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Lactation in the sow during heat stress
Livestock Production Science, 35 ( 1993 ) 153-170
153
Elsevier Science Publishers B.V., Amsterdam
Lactation in the sow during heat stress
J.L. Black a, B.P. Mullan b, M.L. L o r s c h y c a n d L . R . G i l e s c RCSIRO, Division of Animal Production, P.O Box 239, Blacktown, NSW,, Australia, bDepartment of Food and Agriculture, Victorian Institute of Animal Science, Werribee, Vic., Australia, CDepartment of Animal Science, Universityof Sydney, Camden, NSW, Australia (Accepted 1 December 1992)
ABSTRACT Ambient temperatures above the evaporative critical temperature (ECT) of lactating sows lead to a reduction in food intake, milk yield, reproductive performance and growth rate of piglets. The fall in food intake of the lactating sow is closely associated with a rise in deep body temperature as is also observed in growing pigs. Evidence is presented indicating that the reduction in milk yield of sows exposed to high temperatures may be greater than would be expected from an equivalent decline in food intake for sows housed under thermoneutral conditions. It is suggested that the direct effect of high temperatures on milk yield may result from a redirection of blood flow to the skin and away from other tissues, including the mammary gland. Experimental observations showing that skin temperalure is maintained while deep body temperature declines after initially rising rapidly in lactating sows exposed to 28°C for four days lends indirect support to this theory. Oxygen uptake of lactating sows was observed to decrease from 523 to 411 ml/min when ambient temperature was increased from 18 to 28°C. This decline of 20% in heat production was associated with a 25% decline in milk yield and a 40% reduction in food intake. The oxygen uptake of lactating sows was found to be similar to that of non-breeding animals with an equivalent food intake. Consequently, ECT for lactating and nonlactating animals was found to be similar. Performance of lactating sows exposed to high temperatures can be improved by reducing the animal's heat production through decreasing the fibre and increasing the fat content of the diet. However, increasing heat loss from the sow, particularly through increasing the area of wet skin, has a greater positive effect on animal performance than modifying the diet.
Keywords: lactating sow, heat-stress, food intake, milk yield.
INTRODUCTION
The inability o f sows to consume sufficient food to meet the nutrient requirements o f lactation is a c o m m o n problem in commercial pig production Correspondence to: J.L. Black, CSIRO, Division of Animal Production, P.O Box 239, Blacktown, New South Wales, 2148, Australia.
0301-6226/93/$06.00
© 1993 Elsevier Science Publishers B.V. All rights reserved.
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(Cole, 1990). Sows mobilise body reserves to support milk production when nutrient intake during lactation is low and, if the loss of body reserves is excessive, especially with young sows, piglet growth may be reduced and reproductive performance impaired (King and Williams, 1984a; Mullan and Williams, 1989 ). High environmental temperature is one of a number of factors that may reduce the voluntary food intake (VFI) of the lactating sow (O'Grady et al., 1985 ). Ambient temperatures are high in many areas of the world where pigs are raised and, even in situations where piggery temperatures are controlled, the wide difference in the optimum temperature requirements between the lactating sow and the piglet often leads to heat stress in the sow. The objectives of this paper are to review research describing the effects of high temperature on VFI, milk yield and reproductive performance of sows, to examine evidence suggesting that the effect of depressed food intake on milk yield may be greater in heat-stressed than in normal sows, to present experimental results on physiological responses of sows to high temperatures and to discuss ways of maintaining the performance of sows exposed to high temperatures.
EFFECT OF HIGH TEMPERATURE ON VFI AND PERFORMANCE OF SOWS
Definition of heat exposure Animals are considered to be exposed to heat when ambient temperature is above the zone of thermal comfort and energy is expended to maintain body temperature. The zone of thermal comfort covers the range of temperatures between the animal's lower critical temperature (LCT) and its evaporative critical temperature (ECT). LCT is the temperature below which the animal must increase heat production through shivering and other metabolic processes to maintain body temperature. ECT in the pig signifies the temperature at which evaporative heat loss begins to increase, particularly from the lungs, through increased respiration (Black et al., 1986). ECT marks also the ambient temperature near which the growing pig commences to reduce VFI (Giles and Black, 1991 ). The ambient temperatures coinciding with LCT and ECT are not constant for an animal, but are affected by the heat produced through normal metabolic processes and by environmental factors such as air speed, humidity, floor type, radiant heat incidence, number of animals in a group and the area of wet skin. The zones of thermal comfort for the lactating sow and piglet differ markedly; between 12 and 22 °C for the sow and 30 and 37°C for the piglet (Mullan, unpublished). This difference presents an important challenge to the piggery manager when attempting to maintain production efficiency of both the sow and piglets.
LACTATION IN THE SOW DURING HEAT STRESS
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Day of t h e m o n t h Fig. 1. A m b i e n t t e m p e r a t u r e s i n s i d e a f a r r o w i n g s h e d - S h e p p a r t o n , Victoria, Australia; February 1991 ( M u l l a n , 1991 ).
Effect of high temperatures in commercial piggeries There is unfortunately a paucity of published information from commercial piggeries relating environmental temperature in farrowing houses to animal performance. In a study by Cox et al. (1983) at a commercial piggery in North America, no difference was observed in total litter weight at weaning or weight loss of the sow during lactation between summer and winter but the time between weaning and oestrus was longer during summer. However, food intake was not measured and the difference in ambient temperatures between summer and winter was less than 3°C. Alternatively, O'Grady et al. ( 1985 ) observed lower intakes in sows during summer than during winter but no animal performance characteristics were reported. The extent to which high ambient temperatures may affect the performance of sows in Australian piggeries can be gauged from measurements taken in a commercial piggery during summer (Fig. 1 ). Temperature sensors were positioned approximately 1 m above floor level and away from the influence of radiant heaters. Mean temperature at sow level for the month of February was 26 °C (range of 17 to 39 °C) and hourly values were frequently greater than 22 °C which was estimated to coincide approximately with the animal's ECT.
Experimental evidencefor effects of high temperatures Food intake and milk yield. The effect of an increase in ambient temperature on voluntary energy intake during lactation has been investigated in several experiments (Fig. 2). Results from these 9 experiments indicate that, for each
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J.L BLACKET AL. 120
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T e m p e r a t u r e (°C) Fig. 2. Effect of ambient temperature (x) on the voluntary digestible energy intake (DE, y) of lactating primiparous sows (Barb et al., 1991; Cole, 1990; Lynch, 1977; Lynch, 1989; Schoenherr et al., 1989a,b; Stansbury et al., 1987; Vidal et al., 1991; Yen and Cheng, 1990). Mean relationship is given by the bold line (y = 126.6 - 2.4x ).
1 oC increase in ambient temperature above 16 ° C, daily voluntary energy and food intake decline by 2.4 MJ DE and 0.17 kg, respectively. However, only one of these experiments included an intermediate temperature and, as suggested by this result, it is unlikely that the relationship between ambient temperature and food intake is linear over the range of temperatures presented. Rather, as eluded to by Giles and Black ( 1991 ), the decrease in intake may depend on the extent to which ambient temperature exceeds the animal's ECT. The reduction in the growth rate of piglets suckling sows maintained at high temperatures has been assumed to reflect a reduction in milk yield. Schoenherr et al. (1989b) and Vidal et al. (1991) recorded decreases in milk yield of 10 and 35% and associated declines in piglet growth rate when ambient temperature was increased by 10°C and 8°C, respectively (Table 1 ). Milk yield of sows exposed to high temperatures and represented in Fig. 2 can be calculated using the relationship of King et al. ( 1989 ) between piglet growth rate and milk intake for all those experiments in which piglet growth rate was available. Although there is considerable variation between experiments in the response of milk yield to energy intake (Fig. 3 ), there is a consistent decline in milk yield in the heat-stressed animals that had a reduced food intake. However, it is not known whether this reflects a direct effect of high temperature on milk synthesis and/or whether the decrease in the supply of substrates due to the low VFI of the sow is responsible for the decrease in milk production.
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LACTATIONIN THE SOWDURINGHEATSTRESS TABLE 1 The effect of ambient temperature on voluntary food intake and milk production of the sow Reference 1
Ambient temperature (°C) Length of lactation (days) Sow feed intake (kg/day) change in body weight (kg) change in back fat (mm) milk yield (kg/day) Piglet average daily gain (g) litter weight gain (kg/day)
1
20 22
2
30 22
2
22 27
30 27
5.90 - 2.6 8.34
3.36 - 15.9 7.47
7.72 -6.4 +2.2 10.27
4.95 -21.0 -3.0 6.64
206 1.88
182 1.64
226 2.21
167 1.53
1. Schoenherr et al. (1989b). 2. Vidal et al. (1991).
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Fig. 3. Influence of digestible energy intake (DE, x) on milk production (y) of primiparous sows when energy intake is determined by ambient temperature (Barb et al., 1991; Lynch, 1977; Schoenherr et al., 1989b; Stansbury et al., 1987; Vidal et al., 1991; Yen and Cheng, 1990). Mean relationship is given by the bold line (y = 4.08 + 0.05x).
Reproductive performance. T h e
effect of high ambient temperatures on VFI also has important consequences for reproductive performance. Primiparous sows lactating during the summer have a longer interval between weaning and m a t i n g t h a n d o t h o s e d u r i n g w i n t e r ( C o x et al., 1983; C l a r k et al., 1 9 8 6 ) . Season does not, however, appear to have the same effect with multiparous
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sows (Clark et al., 1986; Fernandes et al., 1990) and it has been suggested (Fernandes et al., 1990) that this is because the younger animal mobilises a greater proportion of its more limited body reserves if VFI is low during lactation. There is conflicting evidence, however, as to the mechanism by which season influences reproductive function. Evidence from primiparous sows suggests that high ambient temperatures during lactation cause a decrease in luteinising hormone (LH) pulse frequency and that this is responsible for the delay in rebreeding after weaning (Barb et al., 1991 ). This may explain the result of Cox et al. (1983) who observed no difference in the weight loss of sows during lactation between winter and summer but the weaning-to-oestrus interval increased from I 0 to 23 days. However, there is also evidence that low nutrient intakes in primiparous sows, housed under standard conditions during lactation cause an increase in the weaning-to-mating interval (King and Williams, 1984a; Mullan and Williams, 1989 ) and that this is due partially to a disruption of the normal secretory pattern of LH (Mullan et al., 1991 ). Hence, low nutrient intake and not high ambient temperatures p e r se may be the cause of poor fertility. Feed intake in the experiment of Barb et al. ( 1991 ) was reduced from 6.1 to 2.9 kg/day due to the effect of high ambient temperatures. ENERGY INTAKE - MILK YIELD RELATIONSHIP IN HEAT-EXPOSED SOWS
Limited evidence suggests that the decline in milk yield may be greater in heat-exposed animals than can be accounted for directly by the associated decline in energy intake. Webster (1974) cites evidence from Johnson et al. (1966) showing that milk yield in dairy cattle exposed to hot environments remained depressed despite feeding to thermoneutral intake through a rumen fistula. To examine whether such an effect is apparent in sows, a comparison was made between experiments in which feed intake was deliberately restricted and those in which it was depressed through exposure to high temperatures (Fig. 4 ). Although there was a large variation between experiments in the response of milk yield to energy intake when it was controlled by the amount of food offered, the mean slope of the relationship differs from that found in Fig. 3 for heat-exposed sows. The decline in milk yield associated with a reduction in energy intake appears to be greater in the heat-exposed animals than in sows housed under lower temperature conditions. Despite the variation between experiments, this comparison adds some support to the theory that there is a direct effect of high ambient temperature on milk production of sows. Reduced food intake is a primary mechanism used by animals to limit heat production when they are exposed to high temperatures. However, this strategy may be less effective during lactation than at other times. Sows housed under normal temperature conditions have the capacity to maintain high rates
LACTATION
IN THE SOW DURING
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HEAT STRESS
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DE intake (MJ / d a y ) Fig. 4. Influence of digestible energy intake (DE, x) on milk production (y) of primiparous sows offered different amounts of food during lactation (Brendemuhl et al., 1987; Brcndemuhl et al., 1989; Hoppe et al., 1990; King and Dunkin, 1986a,b; King and Williams, 1984a,b; Mullan and Close, 1989; Mullan and Williams, 1989; Nelssen et al., 1985; Noblet and Etienne, 1987; O'Grady et al., 1973; Rcesc et al., 1982). Mean relationship is given by the bold line (y = 6.62 + 0.01 x) and mean relationship for animals exposed to high ambient temperatures is given by the bold dashed line (from Fig. 3 ).
of milk synthesis, particularly in early lactation, despite low feed intakes, through the mobilisation of body tissue. For example, in the experiment of King and Dunkin (1986a), lactating sows were fed between 19 and 60 MJ DE/day with little effect on piglet growth rate during the first 3 weeks of lactation. This was because sows mobilised maternal body reserves of fat and protein to support milk production and it was only during the fourth week of lactation, when maternal reserves had been depleted substantially, that milk yield was affected by the low energy intake. Thus, in comparison with nonlactating animals, a reduction in energy intake would be expected to have little effect on the heat produced by metabolic processes. There is little experimental evidence to support this contention, but predictions from the AUSPIG model (Black et al., 1986), which are based largely on current understanding of energy metabolism in pigs, indicate that the depression in heat production resulting from a 25% reduction in food intake from ad lib. is approximately 2% in a 150 kg lactating sow compared with nearly 18% in a nonlactating animal of the same weight and body composition. Lactating animals may, therefore, have special mechanisms for reducing milk output when suffering from heat exposure. One possibility may be an increase in blood flow to the skin to assist heat
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loss, at the expense of blood flow to the mammary gland and other organs. Such a mechanism is known to occur with heat-stressed pregnant sheep, where there is a marked diversion of blood flow from the conceptus to the skin that results in a depression in conceptus growth and in low birth weight lambs (Alexander et al., 1987 ). An experiment has recently been conducted to further evaluate the hypothesis that high temperature has a direct effect on milk production, in addition to its indirect effect through reduced energy intake (Mullah et al., 1992). Groups of 6 primiparous sows with litters standardised to 8 piglets were housed at either 20°C or 30°C throughout a 28-day lactation. One group at each temperature was offered ad lib. a diet containing 14.3 MJ DE and 160 g TABLE 2 The effect of ambient temperature on voluntary food intake and milk production of first-parity sows during a 28-day lactation Treatment Temperature ( ° C): Feeding level: Diet: No. of sows Sow post farrowing wt. (kg) backfat 3 ( m m ) change during lactation wt. (kg) backfat 3 ( m m ) food intake (kg/day) Piglets total born total born alive number weaned birth wt. (kg) wean wt. (kg) average daily gain (g) day 0 to 14 day 14 to 28 day 0 to 28 Milk yield (kg/day) 4 day 0 to 14 day 14 to 28 day 0 to 28
20 ad-lib, control ~ 6
20 restricted control ~ 6
30 ad-lib, control ~ 6
30 ad-lib. high-spec 2 6
LSD
145.8 24.9
147.2 25.3
145.3 22.9
151.2 22.9
14.6 3.9
-9.6 - 5.6 4.05
- 19.0 - 5.5 3.16
- 16.0 - 4.6 3.16
-21.8 - 3.4 2.95
14.4 3.0 0.79
8.2 7.3 7.8 1.51 8,18
9.8 9.8 7.8 1.30 7.63
9.3 8.2 8.0 1.50 7,44
7.4 7.4 7.8 1.46 7.32
2.7 3.2 0.95 0.23 1.61
235 248 241 8.88 9,54 9.21
211 245 228 7.86 9.47 8.67
202 209 206 7,53 7,83 7.68
Formulated to contain 14.3 MJ DE, 160 g CP and 0.50 g AvLys per kg DM. 2Formulated to contain 15.2 MJ DE 210 g CP and 0.80 g AvLys per kg DM. 3Depth ofbackfat measured by ultrasound at the P2. 4Calculated according to King et al. (1989).
205 225 215 7.54 8.30 7.92
51 67 53 2.11 2.88 2.18
LACTATION IN THE SOW D U R I N G HEAT STRESS
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crude protein per kg food dry matter. A second group at 20°C was offered the same a m o u n t of food as was eaten by the sows at 30°C. In addition, a second group at 30°C was offered ad lib. a diet containing 15.2 MJ DE and 210 g crude protein per kg food dry matter. All sows entered the experimental facility on day 105 of gestation and supplementary heat was provided for piglets in a creep area designed to minimise the influence of radiant heat on the sow. Feed intake was depressed by approximately 25% and milk yield by 15% for the sows housed at 30°C compared with those housed at 20°C. Although the difference in milk yield and piglet growth rate was not significant during either early or late lactation for the pair-fed sows housed at the two temperatures, there was a strong indication that high ambient temperature was having a direct effect on milk yield (Table 2 ). The average birth weight of piglets from the sows pair-fed at 20°C was significantly less than for the piglets held at 30 °C and this may have contributed to the lack of significance in the initial phase of lactation. Increasing the protein content of the diet offered to sows maintained at 30°C had no significant effect on either feed intake or milk yield. The experiment is continuing with additional sows. The results of this experiment support the observations with lactating cattle and suggest that the depression in milk yield resulting from a decline in food intake is greater in heat-stressed animals than in those held under thermoneutral conditions. It is important, therefore, to examine the effects of heat exposure on the physiological responses and heat production of the lactating SOW. PHYSIOLOGICAL RESPONSES OF LACTATING SOWS TO HEAT EXPOSURE
The effects of high ambient temperatures on food intake, milk yield, oxygen consumption and several physiological characteristics of lactating sows have been investigated (Lorschy et al., 1992 ). Primiparous sows with litters standardised to 7 piglets were housed in a temperature controlled room with air speed of less than 0.1 m / s e c and relative humidity of less than 50%. The animals were held in farrowing crates with slatted plastic floors and drinking water was provided in a way that prevented the sow from wetting her skin. The sows were offered ad lib. a commercial diet containing 14.3 MJ DE and 160 g crude protein per kg dry matter. Milk intake of the piglets was estimated by the isotope dilution technique using tritiated water (Dove and Freer, 1979 ). Oxygen uptake of the sow was determined continuously using the Fick technique of arterio-venous difference in oxygen flow across the lungs (Giles et al., 1991 ). Continuous measurements were also made of deep body and skin temperature, and respiration rate was measured hourly. In the experiment described, two sows were exposed consecutively to ambient temperatures of 18°C, 28°C, 18°C and 25°C. All periods were for 4 days, except the second period at 18 °C which was for 6 days. Measurements were made for
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either the last 1 or 2 days at 18 oC and for the full 4 days of the high temperature treatments. The effects of ambient temperature on the association between the mean daily deep body temperature and VFI, and respiration rate are shown in Fig. 5. When ambient temperature was increased from 18 °C to 28 oC, food intake fell steeply on the first day but recovered slightly on days 3 and 4. However, when ambient temperature was raised from 18°C to 25°C, food intake fell initially to values similar to that observed at 28°C but recovered strongly • Body temperature
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Fig. 5. Effect of ambient temperature on deep body temperature and VFI (a) and on deep body temperature and respiration rate (b) in lactating primiparous sows (Lorschy et al., 1992 ).
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LACTATION IN T H E SOW D U R I N G HEAT STRESS
over the next two days. Deep body temperature followed the reverse pattern, increasing from 39.2 _ 0.1 oC to 40.6 +_0.1 oC on the first day after ambient temperature changed from 18 °C to 28 °C; it then declined steadily to reach 40.2 +_0.02°C on day 4. Although deep body temperature rose steeply to reach 40.4 +_0.1 oC on the first day of exposure to 25 oC, it returned to 39.5 _+0.1 oC on day two and remained constant thereafter. A similar close inverse relationship between deep body temperature and VFI has been observed in growing pigs (Giles and Black, 1991 ). Respiration rate followed a pattern resembling that of deep body temperature. It remained elevated at between 50 to 60 breaths/rain during the period of exposure to 28 °C but, after an initial increase, it returned to the normal rate of approximately 20 b r c a t h s / m i n by the second day at 25°C. Incomplete results were obtained for skin temperature and oxygen uptake because of damage caused by the piglets to leads connecting the recording equipment to the sensors. However, skin temperature was observed to increase from 30.5+_0.5°C when the sows were maintained at 18°C to 36.8 _+0.1 °C on the first day at 28 oC; it was then maintained at this temperaturc in spite of a reduction of 0.4 °C observed in deep body temperature over the same time (Fig 5 ). Mean oxygen consumption for sows held at 18 oC was 523 _+20.7 m l / m i n and this declined to a mean of 413 +_ 11.0 m l / m i n for the period at 28°C, with little trend over the 4 days. This observed decrease of 20% in oxygen consumption, and thus heat production, corresponding with a decrease of approximately 40% in VFI, is substantially greater than that pre-
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dicted for a lactating sow held under thermoneutral conditions. However, the fact that skin temperature was maintained while deep body temperature decreased during exposure of sows to 28 °C lends some support to the hypothesis that blood flow may have been redirected to the skin from other tissues and thus diverted nutrients from the mammary gland. Milk yield declined as ambient temperature was raised from 18 ° C to 28 ° C, but this fall was proportionally less than that observed for food intake (Fig. 6 ). During the second period at 18 ° C, milk yield increased slightly above that of the first period, probably reflecting the increase in milk yield associated with the change in stage of lactation. An increase in ambient temperature to 25 °C did not significantly alter milk yield, despite an initial decline in food intake, presumably because the rate of tissue mobilisation was sufficient to maintain milk synthesis and the sows appeared to have become acclimatised to the heat.
E C T of lactating sows Although the two levels of heat-stress were directly associated with changes in stage of lactation in the experiment described above, the results indicate that the ECT for lactating sows maintained under the conditions of the experiment was only slightly less than 25 ° C. This is surprising because the ECT of growing pigs weighing approximately 90 kg was estimated to be a similar value (Giles and Black, 1991 ), and feed intake and metabolic activity would be expected to be higher in the lactating sow. However, estimated maintenance energy requirements of 420 kJ ME/kg W °75 (ARC, 1981 ) for the growing 90 kg pig do not differ greatly from those of 440 and 460 kJ ME/kg W °75, respectively, for pregnant and lactating sows (Noblet et al., 1990). Similarly, mean oxygen consumption for sows weighing 150 kg and housed at 18 ° C in the experiment of Lorschy et al. (1992) described above was 523 + 21 ml/ min compared with a value of 511 __+83 ml/min observed by Giles and Black (1991) for growing 90 kg pigs housed at 22°C. On the days when oxygen consumption was measured, VFI was approximately 3.0 kg/day for both the lactating gilt and the growing pig. These results suggest that the heat increment of lactation is similar to that for growth, 0.65 for protein deposition and 0.8 for fat deposition (Close and Stanier, 1984), as would be expected from estimates of the efficiency of utilisation of metabolisable energy of approximately 0.7 for lactation, irrespective of whether the energy was derived from either food or tissue mobilisation (Verstegen et al., 1985; Noblet and Etienne, 1987). MAINTAINING
M I L K Y I E L D IN H O T E N V I R O N M E N T S
Milk yield of sows exposed to high temperatures can be maintained by either reducing normal heat production or increasing heat loss to the environment.
LACTATION IN THE SOW DURING HEAT STRESS
16 5
Heat production can be altered by varying the amount and composition of the diet eaten. However, a high food intake needs to be maintained to allow sustained milk yield throughout lactation. Heat loss to the environment can be increased by changing floor type, increasing air speed and wetting a greater area of skin. Changes to the environment should predominantly affect the sow and not lead to increased heat loss from the piglets because of their relatively higher LCT.
Dietary changes The response of sows to high ambient temperatures can be modified by altering the composition of the diet. In a comprehensive experiment Schoenherr et al. (1989a,b) fed lactating sows housed at either 20 or 32°C basal, high-fibre or high-fat diets. In the hot environment, increasing energy density of the diet resulted in a significant improvement in milk yield at all stages of lactation. Conversely, the addition of fibre in a hot environment depressed milk yield, and hence the weight of piglets at weaning compared to either the basal or high-fat diets. In contrast, sows consumed relatively less of a highprotein diet during summer (3.7 kg/day) than during winter (4.4 kg/day; Lynch, 1989) and might therefore have been expected to produce less milk, although this was not reported. In the experiment of Mullan et al. ( 1992 ) (Table 2 ), milk yield tended to be greater in late lactation when sows were fed a diet containing more energy and protein, although VFI of the sows was reduced. This interaction between diet and environmental temperature can be attributed to the lower heat increment of fat compared with that of protein and carbohydrate. Heat production associated with the microbial fermentation of dietary fibre in the hind-gut accounts largely for the poorer performance of sows fed high fibre diets in hot environments.
Thermal resistance properties of the floor The response of the lactating sow to high temperatures may be influenced significantly by the conductive heat properties of the floor. For example, it can be calculated using the AUSPIG model that the ECT would be reduced from 25 to 22 °C if the floor type was changed from concrete to timber. In the experiment of McGlone et al. (1988), there was a 30% difference in feed intake between sows housed at or above 29°C on either plastic-coated expanded metal or concrete slats during lactation (3.09 to 3.99 kg/day, respectively), and this was reflected in a small and non-significant increase in piglet growth rate. In an experiment over all seasons, Stansbury et al. (1987) recorded no difference in feed intake but a small reduction in weight loss for sows housed on concrete rather than plastic floors during lactation. However, piglet weaning weights were greater for those sows housed on a plastic floor, a result which demonstrates the different thermal requirements of piglets and SOWS.
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Evaporative heat loss The effects of high ambient temperatures on VFI can be overcome if the sow is able to increase evaporative heat loss via an increase in the proportion of skin that is wet. McGlone et al. ( 1988 ) reported that at ambient temperatures above 29°C, drip cooling decreased the weight loss of sows from 27 to 9 kg and improved litter weight gain from 1.47 to 1.85 kg/day during a 28day lactation. Similarly, Maxwell et al. (1990) recorded a reduction in respiration rate and an estimated increase in milk production of approximately 1 1/day for sows that were drip cooled when ambient temperature exceeded 26 ° C. Stansbury et al. ( 1987 ) and McGlone et al. ( 1988 ) have also evaluated the effect of increasing convective heat loss by sows, by provision of snout cooling systems, and both studies reported a significant increase in food intake of sows during heat stress. This is likely to result in an increase in milk yield and, provided the environment of the piglet is not compromised, an increase in the growth rate of piglets. The provision of drip cooling systems is TABLE3
Ambient temperature in relation to evaporative critical temperature (ECT) for the sow and its effect on the predicted~performance of a sow and litter during a 28-day lactation (post-partum body weight o f150 kg, fed a diet containing 13.5 MJ DE and 164 g CP per kg DM, litter size of 9, no creep feed provided ) Treatment
Temperature ( ° C) ambient ECT Sow feed intake 6 (kg/day) DE intake (MJ/day ) wt. change (kg) weaning to mating interval
(days) latent heat loss of evaporation from the skin (M J / d a y )
Thermal comfort (dry)
Proportion of wet skin
Sow2
Piglet3
15 to 304
20 22
33 26
33 25
100 s
33 21
5.26 71 + 1.8
2.61 35 -29.1
4.31 58 -9.6
5.26 71 + 1.4
5.0
19.1
9.6
5.0
10.9
10.5
16.0
25.9
152 5.64
168 6.08
193 6.79
194 6.80
Piglet
average daily gain (g) mean wt. at weaning (kg)
~Predicted by AUSPIG model (Black et al. 1986). 2Zone of thermal comfort for sows. 3Zone of thermal comfort for piglets. 4Similating the effect of drip cooling by increasing the proportion of wet skin for the sow from 15 to 30%. 5 Simulating the situation where the proportion of wet skin for the sow is up to 100%. 6Does not include feed wastage.
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167
now c o m m o n in farrowing accommodation and the effect on the sow and on piglet growth is depicted by the results of an AUSPIG simulation in Table 3. When ambient temperature is set at 20 ° C, VFI is not constrained by environmental factors and the sow consumes sufficient nutrients to maintain body weight during lactation. However, the LCT for the piglet is 29°C and at an ambient temperature of 20°C, even though some supplemental heating is supplied, the growth rate of the piglet is less than the 194 g/day that is achievable if the piglet is housed within its zone of thermal comfort. An ambient temperature of 33 °C is more suitable for the piglet but this is too high for the sow with the result that VFI is depressed, body reserves are mobilised and the interval between weaning and re-mating is extended. Piglet growth rate is only marginally improved even though the piglet is now accommodated within its zone of thermal comfort, because milk yield of the sow has been reduced due to the low intake of nutrients by the sow. The effect on piglet growth rate in this example (from 209 to 181 g / d a y ) is of similar magnitude to the results in Table l and the combined data of Lynch (1977), Stansbury et al. (1987), Yen and Cheng ( 1990 ) and Barb et al. ( 1991 ). When sows had access to drip cooling in this simulation, despite ambient temperature remaining at 33 °C they were able to increase the proportion of their skin that was wet and hence increase the amount of heat loss via evaporation. Although VFI was not sufficient to provide all of the substrates required for milk production, these were adequately supplied from body reserves with minimal effects on reproductive performance. First-parity sows in the commercial piggery referred to in Fig. 1 had access to a drip cooling system and consumed 4.2 kg/day, lost 8 kg of body weight during a 28-day lactation and recorded piglet growth rates of 170 g/day, a result similar to that simulated by AUSPIG. The final treatment in Table 3 demonstrates the extent to which heat loss via evaporation must increase for VFI, and hence reproductive performance, not to be affected by high ambient temperature. CONCLUSIONS
Experimental evidence presented in this paper indicates that the depression in milk yield associated with a decline in food intake may be greater in sows exposed to environmental temperatures above their ECT than in sows housed under thermoneutral conditions. It is suggested that this observation may be due partially to a redistribution of blood flow to the skin, away from other tissues such as the m a m m a r y gland. Results also indicate that the depression in heat production associated with a reduction in food intake is less in lactating sows than in non-reproducing animals because of the capacity of the sow to mobilise large quantities of body tissue to maintain high rates of milk synthesis. However, when both lactating and non-lactating sows are fed ad lib. under thermoneutral conditions, heat production and ECT appear
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to be r e l a t i v e l y similar. T h e d e t r i m e n t a l effects o n m i l k p r o d u c t i o n o f sows e x p o s e d to high t e m p e r a t u r e s c a n b e r e d u c e d b y i n c r e a s i n g the energetic efficiency o f f o o d use t h r o u g h e i t h e r i n c r e a s i n g the p r o p o r t i o n o f fat or decreasing the p r o p o r t i o n o f fibre in the diet. H o w e v e r , the ability to h a v e m a r k e d effects t h r o u g h diet c h a n g e s are s m a l l c o m p a r e d w i t h t h o s e t h a t c a n be a c h i e v e d b y i n c r e a s i n g h e a t loss to the e n v i r o n m e n t , p a r t i c u l a r l y b y selecting floors w i t h low t h e r m a l r e s i s t a n c e a n d b y i n c r e a s i n g e v a p o r a t i v e h e a t loss t h r o u g h d r i p cooling.
REFERENCES ARC, 1981. The Nutrient Requirements of Pigs. (Agricultural Research Council), Commonwealth Agricultural Bureau, Farnham Royal. Alexander, G, Hales, R.J.S., Stevens, D. and Donnelly, J.B., 1987. Effect of acute and prolonged exposure to heat on regional blood flows in pregnant sheep. J. Devel. Physiol., 9:1-15, Barb, C.R., Estienne, M.J., Kraeling, R.R., Marple, D.N., Rampacek, G.B., Rahe, C.H. and Sartin, J.L., 1991. Endocrine changes in sows exposed to elevated ambient temperature during lactation. Domest. Anim. Endo., 8:117-127. Black, J.L., Campbell, R.G., Williams, I.H., James, K.J. and Davies, G.T., 1986. Simulation of energy and amino acid utilisation in the pig. Res. Devel. Agric., 3: 121-145. Brendemuhl, J.H., Lewis, A.J. and Peo, E.R., 1987. Effect of protein and energy intake by primiparous sows during lactation on sow and litter performance and sow serum thyroxine and urea concentrations. J. Anim. Sci., 64: 1060-1069. Brendemuhl, J.H., Lewis, A.J. and Peo, E.R., 1989. Influence of energy and protein intake during lactation on body composition of primiparous sows. J. An im. Sci., 67:1478-1488. Clark, J.R., Komkov, A. and Tribble, L.F., 1986. Effects of parity, season, gonadotrophin releasing hormone and altered suckling intensity on the interval to rebreeding in sows. Theriogenology, 26: 299-308. Close, W.H. and Stanier, M.W., 1984. Effect of plane of nutrition and environmental temperature on the growth and development of the early-weaned piglet. 2. Energy metabolism. Anim. Prod., 38: 221-231. Cole, D.J.A., 1990. Nutritional strategies to optimize reproduction in pigs. In: D.J.A. Cole, G R. Foxcroft and B.J. Weir (Editors), Control of Pig Reproduction III. J. Reprod. Fert, Ltd: Cambridge., pp. 67-82. Cox, N.M., Britt, J.H., Armstrong, W.D. and Alhusen, H.D., 1983. Effect of feeding fat and altering weaning schedule on rebreeding in primiparous sows. J. Anim. Sci., 56:21-29. Dove, H. and Freer, M., 1979. The accuracy of tritiated water turnover rate as an estimate of milk intake in lambs. Aust. J. Agric. Res., 30: 725-739. Fernandes, L.C.O., Altafini, A.C., Kunert, H.C. and Carnino, F., 1990. Summer feed intake and weight loss of primiparous and multiparous sows associated with weaning to estrus interval. Proc. 1 lth Int. Pig Vet. Soc. 412 (Abs). Giles, L.R. and Black, J.L., 1991. Voluntary food intake in growing pigs at ambient temperatures above thermal neutral. In: E.S. Batterham (Editor), Manipulating Pig Production III. Aust. Pig Sci. Assoc., Werribee,pp. 162-165. Giles, L.R., Black, J.L., Gooden, J.M. and Annison, E.F., 1991. Energy expenditure of growing pigs maintained at high ambient temperature. In: C. Wenk and M. Boessinger (Editors), Energy Metabolism in Farm Animals, 12th EAAP Symposium on Energy Metabolism of Farm Animals, Switzerland, pp. 52-55.
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Hoppe, M.K., Libal, G.W. and Wahlstrom, R.C., 1990. Influence of gestation energy level on the production of Large White × Landrace sows. J. Anim. Sci., 68: 2235-2242. Johnson, H.D., Ragsdale, A.C., Berry, I.L. Wayman, O. and Merilan, C.P., 1966. University of Missouri, Exper. Stn. Res. Bull. No. 902. (Cited by Webster, 1974). King, R.H. and Dunkin, A.C., 1986a. The effect of nutrition on the reproductive performance of first-litter sows. 3. The response to graded increases in food intake during lactation. Anim. Prod., 42:119-125. King, R.H. and Dunkin, A.C., 1986b. The effect of nutrition on the reproductive performance of first-litter sows. 4. The relative effects of energy and protein intakes during lactation on the performance of sows and their litters. Anim. Prod., 43:319-325. King, R.H., Toner, M.S. and Dove, H., 1989. Pattern of milk production in sows. In: J.L. Barnett and D.P. Hennessy (Editors), Manipulating Pig Production II. Aust. Pig Sci. Assoc., Werribee, p 98 (Abs) King, R.H. and Williams, I.H. 1984a. The effect of nutrition on the reproductive performance of first-litter sows. 1. Feeding level during lactation, and between weaning and mating. Anim. Prod., 38: 241-247. King, R.H. and Williams, I.H., 1984b. The effect of nutrition on the reproductive performance of first-litter sows. 2. Protein and energy intakes during lactation. Anim. Prod., 38:241-247. Lorschy, M.L., Giles, L.R., Black, J.L. and Gooden, J.M. 1992. Effect of high ambient temperature on voluntary food intake, body temperature and milk production in the lactating sow. Proc. Nutr. Soc. Aust., 17:216 (Abs). Lynch, P.B., 1977. Effect of environmental temperature on lactating sows and their litters. Irish J. Agric. Res., 16: 123-130. Lynch, P.B., 1989. Voluntary food intake of sows and gilts. In: A.T. Smith and T.L.J. Lawrence (Editors), Pig Housing and the Environment. Brit. Soc. Anim. Prod., Edinburgh, pp. 7177. Maxwell, R.C., Cutler, R.S. and Callinan, A.P.L., 1990. Drip cooling lactating sows improves performance and comfort. Proc. 1 lth IPVS. 396. McGlone, J.J., Stansbury, W.F. and Tribble, L.F., 1988. Management of lactating sows during heat stress: Effects of water drip, snout coolers, floor type and a high energy-density diet. J. Anim. Sci., 66: 885-891. Mullan, B.P., 1991. The response of the breeding sow to the climatic environment. In: E.S. Batterham (Editor), Manipulating Pig Production III. Aust. Pig Sci. Assoc., Werribee, pp.167-177. Mullan, B.P., Brown, W. and Kerr, M. 1992. The response of the lactating sow to ambient temperature. Proc. Nutr. Soc. Aust., 17:215 (Abs). Mullah, B.P. and Close, W.H., 1989. The partition of nutrients during lactation and its relationship to reproductive performance. In: J.L. Barnett and D.P. Hennessy (Editors), Manipulating Pig Production II. Aust. Pig Sci. Assoc., Werribee, p.302 (Abs). Mullan, B.P., Close, W.H. and Foxcroft, G.R., 199 I. Metabolic state of the lactating sow influences plasma LH and FSH before and after weaning. In: E.S. Batterham (Editor), Manipulating Pig Production III. Aust. Pig Sci. Assoc., Werribee, p.31 (Abs). Mullah, B.P. and Williams, I.H., 1989. The effect of body reserves at farrowing on the reproductive performance of first-litter sows. Anim. Prod., 48: 449-457. Nelssen, J.L., Lewis, A.J., Peo, J.R. and Crenshaw, J.D., 1985. Effect of dietary energy intake during lactation on performance of primiparous sows and their litters. J. Anim. Sci., 6 l: 1164-1171. Nobler, J. and Etienne, M., 1987. Metabolic utilization of energy and maintenance requirements in lactating sows. J. Anim. Sci., 64: 774-781. Noblet, J., Dourmad, J.Y. and Etienne, M., 1990. Energy utilisation in pregnant and lactating sows: modelling of energy requirements. J. Anim. Sci., 68: 562-572.
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O'Grady, J.F., Elsley, F.W.H., MacPherson, R.M. and McDonald, 1., 1973. The response of lactating sows and their litters to different dietary energy allowances. 1. Milk yield and composition, reproductive performance of sows and growth rate of litters. Anim. Prod., 17: 6574. O'Grady, J.F., Lynch, P.B. and Kearney, P.A., 1985. Voluntary feed intake by lactating sows. Livest. Prod. Sci., 12: 355-365. Reese, D.E., Moser, B.D., Peo, E.R., Lewis, A.J., Zimmerman, D.R., Kinder, J.E. and Stroup, W.W., 1982. Influence of energy intake during lactation on the interval from weaning to first estrus in sows. J. Anim. Sci., 55: 590-598. Schoenherr, W.D., Stably, T.S. and Cromwell, G.L., 1989a. The effects of dietary fat and fibre addition on energy and nitrogen digestibility in lactating, primiparous sows housed in a warm or hot environment. J. Anim. Sci., 67:473-481. Schoenherr, W.D., Stahly, T.S. and Cromwell, G.L., 1989b. The effects of dietary fat or fibre addition on yield and composition of milk from sows housed in a warm or hot environment. J. Anim. Sci., 67: 482-495. Stansbury, W.F., McGlone, J.J. and Tribble, L.F., 1987. Effects of season, floor type, air temperature and snout cooling on sow and litter performance. J. Anim. Sci., 65:1507-1513. Verstegen, M.W.A., Mesu, J., van Kempen, G.J.M. and Geerse, C., 1985. Energy balances of lactating sows in relation to feeding level and stage of lactation. J. Anim. Sci., 60:731-740. Vidal, J.M., Edwards, S.A., MacPherson, O., English, P.R. and Taylor, A.G., 1991. Effect of environmentaltemperature on dietary selection in lactating sows. Anim. Prod., 52:597 (Abs). Webster, A.J.F., 1974. Heat loss from cattle with particular emphasis on the effects of cold. In: J.L Momteith and L.E. Mount (Editors), Heat Loss from Man and Animals. Assessment and Control.Butterworths, London,pp.205-321. Yen, H.T. and Cheng, C.S., 1990. Effects of season on feed intake of sows during lactation in Taiwan. In: D.J.A. Cole, G R. Foxcroft and B.J. Weir (Editors), Control of Pig Reproduction III, J. Reprod. Fen. Ltd, Cambridge, 101 (Abs).
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Elsevier Science Publishers B.V., Amsterdam
Lactation in the sow during heat stress
J.L. Black a, B.P. Mullan b, M.L. L o r s c h y c a n d L . R . G i l e s c RCSIRO, Division of Animal Production, P.O Box 239, Blacktown, NSW,, Australia, bDepartment of Food and Agriculture, Victorian Institute of Animal Science, Werribee, Vic., Australia, CDepartment of Animal Science, Universityof Sydney, Camden, NSW, Australia (Accepted 1 December 1992)
ABSTRACT Ambient temperatures above the evaporative critical temperature (ECT) of lactating sows lead to a reduction in food intake, milk yield, reproductive performance and growth rate of piglets. The fall in food intake of the lactating sow is closely associated with a rise in deep body temperature as is also observed in growing pigs. Evidence is presented indicating that the reduction in milk yield of sows exposed to high temperatures may be greater than would be expected from an equivalent decline in food intake for sows housed under thermoneutral conditions. It is suggested that the direct effect of high temperatures on milk yield may result from a redirection of blood flow to the skin and away from other tissues, including the mammary gland. Experimental observations showing that skin temperalure is maintained while deep body temperature declines after initially rising rapidly in lactating sows exposed to 28°C for four days lends indirect support to this theory. Oxygen uptake of lactating sows was observed to decrease from 523 to 411 ml/min when ambient temperature was increased from 18 to 28°C. This decline of 20% in heat production was associated with a 25% decline in milk yield and a 40% reduction in food intake. The oxygen uptake of lactating sows was found to be similar to that of non-breeding animals with an equivalent food intake. Consequently, ECT for lactating and nonlactating animals was found to be similar. Performance of lactating sows exposed to high temperatures can be improved by reducing the animal's heat production through decreasing the fibre and increasing the fat content of the diet. However, increasing heat loss from the sow, particularly through increasing the area of wet skin, has a greater positive effect on animal performance than modifying the diet.
Keywords: lactating sow, heat-stress, food intake, milk yield.
INTRODUCTION
The inability o f sows to consume sufficient food to meet the nutrient requirements o f lactation is a c o m m o n problem in commercial pig production Correspondence to: J.L. Black, CSIRO, Division of Animal Production, P.O Box 239, Blacktown, New South Wales, 2148, Australia.
0301-6226/93/$06.00
© 1993 Elsevier Science Publishers B.V. All rights reserved.
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(Cole, 1990). Sows mobilise body reserves to support milk production when nutrient intake during lactation is low and, if the loss of body reserves is excessive, especially with young sows, piglet growth may be reduced and reproductive performance impaired (King and Williams, 1984a; Mullan and Williams, 1989 ). High environmental temperature is one of a number of factors that may reduce the voluntary food intake (VFI) of the lactating sow (O'Grady et al., 1985 ). Ambient temperatures are high in many areas of the world where pigs are raised and, even in situations where piggery temperatures are controlled, the wide difference in the optimum temperature requirements between the lactating sow and the piglet often leads to heat stress in the sow. The objectives of this paper are to review research describing the effects of high temperature on VFI, milk yield and reproductive performance of sows, to examine evidence suggesting that the effect of depressed food intake on milk yield may be greater in heat-stressed than in normal sows, to present experimental results on physiological responses of sows to high temperatures and to discuss ways of maintaining the performance of sows exposed to high temperatures.
EFFECT OF HIGH TEMPERATURE ON VFI AND PERFORMANCE OF SOWS
Definition of heat exposure Animals are considered to be exposed to heat when ambient temperature is above the zone of thermal comfort and energy is expended to maintain body temperature. The zone of thermal comfort covers the range of temperatures between the animal's lower critical temperature (LCT) and its evaporative critical temperature (ECT). LCT is the temperature below which the animal must increase heat production through shivering and other metabolic processes to maintain body temperature. ECT in the pig signifies the temperature at which evaporative heat loss begins to increase, particularly from the lungs, through increased respiration (Black et al., 1986). ECT marks also the ambient temperature near which the growing pig commences to reduce VFI (Giles and Black, 1991 ). The ambient temperatures coinciding with LCT and ECT are not constant for an animal, but are affected by the heat produced through normal metabolic processes and by environmental factors such as air speed, humidity, floor type, radiant heat incidence, number of animals in a group and the area of wet skin. The zones of thermal comfort for the lactating sow and piglet differ markedly; between 12 and 22 °C for the sow and 30 and 37°C for the piglet (Mullan, unpublished). This difference presents an important challenge to the piggery manager when attempting to maintain production efficiency of both the sow and piglets.
LACTATION IN THE SOW DURING HEAT STRESS
155
40
55
%~
3O
25
~
20
15
10
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Day of t h e m o n t h Fig. 1. A m b i e n t t e m p e r a t u r e s i n s i d e a f a r r o w i n g s h e d - S h e p p a r t o n , Victoria, Australia; February 1991 ( M u l l a n , 1991 ).
Effect of high temperatures in commercial piggeries There is unfortunately a paucity of published information from commercial piggeries relating environmental temperature in farrowing houses to animal performance. In a study by Cox et al. (1983) at a commercial piggery in North America, no difference was observed in total litter weight at weaning or weight loss of the sow during lactation between summer and winter but the time between weaning and oestrus was longer during summer. However, food intake was not measured and the difference in ambient temperatures between summer and winter was less than 3°C. Alternatively, O'Grady et al. ( 1985 ) observed lower intakes in sows during summer than during winter but no animal performance characteristics were reported. The extent to which high ambient temperatures may affect the performance of sows in Australian piggeries can be gauged from measurements taken in a commercial piggery during summer (Fig. 1 ). Temperature sensors were positioned approximately 1 m above floor level and away from the influence of radiant heaters. Mean temperature at sow level for the month of February was 26 °C (range of 17 to 39 °C) and hourly values were frequently greater than 22 °C which was estimated to coincide approximately with the animal's ECT.
Experimental evidencefor effects of high temperatures Food intake and milk yield. The effect of an increase in ambient temperature on voluntary energy intake during lactation has been investigated in several experiments (Fig. 2). Results from these 9 experiments indicate that, for each
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J.L BLACKET AL. 120
100 ¢1
80 v
60
40
20
-15
i
i
t
i
20
25
30
35
T e m p e r a t u r e (°C) Fig. 2. Effect of ambient temperature (x) on the voluntary digestible energy intake (DE, y) of lactating primiparous sows (Barb et al., 1991; Cole, 1990; Lynch, 1977; Lynch, 1989; Schoenherr et al., 1989a,b; Stansbury et al., 1987; Vidal et al., 1991; Yen and Cheng, 1990). Mean relationship is given by the bold line (y = 126.6 - 2.4x ).
1 oC increase in ambient temperature above 16 ° C, daily voluntary energy and food intake decline by 2.4 MJ DE and 0.17 kg, respectively. However, only one of these experiments included an intermediate temperature and, as suggested by this result, it is unlikely that the relationship between ambient temperature and food intake is linear over the range of temperatures presented. Rather, as eluded to by Giles and Black ( 1991 ), the decrease in intake may depend on the extent to which ambient temperature exceeds the animal's ECT. The reduction in the growth rate of piglets suckling sows maintained at high temperatures has been assumed to reflect a reduction in milk yield. Schoenherr et al. (1989b) and Vidal et al. (1991) recorded decreases in milk yield of 10 and 35% and associated declines in piglet growth rate when ambient temperature was increased by 10°C and 8°C, respectively (Table 1 ). Milk yield of sows exposed to high temperatures and represented in Fig. 2 can be calculated using the relationship of King et al. ( 1989 ) between piglet growth rate and milk intake for all those experiments in which piglet growth rate was available. Although there is considerable variation between experiments in the response of milk yield to energy intake (Fig. 3 ), there is a consistent decline in milk yield in the heat-stressed animals that had a reduced food intake. However, it is not known whether this reflects a direct effect of high temperature on milk synthesis and/or whether the decrease in the supply of substrates due to the low VFI of the sow is responsible for the decrease in milk production.
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LACTATIONIN THE SOWDURINGHEATSTRESS TABLE 1 The effect of ambient temperature on voluntary food intake and milk production of the sow Reference 1
Ambient temperature (°C) Length of lactation (days) Sow feed intake (kg/day) change in body weight (kg) change in back fat (mm) milk yield (kg/day) Piglet average daily gain (g) litter weight gain (kg/day)
1
20 22
2
30 22
2
22 27
30 27
5.90 - 2.6 8.34
3.36 - 15.9 7.47
7.72 -6.4 +2.2 10.27
4.95 -21.0 -3.0 6.64
206 1.88
182 1.64
226 2.21
167 1.53
1. Schoenherr et al. (1989b). 2. Vidal et al. (1991).
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Fig. 3. Influence of digestible energy intake (DE, x) on milk production (y) of primiparous sows when energy intake is determined by ambient temperature (Barb et al., 1991; Lynch, 1977; Schoenherr et al., 1989b; Stansbury et al., 1987; Vidal et al., 1991; Yen and Cheng, 1990). Mean relationship is given by the bold line (y = 4.08 + 0.05x).
Reproductive performance. T h e
effect of high ambient temperatures on VFI also has important consequences for reproductive performance. Primiparous sows lactating during the summer have a longer interval between weaning and m a t i n g t h a n d o t h o s e d u r i n g w i n t e r ( C o x et al., 1983; C l a r k et al., 1 9 8 6 ) . Season does not, however, appear to have the same effect with multiparous
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sows (Clark et al., 1986; Fernandes et al., 1990) and it has been suggested (Fernandes et al., 1990) that this is because the younger animal mobilises a greater proportion of its more limited body reserves if VFI is low during lactation. There is conflicting evidence, however, as to the mechanism by which season influences reproductive function. Evidence from primiparous sows suggests that high ambient temperatures during lactation cause a decrease in luteinising hormone (LH) pulse frequency and that this is responsible for the delay in rebreeding after weaning (Barb et al., 1991 ). This may explain the result of Cox et al. (1983) who observed no difference in the weight loss of sows during lactation between winter and summer but the weaning-to-oestrus interval increased from I 0 to 23 days. However, there is also evidence that low nutrient intakes in primiparous sows, housed under standard conditions during lactation cause an increase in the weaning-to-mating interval (King and Williams, 1984a; Mullan and Williams, 1989 ) and that this is due partially to a disruption of the normal secretory pattern of LH (Mullan et al., 1991 ). Hence, low nutrient intake and not high ambient temperatures p e r se may be the cause of poor fertility. Feed intake in the experiment of Barb et al. ( 1991 ) was reduced from 6.1 to 2.9 kg/day due to the effect of high ambient temperatures. ENERGY INTAKE - MILK YIELD RELATIONSHIP IN HEAT-EXPOSED SOWS
Limited evidence suggests that the decline in milk yield may be greater in heat-exposed animals than can be accounted for directly by the associated decline in energy intake. Webster (1974) cites evidence from Johnson et al. (1966) showing that milk yield in dairy cattle exposed to hot environments remained depressed despite feeding to thermoneutral intake through a rumen fistula. To examine whether such an effect is apparent in sows, a comparison was made between experiments in which feed intake was deliberately restricted and those in which it was depressed through exposure to high temperatures (Fig. 4 ). Although there was a large variation between experiments in the response of milk yield to energy intake when it was controlled by the amount of food offered, the mean slope of the relationship differs from that found in Fig. 3 for heat-exposed sows. The decline in milk yield associated with a reduction in energy intake appears to be greater in the heat-exposed animals than in sows housed under lower temperature conditions. Despite the variation between experiments, this comparison adds some support to the theory that there is a direct effect of high ambient temperature on milk production of sows. Reduced food intake is a primary mechanism used by animals to limit heat production when they are exposed to high temperatures. However, this strategy may be less effective during lactation than at other times. Sows housed under normal temperature conditions have the capacity to maintain high rates
LACTATION
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120
DE intake (MJ / d a y ) Fig. 4. Influence of digestible energy intake (DE, x) on milk production (y) of primiparous sows offered different amounts of food during lactation (Brendemuhl et al., 1987; Brcndemuhl et al., 1989; Hoppe et al., 1990; King and Dunkin, 1986a,b; King and Williams, 1984a,b; Mullan and Close, 1989; Mullan and Williams, 1989; Nelssen et al., 1985; Noblet and Etienne, 1987; O'Grady et al., 1973; Rcesc et al., 1982). Mean relationship is given by the bold line (y = 6.62 + 0.01 x) and mean relationship for animals exposed to high ambient temperatures is given by the bold dashed line (from Fig. 3 ).
of milk synthesis, particularly in early lactation, despite low feed intakes, through the mobilisation of body tissue. For example, in the experiment of King and Dunkin (1986a), lactating sows were fed between 19 and 60 MJ DE/day with little effect on piglet growth rate during the first 3 weeks of lactation. This was because sows mobilised maternal body reserves of fat and protein to support milk production and it was only during the fourth week of lactation, when maternal reserves had been depleted substantially, that milk yield was affected by the low energy intake. Thus, in comparison with nonlactating animals, a reduction in energy intake would be expected to have little effect on the heat produced by metabolic processes. There is little experimental evidence to support this contention, but predictions from the AUSPIG model (Black et al., 1986), which are based largely on current understanding of energy metabolism in pigs, indicate that the depression in heat production resulting from a 25% reduction in food intake from ad lib. is approximately 2% in a 150 kg lactating sow compared with nearly 18% in a nonlactating animal of the same weight and body composition. Lactating animals may, therefore, have special mechanisms for reducing milk output when suffering from heat exposure. One possibility may be an increase in blood flow to the skin to assist heat
160
J.L. BLACKET AL.
loss, at the expense of blood flow to the mammary gland and other organs. Such a mechanism is known to occur with heat-stressed pregnant sheep, where there is a marked diversion of blood flow from the conceptus to the skin that results in a depression in conceptus growth and in low birth weight lambs (Alexander et al., 1987 ). An experiment has recently been conducted to further evaluate the hypothesis that high temperature has a direct effect on milk production, in addition to its indirect effect through reduced energy intake (Mullah et al., 1992). Groups of 6 primiparous sows with litters standardised to 8 piglets were housed at either 20°C or 30°C throughout a 28-day lactation. One group at each temperature was offered ad lib. a diet containing 14.3 MJ DE and 160 g TABLE 2 The effect of ambient temperature on voluntary food intake and milk production of first-parity sows during a 28-day lactation Treatment Temperature ( ° C): Feeding level: Diet: No. of sows Sow post farrowing wt. (kg) backfat 3 ( m m ) change during lactation wt. (kg) backfat 3 ( m m ) food intake (kg/day) Piglets total born total born alive number weaned birth wt. (kg) wean wt. (kg) average daily gain (g) day 0 to 14 day 14 to 28 day 0 to 28 Milk yield (kg/day) 4 day 0 to 14 day 14 to 28 day 0 to 28
20 ad-lib, control ~ 6
20 restricted control ~ 6
30 ad-lib, control ~ 6
30 ad-lib. high-spec 2 6
LSD
145.8 24.9
147.2 25.3
145.3 22.9
151.2 22.9
14.6 3.9
-9.6 - 5.6 4.05
- 19.0 - 5.5 3.16
- 16.0 - 4.6 3.16
-21.8 - 3.4 2.95
14.4 3.0 0.79
8.2 7.3 7.8 1.51 8,18
9.8 9.8 7.8 1.30 7.63
9.3 8.2 8.0 1.50 7,44
7.4 7.4 7.8 1.46 7.32
2.7 3.2 0.95 0.23 1.61
235 248 241 8.88 9,54 9.21
211 245 228 7.86 9.47 8.67
202 209 206 7,53 7,83 7.68
Formulated to contain 14.3 MJ DE, 160 g CP and 0.50 g AvLys per kg DM. 2Formulated to contain 15.2 MJ DE 210 g CP and 0.80 g AvLys per kg DM. 3Depth ofbackfat measured by ultrasound at the P2. 4Calculated according to King et al. (1989).
205 225 215 7.54 8.30 7.92
51 67 53 2.11 2.88 2.18
LACTATION IN THE SOW D U R I N G HEAT STRESS
161
crude protein per kg food dry matter. A second group at 20°C was offered the same a m o u n t of food as was eaten by the sows at 30°C. In addition, a second group at 30°C was offered ad lib. a diet containing 15.2 MJ DE and 210 g crude protein per kg food dry matter. All sows entered the experimental facility on day 105 of gestation and supplementary heat was provided for piglets in a creep area designed to minimise the influence of radiant heat on the sow. Feed intake was depressed by approximately 25% and milk yield by 15% for the sows housed at 30°C compared with those housed at 20°C. Although the difference in milk yield and piglet growth rate was not significant during either early or late lactation for the pair-fed sows housed at the two temperatures, there was a strong indication that high ambient temperature was having a direct effect on milk yield (Table 2 ). The average birth weight of piglets from the sows pair-fed at 20°C was significantly less than for the piglets held at 30 °C and this may have contributed to the lack of significance in the initial phase of lactation. Increasing the protein content of the diet offered to sows maintained at 30°C had no significant effect on either feed intake or milk yield. The experiment is continuing with additional sows. The results of this experiment support the observations with lactating cattle and suggest that the depression in milk yield resulting from a decline in food intake is greater in heat-stressed animals than in those held under thermoneutral conditions. It is important, therefore, to examine the effects of heat exposure on the physiological responses and heat production of the lactating SOW. PHYSIOLOGICAL RESPONSES OF LACTATING SOWS TO HEAT EXPOSURE
The effects of high ambient temperatures on food intake, milk yield, oxygen consumption and several physiological characteristics of lactating sows have been investigated (Lorschy et al., 1992 ). Primiparous sows with litters standardised to 7 piglets were housed in a temperature controlled room with air speed of less than 0.1 m / s e c and relative humidity of less than 50%. The animals were held in farrowing crates with slatted plastic floors and drinking water was provided in a way that prevented the sow from wetting her skin. The sows were offered ad lib. a commercial diet containing 14.3 MJ DE and 160 g crude protein per kg dry matter. Milk intake of the piglets was estimated by the isotope dilution technique using tritiated water (Dove and Freer, 1979 ). Oxygen uptake of the sow was determined continuously using the Fick technique of arterio-venous difference in oxygen flow across the lungs (Giles et al., 1991 ). Continuous measurements were also made of deep body and skin temperature, and respiration rate was measured hourly. In the experiment described, two sows were exposed consecutively to ambient temperatures of 18°C, 28°C, 18°C and 25°C. All periods were for 4 days, except the second period at 18 °C which was for 6 days. Measurements were made for
162
J . L S L A C K ET A L
either the last 1 or 2 days at 18 oC and for the full 4 days of the high temperature treatments. The effects of ambient temperature on the association between the mean daily deep body temperature and VFI, and respiration rate are shown in Fig. 5. When ambient temperature was increased from 18 °C to 28 oC, food intake fell steeply on the first day but recovered slightly on days 3 and 4. However, when ambient temperature was raised from 18°C to 25°C, food intake fell initially to values similar to that observed at 28°C but recovered strongly • Body temperature
(a)
o Voluntary food intake
41.0
o
4000
Q
o
5500
o
8"
40,5
J
40.0
3000
o v
2500
L,
2000
i
E o m
39.5
7,
1500
>
1000 39.0
Ambient temperature
(°C)
500 18 38.5
28
i
14
18
i
i
16
t8
r
!
20
22
25 i
i
24
0
i
26
28
Days p o s t p a r t u m Body o
41.0
temperature
Respiration
rate
60
(b) 50
40.5
o
40
40.0 Q)
30 ,,,
39.5
o~
,,, 20
o m
Ambient temperature
(°C)
rv
39,0
10 18
38.5
i
14
I
i
16
25
1B
28
i
18
i
i
20
22
i
24
t
r
26
i
0
28
Days postpartum
Fig. 5. Effect of ambient temperature on deep body temperature and VFI (a) and on deep body temperature and respiration rate (b) in lactating primiparous sows (Lorschy et al., 1992 ).
163
LACTATION IN T H E SOW D U R I N G HEAT STRESS
over the next two days. Deep body temperature followed the reverse pattern, increasing from 39.2 _ 0.1 oC to 40.6 +_0.1 oC on the first day after ambient temperature changed from 18 °C to 28 °C; it then declined steadily to reach 40.2 +_0.02°C on day 4. Although deep body temperature rose steeply to reach 40.4 +_0.1 oC on the first day of exposure to 25 oC, it returned to 39.5 _+0.1 oC on day two and remained constant thereafter. A similar close inverse relationship between deep body temperature and VFI has been observed in growing pigs (Giles and Black, 1991 ). Respiration rate followed a pattern resembling that of deep body temperature. It remained elevated at between 50 to 60 breaths/rain during the period of exposure to 28 °C but, after an initial increase, it returned to the normal rate of approximately 20 b r c a t h s / m i n by the second day at 25°C. Incomplete results were obtained for skin temperature and oxygen uptake because of damage caused by the piglets to leads connecting the recording equipment to the sensors. However, skin temperature was observed to increase from 30.5+_0.5°C when the sows were maintained at 18°C to 36.8 _+0.1 °C on the first day at 28 oC; it was then maintained at this temperaturc in spite of a reduction of 0.4 °C observed in deep body temperature over the same time (Fig 5 ). Mean oxygen consumption for sows held at 18 oC was 523 _+20.7 m l / m i n and this declined to a mean of 413 +_ 11.0 m l / m i n for the period at 28°C, with little trend over the 4 days. This observed decrease of 20% in oxygen consumption, and thus heat production, corresponding with a decrease of approximately 40% in VFI, is substantially greater than that pre-
1.8
Estimated
milk yield
Voluntary
food intake
1.8
o
1.6
1.6
.~- -~
1.4.
1.4
*~ ~
~.2
1,2
~
~
~.o
1.0
~
~
o.8
~~
"~ :2,
o.6
r. i~
0.4 0.2 0.0
18
28 Temperature
18 treatment
1
0.8 0.6 0.4 0.2 0.0
25
(°C)
Fig. 6. Effect of ambient temperature on VFI and milk yield relative to values during the first period at 18 ° C for lactating primiparous sows (Lorschy et al., 1992 ).
164
J.L. BLACK ET AL.
dicted for a lactating sow held under thermoneutral conditions. However, the fact that skin temperature was maintained while deep body temperature decreased during exposure of sows to 28 °C lends some support to the hypothesis that blood flow may have been redirected to the skin from other tissues and thus diverted nutrients from the mammary gland. Milk yield declined as ambient temperature was raised from 18 ° C to 28 ° C, but this fall was proportionally less than that observed for food intake (Fig. 6 ). During the second period at 18 ° C, milk yield increased slightly above that of the first period, probably reflecting the increase in milk yield associated with the change in stage of lactation. An increase in ambient temperature to 25 °C did not significantly alter milk yield, despite an initial decline in food intake, presumably because the rate of tissue mobilisation was sufficient to maintain milk synthesis and the sows appeared to have become acclimatised to the heat.
E C T of lactating sows Although the two levels of heat-stress were directly associated with changes in stage of lactation in the experiment described above, the results indicate that the ECT for lactating sows maintained under the conditions of the experiment was only slightly less than 25 ° C. This is surprising because the ECT of growing pigs weighing approximately 90 kg was estimated to be a similar value (Giles and Black, 1991 ), and feed intake and metabolic activity would be expected to be higher in the lactating sow. However, estimated maintenance energy requirements of 420 kJ ME/kg W °75 (ARC, 1981 ) for the growing 90 kg pig do not differ greatly from those of 440 and 460 kJ ME/kg W °75, respectively, for pregnant and lactating sows (Noblet et al., 1990). Similarly, mean oxygen consumption for sows weighing 150 kg and housed at 18 ° C in the experiment of Lorschy et al. (1992) described above was 523 + 21 ml/ min compared with a value of 511 __+83 ml/min observed by Giles and Black (1991) for growing 90 kg pigs housed at 22°C. On the days when oxygen consumption was measured, VFI was approximately 3.0 kg/day for both the lactating gilt and the growing pig. These results suggest that the heat increment of lactation is similar to that for growth, 0.65 for protein deposition and 0.8 for fat deposition (Close and Stanier, 1984), as would be expected from estimates of the efficiency of utilisation of metabolisable energy of approximately 0.7 for lactation, irrespective of whether the energy was derived from either food or tissue mobilisation (Verstegen et al., 1985; Noblet and Etienne, 1987). MAINTAINING
M I L K Y I E L D IN H O T E N V I R O N M E N T S
Milk yield of sows exposed to high temperatures can be maintained by either reducing normal heat production or increasing heat loss to the environment.
LACTATION IN THE SOW DURING HEAT STRESS
16 5
Heat production can be altered by varying the amount and composition of the diet eaten. However, a high food intake needs to be maintained to allow sustained milk yield throughout lactation. Heat loss to the environment can be increased by changing floor type, increasing air speed and wetting a greater area of skin. Changes to the environment should predominantly affect the sow and not lead to increased heat loss from the piglets because of their relatively higher LCT.
Dietary changes The response of sows to high ambient temperatures can be modified by altering the composition of the diet. In a comprehensive experiment Schoenherr et al. (1989a,b) fed lactating sows housed at either 20 or 32°C basal, high-fibre or high-fat diets. In the hot environment, increasing energy density of the diet resulted in a significant improvement in milk yield at all stages of lactation. Conversely, the addition of fibre in a hot environment depressed milk yield, and hence the weight of piglets at weaning compared to either the basal or high-fat diets. In contrast, sows consumed relatively less of a highprotein diet during summer (3.7 kg/day) than during winter (4.4 kg/day; Lynch, 1989) and might therefore have been expected to produce less milk, although this was not reported. In the experiment of Mullan et al. ( 1992 ) (Table 2 ), milk yield tended to be greater in late lactation when sows were fed a diet containing more energy and protein, although VFI of the sows was reduced. This interaction between diet and environmental temperature can be attributed to the lower heat increment of fat compared with that of protein and carbohydrate. Heat production associated with the microbial fermentation of dietary fibre in the hind-gut accounts largely for the poorer performance of sows fed high fibre diets in hot environments.
Thermal resistance properties of the floor The response of the lactating sow to high temperatures may be influenced significantly by the conductive heat properties of the floor. For example, it can be calculated using the AUSPIG model that the ECT would be reduced from 25 to 22 °C if the floor type was changed from concrete to timber. In the experiment of McGlone et al. (1988), there was a 30% difference in feed intake between sows housed at or above 29°C on either plastic-coated expanded metal or concrete slats during lactation (3.09 to 3.99 kg/day, respectively), and this was reflected in a small and non-significant increase in piglet growth rate. In an experiment over all seasons, Stansbury et al. (1987) recorded no difference in feed intake but a small reduction in weight loss for sows housed on concrete rather than plastic floors during lactation. However, piglet weaning weights were greater for those sows housed on a plastic floor, a result which demonstrates the different thermal requirements of piglets and SOWS.
166
J.L. BLACKET AL.
Evaporative heat loss The effects of high ambient temperatures on VFI can be overcome if the sow is able to increase evaporative heat loss via an increase in the proportion of skin that is wet. McGlone et al. ( 1988 ) reported that at ambient temperatures above 29°C, drip cooling decreased the weight loss of sows from 27 to 9 kg and improved litter weight gain from 1.47 to 1.85 kg/day during a 28day lactation. Similarly, Maxwell et al. (1990) recorded a reduction in respiration rate and an estimated increase in milk production of approximately 1 1/day for sows that were drip cooled when ambient temperature exceeded 26 ° C. Stansbury et al. ( 1987 ) and McGlone et al. ( 1988 ) have also evaluated the effect of increasing convective heat loss by sows, by provision of snout cooling systems, and both studies reported a significant increase in food intake of sows during heat stress. This is likely to result in an increase in milk yield and, provided the environment of the piglet is not compromised, an increase in the growth rate of piglets. The provision of drip cooling systems is TABLE3
Ambient temperature in relation to evaporative critical temperature (ECT) for the sow and its effect on the predicted~performance of a sow and litter during a 28-day lactation (post-partum body weight o f150 kg, fed a diet containing 13.5 MJ DE and 164 g CP per kg DM, litter size of 9, no creep feed provided ) Treatment
Temperature ( ° C) ambient ECT Sow feed intake 6 (kg/day) DE intake (MJ/day ) wt. change (kg) weaning to mating interval
(days) latent heat loss of evaporation from the skin (M J / d a y )
Thermal comfort (dry)
Proportion of wet skin
Sow2
Piglet3
15 to 304
20 22
33 26
33 25
100 s
33 21
5.26 71 + 1.8
2.61 35 -29.1
4.31 58 -9.6
5.26 71 + 1.4
5.0
19.1
9.6
5.0
10.9
10.5
16.0
25.9
152 5.64
168 6.08
193 6.79
194 6.80
Piglet
average daily gain (g) mean wt. at weaning (kg)
~Predicted by AUSPIG model (Black et al. 1986). 2Zone of thermal comfort for sows. 3Zone of thermal comfort for piglets. 4Similating the effect of drip cooling by increasing the proportion of wet skin for the sow from 15 to 30%. 5 Simulating the situation where the proportion of wet skin for the sow is up to 100%. 6Does not include feed wastage.
LACTATION IN THE SOW DURING HEAT STRESS
167
now c o m m o n in farrowing accommodation and the effect on the sow and on piglet growth is depicted by the results of an AUSPIG simulation in Table 3. When ambient temperature is set at 20 ° C, VFI is not constrained by environmental factors and the sow consumes sufficient nutrients to maintain body weight during lactation. However, the LCT for the piglet is 29°C and at an ambient temperature of 20°C, even though some supplemental heating is supplied, the growth rate of the piglet is less than the 194 g/day that is achievable if the piglet is housed within its zone of thermal comfort. An ambient temperature of 33 °C is more suitable for the piglet but this is too high for the sow with the result that VFI is depressed, body reserves are mobilised and the interval between weaning and re-mating is extended. Piglet growth rate is only marginally improved even though the piglet is now accommodated within its zone of thermal comfort, because milk yield of the sow has been reduced due to the low intake of nutrients by the sow. The effect on piglet growth rate in this example (from 209 to 181 g / d a y ) is of similar magnitude to the results in Table l and the combined data of Lynch (1977), Stansbury et al. (1987), Yen and Cheng ( 1990 ) and Barb et al. ( 1991 ). When sows had access to drip cooling in this simulation, despite ambient temperature remaining at 33 °C they were able to increase the proportion of their skin that was wet and hence increase the amount of heat loss via evaporation. Although VFI was not sufficient to provide all of the substrates required for milk production, these were adequately supplied from body reserves with minimal effects on reproductive performance. First-parity sows in the commercial piggery referred to in Fig. 1 had access to a drip cooling system and consumed 4.2 kg/day, lost 8 kg of body weight during a 28-day lactation and recorded piglet growth rates of 170 g/day, a result similar to that simulated by AUSPIG. The final treatment in Table 3 demonstrates the extent to which heat loss via evaporation must increase for VFI, and hence reproductive performance, not to be affected by high ambient temperature. CONCLUSIONS
Experimental evidence presented in this paper indicates that the depression in milk yield associated with a decline in food intake may be greater in sows exposed to environmental temperatures above their ECT than in sows housed under thermoneutral conditions. It is suggested that this observation may be due partially to a redistribution of blood flow to the skin, away from other tissues such as the m a m m a r y gland. Results also indicate that the depression in heat production associated with a reduction in food intake is less in lactating sows than in non-reproducing animals because of the capacity of the sow to mobilise large quantities of body tissue to maintain high rates of milk synthesis. However, when both lactating and non-lactating sows are fed ad lib. under thermoneutral conditions, heat production and ECT appear
168
J.L. BLACK ET AL.
to be r e l a t i v e l y similar. T h e d e t r i m e n t a l effects o n m i l k p r o d u c t i o n o f sows e x p o s e d to high t e m p e r a t u r e s c a n b e r e d u c e d b y i n c r e a s i n g the energetic efficiency o f f o o d use t h r o u g h e i t h e r i n c r e a s i n g the p r o p o r t i o n o f fat or decreasing the p r o p o r t i o n o f fibre in the diet. H o w e v e r , the ability to h a v e m a r k e d effects t h r o u g h diet c h a n g e s are s m a l l c o m p a r e d w i t h t h o s e t h a t c a n be a c h i e v e d b y i n c r e a s i n g h e a t loss to the e n v i r o n m e n t , p a r t i c u l a r l y b y selecting floors w i t h low t h e r m a l r e s i s t a n c e a n d b y i n c r e a s i n g e v a p o r a t i v e h e a t loss t h r o u g h d r i p cooling.
REFERENCES ARC, 1981. The Nutrient Requirements of Pigs. (Agricultural Research Council), Commonwealth Agricultural Bureau, Farnham Royal. Alexander, G, Hales, R.J.S., Stevens, D. and Donnelly, J.B., 1987. Effect of acute and prolonged exposure to heat on regional blood flows in pregnant sheep. J. Devel. Physiol., 9:1-15, Barb, C.R., Estienne, M.J., Kraeling, R.R., Marple, D.N., Rampacek, G.B., Rahe, C.H. and Sartin, J.L., 1991. Endocrine changes in sows exposed to elevated ambient temperature during lactation. Domest. Anim. Endo., 8:117-127. Black, J.L., Campbell, R.G., Williams, I.H., James, K.J. and Davies, G.T., 1986. Simulation of energy and amino acid utilisation in the pig. Res. Devel. Agric., 3: 121-145. Brendemuhl, J.H., Lewis, A.J. and Peo, E.R., 1987. Effect of protein and energy intake by primiparous sows during lactation on sow and litter performance and sow serum thyroxine and urea concentrations. J. Anim. Sci., 64: 1060-1069. Brendemuhl, J.H., Lewis, A.J. and Peo, E.R., 1989. Influence of energy and protein intake during lactation on body composition of primiparous sows. J. An im. Sci., 67:1478-1488. Clark, J.R., Komkov, A. and Tribble, L.F., 1986. Effects of parity, season, gonadotrophin releasing hormone and altered suckling intensity on the interval to rebreeding in sows. Theriogenology, 26: 299-308. Close, W.H. and Stanier, M.W., 1984. Effect of plane of nutrition and environmental temperature on the growth and development of the early-weaned piglet. 2. Energy metabolism. Anim. Prod., 38: 221-231. Cole, D.J.A., 1990. Nutritional strategies to optimize reproduction in pigs. In: D.J.A. Cole, G R. Foxcroft and B.J. Weir (Editors), Control of Pig Reproduction III. J. Reprod. Fert, Ltd: Cambridge., pp. 67-82. Cox, N.M., Britt, J.H., Armstrong, W.D. and Alhusen, H.D., 1983. Effect of feeding fat and altering weaning schedule on rebreeding in primiparous sows. J. Anim. Sci., 56:21-29. Dove, H. and Freer, M., 1979. The accuracy of tritiated water turnover rate as an estimate of milk intake in lambs. Aust. J. Agric. Res., 30: 725-739. Fernandes, L.C.O., Altafini, A.C., Kunert, H.C. and Carnino, F., 1990. Summer feed intake and weight loss of primiparous and multiparous sows associated with weaning to estrus interval. Proc. 1 lth Int. Pig Vet. Soc. 412 (Abs). Giles, L.R. and Black, J.L., 1991. Voluntary food intake in growing pigs at ambient temperatures above thermal neutral. In: E.S. Batterham (Editor), Manipulating Pig Production III. Aust. Pig Sci. Assoc., Werribee,pp. 162-165. Giles, L.R., Black, J.L., Gooden, J.M. and Annison, E.F., 1991. Energy expenditure of growing pigs maintained at high ambient temperature. In: C. Wenk and M. Boessinger (Editors), Energy Metabolism in Farm Animals, 12th EAAP Symposium on Energy Metabolism of Farm Animals, Switzerland, pp. 52-55.
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Hoppe, M.K., Libal, G.W. and Wahlstrom, R.C., 1990. Influence of gestation energy level on the production of Large White × Landrace sows. J. Anim. Sci., 68: 2235-2242. Johnson, H.D., Ragsdale, A.C., Berry, I.L. Wayman, O. and Merilan, C.P., 1966. University of Missouri, Exper. Stn. Res. Bull. No. 902. (Cited by Webster, 1974). King, R.H. and Dunkin, A.C., 1986a. The effect of nutrition on the reproductive performance of first-litter sows. 3. The response to graded increases in food intake during lactation. Anim. Prod., 42:119-125. King, R.H. and Dunkin, A.C., 1986b. The effect of nutrition on the reproductive performance of first-litter sows. 4. The relative effects of energy and protein intakes during lactation on the performance of sows and their litters. Anim. Prod., 43:319-325. King, R.H., Toner, M.S. and Dove, H., 1989. Pattern of milk production in sows. In: J.L. Barnett and D.P. Hennessy (Editors), Manipulating Pig Production II. Aust. Pig Sci. Assoc., Werribee, p 98 (Abs) King, R.H. and Williams, I.H. 1984a. The effect of nutrition on the reproductive performance of first-litter sows. 1. Feeding level during lactation, and between weaning and mating. Anim. Prod., 38: 241-247. King, R.H. and Williams, I.H., 1984b. The effect of nutrition on the reproductive performance of first-litter sows. 2. Protein and energy intakes during lactation. Anim. Prod., 38:241-247. Lorschy, M.L., Giles, L.R., Black, J.L. and Gooden, J.M. 1992. Effect of high ambient temperature on voluntary food intake, body temperature and milk production in the lactating sow. Proc. Nutr. Soc. Aust., 17:216 (Abs). Lynch, P.B., 1977. Effect of environmental temperature on lactating sows and their litters. Irish J. Agric. Res., 16: 123-130. Lynch, P.B., 1989. Voluntary food intake of sows and gilts. In: A.T. Smith and T.L.J. Lawrence (Editors), Pig Housing and the Environment. Brit. Soc. Anim. Prod., Edinburgh, pp. 7177. Maxwell, R.C., Cutler, R.S. and Callinan, A.P.L., 1990. Drip cooling lactating sows improves performance and comfort. Proc. 1 lth IPVS. 396. McGlone, J.J., Stansbury, W.F. and Tribble, L.F., 1988. Management of lactating sows during heat stress: Effects of water drip, snout coolers, floor type and a high energy-density diet. J. Anim. Sci., 66: 885-891. Mullan, B.P., 1991. The response of the breeding sow to the climatic environment. In: E.S. Batterham (Editor), Manipulating Pig Production III. Aust. Pig Sci. Assoc., Werribee, pp.167-177. Mullan, B.P., Brown, W. and Kerr, M. 1992. The response of the lactating sow to ambient temperature. Proc. Nutr. Soc. Aust., 17:215 (Abs). Mullah, B.P. and Close, W.H., 1989. The partition of nutrients during lactation and its relationship to reproductive performance. In: J.L. Barnett and D.P. Hennessy (Editors), Manipulating Pig Production II. Aust. Pig Sci. Assoc., Werribee, p.302 (Abs). Mullan, B.P., Close, W.H. and Foxcroft, G.R., 199 I. Metabolic state of the lactating sow influences plasma LH and FSH before and after weaning. In: E.S. Batterham (Editor), Manipulating Pig Production III. Aust. Pig Sci. Assoc., Werribee, p.31 (Abs). Mullah, B.P. and Williams, I.H., 1989. The effect of body reserves at farrowing on the reproductive performance of first-litter sows. Anim. Prod., 48: 449-457. Nelssen, J.L., Lewis, A.J., Peo, J.R. and Crenshaw, J.D., 1985. Effect of dietary energy intake during lactation on performance of primiparous sows and their litters. J. Anim. Sci., 6 l: 1164-1171. Nobler, J. and Etienne, M., 1987. Metabolic utilization of energy and maintenance requirements in lactating sows. J. Anim. Sci., 64: 774-781. Noblet, J., Dourmad, J.Y. and Etienne, M., 1990. Energy utilisation in pregnant and lactating sows: modelling of energy requirements. J. Anim. Sci., 68: 562-572.
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