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J. Anim Sci. 2006. 84:2224-2231. doi:10.2527/jas.2005-521
© 2006 American Society of Animal Science
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ANIMAL PRODUCTION

Temperature and body weight affect fouling of pig pens1

A. J. A. Aarnink2, J. W. Schrama, M. J. W. Heetkamp, J. Stefanowska and T. T. T. Huynh

Animal Sciences Group of Wageningen University and Research Centre, P.O. Box 65, 8200 AB, Lelystad, the Netherlands


   Abstract
Top
 Abstract
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
Fouling of the solid lying area in pig housing is undesirable for reasons of animal welfare, animal health, environmental pollution, and labor costs. In this study the influence of temperature on the excreting and lying behavior of growing-finishing pigs of different BW (25, 45, 65, 85, or 105 kg) was studied. Ten groups of 5 pigs were placed in partially slatted pens (60% solid concrete, 40% metal-slatted) in climate respiration chambers. After an adaptation period, temperatures were raised daily for 9 d. Results showed that above certain inflection temperatures (IT; mean 22.6°C, SE = 0.78) the number of excretions (relative to the total number of excretions) on the solid floor increased with temperature (mean increase 9.7%/°C, SE = 1.41). Below the IT, the number of excretions on the solid floor was low and not influenced by temperature (mean 13.2%, SE = 3.5). On average, the IT for excretion on the solid floor decreased with increasing BW, from approximately 25°C at 25 kg to 20°C at 100 kg of BW (P < 0.05). Increasing temperature also affected the pattern and postural lying. The temperature at which a maximum number of pigs lay on the slatted floor (i.e., the IT for lying) decreased from approximately 27°C at 25 kg to 23°C at 100 kg of BW (P < 0.001). At increasing temperatures, pigs lay more on their sides and less against other pigs (P < 0.001). Temperature affects lying and excreting behavior of growing-finishing pigs in partially slatted pens. Above certain IT, pen fouling increases linearly with temperature. Inflection temperatures decrease at increasing BW.

Key Words: behavior • body weight • fouling pig • temperature


   INTRODUCTION
Top
 Abstract
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
By nature pigs are clean animals that keep their excreting and lying locations separate (Signoret et al., 1969Go). In modern pig housing, however, a large part of the pen floor is slatted because farmers do not wish to have dirty pens. Pen fouling requires extra labor for cleaning, increases the risk of health problems, and increases emission of ammonia and odor to the environment. When pigs are placed in a new pen, they first choose their lying area (Marx and Buchholz, 1989Go). At low and moderate temperatures pigs prefer to lie on a warm insulated solid floor rather than on a cool slatted floor. The excreting area is located as far away as possible from the lying area (Steiger et al., 1979Go; Buré, 1986Go). However, various researchers found at high ambient temperatures pigs alter their behavior, avoid body contact, seek out a cooler place (slatted floor) to lie, and the distinct areas for the different activities found at low and moderate temperatures disappear (Steiger et al., 1979Go; Fraser, 1985Go; Huynh et al., 2005Go).

Little is known about the exact temperatures above which pigs alter their lying and excreting behavior. These temperatures may depend on BW; heavy pigs become heat-stressed at lower temperatures (Nienaber et al., 1999Go). Therefore, the objective of the study described here was to determine the effect of increasing ambient temperatures on the excreting and lying behavior of pigs at different BW. Similar to the thermoneutral zone in the general heat balance model of Mount (1979)Go, it was expected that pigs would not change their behavior until a certain inflection temperature (IT). Above this temperature behavior is changed, and these IT can be used to improve the climate control of houses for growing-finishing pigs.


   MATERIALS AND METHODS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
Animals and Housing

Animals were maintained and treated under approved animal use and care guidelines of the Animal Experiments Committee of Wageningen University, the Netherlands. The study was conducted in accordance with the Dutch Law on Experimental Animals.

Ten groups of commercial pigs were used, and each group consisted of 5 barrows. Mean BW of the groups was approximately 25, 45, 65, 85, and 105 kg, and there were 2 groups per BW class. The 5 pigs within each group came from different litters. After a period of 2 wk to acclimate pigs to the diet and pen, they were put in a climatic respiration chamber. The pen configuration in this chamber was similar to that of the pen in the adaptation period and measured 3.40 x 1.50 m. The pen floor was 60% solid (2.05 x 1.50 m) and 40% slatted (1.35 x 1.50 m). The concrete, solid floor was insulated with 25 mm of extruded polystyrene and had a 2.4% slope toward the slatted floor. The slatted floor was made of metal triangular bars (12-mm bar width, 12-mm slot width). After an adaptation period of 4 d in the chamber, ambient temperatures were gradually increased for 9 d.

Climatic Respiration Chamber Conditions

Air entered the animal room within the chamber via a perforated ceiling. Outgoing air from the animal room was filtered to remove dust and was conditioned by an air conditioner to the set temperature and to maintain a relative humidity of 60% (SD = 0.3). Most (99.0 to 99.6%) of the air was recycled, and the remainder was made up with fresh air. The amount of fresh air entering the room was set at such a rate that the CO2 concentration averaged 0.7 to 0.8% and varied from 2.4 to 6.0 m3·h–1·pig–1. The air velocity at the animal level averaged 0.14 m/s (measured without animals in the room, at 3 locations along the long axis of the pen, and at 3 heights: 5, 25, and 50 cm). A day-night light regimen was maintained, with the day period from 0600 to 1800. During this period, 4 light bulbs were on (3 of 60 W and 1 of 25 W). During the night one light bulb of 25 W and one infrared lamp of 200 W were on. The infrared lamp was directed toward the ceiling to distribute the radiation equally over the pen.

Feeding

Pigs were fed a commercial dry feed ad libitum. The 25-kg groups received starter feed [calculated composition on an as-fed basis (CVB, 2004Go) was NE = 9,665 kJ/ kg; CP = 168.2 g/kg; crude fat = 42.7 g/kg; crude fiber = 36.4 g/kg]. All other groups received a finisher feed (NE = 9,226 kJ/kg; CP = 152.7 g/kg; crude fat = 43.0 g/ kg; crude fiber = 66.8 g/kg. The 1-place dry feeder was placed in one of the corners of the lying area, with the opening facing toward the slatted floor. The nipple-drinking bowl was placed in the diagonally opposite corner at 65 cm above the slatted-floor level. The bowl minimized water spillage and prevented the pigs from using the water nipple to spray water on themselves.

Imposed Temperatures

In Table 1Go, the temperatures of the outgoing air are given for the 9 d of the experimental period. The purpose of increasing the temperatures was to determine the relationship between temperature and excreting and lying behaviors. It was expected that excretion on the solid floor would be low and constant until the temperature reached a certain IT. Above the IT, it was expected that excretions on the solid floor would gradually increase.


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  		Table 1. Mean temperatures (°C) of the outgoing air from the respiration chambers for the different BW classes of pigs during the 9 d of the experimental period
 
Measurements and Observations

Animal Parameters. Pigs were weighed before and after they were put in the animal room of the climate respiration chamber. Total feed intake and water use was measured per group of pigs. Feed intake was measured by automatic registration of the number of feed portions given by the feeder. The amount of feed per portion was regularly checked. Water use was determined daily by registering the level of water in the storage tank located outside the respiration chambers.

Behavioral Parameters. The behavior of the pigs was continuously videotaped. From the videotapes the following was recorded:

  1. Continuously, the excretions on the solid and slatted floors were determined. A distinction was made between defecations and urinations. Sometimes it was uncertain whether the pigs had urinated or defecated. These observations were classified as uncertain.
  2. Every 15 min the number of pigs lying on the solid and slatted floors was determined. Pigs on the border between the solid and slatted floors were assigned to that area in which most of the pig was lying.
  3. Every 15 min the lying posture of the pigs was determined. The following lying postures were distinguished: fully on the side, fully on the belly, half-side/half-belly, and whether the pigs were lying against other pigs over more than 50% of the length of their body.

Climate Parameters. The temperature and relative humidity of the air entering the animal room and the temperature of the outgoing air were measured every minute with a combined temperature and humidity sensor (type Smartlink KNM-THD-RS485-C, Keithly, Gorinchem, the Netherlands).

Heat Production. Heat production was determined by the method of indirect calorimetry (Verstegen et al., 1987Go). In this method, heat production is calculated from the pigs’ measured oxygen consumption and their measured carbon dioxide production. The concentrations of O2 and CO2 were determined in the air entering and leaving the chamber (O2 with a type ADC7000 Oxygen analyzer, ADC (Hoddesdon, UK); CO2 with a URAS 3G, Hartman and Braun (Frankfurt, Germany). The O2 consumption and the CO2 production were used to calculate total heat production (HP) with a modification of the formula of Brouwer (1965)Go, as follows:


Formula

where O2 = oxygen consumption in liters per day; and CO2 = carbon dioxide production in liters per day.

Statistical Analyses

Effects of BW class on production parameters, excretion frequencies, percentage of lying animals, and lying posture were determined by 1-way ANOVA (GenStat, Release 6.1, 2002Go). In these models, the experimental units were groups of 5 pigs. The effect of temperature on total excreting frequency, urinating and defecating frequencies, the percentage of lying animals, the lying posture, and lying against other pigs was determined by including the temperature as a covariate in an ANOVA model, with animal group as the single factor within the model.

At low and moderate temperatures, the pigs excreted on the slatted floor, and excretion on the solid floor was low and independent of temperature. Above a certain temperature, called IT for excretion, pigs began to excrete on the solid floor. Above the IT, excretion on the solid floor increased linearly with temperature (Figure 1Go). A broken-stick analysis was performed to describe this pattern. The broken-stick analyses accounted for a greater proportion of the variance than linear regression; for total excretions on the solid floor, it was 82 vs. 75%, respectively. Within the broken-stick analysis, 3 parameters were estimated: IT; constant c, which estimates the excretion on the solid floor at temperatures below IT; and regression coefficient z, which estimates the increase in excretions on the solid floor at increasing temperatures above IT, as follows:


Figure 1
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  		Figure 1. A typical relationship between temperature and the number of excretions (defecations + urinations) on the solid floor (as a percentage of total excretions); results are given for a group of pigs of the 45 kg of BW class. Dots represent the observed values; the lines represent the results of the broken-stick analysis.

 

Formula

where Y is number of total excretions, number of urinations, or number of defecations on solid floor (as percentage of total number); and T is temperature.

The following broken-stick model was used to describe the relationship between temperature and the number of pigs lying on the slatted floor:


Formula

where Y is number of pigs lying on slatted floor (in percentage of total number of lying pigs).

This broken-stick analysis (Model 2) accounted for a greater percentage of variance than linear regression, 86 vs. 76%, respectively.

Daily averages of the experimental groups of 5 pigs were used to determine the IT. Simple linear regression analyses were performed to determine the relationships between BW, heat production, and IT for excreting and lying behaviors. The effects of BW on the constants c and the regression coefficients z were also determined by linear regression. The broken-stick analyses and simple linear regressions were performed with the Gen-Stat software.


   RESULTS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
Animal Parameters

Pigs in BW classes 25, 45, 65, 85, and 105 kg had mean BW of 24.0, 45.3, 66.7, 80.8, and 102.0 kg (SE = 0.80), mean growth rates of 598, 656, 859, 748, and 603 g/d (SE = 33), mean feed intake of 1.11, 1.51, 2.20, 2.27, and 2.27 kg/d (SE = 0.05), and mean water use of 2.59, 3.38, 5.32, 5.01, and 4.92 L/d (SE = 0.12), respectively.

Excreting Behavior

Pigs in BW classes 25, 45, 65, 85, and 105 kg had mean urinating frequencies of 4.1, 4.2, 4.9, 2.5, and 2.7 times/pig daily, mean defecating frequencies of 7.0, 4.0, 4.7, 3.2, and 3.1 times/pig daily, and mean total number of excretions of 13.9, 9.4, 10.1, 7.2, and 6.2 times/pig daily, respectively. The difference between total number of excretions and the sum of defecations and urinations is the number of uncertain observations. Temperature had significant effects on the defecating and total excreting frequencies (in times/pig daily per °C increase in temperature); the regression coefficients were –0.15 (SE = 0.04) and –0.29 (SE = 0.06), respectively. No temperature effect was found on the urinating frequency (regression coefficient 0.013, SE = 0.032).

In Figure 1Go an example is given of the influence of ambient temperature on the number of total excretions on the solid floor. When temperature increased, the number of excretions on the solid floor remained relatively constant and at a low level until the IT was reached. Above this temperature the number of excretions on the solid floor increased with increasing temperature. Similar patterns were found for urinations and defecations. Using the statistical Model 1, IT, constants, and regression coefficients were calculated for the different BW classes of pigs for number of total excretions, urinations, and defecations on the solid floor (as percentage of total number; Table 2Go). Regression analysis showed that IT decreased with increasing BW (P < 0.05). In Figure 2Go the relationship between BW and IT for excretions on the solid floor is given. The regression line shows that the IT decreased from approximately 25°C for pigs of 25 kg to approximately 20°C for pigs of 100 kg. Regression analysis showed that the constant c from Model 1 for excretions on the solid floor significantly increased with increasing BW (P < 0.001; Table 2Go). The regression coefficient z from Model 1 for excretions on the solid floor was not influenced by BW.


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  		Table 2. Inflection temperatures, constants, and regression coefficients per BW class for the relationships between ambient temperature and frequency of total excretion, urination, and defecation on the solid floor (as a percentage of total frequency), and for the relationship between ambient temperature and the number of pigs lying on the slatted floor (as a percentage of total number of lying pigs)1
 

Figure 2
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  		Figure 2. Relationship between BW and the temperature above which excretion on the solid floor increased (inflection temperature). Regression line: Y = 26.5 (SE, 1.7) – 0.065 (SE, 0.025) X (R2 = 0.40).

 
In Figure 3Go the relationship is given between heat production and IT for excretions on the solid floor. A significant regression coefficient was found (P < 0.01). A larger part of the variation within IT was accounted for by heat production (59%) than by BW (40%).


Figure 3
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  		Figure 3. Relationship between heat production and the temperature above which excretion on the solid floor increased (inflection temperature). Regression line: Y = 29.6 (SE, 2.0) – 0.46 (SE, 0.12) X (R2 = 0.59).

 
Regression lines of IT on BW and on heat production were very comparable for urinating and defecating frequency when compared with total excreting frequency.

The regressions for urinating frequency (%) were as follows: IT related to BW: Y = 26.5 (SE, 1.4) – 0.070 (SE, 0.022) X (R2 = 53%), and IT related to heat production: Y = 29.1 (SE, 1.6) – 0.46 (SE, 0.10) X (R2 = 71%). The regressions for defecating frequency (%) were: IT related to BW: Y = 26.6 (SE, 1.4) – 0.066 (SE, 0.022) X (R2 = 52%), and IT related to heat production: Y = 28.8 (SE, 1.7) – 0.42 (SE, 0.11) X (R2 = 63%).

Regression analysis showed that for urination and for defecation on the solid floor the constant c from Model 1 significantly increased with increasing BW (P < 0.001), but regression coefficient z (see Model 1) was not influenced by BW.

Lying Behavior

Temperature affected percentage of pigs lying. For each degree Centigrade increase in temperature, the percentage of pigs lying increased by 0.50% (SE = 0.05; P < 0.001). At greater temperatures, an increasing number of pigs lay on the slatted floor. However, the maximum number of pigs lying on the slatted floor was physically limited. A typical pattern of the percentage of pigs lying on the slatted floor at increasing temperatures is given in Figure 4Go. Using statistical Model 2, the IT, constants, and regression coefficients were calculated (Table 2Go).


Figure 4
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  		Figure 4. Typical pattern of the percentage of pigs lying on the slatted floor at increasing temperatures; results are given for a group of pigs of the 85 kg of BW class. Dots represent the observed values; the lines represent the results of the broken-stick analysis.

 
The regression analysis showed a clear effect of BW on IT (P < 0.001). The IT for lying on the slatted floor decreased with increasing BW. Animal weight explained 61% of the variation in IT. A clear effect of heat production on IT was also found (P < 0.001). Heat production explained 62% of the variation in IT. In Figures 5Go and 6Go the relationships from BW and heat production with IT are shown. Regression analysis showed that the constant (c in Model 2) decreased significantly with increasing BW (P < 0.001), and there was a tendency toward a lower regression coefficient at increasing BW (P < 0.10).


Figure 5
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  		Figure 5. Relationship between BW and the temperature at which the maximum number of pigs lying on the slatted floor was reached (inflection temperature). Regression line: Y = 29.0 (SE, 1.1) – 0.060 (SE, 0.015) X (R2 = 0.61).

 

Figure 6
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  		Figure 6. Relationship between heat production and the temperature at which maximum number of pigs lying on the slatted floor was reached (inflection temperature). Regression line: Y = 30.8 (SE, 1.5) – 0.37 (SE, 0.09) X (R2 = 0.62).

 
With rising temperatures, pigs lay more fully on their sides and less against other pigs. On average, 67% of the lying pigs were lying fully on their sides and 27% of the lying pigs were lying against other pigs. For every degree Centigrade increase in temperature the percentage of pigs lying fully on their sides increased by 1.9% (SE = 0.2; P < 0.001). Furthermore, the percentage of pigs lying half-side/half-belly decreased by 0.93% (SE = 0.12; P < 0.001), the percentage of pigs lying on their belly decreased by 0.95% (SE = 0.13; P < 0.001), and the percentage of pigs lying against other pigs decreased by 3.7% (SE = 0.3; P < 0.001) for every degree Centigrade increase in temperature.


   DISCUSSION
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
IMPLICATIONS
 LITERATURE CITED
 
This study showed the effect of ambient temperature on the lying and excreting behavior of pigs. Above a certain IT, excretion on the solid floor increased. This IT was related to pig BW and total heat production. As temperature rose, more and more pigs lay on the slatted floor to try to keep cool, and they expanded their body surface (more lying on their side) and less against other pigs. The slatted floor is generally cooler to lie on than the solid floor. Huynh et al. (2004)Go found a difference of 3.6°C between the surface temperature of the solid floor and the slatted floor.

The question is whether with increasing temperatures pigs first alter their lying behavior and then later their excreting behavior, or vice versa. We found greater IT for lying on the slatted floor than for excretion on the solid floor. However, from that it cannot be concluded that pigs alter their excreting behavior before they alter their lying behavior. The IT for lying on the slatted floor reflects the temperatures at which the slatted floor is occupied to a maximum by lying pigs. Our hypothesis is that even when the slatted floor is not occupied to its maximum by lying pigs, there is a switch to excretion on the solid floor. To test this hypothesis, we examined the correlation between the daily averages of excretions on the solid floor and the occupation of the slatted floor by pigs.

The covering rate of the slatted floor can be calculated from the number of pigs lying on the slatted floor and the sum of the estimated areas they occupy when lying on their sides


Formula

(Baxter and Petherick, 1983Go; Petherick, 1983Go), and


Formula

where Ap is the area taken up by a pig lying on its side, calculated as a rectangle around the pig (m2); W is the mean BW of a group of pigs in the respiration chamber; R is the percentage of slatted floor covered with pigs; n is the number of pigs lying on slatted floor; As is the area of slatted floor (m2).

Figure 7Go shows the relationship between the percentage of slatted floor covered by lying pigs and the excretion on the solid floor. This figure shows that the relative number of excretions on the solid floor was low when the percentage of the slatted floor covered by lying pigs was low. Above a certain covering percentage, the number of excretions on the solid floor increased. With Model 1 this covering percentage was estimated to be 28% (SE = 5). Model 1 explained 59% of the variation in excretion on the solid floor. A few data within Figure 7Go give covering percentages of the slatted floor greater than 100%. This is possible because the area of floor occupied was calculated by drawing a rectangle around the pig (Baxter and Petherick, 1983Go), and therefore the free space between the legs and around the head was also included. In reality, other pigs can make use of this free area. The IT for lying occurred when approximately 50% of the lying pigs were lying on the slats (see constant c in Table 2Go). At an average lying rate of 88%, this was equivalent to 44% of the pigs in the pen, or 2.2 pigs. The estimated cover of 28% above which excretion on the solid floor increased was 1.44 pigs for the 25-kg groups and 0.56 pigs for the 105-kg groups. This means that excretion on the solid floor had already increased before the maximum occupation of the slatted floor had been reached. This agrees with the findings of Randall et al. (1983)Go who reported that pigs wishing to excrete seek out an isolated place, probably because of their vulnerability to attack while excreting.


Figure 7
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  		Figure 7. Relationship between the proportion of slatted floor occupied by lying pigs and the number of excretions on the solid floor (as a percentage of total number of excretions).

 
The IT for behavioral changes might depend not only on BW, but also on other animal and ambient factors. Other factors that might be of significant influence on IT are: breed, sex (female, male, castrated male), air velocity, and insulation of floor, roof, and walls. In our study we used barrows, largely because they generally have a greater feed intake than gilts (Latorre et al., 2003Go) and therefore will come into heat stress at lower temperatures. For all groups of pigs we were able to calculate the IT above which the number of total excretions on the solid floor increased. Below this IT the number of excretions on the solid floor was relatively low. The basic level of excretion on the solid floor (c in Model 1) increased with increasing BW (Table 2Go). The main reason for this increase seems to have been the lack of space for the pigs (Huynh et al., 2005Go). Hacker et al. (1994)Go and Aarnink et al. (1996)Go had previously reported more pen fouling with heavier BW. Greater levels of the constant c, and thus greater levels of excretions on the solid floor during the first few days of the experimental periods, made it more difficult to accurately estimate IT. This is probably one of the reasons that the deviations from the regression lines in Figure 2Go and 3Go seemed to increase at heavier BW.

The finding that heat production explained more variation between IT for excretion than BW was expected; heat production has a direct effect on the animal’s thermoregulation. At greater heat productions pigs need to make more effort to get rid of the heat. The influence of BW is indirect and has different, partly contradictory, effects. Heavier pigs generally have a larger feed intake and therefore will produce more heat. On the other hand, heavier animals generally also have a larger area for dissipating heat. Further, the insulation is of importance (Mount, 1979Go). Heavier pigs generally have a thicker layer of fat than lighter pigs and therefore will lose less heat per unit of area. The heat productions found in this study for different BW fit very well with the regression line developed by Brown-Brandl et al. (2004)Go. In our study it varied between 4.25 W/kg for animals of 23.3 kg and 2.30 W/kg for animals of 102.3 kg. For these BW the regression line of Brown-Brandl et al. (2004)Go calculated heat productions of 4.24 and 2.35 W/kg, respectively.

Temperature not only influenced the choice of the lying location, but also the lying posture, as was also found by Close (1981)Go. At high temperatures pigs lay more fully on their sides and less on their bellies, and they lay less against other pigs. The reason for this seems logical: at high temperatures pigs adopt postures that maximize heat loss to the environment (Mount, 1979Go). By lying fully on their side they increase the body area in contact with the floor and thereby increase conductive heat loss. Huynh et al. (2005)Go reported that by keeping a distance from other pigs, pigs increase their own radiation heat loss and decrease radiation heat gain from other pigs.


   IMPLICATIONS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 IMPLICATIONS
LITERATURE CITED
 
Within this study inflection temperatures for growing-finishing pigs were determined above which pen fouling increased. The inflection temperatures are within the range measured in the summer in temperate climates. Because the main cause of pen fouling is that pigs change their lying behavior when hot, preferring to lie on the cool slatted floor than on the insulated solid floor, cooling systems can be used to prevent undesirable lying and excreting behavior. The inflection temperatures determined in this study can be used in climate control systems and when setting these cooling systems.


   Footnotes
 
1 The authors gratefully acknowledge the financial support of the Ministry of Agriculture, Nature, and Food Quality of the Netherlands. Back

2 Corresponding author: andre.aarnink{at}wur.nl

Received for publication September 15, 2005. Accepted for publication March 30, 2006.


   LITERATURE CITED
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 


Aarnink, A. J. A., A. J. van den Berg, A. Keen, P. Hoeksma, and M. W. A. Verstegen. 1996. Effect of slatted floor area on ammonia emission and on the excretory and lying behaviour of growing pigs. J. Agric. Eng. Res. 64:299–310.

Baxter, S. H., and J. C. Petherick. 1983. Space requirements of pigs. Anim. Prod. 36:531. (Abstr.)

Brouwer, E. 1965. Report of sub committee on constants and factors. Pages 441–443 in Energy Metabolism. K. L. Blaxter, ed. EAAP Publ. No. 11. Academic Press, London, UK.

Brown-Brandl, T. M., J. A. Nienaber, H. Xin, and R. S. Gates. 2004. A literature review of swine heat production. Trans. ASAE 47:259–270.

Buré, R. G. 1986. Die auswirkung der buchtenstruktur auf das liege und ausscheidungsverhalten von schweinen aktuelle abeiten zur artgemässen tierhaltung KTBL Schrift. Darmstadt Kranichstein, Germany No. 319:83–91.

Close, W. H. 1981. The climatic requirements of the pig. Pages 149–167 in Environmental Aspects of Housing for Animal Production. J. A. Clark, ed. Butterworths, London, UK.

CVB. 2004. Tabellenboek Veevoeding (Feed Table) 2004, Centraal Veevoederbureau (Animal Feed Product Board), Lelystad, the Netherlands.

Fraser, D. 1985. Selection of bedded and unbedded areas by pigs in relation to environmental temperature and behaviour. Appl. Anim. Behav. Sci. 14:117–123.

GenStat Release 6.1. 2002. Lawes Agricultural Trust (Rothamsted Experimental Station). VSN Int., Wilkinson House, Oxford, UK.

Hacker, R. R., J. R. Ogilviei, W. D. Morrison, and F. Kainst. 1994. Factors affecting excretory behavior of pigs. J. Anim. Sci. 72:1455–1460.[Abstract]

Huynh, T. T. T., A. J. A. Aarnink, W. J. J. Gerrits, M. J. H. Heetkamp, T. T. Canh, H. A. M. Spoolder, M. W. A. Verstegen, and B. Kemp. 2005. Thermal behavioral adaptation of growing pigs as affected by temperature and humidity. Appl. Anim. Behav. Sci. 91:1–16.[CrossRef]

Huynh, T. T. T., A. J. A. Aarnink, H. A. M. Spoolder, M. W. A. Verstegen, and B. Kemp. 2004. Effects of floor cooling during high ambient temperatures on the lying behavior and productivity of growing finishing pigs. Trans. ASAE 47:1773–1782.

Latorre, M. A., R. Lazaro, M. I. Gracia, M. Nieto, and G. G. Mateos. 2003. Effect of sex and terminal sire genotype on performance, carcass characteristics, and meat quality of pigs slaughtered at 117 kg body weight. Meat Sci. 65:1369.[CrossRef]

Marx, D., and M. Buchholz. 1989. Verbesserungsmoglichkeiten der haltung junger schweine im sinner der tiergerechtheit anhand der untersuchungen von einflussfaktoren auf das verhalten. Tierhaltung Band No. 19:55–67. Birkhauser Verlag.

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