements increase and decrease in a manner similar to those of mature cows
as they move through their production cycle.
Table 3. Daily Energy and Protein Requirements f or First and Second Calf Heifers a,b,c Months
First Calf Heifers
Second Calf Heifers
CPe
Since Heifer Wt.
NEmd
Heifer Wt.
NEm
CP
Calving
(lbs)
Mcal
lbs
(lbs)
Mcal
lbs
0 ( calving)
1080
14.8
2.3
1235
14.3
2.2
1
970
14.1
2.3
1110
15.0
2.5
2 ( peak milk)
985
15.0
2.5
1110
16.0
2.8
3
1000
14.6
2.4
1120
15.4
2.6
4
1010
13.9
2.2
1125
14.4
2.4
5
1025
13.1
2.1
1130
13.5
2.2
6
1040
12.5
1.9
1140
12.7
2.0
7 ( weaning)
1060
9.4
1.5
1150
9.0
1.5
8
1080
9.7
1.5
1160
9.3
1.5
9
1105
10.3
1.6
1180
9.9
1.6
10
1140
11.2
1.7
1205
10.8
1.7
11
1180
12.5
1.9
1240
12.1
1.9
a
Adapted from NRC, 1 996 b
c
Assumes a 1 200 l b mature c ow weight a t a BCS = 5 with moderate peak milk production (20 l b/day)
Does not a ccount for i ncreased e nergy needs due t o c old s tress
d
Net e nergy for maintenance, Mcal/day
e
Crude protein, l b/day
Table 4 shows the energy required to change one BCS in a mature, 1200 pound cow. The
energy requirements are expressed in Mcal of Net Energy for Maintenance (NEm). Thin cows
require less energy to gain one BCS than fatter cows because they are depositing more protein and
water. Accretion of protein is energetically more efficient and requires less energy than accretion
of fat (adipose), which is more energy dense. As fat cows gain weight and condition, they deposit
proportionally less protein and more fat than their thinner counterparts. This can be seen in a
comparison between a BCS 4 cow who requires a total of 158 Mcal of NEm above maintenance to
change one condition score vs. a BCS 6 cow who requires 210 Mcal of NEm.
Thumb rule 1: It takes about 80 pounds for a mature cow to change one (±) BCS. First-calf
heifers, on the other hand, require about 150 pounds to increase one BCS. The difference in
weight required to change one BCS can be explained by the fact that first-calf heifers must
continue to grow before they can begin depositing body condition (fat).
Table 4. Energy Required to Change a Mature 1200 l b, BCS 5 Cow One Condition Score a,b
BCS
1
2
3
4
5
6
7
8
9
Approx. Shrunk Body Wt., l bs
880
960
1040
1120
1200
1280
1360
1440
1520
Mcal NEmc to
Change 1 BCS
78
105
131
158
184
210
238
264
290
a
Adapted from Purdue research published by Buskirk e t a l., 1 992 J . Anim. Sci. 7 0:3867-­‐3 876.
b
c
Feeding 1 l b of dry s helled c orn provides a pproximately 1 Mcal NE m
Net e nergy for maintenance, Mcal/day
Table 5 shows the nutrient requirements for growing heifer calves housed in a thermoneutral
environment and not implanted. This table can be used to develop preconditioning, heifer
development, and feedlot rations. It is consistent with the previous tables and uses a 1200 pound
mature weight for heifers and a 1200 pound harvest weight for finished steers expected to grade
USDA low choice. As animals increase in weight, the maintenance requirements for both energy
(NEm) and crude protein increase. Similarly, as average daily gain of the calf increases, the net
energy for gain (NEg) and protein requirements also increase.
Table 5. Daily Energy and Protein Requirements for Growing and Finishing Cattle a,b,c
Weight
(lbs)
525
650
775
900
1025
1150
Gain
lbs/day
1.0
1.8
2.5
3.3
4.0
1.0
1.8
2.5
3.3
4.0
1.0
1.8
2.5
3.3
4.0
1.0
1.8
2.5
3.3
4.0
1.0
1.8
2.5
3.3
4.0
1.0
1.8
2.5
3.3
4.0
NEmd
Mcal/day
4.7
4.7
4.7
4.7
4.7
5.5
5.5
5.5
5.5
5.5
6.3
6.3
6.3
6.3
6.3
7.0
7.0
7.0
7.0
7.0
7.7
7.7
7.7
7.7
7.7
8.4
8.4
8.4
8.4
8.4
NEge
Mcal/day
1.28
2.37
3.50
4.67
5.86
1.50
2.78
4.11
5.48
6.88
1.72
3.17
4.69
6.25
7.85
1.92
3.55
5.24
6.99
8.78
2.12
3.91
5.78
7.71
9.68
2.31
4.26
6.30
8.40
10.55
CPf
lbs/day
1.22
1.55
1.87
2.18
2.49
1.36
1.69
2.01
2.32
2.62
1.49
1.82
2.13
2.43
2.73
1.57
1.86
2.14
2.40
2.66
1.65
1.91
2.15
2.38
2.60
1.72
1.95
2.16
2.36
2.54
a
Adapted from NRC, 1 996
b
c
Assumes a nimals weigh 1 200 l bs when t hey reach l ow c hoice (Small) marbling
Does not a ccount for i ncreased e nergy needs due t o c old s tress
d
Net e nergy for maintenance, Mcal/day
e
Net e nergy for gain, Mcal/day
f
Crude protein, l b/day
Table 6 shows the energy and protein requirements for growing bulls with a mature weight of 2000
pounds. The requirements relate to weight and gain in a manner similar to those of growing and
finishing calves.
Table 6. Daily Energy ( NEg and NEg)and Protein ( CP) Requirements for Growing Bulls a,b,c
Weight
(lbs)
500
800
1100
1400
1700
2000
Gain
lbs/day
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
NEmd
Mcal/Day
5.2
5.2
5.2
5.2
5.2
7.4
7.4
7.4
7.4
7.4
9.4
9.4
9.4
9.4
9.4
11.2
11.2
11.2
11.2
11.2
13
13
13
13
13
14.6
14.6
14.6
14.6
14.6
NEge
Mcal/day
0.00
1.23
2.64
4.12
5.65
0.00
1.76
3.76
5.86
8.04
0.00
2.23
4.77
7.45
10.21
0.00
2.67
5.72
8.92
12.23
0.00
3.09
6.61
10.32
14.15
0.00
3.49
7.47
11.66
15.98
CPf
lbs/day
0.73
1.19
1.63
2.05
2.47
1.04
1.51
1.93
2.33
2.71
1.32
1.69
2.01
2.30
2.57
1.58
1.87
2.09
2.27
2.43
1.83
2.03
2.16
2.24
2.30
2.07
2.19
2.22
2.25
2.28
a
Adapted from NRC, 1 996
b
c
Assumes a nimals weigh 2 000 l bs when t hey reach maturity with a BCS = 5
Does not a ccount for i ncreased e nergy needs due t o c old s tress
d
Net e nergy for maintenance, Mcal/day
e
Net e nergy for gain, Mcal/day
f
Crude protein, l b/day
When forage quantity (supply) is low, alternative feeding strategies must be considered.
Utilization of crop residues is a strategy that can reduce cost by increasing grazing days and
reducing the amount of hay needed. Corn stalks can provide 30-60 days of grazing depending on
soil types and rainfall. Dropped ears will be gleaned from the field first by cows, but today’s corn
genetics and harvesting equipment are designed to minimize ear drop. Leaves and shucks are the
highest quality forage parts of the plant and will be consumed next. As the time after grain harvest
increases, however, forage quality decreases rapidly due to weathering. For moderately
conditioned cows in the middle trimester of pregnancy, they may only need a good vitaminmineral supplement for the first 20-30 days of corn stalk grazing. After 30 days on stalks,
however, a protein supplement is usually recommended to improve fiber digestibility and forage
intake. The key to successful corn stalk grazing is to monitor cow body condition. If cows begin
losing weight and condition, either more supplementation (protein and/or energy) is needed, or
cows should be provided an alternative diet.
Utilizing low quality forages (baled corn stalks, wheat straw or mature hay) as a primary
forage resource can add significant cost to the ration because of harvest and supplementation costs
compared to the nutrients provided. This is especially true when low quality forages are used in
rations for animals that have higher nutrient requirements (growing/developing, late gestation and
early lactation). To be most cost effective, low quality forages should be fed to animals with the
lowest requirements, such as dry cows in mid-gestation after calves are weaned. Low quality
forages are typically low in protein (3-7% crude protein) and require protein supplementation to
optimize fiber digestion and dry matter intake.
Supplementation. The only way to get an accurate assessment of forage quality is to have the
forage analyzed for nutrient content. If forages are analyzed and it is determined that the available
forage cannot meet the animal’s requirements, then a cost-effective supplementation strategy
should be developed. A wide range of supplements can be used with existing forage to meet
requirements for different production situations. Typically this means utilizing feeds that are high
in one or more nutrient categories (energy, protein, vitamins, minerals) that are deficient in the
forage. It should be noted that mineral and vitamin requirements are not altered significantly by
cow BCS, but energy requirements are dramatically affected by BCS. Factors that affect
supplement selection are nutrient content of the forage, nutrient profile of the supplement, cost and
availability of supplement, as well as cow nutrient needs based on BCS and stage of production.
Type of supplement will then be dictated by how much protein and energy supplementation
is required per day to reach the desired performance level. If energy is the only limiting nutrient,
high energy supplements such as corn grain, soybean hulls or wheat midds will usually be the most
economical in the Cornbelt. If both energy and protein are required, then a co-product with a
higher level of protein such as corn gluten feed, distiller’s grains or soybean meal could be
considered.
Balancing rations when forages are fed free-choice requires an estimate of dry matter
intake (DMI). It should be noted that DMI is different than feed disappearance because cows often
sort, soil, and waste forage when they have unlimited access. Once DMI is determined, the
amount of nutrients can be calculated and a supplement can be formulated that will correct any
deficiencies. Table 7 provides a guide to estimated DMI of forages as a percent of body weight
when intake is not known. This table shows that dry, gestating beef cows eat less dry matter than
lactating cows, DMI goes up as forage quality increases, and protein supplementation increases dry
matter intake when the forage quality (low and average quality) does not provide enough protein to
optimize rumen fermentation and forage digestion. In contrast, protein does not increase DMI
when forages contain adequate protein (high quality). Energy supplementation at low levels does
not increase DMI, regardless of forage quality. It is important to realize that these estimates of
forage intake are not as accurate as measuring actual intake or as using the neutral detergent fiber
(NDF) value from a forage analysis to predict intake. It should be noted that higher levels of
supplementation, especially energy supplementation, can actually decrease DMI.
Table 7. Estimated Dry Matter Intake As determined By Forage Quality (Percent Of Body
Weight)a
No
Supplemented with
Item
Supplement
Protein
Energy
Dry, Gestating Cow
Low Quality Forage
1.5
1.8
1.5
Average Quality Forage
2.0
2.2
2.0
High Quality Forage
2.5
2.5
2.5
Lactating Cow
Low Quality Forage
2.0
2.2
2.0
Average Quality Forage
2.3
2.5
2.3
High Quality Forage
2.7
2.7
2.7
a
Example: 1290 lb, BCS 5 cow in late gestation consuming average quality hay, appropriately
supplemented with protein, would consume about: 1290 x 0.22 = 28.4 lb of hay on a dry matter (DM) basis
daily. That same BCS 5 cow just after calving would weigh approximately 1200 lb and would consume
about: 1200 x .025 = 30 lb of average quality hay (DM matter basis) daily when appropriately
supplemented with protein.
The best single indicator of forage quality is neutral detergent fiber (NDF) and it can be
used to predict dry matter intake. As the NDF percentage in a forage increases with advancing
maturity, dry matter intake will generally decrease. Many laboratories analyze for acid detergent
fiber (ADF), but the analysis may or may not include an NDF value unless requested.
Cow weight varies during the year due to changes in body condition score, pregnancy, and gut
fill. Normal weight fluctuations resulting from a change in body condition (above or below a BCS
5) do not significantly alter the cow’s ability (volume and capacity) to consume forage. Therefore,
a BCS 4 cow weighing 1120 pounds, a BCS 5 cow weighing 1200 pounds, and a BCS 6 cow
weighing 1280 pounds will all consume similar amounts of forage. Similarly, a 1200 pound cow
in a BCS 5 just after calving will consume a similar amount of forage as a 1290 pound, BCS 5 cow
during the eighth month of pregnancy. Therefore, when using the NDF value from a forage
analysis to predict dry matter intake, the correct base weight for a cow is her weight shortly after
calving when pregnancy does not impact her scale weight. Cow weight after calving should be
adjusted to a BCS 5 using Thumb Rule 1.
Limit feeding hay. Research at Purdue University has shown that limiting the time cows have
access to large round bales to 1, 2, or 4 hours per day reduced forage disappearance by 72, 50, and
22%, respectively, compared to estimated free-choice hay intake. This reduced disappearance
reflects both a reduction in hay intake and wastage. Limiting access time to large round bales may
be a valuable tool for producers when hay prices are expensive relative to grain and co-product
feeds. When properly supplemented to meet the requirements, cow performance should not be
negatively impacted. The feed ingredients and level of nutrients needed to supplement the cows
should be determined by the nutrient analysis of the forage, ingredient availability and cost, and
the animal’s requirements.
How-to-Calculate
Conversion of dry matter (DM) to as-fed
Dry matter is an important concept because the beef industry utilizes a number of feeds that
have significantly different amounts of moisture. When balancing diets and determining how
much an animal will eat, always use DM values and then convert the value to an as-fed basis for
feeding. An easy way to visualize a dry matter to as-fed conversion is to imagine a cup of freeze
dried coffee. The cup with dry coffee has weight and the coffee is very concentrated. This is a dry
matter weight and a dry matter concentration of coffee. When water is added to the cup, two
things happen; weight is added to the cup, and the coffee is diluted. The cup with water now has
an as-fed weight and an as-fed concentration of coffee. The same thing happens with feeds when
water is present.
Water
Example: Assume a sample of corn silage is analyzed for nutrient profile and the analysis comes
back containing 35% dry matter (65% moisture) and 8% crude protein (CP) on a DM basis. Also
assume that 17.5 pounds of DM per head daily needs to be fed. What would the CP content be if
the DM was 35%, and how much silage needs to be fed on an as-fed basis?
Thumb Rule 2: Always use the percent dry matter value of a feed (not the percent moisture
value) when balancing diets and determining dry matter intake.
Step 1. Convert CP from a DM basis to an as-fed basis.
The crude protein value will be diluted (get smaller) when going from dry matter to asfed. By multiplying the CP value of a feed by a number smaller than 1.0, it will get
smaller. In this example, there is 8% CP and 35% DM. Therefore, thinking of the DM
value of the feed as a decimal (.35 = .35 lb of dry feed/lb of as-fed feed), instead of a
percent (35%), the number is smaller than 1.0, and can be used as follows;
8% CP in corn silage (DM basis) x .35 = 2.8% CP in corn silage (as-fed basis)
Step 2. Convert pounds of corn silage from a DM basis to an as-fed basis.
The pounds of corn silage on a DM basis will increase as water is added to the feed. In
this case, 17.5 pounds of corn silage on a DM basis needs to get bigger as it is
converted to an as-fed basis. Therefore, if 17.5 is divided by a number smaller than 1.0,
it will get bigger. In this example, the dry matter value of 35% converted to a decimal
(.35 = .35 lb DM/lb of as-fed feed) can be used as follows;
17.5 lb silage (dry matter basis) ÷ .35 = 50 lb silage (as-fed basis)
Conversion of as-fed to dry matter (DM)
The freeze dried coffee example can also be used to visualize conversion of as-fed to dry
matter. As before, the cup filled with freeze dried coffee plus water can be thought of as an as-fed
feed. To convert to DM, think of removing all the water so that only dried coffee remains. The
weight of the cup decreases, and the concentration of the coffee increases when changing from an
as-fed to a DM basis. The same thing happens with feeds.
Water
Example: Assume a modified dry distiller’s grains with solubles (MDDGS) that contain 48% DM
(52% moisture) and 14.5% CP on an as-fed basis. Assume that 9 pounds of MDDGS are being fed
on an as-fed basis per head daily. What would the CP content be on a DM basis, and how much
DM is being fed?
Thumb Rule 3: Rations are almost always formulated on a dry matter (DM) basis in the beef
industry because of the significant variation in water content of feeds that are used. In
addition, feed intakes are more accurately estimated on a DM basis (vs. an as-fed basis).
Step 1. Convert the CP from an as-fed to a DM basis.
The example above showed that the CP value of the feed will be more concentrated (get
larger) when going from as-fed to DM. Dividing the CP value of the feed on an as-fed
basis by the DM value of the feed as a decimal (a number smaller than 1.0) will result
in the number getting larger. In this example, there is 14.5% CP and 48% DM.
Therefore, thinking of the DM value of the feed as a decimal (.48 = .48 lb of dry feed/lb
of as-fed feed), instead of a percent (48%), the number is smaller than 1.0 and can be
used as follows;
14.5% CP in MDDGS (as-fed basis) ÷ .48 = 30.2% CP in MDDGS (DM basis)
Step 2. Convert the pounds of MDDGS from an as-fed basis to a DM basis.
In this case, 9 pounds of MDDGS on an as-fed basis needs to get smaller as it is
converted to a DM basis. Therefore, when 9 is multiplied by a number smaller than
1.0, it will get smaller. In this example, the dry matter value of 48% can be converted
to a decimal (.48 = .48 lb DM/ lb of as-fed feed) and can be used as follows:
9 lb of MDDGS (as-fed basis) x .48 = 4.32 lb of MDDGS (DM basis)
Dry matter intake (DMI) of a forage
To balance forage-based diets, both the nutrient content of the forage and dry matter intake
must be determined. These two factors can then be used to determine if the forage can supply the
required nutrients, or if the diet needs to be fortified with a supplement. Neutral detergent fiber
(NDF) is one of the best predictors of forage quality and dry matter intake.
Thumb Rule 4: Dry matter intake (DMI) can be predicted using the NDF value(DM basis) from
a forage analysis as follows: 120 ÷ %NDF of the forage = DMI as a percent of body weight
Thumb Rule 5: When estimating forage intake, a cow’s weight should be adjusted to a BCS 5
(using Thumb Rule 1) and either a non-pregnant, or very early gestation scale weight. As
pregnancy progresses, the products of conception (gravid uterus, fluids, and membranes)
result in an increase in cow weight across the scale, but do not result in an increase in dry
matter intake. Similarly, as cows add or lose weight based on change in body condition, their
intake does not change significantly.
Example: Assume a 1200-pound base-weight cow is consuming hay with an NDF value of 50%
(DM basis) and a dry matter value of 88%. How much hay will this cow consume per day?
Step 1. Determine daily DMI of this cow as a percent of body weight using Thumb Rule 4.
120 ÷ 50% NDF = DMI of 2.4% of body weight
Step 2. Determine daily DMI in pounds of hay per day for this 1200 pound cow.
Convert DMI as a percent of body weight to a decimal equivalent.
1200 lb cow x .024 = 28.8 lb of hay DMI/day (DM basis)
Step 3. Convert daily DMI to an as-fed basis for feeding.
28.8 lb hay DMI/day ÷ .88 = 32.7 lb of hay/day (as-fed basis)
Caution: Not all hay that is fed is actually consumed. Hay disappearance is a combination of hay
intake plus waste due to sorting and soiling.
Dry matter intake (DMI) of hay when daily access-time is limited
There may be times when limiting forage intake and supplementation of the needed nutrients is
cheaper than feeding hay free-choice. For example, a drought could cause hay prices to be high
relative to that of grain and/or by-products, or when nutrient requirements are low and available
high quality forage exceeds the nutrient requirements.
Thumb Rule 6: When cows have limited access-time to forage, an equation developed from
research conducted at Purdue University can be used to determine the amount of forage dry
matter intake (DMI) that that will be consumed per day: Hay DMI (% of BW) = 0.30 x Hours
access – (.02 x Hay NDF%) + 1.34, where NDF of the forage is on a DM basis.
Example: Assume a group of 1200 pound base-weight, BCS 5 cows consuming hay that contains
50% NDF (DM basis) and 85% dry matter. Cows will be limited to 4 hours of access-time to large
round bales each day. How much hay will a 1200 pound cow consume per day?
Step1. Determine daily DMI of this cow, as a percent of her body weight, using Thumb Rule 6.
0.30 x 4 hr access-time – (.02 x 50% NDF) + 1.34 = DMI of 1.54% of body weight/day
Step 2. Determine daily DMI in pounds per day for this 1200 pound cow.
1200 lb x .0154 = DMI of 18.5 lb/day of hay
Note: In this example, these same cows would consume about 120 ÷ 50 = 2.4% of their body
weight on a DM basis, or 1200 lb x .024 = 28.8 lb of hay DM per day, if they were allowed freechoice access to large round bales. In this limited access-time feeding example, the producer could
expect to reduce the amount of hay consumed (not including differences that might also exist in
waste) by almost 36% (28.8 – 18.5 ÷ 28.8 = 35.76%). Research at Purdue found that when cows
have limited access time to large round bales each day, they waste less hay because they spend
more time eating, and less time sorting than cows with 24 hour per day access.
Step 3. Convert DMI to an as-fed basis for feeding.
18.5 lb of hay intake (DM basis) ÷ .85 = 21.8 lb of hay (as-fed)
Protein supplementation when energy deficiencies are minimal
Producers often must supplement low quality forages to meet the protein requirement of the
cow herd. An example would be when mid-gestation cows are consuming low quality forages that
contain less than 8% crude protein (corn stalks, wheat straw, mature grass hay).
Thumb rule 7: When the CP concentration of the diet drops below 8% on a dry matter basis, the
microbes in the rumen will not have enough nitrogen to optimize fiber digestion and dry matter
intake.
Thumb rule 8: CP requirements for cows in mid-gestation, late gestation, and early lactation,
respectively, are 8 – 10 – 12% (DM basis). If the diets do not supply these levels of CP, then
supplementation is justified.
Example. Assume a herd of 1200 pound, BCS 5, non-lactating, mid-gestation beef cows have
free-choice access to a low quality forage and a good quality commercial vitamin/mineral
supplement. How much soybean meal (SBM) supplement is required per cow daily to meet their
CP requirement? Table 8 shows the CP and NEm content of the feeds and this cow’s requirements.
Table 8. Crude protein (CP) and net energy for maintenance (NEm) values.
Dry matter basis
Item
CP, %
NDF, %
NEm, Mcal/lb
Soybean meal, 44%
49.9
-.95
Low quality forage
4.0
66.7
.44
CP, lb
NEm, Mcal/day
a
Cow requirements
1.6
-9.8
a
From Table 2, month 9 of the production cycle, month 6 of pregnancy. Cow is a 1200 lb cow that now weighs 1240
lb due to pregnancy.
Step 1. Make sure the vitamin and mineral requirements are met from the free-choice access to a
good commercial vitamin/mineral mix. This example assumes the vitamin/mineral
requirements are met.
Step 2. Determine daily DMI as a percent of body weight using Thumb Rule 4.
120 ÷ 66.7 = DMI of 1.8% of body weight
Step 3. Determine daily DMI in pounds per day for this 1200 pound cow.
1200 lb cows x .018 = 21.6 lb of low quality forage intake/day (DM basis)
Step 4. Calculate how much protein and energy are provided per day by the forage.
21.6 lb x .04 lb protein/lb of low quality forage = .86 lb of protein intake from forage
21.6 lb x .44 Mcal NEm/lb of low quality forage = 9.5 Mcal NEm intake from forage
Step 5. Calculate the daily deficiency in CP and energy.
1.6 lb of protein required - .86 lb of protein supplied by forage = 0.74 lb protein
deficient
9.8 Mcal NEm required – 9.5 Mcal NEm supplied by forage = 0.3 Mcal NEm deficient
Step 6. Calculate how much SBM is needed per cow daily to correct the protein deficiency.
.74 lb CP deficient ÷ .499 lb CP/lb of SBM = 1.48 lb/day of SBM (DM basis) needed
Note: Adding 1.48 lb of SBM daily (DM basis) will add energy to the diet which will result in a
slight excess in energy compared to the requirement. This is not necessarily a bad thing. In this
case, energy is over-fed by 1.1 Mcal NEm/d (calculated as 1.48 lb/d x .95 Mcal NEm/lb = 1.4 Mcal
NEm per cow daily provided by SBM, and 1.4 Mcal NEm provided by supplementation – 0.3 Mcal
NEm deficiency before supplementation = 1.1 Mcal NEm/d excess).
Thumb rule 9: On a practical basis, assume a 90% dry matter value for feeds taken out of a bag
or a bin. While this value may not be absolutely correct, it is usually pretty close and easy to
remember.
Step 7. Convert the DM value to an as-fed value for feeding using Thumb Rule 9
1.48 lb of SBM (DM basis) ÷ .90 = 1.65 lb of SBM (as-fed basis) per cow daily
Calculating a supplement when both energy and protein are required.
When rations are balanced for growing/developing cattle, young cows, and cows in either
late gestation or early lactation; an energy deficiency will often exist after the protein requirement
has been satisfied. In this case, a feed containing both high energy and protein, such as distiller’s
grains plus soluble (DGS) or corn gluten feed (CGF), can be considered. These two feeds have the
advantage over corn because their energy comes in the form of a highly digestible fiber instead of
starch. When starch is added to a forage-based diet at levels above 0.3% of body weight on a dry
matter basis, it can reduce digestibility of the forage component in the diet. This is called a
negative associative effect.
Thumb rule 10: The recommended upper limit of supplementation (DM basis) is 0.3% of body
weight for corn, 0.5% of body weight for the corn co-products (DGS and CGF) and 1.0% for
pelleted soybean hulls.
Thumb rule 11: The calcium:phosphorus ratio must be considered when adding either
distiller’s grains or corn gluten feed to the diet because they contain high levels of phosphorus.
Many commercial feed companies offer a high calcium vitamin/mineral supplement to be fed
with these corn co-products. Feed grade limestone can also be fed as a calcium source. The
recommended Ca:P ratio of the total diet is at least 1.5:1.
Example. Assume a first-calf, BCS 5 heifer weighing 1080 just after calving is consuming a
low/moderate quality hay and provided a high quality, free-choice vitamin/mineral supplement.
How much dry corn gluten feed is needed per day to meet the protein and energy requirements.
Table 9 provides the crude protein and energy values for feeds and this cow’s requirements.
Table 9. Crude protein (CP) and net energy for maintenance (NEm) values.
Dry matter basis
Item
CP, %
NDF, %
NEm, Mcal/lb
Low quality forage
9.0
54.5
.52
Dry distiller’s grains +
28.0
-.98
soluble
Dry corn gluten feed
23.0
-.88
CP, lb
NEm, Mcal/day
a
Cow requirements
2.3
-14.80
a
From Table 3, month 0 of the production cycle just after calving.
Step 1. Make sure the vitamin and mineral requirements are met from the free-choice access to a
good commercial vitamin/mineral mix. In this example, we will assume the
vitamin/mineral requirements are met.
Step 2. Determine daily DMI in pounds per day for this 1200 pound cow using Thumb Rule 4 and
the hay NDF value from Table 9.
120 ÷ 54.5 = 2.2% of her body weight (DM basis)
1080 lb first calf heifer x .022 = 23.76 lb of hay intake (DM basis)
Step 3. Calculate how much protein and energy are provided per day from forage.
23.76 lb x .09 lb protein/lb of low quality forage = 2.14 lb of protein intake
23.76 lb x .52 Mcal NEm/lb of low quality forage = 12.35 Mcal NEm intake
Step 4. Calculate the daily deficiency in CP and energy using the cow requirements listed in Table
9.
2.3 lb of protein required – 2.14 lb of protein supplied by forage = 0.16 lb protein
deficient
14.8 Mcal NEm required – 12.35 Mcal NEm supplied by forage = 2.45 Mcal NEm
deficient
Step 5. Calculate how much dry CGF will be needed per cow daily to correct the energy
deficiency using the NEm value for CGF in Table 9.
2.45 Mcal NEm deficient ÷ .88 Mcal NEm/lb of supplement = 2.78 lb of CFG (DM
basis) needed per cow daily.
Step 6. Check to make sure this level of supplementation meets the CP deficiency.
2.78 lb of CGF (DM basis) x .23 lb of CP/lb of feed = .64 lb of CP provided by CGF
.64 lb of CP provided - .16 lb CP deficiency = .48 lb of CP excess.
Note: Feeding 2.78 lb of CGF (DM basis) meets the energy needs of the cow and exceeds the
protein requirement, but it does not exceed the recommended upper level of dietary corn coproduct inclusion (Thumb Rule 10) of 0.5% of body weight (.005 x 1080 lb heifer = 5.4 lb) on a
DM basis. The excess protein will be used as an energy source and cows may increase slightly in
weight and BCS.
Step 7. Convert from a DM to an as-fed basis for feeding using Thumb Rule 9.
2.78 lb ÷ .90 = 3.08 lb of CFG needed per cow daily on an as-fed basis
Cost per unit of protein
Producers are often faced with deciding which protein source to use. Ease of supplementation
(convenience factor) and cost per unit of protein are two factors that need to be considered. Freechoice supplements that come in the form of blocks or tubs are convenient, but they typically
increase the cost of supplementation compared to other protein sources that have less
manufacturing cost, such as wet or dry corn co-products, soybean meal, and alfalfa hay. The price
used for each protein source needs to reflect the cost to deliver it to the cows. This means that cost
of storage, transportation, and delivery to the bunk must to be included to accurately compare
protein sources.
Example. Assume that three protein sources are available to a cow-calf producer; soybean meal
(SBM), wet corn gluten feed (WCGF), and dry distiller’s grains plus solubles (DDGS). The cost
per ton, dry matter analysis, and crude protein content on a dry matter basis for these protein
sources are shown in Table 10.
Table 10. Supplemental feed cost and nutrient profile of selected protein sources.
Item
Soybean meal, 44%
Wet corn gluten feed
Dry distiller’s grains + solubles
Cost, $/tona
300
40
120
Dry Matter, %
88
40
89
% CP, DM basis
50
23
28
a
This cost is an as-fed basis and needs to reflect cost delivered to the feed bunk. Cost of transportation,
storage and feeding (labor, equipment and spoilage) of high moisture feeds can be significantly different
than dryer feeds.
Step 1. Feed prices are typically expressed on an as-fed basis, but the CP analysis is typically
reported on a DM basis, therefore, the first step is to determine pounds of DM in a ton.
SBM: 2000 lb/ton x .88 lb of dry matter/lb of feed = 1760 lb of DM per ton
WCFG: 2000 lb/ton x .40 lb of dry matter/lb of feed = 800 lb of DM per ton
DDGS: 2000 lb/ton x .89 lb of dry matter/lb of feed = 1780 lb of DM per ton
Step 2. Calculate the pounds of protein per ton.
SBM: 1760 lb of DM/ton x .50 lb of protein/lb of DM = 880 lb of protein per ton
WCGF: 800 lb of DM/ton x .23 lb of protein/lb of DM = 184 lb of protein per ton
DDGS: 1780 lb of DM/ton x .28 lb of protein/lb of DM = 498 lb of protein per ton
Step 3. Calculate the cost per unit ($/lb) of protein
SBM: $300/ton ÷ 880 lb of protein/ton = $0.34/lb of protein
WCGF: $40/ton ÷ 184 lb of protein/ton = $0.22/lb of protein
DDGS: $120/ton ÷ 498 lb of protein/ton = $0.24/lb of protein
Step 4. Determine which is the best option. The cheapest source of protein in this example is
provided by WCGF. The primary disadvantage with this protein source, however, is that it
has a shorter shelf-life than the two other protein sources used in this example. While price
per pound of protein is slightly higher for the DDGS in this example, the final cost of
supplementing cows could be cheaper if a portion of WCGF is discarded because of
spoilage.
Adjusting energy level in the diet for cold stress.
During the winter, cold stress can be both a nutritional and management issue. Cows
consuming low to moderate quality forages late in gestation, or during early lactation, may not be
able to eat enough forage to meet the increased energy requirements caused by cold stress. Putting
out another big round bale of low to moderate quality hay typically will not meet the cow’s
increased energy requirements. The energy requirements of a cow increase in direct proportion to
wind chill. If cows are housed in an open lot, their energy requirements are not based on
thermometer temperature, but rather wind chill temperature. If wind breaks are provided,
thermometer temperatures become more useful. Wind breaks may be more cost effective in the
long term compared to the cost of supplementing expensive high energy feeds.
Thumb rule 12: For each 10o F drop below a wind chill of 30o F, the energy requirements
increase 13% for cows in good body condition with a dry, winter hair coat; and 30% for thin
cows, or cows with a wet or summer hair coat.
Example. Assume a 1200 lb base-weight , BCS 5 cow during the last trimester of gestation is in a
-10o F wind chill environment. The cow has free choice access to moderate quality hay that
analyzes 85% DM and a good quality commercial vitamin/mineral supplement. How much
pelleted soybean hulls will be needed per day to meet this increased requirement? Table 11 shows
the crude protein and energy values for the feeds and this cow’s requirements.
Table 11. Crude protein (CP) and net energy for maintenance (NEm) values.
Dry matter basis
Item
CP, %
NDF, %
NEm, Mcal/lb
Moderate quality forage
11.7
54.5
.53
Pelleted soybean hulls
12.0
-.88
CP, lb
NEm, Mcal/day
a
Cow requirements
2.3
-12.0
a
From Table 2, scale weight of this 1200 lb, BCS 5 cow in late gestation is about 1290 lb.
Step 1. Calculate the increase in energy required above maintenance (Table 11) for this cow under
cold stress using Thumb Rule 12.
A wind chill factor of -10 o F is 4 units of 10 below 30 o F, therefore:
4 x .13 = 52% increase in energy required above maintenance
12 Mcal NEm/day required x .52 increase = 6.24 Mcal NEm/day increase in energy
needed above maintenance due to cold stress
Step 2. Calculate the total energy required per day for this 1200 lb cow under cold stress.
12 Mcal NEm/day requirement + 6.24 NEm/day increase = 18.24 Mcal NEm/day
Step 3. Calculate how much hay this cow would be expected to consume using Thumb
Rule 4 and the 54.5% NDF forage analysis.
Note: The crude protein (DM basis) in hay (11.7%) and soybean hulls (12%) both exceed 10% CP
for cows in late gestation (Thumb Rule 8), therefore, no additional protein supplementation is
needed.
120 ÷ 54.5 = DMI of 2.2% of her body weight/day
1200 lb x .022 = 26.4 lb of hay DM/day
Step 4. Calculate how much energy this cow would be expected to consume per day.
26.4 lb of hay DM x .53 Mcal NEm/lb of hay = 13.99 Mcal/d
Step 5. Calculate the deficiency in energy intake per day.
18.24 Mcal NEm required/day – 13.99 Mcal NEm/day provided by hay = 4.25 Mcal
NEm deficient/day
Step 6. Calculate the amount of a high energy feed needed to meet this deficiency using SBH.
4.25 Mcal NEm/day deficiency ÷ .88 Mcal NEm/lb in SBH = 4.83 lb SBH (DM basis)
Caution: Corn (1.01 Mcal NEm/ lb) was an optional high energy feed, but the amount needed
(4.25 ÷ 1.01 = 4.2 lb of corn) would exceed Thumb Rule 10 of 0.3% of body weight (1200 x .003
= 3.6 lb) on a DM basis.
Step 7. Convert from a DM basis to an as-fed basis for feeding using Thumb Rule 9.
4.83 lb of SBH (DM basis) ÷.90 = 5.4 lb of SBH (as-fed basis)
Adjusting energy in the diet to increase body condition.
Thin cows require more energy than moderately conditioned cows to maintain body
temperature and an acceptable level of productivity. It is possible to program cows to gain weight
and body condition to reach a body condition score of 5 by a target date using Tables 2 and 4.
Example. Assume a typical 1200 lb cow in a BCS 4 at the beginning of the last trimester of
gestation consuming moderate quality forage with a goal of having this cow calve in a BCS 5 in 90
days. How much pelleted soybean hulls will be needed per day to meet this increased energy
requirement? Table 10 shows the crude protein and energy values for the feeds and this cow’s
maintenance requirements.
Note: This BCS 4 cow would weight about 1180 lb at this point in the production cycle (Month
10, Table 1), but she should weigh about 1180 + 80 = 1260 if she was in a BCS 5 (Thumb Rule 1).
She has the volume and capacity to eat like she was a 1200 lb, base-weight cow (Thumb Rule 5).
Table 12. Crude protein (CP) and net energy for maintenance (NEm) values.
Dry matter basis
Item
CP, %
NDF, %
NEm, Mcal/lb
Moderate quality forage
10.7
54.5
.53
Pelleted soybean hulls
12.0
-.88
CP, lb
NEm, Mcal/day
a
Cow requirements
1.9
-12.0
a
From Table 2, month 11 of the production cycle, month 8 of pregnancy.
Step 1. Calculate the energy required above the maintenance requirement per day for this cow to
move from a BCS of 4 to 5 in 90 days using Table 4.
It will take a total of 158 Mcal NEm above the maintenance energy requirement to
change from a BCS 4 to a BCS 5, therefore:
158 Mcal NEm ÷ 90 days = 1.75 Mcal NEm/day to move one BCS in 90 days
Step 2. Calculate the total energy required/day for this cow.
12 Mcal NEm/day required + 1.75 Mcal NEm/day = 13.75 Mcal NEm needed per day
Step 3. Determine hay intake using the Thumb Rule 4 and the hay’s NDF value from Table 12.
120 ÷ 54.5% NDF = DMI of 2.2% of body weight/day
1200 lb cow x .022 = DMI of 26.4 lb/day of hay
Step 4. Calculate how much energy will be consumed from the forage daily.
26.4 lb/day of hay dry matter x .46 Mcal NEm/lb = 12.14 Mcal NEm/day
Step 5. Calculate the daily energy deficiency.
13.75 Mcal NEm required/day – 12.14 Mcal NEm provided by hay intake/day = 1.61
Mcal NEm/day deficient
Step 6. Calculate the amount of SBH/day needed to balance the energy requirement.
1.61 Mcal NEm deficient/day ÷ .88 Mcal NEm/lb of SBH = 1.83 lb/day of SBH (DM
basis)
Step 7. Convert from a DM basis to an as-fed basis using Thumb Rule 9 for feeding.
1.83 lb/day of SBH (DM basis) ÷ .90 = 2.0 lb/day of SBH (as-fed basis)
Caution: Thin cows should be identified shortly after weaning when the cow is in the middle third
of gestation and the nutrient requirements are low. As cows progress into late pregnancy and on
into lactation, it becomes more difficult to formulate practical and economical forage-based rations
that will provide a significant increase in weight and BCS. A little supplementation when
requirements are low, that can be fed in small quantities over a longer period of time, is more
economical than trying to make large changes in a short period of time when requirements are
high.
References:
Ammerman, C.B., D.H. Baker, and A.J. Lewis. 1995. Bioavailability of Nutrients for
Animals. New York: Academic Press; National Research Council. 1998. Nutrient
Requirements of Swine, 10th revised edition. Washington, D.C.: National Academy
Press;
Buskirk, D.D., R.P. Lemenager, and L.A. Horstman. 1992. Estimation of Net Energy
Requirements (NEm and NE ) of Lactating Beef Cows. J. Anim. Sci. 70:3867-3876.
Δ
Hale, C. and K.C. Olson. Mineral Supplements for Beef Cattle. University of Missouri
Extension. http://extension.missouri.edu/publications/DisplayPub.aspx?P=G2081
National Research Council (NRC). 1996. Nutrient Requirements of Beef Cattle:
Seventh Revised Edition: Updated 2000. National Academy Press.