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OCR for page 96
Swine
INTRODUCTION
Stresses from the thermal environment influence productivity of swine by al-
tering their heat exchange with the environment, feed intake rate, average
daily gain, and dietary protein-concentration requirement, among other
things. Effects on dietary requirements for other nutrients are less important.
Cold stress and heat stress will be considered separately.
LOWER CRITICAL TEMPERATURE
In thermoneutral conditions, heat-production rate depends mostly on feed in-
take and metabolic body size. Physical activity is a lesser factor. By defini-
tion, within the thermoneutral zone, ambient temperature does not affect
heat-production rate (Blaxter, 1977~. By expressing lower critical tempera-
ture (LCT) in terms of effective ambient temperature (EAT), effects of various
combinations of thermal-environmental factors on the- pig can be evaluated
and compared.
Holmes and Close (1977) summarized data from several sources regarding
swine energetics. They concluded that thermoneutral heat-production rate of
pigs weighing 20 to 180 kg can be predicted as:
HE = 64.5 + 0.32 ME,
where
HE = heat production (kcal/W~ '5/day),
ME = metabolizable energy intake (kcal/W075/day).
96
OCR for page 97
Swine
97
When conditions change the animal's maintenance requirement, the factor
64.5 in this formula may have to be changed.
Maintenance energy requirement (HE) is defined as the metabolizable en-
ergy needed for the animal to maintain a constant body-energy content. At
thermoneutrality, the maintenance-fed animals' heat-production rate is equal
to ME intake, so the latter may be calculated using the above formula by sub-
stituting it for both heat production and rate of ME intake. Thus,
MEm = 64.5 + 0.32 MEm = 95 kcal/W075/day,
where
MEm = maintenance metabolizable energy (kcal/W075/day).
Recent reviews by Van Es (19723 and Close et al. (1973) show that a more
accurate estimate for growing-finishing hogs, based on the average of var-
ious data, is about 100 kcal/W0 75/day.
Values for the lower critical temperature when pigs are at maintenance
level of intake are given in Table 28. Pigs in groups huddle in the cold, so
their heat-loss rate is less (and their ACT lower) than that of those held singly.
If specific alterations in animal activity or productivity raise or lower metab-
olizable energy intake at maintenance, the lower critical temperature will
TABLE 28 Lower Critical Temperatures (LCT) in Pigs of Different Body
Weights Fed at Maintenance (100 kcal/W075/day), 2 Times Maintenance, and
3 Times Maintenance
Lower Critical Temperature (°C)
Feeding Level
Kind of Animal
Weight 1 x 2x 3x
(kg)
Maintenance Maintenance Maintenance
Baby pig (single) 2 31 29 29
(group) 2 27 24 24
Growing pig (single) 20 26 21 17
(group) 20 24 19 15
Finishing pig (single) 60 24 20 16
(group) 60 23 18 13
Finishing pig (single) 100 23 19 14
(group) 100 22 17 12
Sow (thin) 140 25 20 14
(fat) 140 23 18 12
SOURCE: Holmes and Close, 1977.
OCR for page 98
98 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
change accordingly. Moreover, it should be observed that these estimates are
derived from relatively short-term exposures to cool temperatures (up to 3
weeks at an EAT of 5 to 10°C below ~CT). Long-term exposure to much
colder conditions may engender further acclimation and result in a much
lower value (Irving, 1964~.
A pig normally consumes feed at a rate 2 to 4 times the maintenance level.
As a consequence, its heat-production rate is higher and its LCT iS lower than
at maintenance intake. The lower critical temperature of pigs in groups fed at
2 or 3 times maintenance is also given in Table 28. According to these
values, for each extra 100 kcal ME consumed per kg0 75 per day, LCT falls by 1
or 2°C in sows.
Lower critical temperature (Table 28) can be assumed to be the lowest ef-
fective ambient temperature for optimal management of pigs at the feed in-
take rate specified. This can be estimated for ad libitum-fed animals only if
their actual feed intake rate is known. Assuming the feed contains 2.8 kcal
ME/g, feed intake rate for which the LCT estimates in Table 28 are valid are
given in Table 29.
According to NRC requirements, a pig fed ad libitum in thermoneutral sur-
roundings will have an average feed-intake rate of 2.8 kg per day over the
weight range 20 to 100 kg. Lighter pigs, of course, eat less; heavier ones
more. For example, those weighing 20 to 35 kg will eat about 1.7 kg of feed
per day, which is about 3.2 times maintenance, according to NRC standards.
In the weight range 60 to 100 kg, feed-intake rate is about 3.5 kg daily, or
3.7 times maintenance. The LCT of ad libitum-fed pigs is thus 1 to 4°C, still
lower than that of pigs with a feed-intake rate 3 times maintenance (Table
28), provided they eat the amount of feed predicted by NRC.
For the average lower critical temperature of pigs in a group on a well-
insulated floor, one can expect 13 to 14°C in the growing period and 10 to
TABLE 29 Amounts of Feed (g/day) Given Pigs of Various Weights at 1 to
3 Times Maintenance (Assuming Feed Contains 2.87 kcal ME/g)
Weight
(kg)
Metabolic
Weight 1 x
(kg0 75)
Maintenance
2x
3x
Maintenance Maintenance
20 9.46 330 660 989
40 15.91 554 1,109 1,663
60 21.56 751 1,502 2,254
80 26.75 932 1,864 2,796
100 31.62 1,102 2,703 3,305
140 (sow) 40.70 1,418 2,836 4,254
OCR for page 99
Swine
99
11°C during finishing. If the pigs are kept on a slotted floor, 3 or 4°C must
be added to the LCT to account for the extra heat-loss rate to the floor
(Mount, 1975; Verstegen and van der Hel, 1974~. A wet floor increases LCT
even further. In Table 30 a survey of the effects of some environmental fac-
tors on LCT, derived mainly from Mount (1975), is given.
It may be assumed that the lower critical temperature of ad libitum-fed
pigs on a partly wet, slotted floor will be about 18 to 19°C during the gro-
wing phase and 14 to 15°C for finishing hogs. Thus, 18°C will be used as the
reference point for effects of cold stress on ad libitum-fed growing pigs and
14°C on ad libitum-fed finishing hogs.
Yet another factor is that individual pigs eat different amounts of feed, and
those with a relatively low feed-intake rate have higher LCT and therefore are
especially vulnerable to cold stress. Also, since many studies are camed out
TABLE 30 Changes in Lower Critical Temperature (LCT) in Pigs at
Various Housing, Management, and Climatic Conditions
Weight Change in
Condition Specification (kg) LCT (°C) Reference
Airspeed (kmlh)
2.4 individual pigs - + 4 Mount, 1975
1.8 individual pigs - + 7 Mount, 1975
5.5 individual pigs - + 10 Mount, 1975
1.6 group of nine 40 + 1.5 Verstegen and
van der Hel, 1976
Floor
, Ado.
Concrete vs. straw at 10°C piglet +8 Stephens, 1971
Concrete vs. straw at 30°C piglet +2 Stephens, 1971
Straw group of nine 35 -4 Mount, 1975
Concrete slats group of nine 35 +5 Mount, 1975
Wet surface group of nine 35 +5- + 10 Mount, 1975
Draft
Draft insulation - +6 Mount, 1975
Draft uninsulated +8 Mount, 1975
(winter)
No draft uninsulated - +2 Mount, 1975
(winter)
No draft uninsulated -4 Mount, 1975
with straw
Radiant temperature
+ 1°C individual piglet - 1 Mount, 1964
Reflective wall Holmes and
and ceiling group 11 -2 McLean, 1977
OCR for page 100
100 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
with relatively fat hogs, meat-type pigs are more sensitive to cold stress
(Comber" et al., 19671.
Further, vapor pressure is a consideration. Significant effects of high hu-
midity on pig performance at low environmental temperatures are not ex-
pected. However, some negative effects can nonetheless occur. High vapor
pressure reduces the evaporation of water, so floor and walls tend to be wet-
ter. This may influence the pigs' heat balance, because their body surfaces
may be wetter, and it may indirectly reduce performance because it favors
survival of pathogenic microbes in the environment.
COLD STRESS
The cold-stressed pig's extra thermoregulatory heat-production rate is based
on data in Table 28 and the formula given on page 96. From measurements
of heat-production rate in both cold and thermoneutral conditions, Holmes
and Close (1977) calculated the extra feed needed to compensate for the in-
creased rate of heat loss. They made separate calculations based on measure-
ments near the lower critical temperature, as well as at 10°C of coldness.
(Coldness is a term used to describe the magnitude of difference between
lower critical temperature and existing thermal conditions.) The latter
condition an EAT only a few degrees above the freezing point is often en-
countered in practice. These data can be used to calculate the extra feed re-
quired to compensate for the heat needed to keep the body warm.
In Table 31 estimates are given of this extra heat to keep the body warm
and the extra feed requirement. There is a difference in the additional feed
needed per °C of coldness between grouped pigs and those kept singly.
Those in groups can huddle, thereby reducing body-surface exposure and
TABLE 31 Extra Heat Required per °C Coldness and Feed Equivalent
Required to Compensate (Assuming Feed Contains 2.9 kcal ME/g)
Feed
Weight Extra Heat Equivalent
Kind of Animal(kg) (kcal/°C/day) (g/°C/day)
Baby pig2 11 4
Individual20 39 14
Group20 38 13
Individual100 103 36
Group100 100 35
Sow: thin140 170 59
Sow: fat140 98 34
SOURCE: Holmes and Close, 1977.
OCR for page 101
Swine
101
heat loss to the cold environment. One can conclude that a pig weighing 20
kg should consume additional feed at the rate of 1~3 g/day/°C of coldness, and
for pigs weighing 100 kg up to 35 g/day/°C. In Table 32 are given some ex-
amples of the extra feed requirement. Of course, compensation for coldness
by extra feed intake as outlined results in an average daily gain comparable
to that at thermoneutrality only if body composition is not influenced by en-
vironmental conditions.
Extra feed intake is required during cold to compensate for reduced gain in
restricted-fed pigs. For example, if rHE intake remains the same, body energy
gain is reduced at environmental temperatures below the lower critical tem-
perature due to extra heat to keep the body warm. However, daily gain-
even with extra feed-intake rate still may not be the same in the cold as at
thermoneutrality .
Fat, protein, water, and ash gains, which together comprise body weight
gains, may be reduced in several ways. Fat gain will be more reduced in the
cold than protein gain, because fat is used primarily as fuel (Masoro, 19661.
In growing pigs, the same has been found by Hacker et al. (1973), Verstegen
et al. (1973), and Brown et al. (19761. Close and Mount (1976) showed that
reduction in protein gain in the cold depended on feeding level. Sorensen's
data (1962), on the other hand, suggest that protein gain was reduced more
than fat gain in severe cold (below 8°C). Also, the protein-gain/water-gain
ratio is increased in young pigs in the cold (Brown et al., 1976~. Conse-
quently, body weight gain may be less than expected from protein- and fat-
deposition measurements alone (Verstegen et al., 1973~.
From results of a number of growth trials in various environmental condi-
tions, calculations show average daily gain is reduced from 13 to 19 g/°C of
coldness over the entire growing-finishing period (Fuller and Boyne, 19711.
From review of many trials, Verstegen et al. (1978) concluded that average
daily gain is depressed by 15 g/°C of coldness when feed-intake rate remains
constant. When the feed requirement needed to compensate for this reduction
in gain was calculated from results of growth trials, values of 30 to 45 g/°C
TABLE 32
(g/day)
Estimates of Extra Feed Required at Various Temperatures
Effective Ambient Temperature (EAT)
1 5°C 1 0°C 5°C
Animals of 20 kg
(UCT = 18 C) 39 104 169
Animals of 100 kg
(ACT = 14 C) 0 144 334
OCR for page 102
102 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
of coldness per day were obtained (Verstegen et al., 1977, 19781. There
was, however, much variation in this extra feed requirement across experi-
ments; these values are averages from numerous investigations.
For ad libitum-fed pigs, additional feed must be consumed during cold to
maintain daily gain. There is, however, less information on voluntary feed-
intake rate in pigs under standardized environmental conditions than on heat-
production rate. In some experiments, cold-stressed pigs ate so much addi-
tional feed that daily gain was actually higher than at thermoneutrality.
Sugahara et al. (1970) found this with pigs weighing 6 to 35 kg. Holme and
Coey (1967) noted the same thing in heavier pigs. Seymour et al. (1964),
who studied protein-concentration/environmental-temperature interaction,
noticed that pigs had, if any, only a slight reduction in gain, but increased
feed intake at environmental temperature below 16.5°C.
Ultimately, there are three possibilities in regard to feed intake and gain in
ad libitum-fed pigs in the cold. Each is supported to some extent by reports
in the literature. These are:
1. A small change, if any, in gain, but an increase in feed intake. Sey-
mour et al. (1964) found an increase in daily feed intake amounting to 11 to
16 g/°C of coldness between 16 and 2.5°C (three pigs in a group). Mangold
et al. (1967) calculated from three winter trials that extra feed requirement
per °C of coldness below 15.5°C increased with body weight (Table 33), but
they worked with fluctuating temperature. Jensen et al. (1969) reported simi-
lar findings with pigs in groups of 7 to 12.
2. A reduced gain accompanied by no change or an increased feed intake.
In trials of 1 week's duration, Heitman and Hughes (1949) found an in-
creased feed intake in the cold. In weight ranges of 32 to 65 kg and 75 to 100
kg, they observed increased daily feed intake of about 53 and 41 g/°C of
coldness, respectively, when environmental temperature fell from 15.5 to
4.5°C. However, despite the higher rate of feed intake, there remained a re-
duction in daily gain of about 12 and 54 g/°C of coldness per day, respec-
tively.
From a review of the literature, Verstegen et al. (1978) calculated a reduc-
tion in daily gain of 8.1 g/°C of coldness below 15°C, with an increase in
daily feed intake of 19.5 g/°C. Corrected toward a daily gain comparable to
that at thermoneutrality (assuming a gain/feed ratio of 0.31), this would re-
quire approximately 19.5 + (8.1/0.31), or 46 g/°C per day.
Mangold et al. (1967) found only a small change, if any, in feed intake in
pigs weighing about 65 kg over the environmental-temperature range, 9 to
15.5°C. They noticed a decrease in daily gain of 13 g/°C of coldness. In an
earlier report, Hazen and Mangold (1960) also used this approach to deter
OCR for page 103
Swine
103
TABLE 33 Extra Feed Intake and Reduction in Gain per °C Temperature
Change in the Cold
Increase
Temperature Range Weight in Intake
(°C) (kg) (g/°C) Reference
Constant gain, but increase in feeding level
15.5-2.5 7-92 11-16
15.5-6.5 13.6 4
15.5-5.5 34 20
15.5-5.5 65 37
1~- 1 5~86 33
22--10 14 48 14
Constant feed intake, but decrease in body weight gain
, , ~
Decrease
Temperature Range Weight in gain
(°C) (kg) (g/°C)
Seymour et al., 1964
Mangold et al., 1967
Mangold et al ., 1967
Mangold et al., 1967
Jensen et al., 1969
Jensen et al., 1969
15.5-9.0 65 13 Mangold et al., 1967
15.5-5.5 22-92 9- 11 Hazen and Mangold, 1960
13.~5.0 22-90 17.8 Fuller and Boyne, 1971
Changes in both intake and gain (g/°C)
Decrease
Temperature Range Weight in gain
(°C) (kg) (g)
Increase
in intake
(g)
15.~5.0 2~110 8.1 19.1 Verstegen et al., 1978
15.5-4.5 32-64 12 53 Heitman and Hughes, 1949
15.5~.5 75-95 54 41 Heitman and Hughes, 1949
mine that during cold pigs are able to increase feed intake so that body
weight gain remains the same as at thermoneutrality.
Another factor that affects estimates of extra feed required by cold expo-
sure is a small, but consistent, reduction of dietary-energy digestibility in
cold-stressed animals. From the data of Hicks (1966) and Fuller and Boyne
(1971) on pigs, Ames and Brink (1977) on lambs, and Young and Chris-
topherson (1974) on cattle, a digestibility coefficient drop of 0.1 to 0.4 per-
cent per °C may be expected. Lowered digestibility is less predictable for
swine, since some investigations did not find a significant alteration in di-
gestibility with temperature in pigs (Table 2~.
In summary, the extra feed intake required by cold-stressed pigs when rate
of gain remains comparable to that at thermoneutrality can be calculated
from results of a few ad libitum feeding trials (Table 33) and averages about
25-30 g/°C of coldness per day for pigs weighing between 20 and 100 kg.
OCR for page 104
104 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
On the basis of energy balance trials, 25 g of feed intake per °C coldness for
the growing-finishing period may be the best estimate. However, in growth
trials with relative short-term cold exposure, a much higher value is ob-
tained. This value would, of course, vary directly with body weight. From a
more extensive review of the literature, when data were corrected to a con-
stant daily gain basis, the estimate of extra feed required was 45 g of feed per
°C of coldness per day (Verstegen et al., 1978~.
From all this evidence, it seems justifiable to estimate arbitrarily that the
extra feed requirement for pigs under cold stress in practical conditions aver-
ages in the range of 30 to 40 g/°C of coldness per day for the
growing-finishing period when body weight ranges from 20 to 100 kg.
In 1968, NRC estimated that ad libitum-fed pigs weighing from 20 to 100
kg have an average daily gain of 800 g and a gain/feed ratio of 0.29, pro-
vided they have a daily feed intake of 2.8 kg under optimal conditions. Rec-
ommended crude-protein concentration averages 13.52 percent. There is no
clear evidence that total dietary crude-protein requirement varies with envi-
ronmental temperature. Data of Seymour et al. (1964) showed similar reduc-
tion in gain in the cold when pigs were fed diets with high (16 percent) or
low (13 percent) crude-protein levels. Thus, crude-protein concentration in
the diet may be reduced when the pigs are cold-stressed. Example calcula-
tions for adjusting crude-protein concentration for cold-stressed pigs follow.
Assume extra daily feed intake per pig required per °C of coldness is 40 g.
(This amounts to 40/2,800, or 1.4 percent extra feed per °C of coldness.) As-
sume further that dietary crude-protein concentration is adjusted by diluting a
basic corn-soy diet, containing 13.5 percent crude protein, with corn, con-
taining 8 percent crude protein. [Crude-protein intake at optimal conditions
would be (2,800 g/day)~0.135) = 378 g/day.]
If, for example, a practical environment is at 5°C of coldness (in the range
of 5 to 10°C), each pig will need an extra feed intake of (40 g/°C)~5°C) =
200 g daily. In such a case, total feed intake would be 2,800 + 200 =
3,000 g/day. If total crude-protein intake is to remain equal (378 g/day), the
adjusted dietary-crude-protein concentration (X) would be calculated as:
(2,8001~0.135) = (3,000~(X)
X = 12.6 percent.
And the replacement (Y) of the basic diet by corn to achieve this would be
calculated as:
t(3,0004~1 - Y)~0.1354] + ~3,000 Y)~0.081] = (2,800~0.135)
Y= 0.164.
OCR for page 105
Swine
105
That is, the adjusted diet would contain 16.4 parts corn and 83.6 parts basic
diet. A similar approach for cattle and sheep exposed to both heat and cold
stress has not altered daily gain but improved protein efficiency ratio (Ames
et al., 19801.
HEAT STRESS
Various definitions and concepts have been used to describe the reactions of
pigs to heat stress. From the concept of thermoneutrality, as proposed by
Mount (1974), the upper critical temperature is defined as the effective ambi-
ent temperature above which total heat-production rate at a given feed intake
will rise. As a working hypothesis, Holmes and Close (1977) described up-
per critical temperature as that point at which a pig with dry skin can main-
tain maximal rate of heat loss. One may also use the point at which there is a
rise in core temperature or frequency of respiration, as suggested by Heitman
and Hughes (19491.
Since the pig must rely on evaporative heat loss in the heat, vapor pressure
is more important during heat stress than in the cold. When considering heat
loss, it has been suggested by Holmes and Close (1977) that at 30°C an in-
crease of 18 percent in relative-humidity value is equivalent to an increase in
air temperature of 1°C for swine. One may extract various other values of the
temperature equivalence of relative humidity from other data. Morrison et al.
(1967) pointed out that at 22.8°C, an increase in relative humidity from 45 to
95 percent was comparable in its effect on the pigs' heat balance of 2.2°C
temperature increase. Using rise in core temperature as an index, one can
calculate a value for EAT when relative humidity is greater than 30 percent as:
EAT = (0.35) (wet-bulb temperature) + (0.65) (dry-bulb temperature).
However, to be useful, such indices must be predictive of feed intake and
weight gain. Thus, effects of air speed, thermal radiation, housing and
equipment factors, and group size must ultimately be accounted for and
should be included to improve the accuracy of calculating EAT.
Data relating heat-induced decreases in feed intake and rate of gain in
groups of pigs to effective ambient temperature and body weight are shown
in Table 34. Compared with cold stress, much less information is available
relating effects of heat stress on production traits. Houses for swine produc-
tion during hot weather have been only partially evaluated in terms of effects
on specific performance characteristics. For these comparisons, EAT, includ-
ing at least dry- and wet-bulb temperatures, should be used as the standard.
If not stated otherwise, EAT iS assumed in the following discussion.
From data on short-term exposure to hot conditions (Heitman and Hughes,
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106 APPROACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
TABLE 34 Decrease in Feed Intake and Rate of Gain During Heat Stress
Decrease
Temperature Weight Intake Gain
(°C) (kg) (g/°C) (g/°C) Reference
21-32 35-65 60 33 Heitman and Hughes, 1949
21-32 75-100 120 57 Heitman and Hughes, 1949
2~32 45 38 Heitman et al., 1958
22-32 68 60 Heitman et al., 1958
22-27 91 60 Heitman et al., 1958
19-27 114 60 Heitman et al., 1958
18-32 2~100 30 7 Hazen and Mangold, 1960
16.5-32 7-92 14 3 Seymour et al., 1964
22-38 13.5 8 4 Seymouretal., 1964
22-38 35 33 8 Seymour et al., 1964
22-38 65 41 12 Seymour et al., 1964
23-33 ~35 42 21 Sugahara et al., 1970
1949; Heitman et al., 1958), it can be calculated that pigs ate about 60 to
100 g less feed each day per °C of heat stress (32 as opposed to 21°C). This
decline in feed intake resulted in a reduction in daily gain of 35 to 57 g/°C of
heat stress.
Heavier pigs are more sensitive to heat stress than lighter ones (Ingram,
1974~. Performance data bear this out. In one experiment, finishing hogs did
not grow at all in a 39°C environment, whereas those weighing around 45 kg
continued to gain weight (Heitman and Hughes, 1949~. Mangold et al.
(1967) found the same thing in their three summer trials: heavy pigs showed
greater reduction in both intake and gain than did growing pigs. Previous
Iowa studies had also shown that pigs weighing less than 20 kg had much
less decrease in feed intake than older pigs (Hazen and Mangold, 1960~.
They reported figures from which a reduction of 30 and 7 g/°C of heat load
in intake and gain, respectively, in the body weight range of 20 to 100 kg
were calculated when 32 and 18°C were compared. Of course heavier pigs
are usually fatter and have a lower UCT. Consequently, heavier pigs are more
heat stressed at the same temperature because they are exposed to a greater
heat load.
It is logical that adding fat to swine diets may be advantageous during heat
stress, because fat has a lower heat increment than either carbohydrate or
protein. In addition, fat has a high caloric density that helps offset lowered
caloric intake during heat exposure. Stably et al. (1979a) have reported ad-
vantages for adding fat to diets of heat-stressed pigs. These same workers
OCR for page 107
Swine
107
(1979b) report an advantage for feeding synthetic lysine instead of natural
protein, which reduces heat increment of the diet.
Some investigators have found that dietary vitamin and mineral concentra-
tions may need to be increased under heat-stress conditions. But there is little
evidence indicating that total daily requirements for these nutrients are af-
fected by effective ambient temperature. Of course, as high temperatures re-
duce feed intake, it may be advisable to increase the concentration of certain
vitamins and minerals in the diet to compensate. Peng and Heitman (1974)
found that dietary thiamine requirement may be greater at 30-and 35°C com-
pared with thermoneutral. Holmes and Grace (1975) found more potassium
in the urine of heat-stressed pigs, but calcium retention was not affected.
Holmes and Grace (1975) and Gray and McCracken (1974) noticed an in-
crease in the nitrogen content of the urine of pigs held at relatively high am-
bient temperatures, suggesting protein retention may be decreased somewhat
in heat-stressed pigs. There has also been the suggestion that backfat thick-
ness is increased at high temperatures (Holmes, 1971~. Information on the
magnitude and cause of body-composition changes in pigs in hot conditions
is far from complete. Yet, with some exceptions (Firmer and Curran, 1977),
there is reason to believe an increase in dietary crude-protein content during
hot periods may be justified.
To get some idea of the normal magnitude of environmental heat stress
during hot weather, one must know both the optimum temperature range and
the average EAT. Most investigators have found the range, 18 to 21°C, opti-
mal for growing-finishing pigs. Heat exposures have mostly been made at 32
to 38°C. However, these temperatures more likely reflect daily maximums in
practice rather than daily means. It has been found that cyclic temperatures
have about the same effect on physiological responses and performance traits
as the mean of the cycle. Mean daily temperature is usually less than 30°C
(Bond et al., 1967; Morrison et al., 19751; therefore, it seems justifiable for
illustrative purposes to assume a mean heat load (difference between EAT and
the pig's UCT) of 5°C during summertime. Of course, not all animals will al-
ways be under this much heat stress. By facilitating wallowing, providing
shade, increasing air velocity (Bond et al., 1965), and taking other mea-
sures, heat load can be varied, and this in turn influences feed intake and
daily gain. Quantitative effects of environmental modifications are difficult
to evaluate for practical situations, thus the 5°C heat load seems reasonable.
During heat stress dietary crude protein may require adjustment. If the av-
erages of the data of Hazen and Mangold (1960), Mangold et al. (1967), and
Heitman et al. (1958) are used, daily feed intake will decrease about 40 g/°C
of heat stress, and this is paralleled by a daily gain depression of 10 to 20 g/
°C. In pigs held at 5°C above optimum, daily intake will be (40 g/°C)~5°C)
= 200 g lower (2,800 - 200 = 2,600 g/day), but the animals should still
consume a total of 375 g of digestible crude protein daily (NRC, 19731.
OCR for page 108
108 APP8QACHES FOR PRACTICAL NUTRITIONAL MANAGEMENT
For example calculations, two feeds will be considered: a basic corn-soy
diet, containing 13.5- percent crude protein, and a supplement containing 40
percent crude protein. Combination af the two is expected to- yield a daily in-
take of 37S g of crude protein at the specified feed-intake rate (2,600 g/day).
The crude-protein content (X) of the adjusted diet will be greater than 13.5
percent:
(2,8001~0.135) = (2,600~(X)
X = 14.42 percent.
And the replacement (Y) of the basic diet by protein supplement to achieve
this would be calculated-as:
t(2,600~1 - Y)~0.135~] + [~2,600 Y)~0.40~] = (2,800~0.135)
Y= 0.039.
That is, the adjusted diet would contain 3.9 parts protein supplement and
96.1 parts basic diet.
In summary, dietary crude protein should be provided in accordance with
requirement for gain and not simply fed as a constant percentage of a
temperature-dependent daily intake.
SUMMARY
Swine, like other species, are sensitive to changes in the effective thermal
environment. Size is highly correlated with fatness and therefore related to
rate of heat loss. Consequently, responses of swine to specific environmental
conditions are largely dependent on size. Increased heat loss during cold
stress reduces rate of performance unless compensation is made by increas-
ing rate of feed intake. During heat stress, rate of intake is depressed, result-
ing in lowered performance. Dietary adjustments designed to provide each
nutrient as required but to avoid over- or underfeeding when intake varies
with environmental condition are discussed. Although more quantitative data
are needed for accurate nutrient adjustments in response to thermal stress,
swine producers should consider the impact of environmental conditions
when developing diets for pigs.
Representative terms from entire chapter:
daily gain
