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ANIMAL PRODUCTION |
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* Prairie Swine Centre, Saskatoon, Saskatchewan, Canada S7H 5N9;
and
USDA-ARS, Swine Odor and Manure Management Research Unit, Ames, IA 50011-3310; and
and
Mississippi State University, Mississippi State 39762-9665
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: behavior • handling • pig • tryptophan
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Heine et al., 1995; Sainio et al., 1996).
In domestic animals, therapeutic functions of Trp include decreasing feed intake, modifying aggression, suppressing hysteria, and inhibiting the response to stress (Laycock and Ball, 1990; Chung et al., 1991; Shea et al., 1991; Adeola and Ball, 1992). In swine production units and slaughter facilities, there are potentially high levels of stress as animals are moved from their home pens, regrouped, transported, and handled in the packing plant. Suppression of the stress response and associated aggression may be beneficial in terms of animal welfare and meat quality. The present experiments were conducted to determine the effect of short-term supplementation of Trp on behavioral and physiological responses and meat quality indices of pigs within a stable social group, when regrouped, and during handling.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Experiment 1
One hundred pigs (Cambrough 15 x Canabrid) from the Prairie Swine Centre, Inc. in Saskatoon, SK, Canada, averaging 70 kg at the initiation of the study, were assigned randomly to 20 pens of 5 pigs each, segregated by sex (10 pens of each). The pigs were deemed to be in excellent health. Pens were 1.8 m x 2.4 m, fully slatted, and equipped with one single space feeder and one nipple drinker. Pen walls were solid polyvinyl chloride (PVC) panels. The 20 pens were in one negatively ventilated room with end-wall fans and ceiling inlets.
The control diet was a corn-corn gluten meal-based diet and was balanced to meet NRC (1998) nutrient requirements for pigs of this size (3,300 kcal/kg), including 13.7% CP and 0.11% Trp (as-fed basis; Table 1). The experimental diets, 2x and 4x (dietary Trp levels at 2 and 4 times the standard requirement for growth, respectively), were similar to the control diet except that crystalline L-Trp (98% L-Trp; BioKyowa Inc., Chesterfield, MO) was supplemented to achieve 0.23 and 0.43% Trp, respectively. In this and subsequent experiments, "high" tryptophan diets refer to both 2x and 4x treatments unless further qualified. All pens were fed control diets for 7 d. During the subsequent 7 d, pens were assigned randomly within sex to the 3 diets, such that 6 pens were fed control, 7 pens were fed 2x, and 7 pens were fed 4x. All pens were subsequently returned to the control diet for an additional 7 d. Feed was added, weighed back, and changed at approximately 1100 when necessary.
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). During 2 complete rotations among the pens, each pen was observed for a total of 50 min (50 scans). During the observations used for determining time budgets, continuous behavior sampling was conducted for frequency and duration of aggression during the 4 min between the first and last scan sample of each 5-min observation period (Martin and Bateson, 1993). This represented 40 min of continuous observation (10 x 4-min periods) per pen daily. Aggression was defined as open-mouthed contact with another pig, and a bout of aggression was deemed to have ended when pigs separated for a period of 
5 s. Beginning on d 0 and continuing on each subsequent observation day, the response of pigs to a frightening stimulus was determined during an additional round of observations. Base activity levels were determined during the initial 10 min (5 min per pen) in the same manner described earlier for time budgets. The animals were then startled by the simultaneous blasting of 4 air horns within the room. A complete round of observations followed immediately, lasting 50 min, with 25 min (25 scans) of observations per pen. The experimental unit was the pen. Data were classified as pretreatment (d –4, –3, and 0), during treatment (d 1, 2, 3, 4, and 7), and posttreatment (d 8, 14, and 15) periods. Within each period, data were averaged to obtain a single value for each pen. Data were then analyzed within these periods (3 analyses) for the effects of treatment, sex, and the treatment x sex interaction tested against pen (treatment sex) using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC).
Experiment 2
In each of 2 identical trials, 120 pigs (Cambrough 15 x Canabrid) were allotted to 6 (2.8 m x 4 m) pens. The pigs were deemed to be in excellent health. Following a 2-wk social adjustment period during which all pigs were fed the control diet, each of the 3 dietary treatments (Table 1
) was imposed on 2 randomly allotted pens of pigs for 4 consecutive days. Tests on aggression were conducted on the final 2 d of the dietary treatment. The average BW of the pigs at the time of the aggression tests was 29.2 ± 3.6 kg in Trial 1 and 27.3 ±4.9 kg in Trial 2. Aggression tests were conducted in a separate room equipped with 20 pens (1.8 m x 2.4 m). The pens were fully slatted with solid PVC walls. The room was negatively ventilated with end-wall fans and ceiling inlets.
The test situation consisted of a pair of unfamiliar pigs, of opposite sex, that had been fed the same experimental diet. Six pairs, 2 from each dietary treatment, were tested simultaneously in adjacent pens and were referred to as a block. Four blocks of testing were conducted each day. Testing was conducted on 2 d in each of 2 trials. In total, 192 pigs were involved in the testing, in 96 pairs, with 32 pairs per dietary treatment. Two blocks were conducted within the room between washings, so that no tests were conducted in unwashed pens. Once tested, pigs were moved to a third room to prevent contact with untested pigs. Pigs were maintained in familiar groups when moved to the test room and were regrouped as quickly as possible (< 5 min) at the beginning of the test.
Aggressive behaviors of the pigs were recorded for 1 h after regrouping. Six trained observers rotated among the pens at 10-min intervals, observing the pen continuously during the interval between rotations. Thus, each pen was observed continuously for 60 min. Observers recorded the frequency and duration of behaviors of interest within each pen. The observed behaviors included the following:
Data were analyzed using the GLM procedure of SAS (SAS Inst., Inc.), with each pair of regrouped pigs as an experimental unit. Trial, day within trial, block within day, and their interactions were included in the model. Treatment and all interactions of trial, day within trial, and block within day with treatment were tested against the residual term (df = 48).
Experiment 3
One hundred twenty pigs (Cambrough 15 x Canabrid) were weighed and assigned to 20 pens of 6 pigs each at approximately 140 to 150 d of age. The pigs were deemed to be in excellent health. There were 6 pens of males and 14 pens of females. The 1.8-m x 2.4-m pens were equipped with fully slatted floors, a single-space dry feeder, and a nipple drinker. Following 2 wk on the control diet, 6 pens (2 pens of males and 4 pens of females) were assigned to each of the 3 dietary treatments (control, 2x, and 4x; Table 1) for 4 d.
Meat Quality.
After 3 d on the experimental diets, 2 or 3 larger pigs from each pen were selected for marketing, such that a total of 16 pigs (6 males and 10 females) for each treatment group were obtained. Pigs were tattooed in their home pen and moved to the loading area pen by pen, taking approximately 1 min to move a pen to the loading area. Within the loading area, the pigs were held in a common pen. Animals were loaded onto a truck with the use of electric prods and transported approximately 40 km to a slaughter facility. All pigs received one short electric shock while being loaded to provide a base level of stressful handling. Additional shocks were given at the discretion of the loading and unloading crews as per normal handling procedures. The pigs were slaughtered after unloading without rest. The total time between moving pigs from their home pens to slaughter was approximately 1 h. These handling procedures were employed to induce a relatively high incidence of PSE and, thereby, provide a strong test of the treatments.
After slaughter, carcasses were chilled at 2°C for approximately 24 h, after which a 10-cm sample was collected from the middle of the loin and cut into 2.5-cm-thick chops. Ultimate pH and color were determined, in duplicate, at 24 h postmortem. Ultimate pH was measured with a pH meter (Accumet pH meter 910; Fisher Scientific, Edmonton, Alberta, Canada) fitted with a pear-tip glass probe (Sealed Ag/Agcl; Cole-Parmer, Vernon Hills, IL). Color was determined after a 30-min bloom period (30 min after cutting) by measuring Hunter L*, a*, and b* with a Minolta Chroma Meter (Portable CR-200b 212410390 Colorimeter, Osaka, Japan; Hill et al., 1998). In addition, duplicate standard samples of approximately 50 g were taken from the center of the LM for measurement of 48-h drip loss (Kauffman et al., 1986). Muscle samples were weighed, and 2 filter papers (P5, 5.5-cm diameter, Cat. No. 09-801K; Fisher Scientific) were placed on their transactional surfaces. Samples were then lightly wrapped in a paper towel and individually sealed in air-inflated, zip-polyester bags and subsequently stored at 0°C for 48 h and weighed. Drip loss was determined by calculating total weight loss as a percentage of initial sample weight. Pigs were classified as PSE if their drip loss was >5% and Hunter L* was >58 (Kauffman et al., 1993).
Handling.
The handling test was conducted on non-marketed pigs in 4 blocks after the third day of the dietary treatments. Within each block, 4 pigs on each treatment from different pens were tested for their response to routine handling, such as moving, weighing, isolation, and regrouping. Before being handled, skin surface temperature and heart rate were determined in their home pens. Skin temperature was measured by an infrared thermometer (Oakton Infrapro 3; Cole-Parmer) at 2 locations, behind the ear and on the rump. Polar heart rate monitors (Polar Beat HRM; Polar Electro Oy, Kempele, Finland), designed to monitor heart rate during human exercise, were used to determine the heart rate. These monitors consisted of 2 electrical sensors attached to an elastic belt, strapped around the thorax of the pig. Input to these sensors was integrated to determine heart rate, which was transmitted a short distance to a hand-held monitor and recorded. Following the prehandling measurements, the pigs were moved from their home pens to another test room. The individual pigs were brought to a start point at the exit of their home room and then herded, without prodding, to the end point, close to the scale and holding pens. The distance between the start point and the end point was 20 m. The walking time was recorded. The pigs were then weighed on a crate scale and put into individual pens for 5 min of isolation. One-half the pigs from each treatment group received 2 short-duration electric shocks from a prod while in the scale. The 1.8-m x 2.4-m isolation pens had solid walls that prevented pigs from contacting each other. Skin temperatures were measured in the scale immediately before release (after prodding, if applied). Heart rate was determined at the end of the walk, in the scale, immediately before release (after prodding, if applied), and after 5 min of isolation. Four unfamiliar pigs on the same treatment were regrouped in a holding pen for 1 h, during which the behavior of the pigs was videotaped. Immediately following the last pig entering the holding pen, fighting incidence (frequency and duration) was recorded by continuous observation. Meanwhile, lying and exploratory (sniffing, biting, rooting, and chewing walls and floor) behaviors were recorded by instantaneous scan sampling at 5-min intervals.
For the meat quality portion of the experiment, the individual pig was considered the experimental unit. Data were analyzed by one-way ANOVA using GLM. The main factor was dietary Trp level with individual pig as the experimental unit. The incidence of PSE was tested by 
2 analysis. For the handling test, a randomized block ANOVA was employed, with individual animal as the experimental unit. For all variables assessed before weighing, the model included dietary Trp treatment only. For the variables measured after weighing, the main factors of dietary Trp and prodding, and their interaction, were tested by the residual error. To test changes in skin temperature and heart rate from pre-handling to posthandling, repeated measures analysis of variance was used.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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No differences in activity time budgets were observed among dietary treatments during either the pre- or posttreatment periods. During the treatment period, pigs fed high-Trp diets spent less time eating (P = 0.05) and more time lying (P = 0.04) during the prestartle phase (Table 2). The response of the animals to the startling stimulus was to become alert and stand. Gradually, the animals returned to their previous activity over the course of the observations. Subsequent to being startled, those pigs fed high-Trp diets continued to spend more time lying (P = 0.01) and also spent less time standing (P = 0.01) than the pigs fed the control diet. Neither the frequency nor the duration of aggression differed among dietary treatments during any of the 3 periods of the study.
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).
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). Approximately 20% of pigs did not fight during the first hour of regrouping in either of the treatment or control groups. Pigs involved in fighting usually had one long and intense fight, which was followed by a few short fights.
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Experiment 3
For the meat quality portion of the experiment, the preslaughter handling resulted in lower pH and higher drip loss than expected in normal pork (Kauffman et al., 1993). The color scores were not extreme, resulting in only a moderate number of pigs exhibiting PSE (3, 1, and 5 for control, 2x, and 4x, respectively). There were no differences among dietary treatments for any of the meat quality characteristics measured (Table 5).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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evaluated the effect of 0.5% Trp on regrouping and heat stress in 8.6-kg pigs and reported that regrouped pigs fed Trp had improved ADG and ADFI during the first 7 d of the trial; however, neither regrouping nor Trp alone affected growth. Seve et al. (1991) evaluated brain metabolites and behavior (grunts, squeals, ambulation, and exploration in an open-field test on d 5, 23, and 45 after weaning) in weanling pigs given varied levels (0.14, 0.23, or 0.32%) of dietary Trp and reported that behavioral reactivity was not influenced by dietary Trp, even though brain Trp and Trp:large neutral amino acids were increased. In a companion paper, Meunier-Salaün et al. (1991) reported that dietary Trp levels induced large variations in brain amino acids and indole concentrations, but changes in behavioral responses were minor. Adeola and Ball (1992) and Adeola et al. (1993) reported that concentrations of serotonin in the hypothalamus of stress-susceptible pigs were less than in stress-tolerant pigs, even though Trp concentrations did not differ. These studies did not evaluate the effect of dietary Trp on any behavioral variables. A differential response to supplemental Trp, in which aggression was decreased to a greater extent in dominant than in subordinate chickens, was reported by Shea et al. (1991). Recently, Peeters et al. (2004) reported that pigs provided with Trp in their drinking water spent more time lying during simulated transport than control pigs, but no other differences were observed.
In Exp. 1, intakes were low during the week of adaptation to the control diet relative to feed intake the previous week, potentially because of the change from the standard farm diet or the change in environment with movement to the experimental room. Intakes during the experimental period returned to normal and did not differ among treatments. This result suggests that at levels of 0.43% Trp did not have an intake suppressing effect as did higher levels reported in previous studies (Chung et al., 1991; Rosebrough, 1996). The time spent eating was decreased on the high-Trp diets during treatment, although these observations were only for a short portion of the day. More importantly, pigs fed high levels of Trp (both 2x and 4x) spent more time lying and tended to spend less time standing during the treatment period. An increase in lying in response to Trp supplementation also was reported by Peeters et al. (2004). In our study, these differences also extended into the startle recovery period. The change in behavior on treatment was generally evident on the first observation day after diet change. Behavioral differences disappeared within 3 d of Trp removal. No differences were observed between the 2 levels of Trp supplementation. These results suggest that the levels of supplementation used were effective within a few days of application.
Aggression was studied in all 3 experiments, although it was the principle focus only in Exp 2. The use of familiar social groups in Exp. 1 and the inadequate number of experimental units in Exp. 3 precluded obtaining significant differences among treatments unless dramatic effects were present. The results of Exp. 2 indicated that aggression was decreased in both high-Trp treatments. No effect was found in the preliminary investigative (nosing) phase of the fight, but rather in the intensely aggressive portion. The most severe form of fighting is inverse parallel pressing, where neither pig has gained an advantage and their efforts are intense. This portion of the fight was decreased in the Trp-treated animals. The effect was a decrease in the total duration of fighting and in the intensity of those fights. A similar differential response in aggression to Trp supplementation was reported in mature broiler breeder males, in which threatening behavior was less affected than was actual pecking (Shea et al., 1990).
High levels of dietary Trp, 2x and 4x, slowed the movement of pigs in a minimal-force handling situation; however, the same pigs responded to weighing and electrical shock to the same degree as pigs fed the control diet. The response of pigs to high levels of dietary Trp may be situation-specific. When pigs are in a situation that they control, such as resting in a pen, when choosing to terminate a fight, or when handled with minimal force, they were less excitable when fed high levels of Trp. When stress was unavoidable, such as with confinement in a scale or receiving an electric shock, pigs responded similarly regardless of the dietary Trp level. This result is similar to that of Meunier-Salaün et al. (1991), who reported that dietary Trp could induce large variations in brain serotonin levels, but minor changes in behavioral or cortisol responses to stress. Peeters et al. (2004) reported no differences in responses to simulated transport in Trp-supplemented pigs, with the exception that they resumed lying earlier than did control pigs, somewhat similar to our pigs’ recovery from the startle application.
Because of the metabolic relationships between dietary Trp and serotonin, behavior type and serotonin production, stress at slaughter and hypothalamic neurotransmitter concentration, and stress at slaughter and the production of PSE pork, it could be hypothesized that supplemental Trp may help alleviate the negative effects of stress at slaughter on meat quality. Meat quality, however, is complex, as there are many contributing factors including environment, genetics, and plane of nutrition (Forrest et al., 1975). Adeola and Ball (1992) evaluated the use of supplemental Trp in 92-kg pigs and reported that Trp supplementation lowered the incidence of PSE pork compared with pigs fed the control diet (27 vs. 33%, respectively); however, the pH, color, and structure of the loins and hams were similar regardless of the diet fed. Henry et al. (1992) fed 44- to 90-kg pigs a diet deficient in Trp and reported that pigs fed the Trp-deficient diet had higher initial pH levels in the ham and loin. In turkeys, Denbow et al. (1993) tested the effect of 0.1, 0.2, and 0.3% supplemental Trp for 2 wk before market and reported that dietary Trp had no effect on muscle shear, color, or pH (0, 4, or 24 h), despite changes in brain metabolite concentrations. Although we did not measure plasma Trp or its metabolites, we did not see any effect of supplemental Trp on meat quality characteristics.
There are 2 possibilities that may explain the lack of differences in meat quality among the treatments. The first is that our handling regimen was too severe and overwhelmed any possibility of differential response. Specifically, the animals were rested for a shorter period of time at the packing plant than normal; however, this resting time relates to metabolic recovery from the stress response and not to a decrease in the stress itself. In other words, the results suggest that all pigs reacted similarly to the stressful conduction. Our hypothesis was that high-Trp diets would decrease the reaction of animals to these stressors, not that it would improve their recovery. The second possibility is that Trp is effective in nonforced or animal-controlled situations. In Exp. 2, the length of a fight was decreased, as the fight itself and its length were under the control of the animal. In addition, under nonforced movement, high-Trp pigs moved more slowly. When movement was forced (moving to loadout), they moved as quickly as the control pigs. Similarly, when animals had an electric shock forced on them, all physiological responses were similar across diets.
In conclusion, behavioral effects were evident in pigs within a few days of being fed diets that provided Trp at 2x and 4x. These behavioral differences included a greater time spent lying and less time spent eating. Aggression among unfamiliar pigs was decreased in duration and intensity, but not frequency, when pigs were fed high-Trp diets. The responses of pigs to the stressors of handling, including electric shock, were unaffected by Trp treatment. Short periods of high dietary levels of Trp could be used to decrease aggression, but high levels of Trp seem to have little effect on the response to handling. Therefore, it is suggested that high levels of Trp may result in avoidance of stressful situations, if possible. However, these higher levels of Trp have no effect on response to stressors that are forced on the animal. High dietary Trp had no effect on the color, pH, or drip loss of the LM.
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Footnotes |
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2 Corresponding author: kerr{at}nsric.ars.usda.gov
Received for publication November 23, 2004. Accepted for publication August 1, 2005.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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