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OCR for page 85
Sheep
INTRODUCTION
Comparatively speaking, sheep are more tolerant of climatic extremes than
other farm animals. For example, Alexander (1974) calculated from data of
Bennett (1972) that adult sheep when dry, in a calm wind, and with 10 cm of
fleece could withstand -120°C ambient temperature. However, calculated
cold tolerance for freshly shorn adult sheep (7 mm of fleece) would be only
-15°C, and, when wet and exposed to a 7 m/s wind, similar sheep could
withstand only 13°C. For lambs, Alexander (1968) suggested that heat loss
would exceed summit metabolism at 23°C when a small lamb with a short
fleece was wet and exposed to a 5.5 m/s wind. For a large lamb in the same
condition, metabolism would match heat loss at approximately 4°C. Obvi-
ously, survival during cold is highly variable, depending on both the ani-
mal's ability to increase heat production and aspects of the environment af-
fecting effective ambient temperature (EAT).
Heat tolerance is even less defined than cold tolerance for sheep. Mount
(1979) reports that sheep will survive acute periods of 40-60°C dry-bulb
temperature, but successful growth and reproduction at that temperature has
not been demonstrated and is unlikely. Since homeotherms must rely more
on evaporative heat loss when exposed to effective ambient temperatures
(EAT) in the hot zone, factors affecting rate of evaporation (wind and humid-
it~y) are of major importance in determining survival during heat.
85
OCR for page 86
86 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
THERMAL ZONES FOR SHEEP
The thermal zones for sheep depend largely on amount of external insulation
provided by the fleece. As noted in Table 1, shorn growing lambs on a main-
tenance ration have a lower critical temperature (LCT) of about 25°C, but LCT
is estimated as low as -20°C in similar fleeced animals (Blaxter, 1967;
Webster et al., 1969~. Calculation of lower critical temperature indicates that
increased intake and the concurrent increase in heat production reduces LCT
as much as 20°C from ad libitum feeding to fasting. Clearly, LCT can be in-
fluenced by wetting of fleece because insulatory value is reduced by wetting
(Bennett, 19724. Wind increases rate of heat loss (Joyce and Blaxter, 1964)
and has an additive effect when sheep are wetted (Blaxter et al., 19661.
Since TNZ iS, in fact, a descriptive term relating balance of heat production
and loss, it is obvious that any factor affecting either of these two determi-
nants will consequently alter TNZ and LCT.
ENVIRONMENTAL EFFECTS ON FEED ENERGY INTAKE
Many factors affect the amount of metabolizable energy (ME) available to the
animal. These include digestibility of the foodstuff, amount of the foodstuff
consumed, and/or the ability of the animal to acquire food. In general, fac-
tors influencing amount of metabolizable energy available for consequent use
may be considered as feed intake.
Exposure to heat reduces voluntary intake in sheep as it does in other spe-
cies (see page 31~. Bhattacharya and Hussain (1974) reported that high ambi-
ent temperatures coupled with high humidity during the day, reaching a max-
imum of 32°C and 98 percent, respectively, reduced ad libitum intake in
sheep, with depression most severe when the diets contained high levels of
roughage. However, a later report from the same laboratory (Bhattacharya
and Uwayjan, 1975) showed conflicting results when temperature was cou-
pled with low humidity. The diversity of those findings supports the need to
discuss results in terms of EAT rather than separating the effects of tempera-
ture and humidity during heat stress.
In general, voluntary intake increases during cold compared to TNZ. Web-
ster et al. (1969) reported increased food intake of sheep housed indoors as
still air temperature fell. Soderquist and Knox (1967) reported higher dry
matter intake at 0°C compared to 23°C in growing lambs. Ames and Brink
(1977) reported increased dry matter consumption in lambs, whose critical
temperature was 13°C, to be statistically higher at 10, 5, and 0°C, but no fur-
ther increase was noted at - 5°C. Those data clearly show that voluntary in-
take is increased above thermoneutral values during mild cold, but that a
limit in voluntary intake was reached before animals were severely cold
stressed.
OCR for page 87
Sheep
87
For shorn lambs whose lower critical temperature was 13°C, Brink (1975)
related dry matter consumption over a wide range of ambient temperatures
- 5 to 35°C) and found the following linear relationship:
DMl = 1 1 1.3 - 0.52 T.
where
DMI = daily intake (g/W075),
T = temperature (°C).
The impact of fluctuating temperatures and the validity of mean daily tem-
perature when the standard deviation of daily temperature is high has been
recently studied by Giacomini (1979) where lambs exposed to a constant
thermoneutral temperature (15°C) were compared with lambs in fluctuating
environments with a mean temperature of 15°C. When fluctuations were
from 10 to 20°C, 5 to 25°C, or 0 to 30°C, feed intake was not different from
constant 15°C temperature. Certainly the area of fluctuating temperature de-
serves more study. This is particularly important in the design of confine-
ment systems where constant temperatures may result in patterns of feed in-
take different from those established during fluctuating temperatures.
Assuming some nutrient (e.g., vitamin and mineral) requirements are con-
stant over wide ranges in temperature raises a concern that difference in in-
take can have a major effect on vitamin and mineral intake. When these nu-
trients represent a constant percentage of the diet, it is obvious that reduced
intake may lead to deficiency. Care must be taken (and adjustments made
when necessary) to meet the animal's requirement when intake varies. The
same consideration must be made for various nonnutrient additives (antibi-
otic premixes, growth promotants, etc.) when they are to be ingested at a
constant rate daily. Obviously, diet adjustment for components that are not
affected by the thermal environment should be altered in proportion to
changes in rate of feed consumption. For this reason it is important that accu-
rate estimates of voluntary intake during thermal stress be established.
NUTRIENT DIGESTIBILITY
Many authors have reported a positive relationship between ambient temper-
ature and nutrient digestibility for ruminants (see Table 21. A general discus-
sion is found on page 31.
Although information relating diet digestibility to heat stress is not in total
agreement, most data tend to support the hypothesis that digestibility is in-
creased during heat stress. Some believe this results from decreased volun-
tary intake rather than from a direct effect of increased EAT. Ames and Brink
OCR for page 88
88 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
(1977) used shorn lambs fed in controlled environmental chambers to deter-
mine the effect of ambient temperature on the digestibility of diet compo-
nents. This study showed increased digestibility of dry matter, crude protein,
and nitrogen-free extract as temperature rose from 15°C to 35°C (LCT was
13°C). Crude fiber digestibility increased when temperature rose from 15 to
30°C, but was not increased at 35°C. No difference was found in ether ex-
tract digestibility during heat stress. Bhattacharya and Hussain (1974) re-
ported that during heat stress sheep diets had lower digestibilities except for
crude fiber and nitrogen-free extract. They found that higher roughage (75
percent) diets were most affected. Perhaps variations in findings relating heat
stress to diet digestibility are altered by roughage to concentrate ratio. Obvi-
ously, more knowledge is needed for different rations.
Several authors (Blaxter and Wainman, 1961; Christopherson, 1976;
Kennedy and Milligan, 1978; Young and Christopherson, 1974) indicate
lowered dry matter digestibility during cold. Examples of depressed digest-
ibilities during cold are shown in Table 2. Consequently, when increased in-
take during cold is considered in combination with decreased digestibility,
the advantages of the former would be partially offset by the latter. For ex-
ample, increased consumption per unit of metabolic size from 10 to 0°C is
5.3 percent, but with decreased digestibility the net increase in digestible en-
ergy was only 2.7 percent. Christopherson (1976), who conducted extensive
studies of digestibility during prolonged cold with both sheep and cattle, also
found a temperature effect independent of level of intake. He reported that
dry matter digestibility had a 0.31, 0.21, and 0.08 percent decline per °C
cold stress for sheep, calves, and steers, respectively. Kennedy and Milligan
(1978) suggest that cold effect on digestibility is greater with higher levels of
food intake.
ENERGY REQUIREMENT DURING COLD STRESS
When exposed to effective ambient temperatures (EAT) below the cool zone,
energy expenditure must compensate for increased energy loss. Increased
heat production during cold stress has been measured by several authors
(Alexander, 1962; Bennett, 1972; Blaxter et al., 1966; Graham et al., 1959~.
Wind (Joyce and Blaxter, 1964), rain (Panaretto et al., 1968), and wind and
rain combinations (Blaxter et al., 1966) have been shown to increase further
the rate of heat loss of sheep exposed to cold. These findings suggest an ap-
parent reduction in external insulation and result in increased rate of sensible
heat loss when measured at a given thermal gradient. Use of effective ambi-
ent temperature measures such as wind-chill temperature (Ames and Insley,
1975) and insulatory value of the wetted fleece should provide similar esti-
mates of heat loss.
OCR for page 89
Sheep
89
When sheep are exposed to temperatures below the LCT, there are two fac-
tors that determine rate of heat loss: (1) thermal gradient between core tem-
perature and ambient temperature, and (2) amount of insulation provided by
tissue, fleece, and air. These factors can be used to estimate rate of heat loss,
and therefore maintenance energy requirement for animals exposed to cold,
by the following equation:
MEm = a\;V1~75 + b l-
where
MEm = metabolizable energy for maintenance corrected for effec-
tive temperature (kcal ME/day),
T = magnitude of cold (°C), i.e., difference between animal's
lower critical temperatures and effective temperature,
I = total insulation (0C/kcal/m2/day),
a = coefficient of maintenance requirement for animal in zone
of thermoneutrality (kcal ME/day),
b = surface area of animal (my.
NOTE: Coefficient a is 127.8 kcal ME/day for sheep.
Table 24 shows the percentage increase in maintenance energy require-
ments at seven levels of insulation for five different weights. The system to
estimate the listed values does have shortcomings and cannot be calculated
without some error. First, the insulative value of the animal is not uniform
over its entire surface. Indeed, small errors in estimating insulative value will
lead to relatively large differences in energy required during cold. For exam-
ple, Blaxter et al. (1959) reported conductive value of fleece (in similar
sheep) ranges from 122 to 149 kcal/m2/24 h/°C/cm. That variation would
result in a 111 percent error for a 50-kg lamb with 0.04°C/kcal/m2/day total
insulation when estimating additional heat loss during cold. Second, the ani-
mal's ability to shunt blood to and from specific areas of the body reduces
the accuracy of the estimates of insulation. Third, calculations assume that
the animal is a sphere with no facing surfaces.
When the system presented in Table 24 is compared to observed increases
in heat production for rather well defined sheep, the utility of the system can
be assessed (Table 251. Variations of up to 19.7 percent are noted; however,
in each case insulative value was estimated, and increased metabolic heat
production during cold exposure was measured for a relatively short period.
While more work is obviously needed to improve estimates of insulation, in
a general sense these data support increasing energy requirements of sheep
based on estimated insulation and body weight.
OCR for page 90
90 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
TABLE 24 Percent Increase in Maintenance Energy Cost per Degree
Centigrade Below Lower Critical Temperature
Sheep Weight (kg)
Insulation
(0C/kcal/m2/day)a 40 50 60 70 80
0.01 5.8 5.4 5.2 4.7 4.6
0.02 2.9 2.7 2.6 2.4 2.3
0.03 1.9 1.8 1.7 1.6 1.5
0.04 1.4 1.3 1.3 1.2 1.2
0.05 1.2 1.1 1.0 0.9 0.9
0.06 1.0 0.9 0.8 0.8 0.7
0.07 0.8 0.7 0.7 0.7 0.6
a Wool provides about 0.007°C/kcal/m2/day per cm depth (Blaxter et al., 19S9).
The options for increasing energy intake for animals fed ad libitum are
more limiting than for those where adjustment can be accomplished by sim-
ply providing more feed. In this situation increasing caloric density by addi-
tion of fat or increasing relative amounts of feedstuffs with higher caloric
densities (i.e., replacing roughages with concentrates) are possibilities. The
former approach, however, is contrary to the notion of increasing heat incre-
ment during cold since fat is lower in heat increment compared with most
feedstuffs. However, its high caloric density may still be used advanta-
geously. The value of heat increment during cold has been mentioned previ-
ously, and, while this may prove valuable, it should be understood that heat
increment for each unit of weight of roughages and concentrates is similar.
However, heat increment is a higher percentage of digestible energy for
roughages as compared with concentrates, so that calories supplied by the
heat increment of a roughage may be less expensive than those from a con
TABLE 25 Comparison of Measured and Estimated Heat Loss (kcal/m2/
day/°C) During Cold
Difference
Measured Estimated (%) Data Sourcea
115 96 + 11.9 Graham et al.,
1959
77 96 - 19.7 Webster et al.,
1969
84.2 75 + 11.2 Webster and Blaxter,
1966
a Description of sheep used in these studies were used to obtain estimated values from Table 24.
OCR for page 91
Sheep
91
centrate source. Consequently, from an economic standpoint energy costs
may be reduced during cold by increasing roughages at the expense of con-
centrates. Moose et al. (1969) reported that feeds with high heat increments
fed during cold have a sparing effect on net energy for production, thus al-
lowing their use for gain. Brokken (1971) developed a model using the data
of Lofgreen and Garrett (1968) to blend rations to improve performance dur-
ing cold stress. This model is based on maximum use of heat increment and
concludes that economic advantages do exist for altering rations during cold.
The magnitude of this advantage is dependent upon relative ingredient
prices, magnitude of cold, and the effect of cold on intake.
ENERGY REQUIREMENT DURING HEAT STRESS
Expected increases in heat production during heat exposure of sheep have
been reported, but little quantification of increased energy needs is availa-
ble. During heat exposure, energy requirement increases because of energy
expended during panting (Kibler, 1957), sweat gland activity (Macfarlane,
1964), and the calorigenic effect of hormones (Whittow and Findlay, 1968~.
An additional factor when core temperature rises is the Q,O effect (Schmidt-
Nielsen et al., 19674. Whittow and Findlay (1968) calculated that the effect
of increased rectal temperature accounted for as much as 62 percent of in-
creased O2 consumption in cattle, and Ames et al. (1971) measured a 41 per-
cent increase in O2 consumption of sheep assuming Q,O = 2.0. It is apparent
that increases during heat are nonlinear (Graham et al., 1959) as opposed to
linear increases during cold. Nonlinearity is to be expected because of the
decreased efficiency of evaporative mechanisms and increased Q,O effect as
heat stress becomes more severe (Ames et al., 1971~. More precise estimates
of increased energy requirement during heat are further confounded by de
pressed appetite.
The adjustment of diets during heat stress can be a practical approach to-
ward minimizing the effect of heat even though increased maintenance re-
quirement during heat is difficult to estimate accurately. McDowell (1972)
and Brink and Ames (1978a) have reported nonlinear increases in mainte-
nance energy during heat when maintenance is calculated as the difference
between intake and gain.
Lofgreen (1974) confirmed the validity of formulating diets for relief of
heat stress. His adjustment involved lowering heat increment of the diets
while keeping net energy constant. These adjustments were accomplished by
reducing the roughage content of the diets, adding dried beet pulp, and in-
creasing fat content. Moose et al. (1969) found that low-concentrate diets
(35 percent) had lowered heat increment than higher-concentrate diets (70
percent) when fed to lambs and reported that at temperatures above 25°C
high heat increment can seriously impair the efficiency of diets containing
higher percentages of roughage. Rea and Ross (1961), in a growing trial with
OCR for page 92
92 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
lambs, concluded that lambs gain more rapidly in warm temperatures when
given diets of 60 percent concentrate as compared to 40 percent concentrate.
THERMAL EFFECTS ON PROTEIN REQUIREMENT OF SHEEP
Protein requirement includes both that necessary to maintain nitrogen equi-
librium (maintenance protein) and that needed for productive function. Ide-
ally, dietary protein exceeding that needed for maintenance is used only for
production (growth, wool, or milk); however, growth and other productive
functions may be limited by available energy because of increased energy for
maintenance during thermal stress. When energy is limiting, protein may
then be catabolized and serve as an energy source (Crampton and Harris,
1969).
Because of the relationship between energy and protein requirement, the
direct effect of climate on energy requirement has a subsequent effect on pro-
tein required for growth or production. Data suggest no effect of thermal en-
vironment on protein required to maintain nitrogen equilibrium (Brink and
Ames, 1978b). The frequently used protein-to-energy ratio for formulation
of animal diets is not appropriate for describing diets during thermal stress
when maintenance energy requirement and intake vary unless only protein
and calorie values surplus to maintenance are used to calculate a ratio. In-
stead, when formulating diets with respect to the thermal environment, both
energy and protein should be included to meet requirements for each nutrient
separately, and the protein-to-energy ratio of the diet should be ignored.
When energy requirement for maintenance increases (i.e., cold stress),
less energy is available for production, and.con-sequently the protein-to-
calorie ratio above maintenance levels increases. Ames and Brink (1977)
have reported reduced protein efficiency ratio of lambs~during both cold and
heat compared to TNZ. Ames et al. (1980) have suggested a system for ad-
justing protein above maintenance to match expected growth rate of lambs
exposed to thermal stress. When protein is adjusted, growth rate during ther-
mal stress is not altered, but protein efficiency ratio is improved. When pro-
tein is a more expensive nutrient than energy, cost of gain is decreased. Ta-
ble 26 indicates protein adjustments for a 27 kg lamb receiving a diet that is
expected to result in 272-g ADG.
ENVIRONMENTAL EFFECTS ON LAMB PERFORMANCE
Exposure of sheep to thermal stress affects voluntary food intake and mainte-
nance requirement as discussed previously. Obviously, average daily gain
and feed required per unit of gain are also affected by thermal stress. Few
studies have reported the effect of temperature on performance of growing
OCR for page 93
Sheep
TABLE 26 Protein Adjustment for Growing Lambs
93
Deviation from Expected ADO Maintenance Protein for cP
Critical Temperature (g)a Protein (g)b Growth (g) in Ration (%)c
20 54 33.2 17.0 2.4
15 132 33.2 41.0 5.8
10 195 33.2 60.8 8.6
5 236 33.2 73.6 10.4
Critical temperature 272 33.2 84.9 12.0
- 5 222 33.2 69.3 9.8
- 10 181 33.2 56.6 8.0
- 15 136 33.2 42.5 6.0
- 20 95 33.2 29.7 4.2
a Ames et al., 1975.
bPreston, 1966.
c Constant intake.
lambs, but Ames and Brink (1977) conducted growth and efficiency studies
for shorn lambs exposed to a wide range ~ - 5 to 35°C) of ambient tempera-
tures. These lambs received a 50 percent concentrate (grain sorghum), 50
percent roughage (alfalfa) diet. Table 27 relates temperature with ADG and
feed efficiency.
Daily gain for lambs receiving a grain sorghum (50 percent), alfalfa (50
percent) ration can be predicted for the range of temperatures above by the
equation:
ADO(g) = 129.94 + 9.27 T- 0.35 T7,
where
T = temperature in °C.
This equation suggests 13°C as the temperature for maximum ADG of shorn
lambs. For lambs with fleece, temperature for maximum growth and effi-
ciency would be lowered.
MISCELLANEOUS ENVIRONMENTAL FACTORS
AFFECTING NUTRITION OF SHEEP
Knowlton et al. (1969) studied effects of high carbon dioxide levels on nutri-
tion of sheep. They reported reduced feed intake when exposed to 4 percent
OCR for page 94
94 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
TABLE 27 ADG and Feed Efficiency of Lambs Grown at Different
Ambient Temperatures and Fed Ad Libitum
Feed Efficiency
Temperature (°C) ADG (g) (gain/feed)
5 73 0.04
0 1 30 0.08
5 170 0.11
10 192 0.15
15 197 0.14
20 184 0.13
30 107 0.08
35 41 0.04
SOURCE: Ames and Brink, 1977.
CO2, but intake was inversely related to CO, concentration at exposure levels
of 8, 12, 16, and 18 percent. Nutrient digestibility was not influenced by
CO2 levels up to 16 percent, but variable decreases were measured at 16 and
18 percent CO2.
Blaxter (1978) studied the effect of simulated altitude on sheep and found
no difference in heat production or heat increment values when oxygen con-
centration in air was 150 ml/liter compared to 200 ml/liter.
Confinement rearing of sheep has been estimated to reduce energy require-
ments by 30 percent (Parker, 19761. Direct measurement of heat production
in commercial confinement management systems has not been done, but
Young and Corbett (1972) reported that maintenance requirements were gen-
erally 6~70 percent greater for grazing sheep compared to housed animals.
The need for a more precise estimate of effect of confinement on energy re-
quired for maintenance is needed.
Harbers et al. (1975) reported that sheep acclimate to sound of 100 dB or
less; however, intermittent noise was shown to increase nutrient digestibility
during short-term exposure.
Allden (1968) reported that feed consumption and utilization for weight
gain were not affected in the long term by prolonged periods of undernutri-
tion. They reported that lambs subjected to restricted feed levels during the
first 6 months of life failed to fully recover (in terms of weight) compared
with lambs receiving adequate nutrition. However, when unlimited feed was
available, when lambs were 6-12 months old, they completely recovered
earlier weight loss. Graham and Searle (1975) held 4-month-old Tethers at
20-kg live weight for 4 or 6 months then fed ad libitum to recover weight for
age. They reported increased voluntary food intake during rehabilitation
OCR for page 95
Sheep
95
compared to controls. While heat production per unit metabolic size fell dur-
ing weight stasis, it rose in the first month of recovery but remained less than
controls. Gross efficiency was higher during the first week of recovery then
returned to control values. Except for acute changes in intake and consequent
efficiency during the first week of recovery following undernutrition, there
appear to be no long-lasting nutritional differences in sheep receiving re-
stricted levels of nutrition.
SUMMARY
Sheep are probably most tolerant of environmental extremes compared with
swine, cattle, and poultry. They are unique because of the potentially large
insulatory value of the fleece. Responses of sheep to thermal stress in terms
of intake, maintenance energy requirement, and rate of performance are typi-
cal. Nutrient adjustments for changes in voluntary intake, energy adjust-
ments for cold stress, and protein adjustments for differences in thermally in-
duced changes in rate of growth are presented but are based on limited
research data. More basic information is needed to accurately predict the im-
pact of environment on nutrient requirements of sheep.
Representative terms from entire chapter:
heat stress
