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ANIMAL PRODUCTION |
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1 Department of Farm Animal Health, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 7, 3584 CL Utrecht, the Netherlands; and
Adaptation Physiology Group, Department of Animal Sciences, Wageningen University, PO Box 336, 6700 AH Wageningen, the Netherlands
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: activity • extended lactation • intermittent suckling • nursing • pig • piglet behavior
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). To familiarize piglets with solid feed before weaning, the feed is often provided during lactation. However, in conventional housing, preweaning creep feed intake is usually low and highly variable between and within litters (Pajor et al., 1991). Compared with conventional housing, piglets kept in sow-controlled housing combined with a communal piglet area spend more than 40% of their time away from their dam, have a greater preweaning feed intake, and have a lower nursing frequency (Pajor et al., 2002). Similarly, intermittent suckling (IS; separating sow and piglets during a fixed period each day) increases preweaning feed intake (Berkeveld et al., 2007) and improves ADG immediately after weaning (Kuller et al., 2004). However, effects of IS on piglet behavior are currently unknown.
During IS, piglets are involuntarily separated from their sow. Enforced (repeated) maternal deprivation can cause behavioral and physiological changes, both in altricial mammals such as rats (Kuhn and Schanberg, 1998) and in precocious species such as sheep (Napolitano et al., 2003) and calves (Haley et al., 2005). In piglets, a single overnight maternal separation results in greater basal plasma cortisol concentrations (Klemcke and Pond, 1991). Therefore, the extent to which repeated maternal separation (as applied in an IS regimen) affects piglet behavior and induces behavioral patterns associated with postseparation distress (Dybkjaer, 1992) might be questioned.
The objective was to study piglet behavior during IS by determining how IS affects nursing behavior, litter activity, and general behavioral patterns during lactation, and whether IS during an extended lactation results in behavioral patterns associated with piglet distress. In the current study, IS with separation intervals of 6 or 12 h was compared with a conventional treatment (CT).
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The Animal Care and Use Committee of Wageningen University and Research Center approved the following experimental animal use.
A total of 45 multiparous sows (Topigs 40-line, Topigs, Vught, the Netherlands) were used in 3 replicates consisting of 14, 14, and 17 sows, respectively. Sows were randomly selected from a breeding farm, with parities ranging from 3 to 9. Sows were mated at the experimental farm and all were housed individually in the same room during gestation.
Three weeks before farrowing, pregnant sows were individually housed in farrowing pens (2.15 x 2.25 m), consisting of 4.39 m2 of concrete floor and 0.45 m2 of slatted floor. From approximately 1 wk before the expected date of farrowing until weaning, sows were placed in farrowing crates (2.15 x 1.00 m) within the farrowing pen to prevent crushing of piglets. Every morning, the pen was cleaned and the litters were provided with wood shavings. The piglet area was heated by an infrared light from birth until d 10, and by floor heating. Artificial lighting was provided between 0800 and 2000; no additional sunlight was present. During the night, light was dimmed to a level that enabled videorecording of sow and piglet behavior.
Litter size at birth varied from 4 to 18 live piglets and was standardized within 2 d after farrowing by cross-fostering, resulting in an average litter size of 10.9 ± 1.8 piglets. Males were not castrated. In each replicate, the beginning of the experimental procedure (d 0), was designated as the day on which most litters were born. Litters were born from 2 d before to 3 d after d 0.
All piglets were offered creep feed ad libitum from d 7 onward in a piglet feeder with 2 feeding places. During the entire experiment, piglets had unlimited access to drinking water, which was provided by 1 drinking nipple per pen.
Treatments
Before parturition, sows in each replicate were allocated, according to parity, BW, and backfat, to 1 of 3 treatments: 1) CT, 2) intermittent suckling with 12-h intervals (IS12), or 3) intermittent suckling with 6-h intervals (IS6). Litters receiving different treatments were housed in separate rooms.
In CT litters (n = 17), sows were continuously present in the farrowing pen with their litters throughout the 21-d lactation period; CT litters were weaned on d 21 by removal of the sow, and the piglets remained in the farrowing pen until d 41. In the IS treatments, sows were separated from their litters for 12 h/d from d 14 of lactation onward. In both IS groups, sows were present for a total of 12 h/d, but the distribution of these hours differed. In IS12 litters (n = 14) this was a continuous period from 2000 to 0800, and in IS6 litters (n = 14) separation was from 1400 to 2000 and from 0200 to 0800. During separation from their litters, sows were housed individually in a different room. Weaning of IS litters occurred between d 41 and 45 of lactation (mean, d 43 ± 1). Litters were moved from their farrowing pen to a nursery pen at weaning (IS) or on d 41 (CT). The nursery pen consisted of 2.63 m2 of slatted floor and 2.45 m2 of concrete floor. Litters remained in the nursery pens until the end of the experiment at d 55. Two IS litters were weaned earlier than scheduled: 1 litter (IS12) because of a rectal prolapse of the sow (d 27), and 1 litter (IS6) because of mastitis of the sow (d 37). For both of these litters, all data gathered until weaning were included in the analysis.
Measurements
Piglet Performance and Feed Intake.
Individual piglets were weighed 1 d after farrowing and on d 7, 13, 16, 20, 23, 27, 34, 41, and 55. Creep feed residuals were determined per litter simultaneously with BW of the piglets and on d 29. Data on piglet performance and feed intake have been published previously (Berkeveld et al., 2007) but are briefly summarized in the following paragraph.
One week after onset of IS (d 20), creep feed intake was increased in both IS treatments compared with the CT. The ADFI was numerically greater in IS12 litters compared with IS6 litters throughout lactation, resulting in a greater cumulative feed intake in IS12 litters compared with IS6 litters at weaning. Onset of IS induced a growth check in both IS groups, resulting in lower BW at d 20 compared with the CT. A serious growth check was observed after weaning of CT litters (98%), but in IS litters only a mild growth check was observed (14%). Body weights at the end of the experiment were similar among treatments.
Teat Order.
During the first 2 wk of lactation, teat order was assessed for each litter based on the methodology of Kim et al. (2000). In each litter, 2 piglets were selected from cranial (teat pairs 1 and 2), middle (teat pairs 3, 4, and 5) and caudal (teat pairs 6 and 7) teat order (6 in total) and observed during the scan-sampling recordings, as described below.
Behavioral Measurements.
Continuous 24-h, time-lapse videotape recordings were obtained at d 10, 14, 16, 21, and 28. The 24-h recordings were continuously analyzed for litter activity and nursing behavior. Litters were considered active if at least 50% of the piglets demonstrated behavior other than lying. Observations of nursing behavior included beginning time and duration of each nursing, termination by the sow or piglets, and position of the sow (lying or standing) during nursing. A nursing was considered to begin when at least 50% of the piglets were actively massaging the udder and was considered to end when more than 50% of the litter had left the udder or were not actively manipulating the udder (Valros et al., 2002). The nursing was terminated by the sow if she moved from a left or right side lying position to sternal recumbancy or to a standing position. In the case of nursing while standing (motionless), lying down or walking up and down in the farrowing crate were defined as the end of nursing. The nursing was said to be terminated by the piglets if more than 50% of the litter was not actively manipulating the udder. In addition to the duration of nursings, the interval between nursings was calculated as the difference in beginning time between 2 consecutive nursings. Preliminary analyses revealed that the successive nursings shortly after the return of IS sows followed each other rapidly. Because previous research (Weary et al., 1999) indicated that successful nursings rarely occur within 20 min, a nursing in the current study was considered to be unsuccessful when the consecutive nursing began within a 20-min interval. Both total number of nursings and successful nursings were used for analysis of nursing behavior.
On d 17, 24, and 37, the behavior of 6 piglets per pen was observed live for 30 min every 2 h by using 2-min instantaneous scan sampling, thus providing 180 observations per piglet per day. Observations were recorded by using a Psion Organiser II LZ64 handheld data recorder installed with the Observer software package (Noldus Information Technology BV, Wageningen, the Netherlands). Piglets were uniquely marked with color spray for individual identification. The different types of behaviors recorded during scan sampling were based on an ethogram described previously (Bolhuis et al., 2005) and are presented in Table 1.
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Litter activity and nursing behavior were summed per day, and are referred to hereafter as total litter activity and total nursing time or frequency. Furthermore, observations on litter activity were assigned and summed in 4 periods: morning, 0800 to 1400; afternoon, 1400 to 2000; evening, 2000 to 0200; and night, 0200 to 0800. After the onset of IS, sows of IS litters were present in only 2 periods: in the evening and night in IS12 litters, and in the afternoon and night in IS6 litters. In addition to summing activity per period, for IS litters activity was also summed during the periods of absence or presence of the sow. Litter activity during the sow’s absence (or presence; min/12 h) was expressed as the percentage of total litter activity on that day (min/d).
Some of the video observations were missing because of inadequate recordings, mostly caused by technical problems. In the CT, 3 litters had missing values (1.9% of total observations). In the IS12 treatment, 1 litter had missing values (0.4% of total observations). In the IS6 treatment, 2 litters had missing values (1.5% of total observations).
Behavioral observations of each piglet were summed per day. The behavioral elements were summed per category per day (see Table 1
). The proportion of total observations spent per behavioral category was calculated by dividing the number of observations in the behavioral category by the total observations per day.
Statistical Analysis
Unless stated otherwise, data are presented as means ± SE. Effects were considered significant if P < 0.05, and a tendency if 0.05 
P < 0.10; in post hoc testing, the Bonferroni correction was applied. All statistical analyses were performed by using SAS (SAS Inst. Inc., Cary, NC). Normally distributed data were analyzed by using PROC MIXED (Littell et al., 1998). Correlations were calculated by using Pearson correlation coefficients for normally distributed data and Spearman correlation coefficients for skewed data. Correlations between behavioral observations and piglet ADFI or ADG were calculated. Because ADFI and ADG were not determined on a daily basis, the ADFI or ADG of the period closest to the day of behavioral observations was used.
Both data on total nursing time and total litter activity were analyzed by using PROC MIXED, with age on d 0, treatment, day, and their interaction as fixed factors, replicate as a random factor, and sow (nested within treatment and replicate) as a repeated measurement, with an autoregressive covariance structure. The effect of treatment was tested against sow as the error term. Data on litter activity over periods were analyzed by using the same model, but with period and its interaction with treatment and day as additional fixed factors.
Data on the proportion of total observations spent per behavioral category (scan sampling) were analyzed by using PROC MIXED, with age on d 0, treatment, day, and their interaction as fixed factors, replicate and sow (nested within replicate and treatment) as random factors, and piglet (nested within treatment, replicate, and sow) as a repeated measurement, with an autoregressive covariance structure. The effect of treatment was tested against the random sow effect. Because no significant effect of teat-order class on piglet behavior was observed, it was omitted from the analyses. In the case of a skewed distribution of residuals, various proportions of behavior were arcsine square root-transformed to obtain homogeneity of variances. Transformation of proportions did not result in a normal distribution of residuals for eating, aggressive, and manipulative behavior. These behaviors and the percentage of standing nursings were analyzed by using a nonparametric Kruskal-Wallis test. If this test detected an overall treatment effect, data were subsequently tested pairwise.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Total litter activity on d 10, before onset of IS, was similar in all 3 treatments (Table 2). Litter activity was greatest in the morning and decreased gradually in all treatments during the afternoon, evening, and night, resulting in a lower activity at night (P < 0.04) compared with the morning in all 3 groups (Figure 1). This pattern was maintained throughout lactation in litters of the CT. During the first day of the IS regimen (d 14), total litter activity in IS12 litters was greater (P = 0.04) compared with the CT piglets; activity in IS6 litters was intermediate (Table 2). Furthermore, the onset of IS resulted in a marked change in the activity pattern over the 24-h cycle in the IS litters (Figure 1). After 2 d of IS (d 16), litter activity in IS12 litters was lower (P < 0.001) during the absence of the sow (morning and afternoon) and greater (P < 0.001) during the first 6 h after the return of the sow (evening) compared with the CT litters (Figure 1). Similarly, IS6 activity was also lower (P < 0.001) during the absence of the sow (morning and evening) and greater (P < 0.001) during the presence of the sow (afternoon and night) compared with the CT litters (Figure 1). The activity pattern observed on d 16 was maintained on d 21 and 28 for all treatments (data not shown).
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). The CT litters had lower (P < 0.001) activity after weaning at d 21 and 28 compared with preweaning values at d 16. Although total litter activity was similar in both IS groups between d 14 and 28, the proportion of litter activity observed during the absence of the sow was greater in IS12 litters compared with IS6 litters (Figure 2).
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Total nursing time and frequency on d 10, before the beginning of IS, was similar in all 3 treatments (Table 3). On d 14 and 16, total nursing frequency was lower (P < 0.001) in IS litters compared with CT litters, but no differences existed between both IS treatments (Table 3). On d 14 and 16, total nursing time of IS12 litters was also lower (P < 0.001) compared with CT litters; IS6 was intermediate. However, if total nursing time of litters was expressed as nursing time per hour of time together (24 h for CT and 12 h for IS litters), IS6 litters spent more time (14.2 min/h; P < 0.001) nursing on d 16 compared with CT litters (8.5 min/h); IS12 litters were intermediate (11.0 min/h). At d 21 and 28, IS6 litters had a greater (P < 0.02) nursing frequency compared with IS12 litters. Total nursing time, however, was not different between the IS treatments on both days.
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). In contrast to the IS6 treatment, the percentage of successful nursings increased gradually from d 14 to 28 in the IS12 treatment. This resulted in a greater (P < 0.002) percentage of successful nursings in the IS12 treatment on d 21 and 28 (Table 3) compared with the IS6 treatment. However, no differences in frequency of successful nursings existed between the 2 IS treatments (Table 3).
Within each treatment, the percentage of sow-ended nursings was unaltered over time (Table 3). However, on d 16 and 21, a lower percentage (P < 0.04) of sow-ended nursings was observed in IS6 litters compared with IS12 litters.
Effect of IS on Piglet Behavior
The proportion of total observations (n = 180) spent per behavioral category is presented in Table 4. Three days after the onset of IS (d 17), the proportion of total observations spent inactive in IS12 litters was greater compared with IS6 litters (P = 0.04), but inactivity in both IS treatments was similar to the CT. Inactivity of IS12 litters decreased over time (P < 0.001). In contrast, inactivity of IS6 litters increased over time (P < 0.001; d 37), resulting in a greater inactivity compared with IS12 litters on d 37 (P = 0.01). Weaning of CT litters (at d 21) resulted in a transient increase in inactivity (d 24; P < 0.001).
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On d 17 (3 d after beginning IS), the proportion of total observations spent on exploratory behavior was similar for all treatments. In both the IS12 and conventional treatments, exploratory behavior increased over time (P < 0.001), whereas it was relatively stable over time in IS6 litters.
On d 17, time spent on aggressive or manipulative behavior was similar in all treatments. The proportion of total observations spent on manipulative behavior was relatively unaltered over time for both IS treatments but increased over time in CT litters (P < 0.001). On both d 24 and 37, a large proportion of the manipulative behavior in the weaned CT litters consisted of belly nosing (41.5 ± 6.9% and 57.5 ± 4.6% of manipulative behavior, respectively). Belly nosing was not, or only rarely, observed in the unweaned IS litters (0 and 0.1% of total observations for the IS12 and IS6 litters, respectively, on d 37). The incidence of aggressive behavior was quite low and, similarly to manipulative behavior, remained relatively unaltered over time in both IS treatments. Aggressive behavior was greater in CT litters 2 wk after weaning (d 37), compared with preweaning values (d 17; P = 0.01).
A considerable proportion of total observations was spent on nursing behavior in all treatments on d 17. Nursing behavior was lower (P < 0.04) in the IS12 litters compared with both the CT and IS6 litters. Interestingly, IS did not reduce the proportion of total observations spent on nursing behavior in IS6 litters compared with CT litters. The difference in nursing behavior between both IS treatments lasted until d 24 (P = 0.006) but disappeared on d 37 because of a sharp decrease in nursing behavior in IS6 litters (P < 0.001 compared with d 17).
Relationship Between Behavior and Feed Intake or Performance
Feed intake of litters was correlated with eating behavior of the sow’s 6 selected piglets in all treatments on d 17 and 24 (Table 5). On d 37, irrespective of treatment, no correlation between eating behavior and feed intake of litters was observed.
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). This correlation was even greater when only activity during the sow’s absence was used for the calculations on d 21 and 28 and was not found when only activity during the sow’s presence was used.
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The main objective of that study was to investigate whether IS during extended lactation was a suitable management strategy to improve postweaning piglet performance. As a result, the 2 IS regimens were compared with a CT, and the experimental design lacked a weaning regimen with only an extended lactation (without IS). Because CT litters were weaned at d 21, only short-term effects of IS on piglet behavior, during the first week of the regimen, are compared with CT litters. Additionally, differences in piglet behavior throughout lactation between the 2 IS treatments are discussed. However, caution must be used in attributing changes in the behavioral pattern of IS litters entirely to the IS regimen. Age might also have had an influence on the observed piglet behavior during extended lactation.
Under (semi)natural conditions, piglets are weaned gradually, between 11 to 17 wk of age (Jensen, 1986; Stolba and Wood-Gush, 1989; Bøe, 1991). During this period, they make a gradual transition from sow’s milk to a nonmilk diet. During the weaning process, the nursing frequency and the number of sow-initiated nursings decrease, whereas the number of sow-ended nursings and the time spent away from the piglets increase (Jensen, 1988; Jensen and Recen, 1989). Similar to free-ranging sows, both the time spent in the lactation pen and the number of nursings decrease during lactation for sows in a sow-controlled housing system (Rantzer et al., 1995; Weary et al., 2002). As expected, the onset of IS in the current study resulted in a sharp decrease in nursing frequency in both IS treatments. Interestingly, the frequency of successful nursings in both IS treatments after the onset of IS corresponded closely to the decreased nursing frequency found in a sow-controlled housing system during a 12-h presence of the sow (Pajor et al., 2002). In contrast to free-ranging sows, and irrespective of treatment, the percentage of sow-ended nursings was similar over time. Moreover, it was lower (on d 14) compared with the 55% on d 13 postpartum reported previously (Valros et al., 2002). The difference in proportion of sow-ended nursings can probably be attributed to the housing conditions of sows during lactation (with vs. without a farrowing crate). Maternal behavior has a great influence on observed nursing behaviors (Pitts et al., 2002). The small sample size in the current study may have been inadequate to overcome individual differences and reveal differences among treatments in nursing behavior. As a result, the lack of treatment or time effects should be interpreted with care, especially when variation within treatments is high.
The greater nursing frequency in IS6 litters compared with IS12 litters might indicate that IS6 litters maintained a stronger motivation for nursing behavior compared with the IS12 litters, which is supported by the lower proportion of total observations spent on ingestive behavior. The difference in nursing frequency and the greater percentage of unsuccessful nursings were probably caused by a high nursing frequency directly after reunion with their dams, which occurred twice daily in IS6 litters and only once daily in IS12 litters. On the other hand, the IS12 sows may have been less compliant in nursing their litters than IS6 sows, because the percentage of sow-ended nursings was greater in IS12 litters. Although the number of successful nursings was comparable between the 2 IS treatments, the IS6 litters had a greater growth in the first week following IS (Berkeveld et al., 2007) while creep feed intake was still relatively low. Considering this, it might be questioned whether the defined "unsuccesful" nursings (nursings followed by a consecutive nursing within 20 min) were indeed without any milk yield for the piglets. However, the findings are similar to data indicating that a shorter interval between nursings leads to a greater proportion of unsuccessful nursings, but also results in a greater milk yield (27%) and greater growth (44%) of litters compared with litters with longer nursing intervals (
pinka et al., 1997). Furthermore, a greater nursing frequency results in a greater mammary gland mass (Auldist et al., 2000). Interestingly, nonnutritive nursings increase plasma prolactin concentration, which increases proliferation of mammary glands and promotes lactational performance (Rushen et al., 1993; pinka et al., 1999). Plasma prolactin concentrations of sows decrease rapidly within the first 2 h after removal of piglets, but are restored rapidly to normal lactation levels after reunion with their litters (Bevers et al., 1978). After weaning, a reduction in plasma prolactin concentrations to a constant level of 1 to 2 ng/mL takes place within 5 to 6 h (Bevers et al., 1978; Rojkittikhun et al., 1991). To our knowledge, no studies are available describing a diurnal rhythm of prolactin secretion in lactating sows, eliminating a possible effect of differences in light regimen. Therefore, we hypothesize that in the current study, sows in the IS6 treatment might have maintained greater prolactin levels because of shorter separation intervals compared with sows in the IS12 treatment, possibly resulting in greater milk production and subsequently greater litter growth.
The greater total litter activity in IS treatments on d 14 may have been the result of restless activity associated with a sudden, previously unexperienced separation from the sow (Fraser et al., 1998), because this difference completely disappeared on d 16. After the onset of the IS regimens, litter activity coincided closely with the presence of the dam, resulting in dramatic changes in 24-h activity patterns compared with CT litters. Nevertheless, the influence of light and dark on the activity of litters should also be recognized. The IS12 litters were separated from their sows during the day, but one separation period of the IS6 litters was during the evening, when the lights were dimmed. As observed in the CT litters, piglets are normally less active during the dark period compared with the light period. Therefore, some of the observed differences in the litter activity pattern can be attributed to the light regimen. However, as shown in Figure 1 (d 16), the contact frequency with their dams had a stronger influence on the activity pattern of IS litters than the light regimen. In addition, Kirkwood and Smith (1983) demonstrated that piglets separated from their sows overnight consumed similar amounts of feed compared with piglets with daytime separations (12 h). The greater contact frequency of IS6 litters with their dams might account for the attenuated stimulation of eating behavior, and consequently lower feed intake compared with IS12 litters. Interestingly, the observed differences in both eating and nursing behavior between IS treatments agree with a greater estimated dependency on milk for their growth in IS6 litters compared with IS12 litters (Berkeveld et al., 2007). In conclusion, we postulate that the observed differences in piglet behavior of IS litters were mainly attributable to the separation regimen, and were slightly influenced by light conditions.
In line with previous findings, weaning of CT litters resulted in an increased inactivity, which might be considered a symptom of stress (Colson et al., 2006). However, we suspect that the increased inactivity of IS6 litters in late lactation is most probably explained by the decrease in nursing behavior and was an indication of the piglets’ comfort in their environment, rather than a sign of distress. Several behavioral patterns are known to be associated with postseparation piglet distress (Dybkjaer, 1992), including belly nosing, manipulating pen mates, and aggression. Belly nosing is most probably associated with the need to suckle (Metz and Gonyou, 1990; Gonyou et al., 1998). The absence of belly nosing in IS litters, maternally deprived from d 14 onward, might indicate that complete weaning is necessary to induce development of belly-nosing behavior, because weaning of CT litters at d 21 induced belly nosing. Similarly, the weaned CT litters demonstrated a greater proportion of total observations spent on manipulative behavior and aggression, whereas these behavioral patterns were absent in IS litters. Results of the current study indicate that IS during an extended lactation is not associated with the development of behavioral patterns indicative of postseparation piglet distress, suggesting that repeated maternal separation does not induce the behavioral patterns associated with complete separation of the sow and piglets after weaning. However, no conclusions can be drawn for the behavioral development after weaning, because no behavioral observations were made in IS litters after complete weaning.
In conclusion, IS reduced the nursing frequency, and litter activity largely coincided with the presence of the sow during an extended lactation period. Separation of the sow and piglets during 12 consecutive hours in IS12 litters reduced the time spent on nursing behavior and, similarly to weaned CT litters, increased the time spent on exploratory and ingestive behavior during lactation, possibly improving adaptation to the postweaning state. Distribution of total separation time (12h/d) over 2 periods synchronized litter activity even more to the presence of the sow, because they spent more time on nursing and less on eating behavior. Finally, both IS regimens were not associated with any behavioral patterns indicative of piglet distress. As such, an intermittent suckling regimen may provide a gradual adaptation to the postweaning situation without causing obvious behavioral distress.
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Footnotes |
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3 Current affiliation: SARDI Livestock Systems, University of Adelaide, Roseworthy Campus, SA 5371, Australia.
2 Corresponding author: m.berkeveld{at}vet.uu.nl
Received for publication April 19, 2007. Accepted for publication August 16, 2007.
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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pinka, M., G. Illmann, B. Algers, and Z. t
tková. 1997. The role of nursing frequency in milk production in domestic pigs. J. Anim. Sci. 75:1223–1228.pinka, M., G. Illmann, Z. ttková, P. Krej
í, M. Tománek, L. Sedlák, and J. Lidick
. 1999. Prolactin and insulin levels in lactating sows in relation to nursing frequency. Domest. Anim. Endocrinol. 17:53–64.[CrossRef][Medline]pinka, H. Saloniemi, L. Rydhmer, and B. Algers. 2002. Nursing behaviour of sows during 5 weeks lactation and effects on piglet growth. Appl. Anim. Behav. Sci. 76:93–104.[CrossRef]This article has been cited by other articles:
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