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A test of sows’ willingness to nurse
Applied Animal Behaviour Science 94 (2005) 49–58 www.elsevier.com/locate/applanim
A test of sows’ willingness to nurse Karen Thodberg *, Karin H. Jensen Department of Animal Health and Welfare, Foulum, Danish Institute of Agricultural Sciences, P.O. Box 50, DK-8830 Tjele, Denmark Accepted 23 January 2005 Available online 7 March 2005
Abstract The aim of the study was to validate a test for willingness of sows to nurse to find a test situation in which maximum variation in the nursing motivation of sows was displayed. The factors considered were the interval since last successful nursing, level of hunger of the sow and activity of the piglets. Fifty-two sows were assigned to six treatments according to a 2 3 factorial design with two levels of hunger, receiving either a normal feed ration (HL1) or 1/3 (HL2) of their normal feed ration at the morning feeding and three intervals since last nursing (70, 100 or 130 min). During the nursing interval, the sow and piglets were isolated in the home environment, however, still in visual and auditive contact. The sows were reunited with their litters and given a small amount of feed and the latency until lateral recumbency was measured. In sows on HL1, the latency was significantly shorter when the nursing interval was 130 compared to 70 min (x2 = 6.13, P = 0.01). On the contrary, the interval since last nursing did not affect the latency to assume lateral recumbency in the sows on HL2, probably because the motivation to eat exceeded the motivation to nurse in these sows. After a nursing interval of 130 min, sows on HL1 were also significantly faster to assume nursing posture compared to HL2 sows (x2 = 5.87, P < 0.05). Finally, the latency to assume lateral recumbency was shorter when the piglets were more active (x2 = 6.17, P = 0.01). The results show that the sows’ willingness to nurse their piglets could be measured in a test based on a conflict between feeding and nursing motivation, if the sows were not too hungry. # 2005 Elsevier B.V. All rights reserved. Keywords: Maternal behaviour; Nursing; Motivation; Behavioural test; Hunger
* Corresponding author. Tel.: +45 89 99 13 23; fax: +45 89 99 15 00. E-mail address: [email protected] (K. Thodberg). 0168-1591/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2005.01.011
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1. Introduction Maternal ability in sows is highly variable and it is not uncommon to find that a high proportion of the crushed piglets in a herd belong to relatively few sows with poor maternal abilities (Dyck and Swierstra, 1987; De Passille and Rushen, 1989; Jarvis et al., 1998; Kongsted and Larsen, 1999; D’eath and Jarvis, 2002). Since development is towards loose housing in the farrowing as well as the gestating unit, it is important to be able to identify animals suited for this kind of production systems. Being a good mother can be described by a long list of qualities and several studies have focussed on the variation in these traits (Grandinson et al., 2003; Spinka et al., 2000), e.g., during nest building (Thodberg et al., 2002a), farrowing (Thodberg et al., 2002a), nursing (Spinka et al., 1997; Thodberg et al., 2002b; Valros. et al., 2002), risky behaviour by the sow (Cronin and Cropley, 1991; Wechsler and Hegglin, 1997; Marchant et al., 2001; Valros. et al., 2003), and weaning and nest leaving (Bøe, 1991, 1994; Pajor et al., 2000; Pitts et al., 2002). In the present study, we have chosen to focus on how to measure the variability in the willingness of sows to nurse as one out of several important elements describing the maternal ability of sows. Normally a sow will lie down to nurse every 45–80 min depending on the stage of lactation (e.g., Jensen et al., 1991; Spinka et al., 1997; Valros. et al., 2002). The sow’s willingness to nurse is influenced by the balance between several factors. There are factors related to the sow’s own condition such as hunger level and time since last nursing and factors related to the piglets, like their activity and especially activity directed towards the sow which again is influenced by time since their last meal (Pajor et al., 2000). Also stimulation from other sows (Wechsler and Brodmann, 1996) and human disturbance affect when the sow chooses to nurse. A low nursing motivation will probably lower the nursing frequency, which have been shown to lower the milk intake of the piglets (Spinka et al., 1997; Valros. et al., 2002). The motivation to nurse decreases during lactation and the piglets gradually takes over the role of initiating nursings from their second week of life (Valros. et al., 2002; Damm et al., 2003). A mother–young conflict is escalated and the sow hereby saves energy for future reproduction (e.g., Fraser et al., 1995). In the first 2 weeks after farrowing it is, however, important that the sow is motivated to nurse as the sow represents the only food resource for the piglets. Especially in the first days after farrowing sows with a low nursing motivation put their piglets at risk; hungry, unthriving piglets are a problem in itself, but these piglets are also more prone to crushing. Presumably weak piglets do not react as readily to the postural changes of the sow as their well-fed and thriving littermates, as suggested by Bauman et al. (1966). Studies have shown, that piglets with a lower weight gain than their littermates, stay closer to the sow nuzzling her udder, thereby enhancing the risk of being crushed (Weary et al., 1996b). A test for willingness of sows to nurse would be a useful tool to select sows and has already been used by Herskin et al. (1999). The test involves a conflict situation between eating and nursing and is performed within the first week after farrowing. In a pilot study, the test was performed without evoking eating motivation by feed provision, which made the sows start to nurse readily after test start with very little variation between sows. Furthermore, the motivation to nurse inevitably is balanced against other motivations, e.g., eating motivation. When put in the conflict between eating and nursing the sow has to choose between providing for herself and her offspring, which is a realistic choice in modern pig production with few
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daily feedings. The outcome of this conflict could be interpreted as the willingness of the sow to nurse. When eating motivation is part of the test complex it is necessary to validate the effect of different hunger levels on the test response. The aim of the present study was to validate the existing test and to find a test situation in which maximum variation in the willingness of sows to nurse is displayed. Interval since last nursing, level of hunger of the sow and her piglets as well as the piglet activity during the test was considered.
2. Material and methods 2.1. Animals and housing Fifty-two Danish Landrace/Yorkshire sows ranging from parity 1 to 7 were recruited from the experimental herd of Research Centre Foulum. They were stalled in crates from 1 week before farrowing and fed at 07:30 and 14:30 h according to Danish standard. In the home crate, a trapdoor could be lowered between the sow and the piglet area, separating the sow and the piglets, however, still allowing auditive and visual contact. The experimental buildings were insulated and thermostatically controlled to approximately 18 8C. 2.2. Design The experiment had a 2 3 factorial design including level of hunger (HL1 and HL2) and interval since last nursing (70, 100 or 130 min). On the day of testing the sows on hunger level 1 (HL1) were fed normally at the morning feeding, whereas the sows on hunger level 2 (HL2) only received 1/3 of the normal ration (normal ration at the morning feeding: 2 kg) in order to induce a higher level of hunger in the HL2 treatment. 2.3. Test for willingness to nurse The test was performed on either day 5, 6 or 7 pp at 13:30 h. The fixed intervals since last nursing were achieved by manipulating the sows to nurse at specific times. If an interval of 100 min was required the piglets were separated from the sow at 10:50 h and reunited with her after 1 h (11:50 h), which almost always made the sow start a successful nursing right away. The nursing was evaluated as successful if the sow and piglets went through all the phases of a successful nursing (Whittemore and Fraser, 1974). After approximately 1 min of final massage, the sow and piglets were separated for 100 min, after which the test began at 13:30 h. The time schedule for the three different nursing interval treatments is illustrated in Fig. 1. If the sow did not stand up she was made to do so before the start of the test. The piglets were reunited with the sow simultaneously with her being introduced to 1 dl of her normal feed. Latency until she resumed lateral recumbency was recorded within a maximum test time of 10 min. The behaviour of the sow and the piglets was recorded by instantaneous sampling in 15 s intervals (Table 1) until lateral recumbency was assumed by the sow or until the maximum 10 min was reached. There were 20 crates in the stall and therefore sometimes other sows would nurse during the test. The sound of other nursing sows
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Fig. 1. The course of the experiment. The sows were fed at 08:00 and 14:30 h and the test was always performed at 13:30 h. The sows were manipulated into having a successful nursing (SN) at either 11:20, 11:50 or 12:20 h.
stimulates sows to start at a nursing as found by Wechsler and Brodmann (1996) and therefore a tape recording of the sounds of nursing sows was played back during the test to standardise the stimulation from neighbouring sows. 2.4. Statistics Latency to assume lateral recumbency was analysed by Cox’s proportional hazards model for survival data using the PHREG procedure of the computer software Statistical Analysis System (SAS Institute, 1996). This model allowed an unspecified form of the underlying survivor function. Due to dependence of covariates, it was not possible, by the usual graphic methods to test whether the assumption of proportionality between response intensities of all animals was fulfilled. Thus, the proportionality was assumed. In all analyses the model included the two factors: hunger level (1 and 2), and interval since last successful nursing (70, 100 and 130 min) and the covariates: piglet behaviour (active/ lying), parity of the sow, litter size and the test day (day 5, 6 or 7 pp). All elements in the model were reduced when the P-value exceeded 0.05, except for test day. In case of no response, the observations were treated as censored as is usual for survival analysis (Kleinbaum, 1997). The results are presented by the median of response from the survival curve including specific covariates, the inter-quantile range, the x2-value from the asymptotic Wald test and the P-value. Table 1 Behavioural categories used for instantaneous sampling in 15 s intervals during the test Sow behaviour Standing idle Standing and eating Standing in contact with piglet Lying in sternal recumbency
The sow is standing without any other activity The sow is standing while eating from the trough The sow is standing while touching a piglet (<5 cm) with her snout The sow is lying on her udder thereby preventing the piglets from access to the teats
Piglet behaviour (>75% of the litter engaged in the activity) Sow oriented activity Trying to nuzzle the udder or other parts of the sow Lying Lying Non-homogenic activity Less than 75% are lying and less than 75% are directing their activity towards the sow.
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Piglet and sow behaviour during the test was analysed separately for each hunger level by the Kruskal–Wallis test (Siegel and Castellen, 1988) using the Npar1way procedure of SAS (SAS Institute, 1996), based on ranks. The results will be presented by the median, the inter-quantile range, the x2-value from the asymptotic Wald test and the P-value.
3. Results In sows on HL1, the latency to assume lateral recumbency was significantly shorter when the interval from last nursing was 130 compared to 70 min (x2 = 6.13, P = 0.01; Fig. 2). On the contrary, interval from last nursing did not affect latency to assume lateral recumbency in sows on HL2. After 130 min nursing interval, sows on HL1 were significantly faster to assume nursing posture compared to those on HL2 (x2 = 5.87, P < 0.05; Fig. 2). The latency to assume lateral recumbency was shorter the higher proportion of time the piglets spent active (x2 = 6.17, P = 0.01). The activity of the piglets depended on the hunger level of the sows. Piglets from HL2 sows did not alter the proportion of time spent performing different behavioural elements according to the interval since last nursing (Fig. 3b). On the contrary, piglets from HL1 sows spent more time in sow directed activity and less time in ‘‘non-homogenic activity’’ when 130 min had passed since last nursing, compared to piglets experiencing the shortest interval of 70 min (x2 = 6.58, n = 26, P < 0.05 and x2 = 8.12, n = 26, P < 0.05; Fig. 3a). Apart from the latency to assume lateral recumbency the sow behaviour during the test did not vary according to the different treatments.
Fig. 2. The latency to assume lateral recumbency shown for sows at hunger level 1 (HL1) or hunger level 2 (HL2) after a nursing interval of 70, 100 or 130 min, respectively. The bars illustrate the inter-quantile range. Columns with different letters differ significantly.
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4. Discussion The present study shows that the sows’ willingness to nurse their piglets could be measured in a test based on a conflict between feeding and nursing motivation, assuming the hunger level of the sow and the piglets is standardised and not too high. In sows on the lowest hunger level 1, the latency to assume lateral recumbency decreased as the time, since last successful nursing increased. However, the same relationship was not present in the data from sows on the highest hunger level 2. The piglets’ behaviour was affected by the state of hunger of the sow. In HL1 sows, the piglets showed more sow-oriented behaviour the longer since last nursing, whereas piglets from HL2 sows did not. Apart from the latency to assume lateral recumbency the behaviour of the sow during the test was not affected by the treatments. The relationship between duration of the separation time and latency to nurse in HL1 sows suggests that the test, under these circumstances, actually measures the sows’ willingness to nurse. However, when using the test to illustrate the variation between sows, the interval between the last successful nursing and the test should not be too long, as the motivation to nurse will be too strong in most sows to display the individuality, as evident from the reduced inter-quantile range for HL1 sows shown in Fig. 2. When the sows are as
Fig. 3. The behaviour of the piglets from either HL1 (A) or HL2 (B) sows during the test. The bars illustrate the inter-quantile range. Columns with different letters within the same category differ significantly.
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Fig. 3. (Continued ).
hungry as HL2 the eating motivation seems to be so strong, that the motivation to nurse the piglets looses in the competition between the two motivations. This means that the hunger level in the sows should be standardised at an optimal level when performing the test and that HL2 is too high. The dependence of the outcome on hunger level also implies that the test measures the willingness to nurse rather than motivation to nurse. We can standardise the hunger level at test time to a certain point, but there will always be individual differences in hunger that we cannot separate from the individual differences in nursing motivation, as we are measuring the outcome of the conflict between two motivations. However, in terms of maternal quality, willingness to nurse may be a key factor rather than the motivation, as a very strong motivation to eat could be just as unwanted as a low nursing motivation, not only in the test situation but also in the undisturbed nursing situations. It could be argued that the test, to some extend, measures hunger of the piglets, and that the changes in the behaviour of the sow in relation to the interval since last nursing are a consequence of more stimulation from the piglets. The latency to assume nursing posture was indeed affected by the piglets’ behaviour; the more active the piglets the sooner the sow assumed nursing posture. Sow-directed activity could be interpreted as begging as suggested by Jensen et al. (1998). Some studies on pigs demonstrate that elements of sowdirected piglet behaviour increase with time since last nursing (Jensen et al., 1998; Pajor
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et al., 2000), although these and other studies (Lewis and Hurnik, 1986; Appleby et al., 1999) indicate that this mechanism depends both on the specific set-up, the level of hunger as well as the possibility for performing nursing behaviour. It has been shown that the young of several bird species beg more (vocalisation and postural changes) when the hunger level is higher (Redondo and Castro, 1992; Leonard et al., 2003) and the surrounding temperature is lowered (Leonard and Horn, 2001a). In pigs, it has been shown that piglets vocalise more when hungry and cold (Weary et al., 1996a, 1997). In the present study, the piglets presumably felt more hungry after a long nursing interval and it is likely that this ‘‘need’’ was communicated to the sow, making her respond sooner. In the HL1 sows, it could be seen that the piglets made more sow directed activity with an increasing interval from the last nursing and that the sows decreased their latency to nurse. It has been shown that sows react stronger to piglets in need due to either hunger or being cold (Weary et al., 1996a). Many examples of this relationship have been shown in birds, with a higher feeding rate towards young in need, showing more begging (Leonard and Horn, 1998, 2001b), but also in marmosets the young receives the caretaking in relation to their signalling of need (Geiss and Schrader, 1996). The observed effect of piglet activity underlines the importance of standardising the hunger of the piglets by controlling the interval since last nursing. The hunger state of the sow seemed, however, also to influence the behaviour of the piglets during the test. As opposed to piglets from HL1 sows, the piglets from HL2 sows did not increase their proportion of sow directed behaviour with increased nursing interval. These results suggest that the willingness to nurse influences the behaviour of the piglets. It could be that the hunger level of the sow affects the communication between sow and piglets. To our knowledge this has not been investigated. We suggest that the supposed more intense feeling of hunger in the HL2 sows is communicated to the piglets. Instead of inviting them to nurse, the sows perhaps signal that the piglets should stay away or maybe they simply stop communicating and focus on the small amount of feed they received at the start of the test. In the present study, no detailed observations of the vocalisations of sows or piglets or how the sow positions herself in relation to the piglets were made. Inclusion of these elements would be relevant in future studies of nursing motivation. In conclusion, the present test measures the sows’ willingness to nurse at least at a moderate hunger level. The biggest variation between sows was displayed when the interval from last nursing was 70 and 100 min, and intervals longer than this should be avoided in a test situation. However, sow hunger disrupts the test result and influences the behaviour of the piglets during the test and therefore hunger should be optimized and standardised during testing. A practical implication of the study is to prevent having too hungry sows in the production, as it could lower their willingness to nurse.
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Spinka, M., Illmann, G., de Jonge, F., Anderson, M., Schuurman, T., Jensen, P., 2000. Dimensions of maternal behaviour characteristics in domestic and wild domestic crossbred sows. Appl. Anim. Behav. Sci. 70, 99– 114. Thodberg, K., Jensen, K.H., Herskin, M.S., 2002a. Nest building and farrowing in sows: relation to the reaction pattern during stress, farrowing environment and experience. Appl. Anim. Behav. Sci. 77, 21–42. Thodberg, K., Jensen, K.H., Herskin, M.S., 2002b. Nursing behaviour, postpartum activity and reactivity of sows: effects of farrowing environment, previous experience and temperament. Appl. Anim. Behav. Sci. 77, 53–76. Valros, A., Rundgren, M., Spinka, M., Saloniemi, H., Rydhmer, L., Algers, B., 2002. Nursing behaviour of sows during 5 weeks lactation and effects on piglet growth. Appl. Anim. Behav. Sci. 76, 93–104. Valros, A., Rundgren, M., Spinka, M., Saloniemi, H., Algers, B., 2003. Sow activity level, frequency of standingto-lying posture changes and anti-crushing behaviour—within sow-repeatability and interactions with nursing behaviour and piglet performance. Appl. Anim. Behav. Sci. 83, 29–40. Weary, D.M., Lawson, G.L., Thopson, B.K., 1996a. Sows show stronger responses to isolation calls of piglets associated with greater levels of piglet need. Anim. Behav. 52, 1247–1253. Weary, D.M., Pajor, E.A., Thompson, B.K., Fraser, D., 1996b. Risky behaviour by piglets: a trade off between feeding and risk of mortality by maternal crushing. Anim. Behav. 51, 619–624. Weary, D.M., Ross, S., Fraser, D., 1997. Vocalisations by isolated piglets: a reliable indicator of piglet need directed towards the sow. Appl. Anim. Behav. Sci. 53, 249–257. Wechsler, B., Brodmann, N., 1996. The synchronization of nursing bouts in group-housed sows. Appl. Anim. Behav. Sci. 47, 191–199. Wechsler, B., Hegglin, D., 1997. Individual differences in the behaviour of sows at the nest-site and the crushing of piglets. Appl. Anim. Behav. Sci. 51, 39–49. Whittemore, C.T., Fraser, D., 1974. The nursing and suckling behaviour of pigs. II. Vocalization of the sow in relation to suckling behaviour and milk ejection. Br. Vet. J. 130, 346–356.
A test of sows’ willingness to nurse Karen Thodberg *, Karin H. Jensen Department of Animal Health and Welfare, Foulum, Danish Institute of Agricultural Sciences, P.O. Box 50, DK-8830 Tjele, Denmark Accepted 23 January 2005 Available online 7 March 2005
Abstract The aim of the study was to validate a test for willingness of sows to nurse to find a test situation in which maximum variation in the nursing motivation of sows was displayed. The factors considered were the interval since last successful nursing, level of hunger of the sow and activity of the piglets. Fifty-two sows were assigned to six treatments according to a 2 3 factorial design with two levels of hunger, receiving either a normal feed ration (HL1) or 1/3 (HL2) of their normal feed ration at the morning feeding and three intervals since last nursing (70, 100 or 130 min). During the nursing interval, the sow and piglets were isolated in the home environment, however, still in visual and auditive contact. The sows were reunited with their litters and given a small amount of feed and the latency until lateral recumbency was measured. In sows on HL1, the latency was significantly shorter when the nursing interval was 130 compared to 70 min (x2 = 6.13, P = 0.01). On the contrary, the interval since last nursing did not affect the latency to assume lateral recumbency in the sows on HL2, probably because the motivation to eat exceeded the motivation to nurse in these sows. After a nursing interval of 130 min, sows on HL1 were also significantly faster to assume nursing posture compared to HL2 sows (x2 = 5.87, P < 0.05). Finally, the latency to assume lateral recumbency was shorter when the piglets were more active (x2 = 6.17, P = 0.01). The results show that the sows’ willingness to nurse their piglets could be measured in a test based on a conflict between feeding and nursing motivation, if the sows were not too hungry. # 2005 Elsevier B.V. All rights reserved. Keywords: Maternal behaviour; Nursing; Motivation; Behavioural test; Hunger
* Corresponding author. Tel.: +45 89 99 13 23; fax: +45 89 99 15 00. E-mail address: [email protected] (K. Thodberg). 0168-1591/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2005.01.011
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1. Introduction Maternal ability in sows is highly variable and it is not uncommon to find that a high proportion of the crushed piglets in a herd belong to relatively few sows with poor maternal abilities (Dyck and Swierstra, 1987; De Passille and Rushen, 1989; Jarvis et al., 1998; Kongsted and Larsen, 1999; D’eath and Jarvis, 2002). Since development is towards loose housing in the farrowing as well as the gestating unit, it is important to be able to identify animals suited for this kind of production systems. Being a good mother can be described by a long list of qualities and several studies have focussed on the variation in these traits (Grandinson et al., 2003; Spinka et al., 2000), e.g., during nest building (Thodberg et al., 2002a), farrowing (Thodberg et al., 2002a), nursing (Spinka et al., 1997; Thodberg et al., 2002b; Valros. et al., 2002), risky behaviour by the sow (Cronin and Cropley, 1991; Wechsler and Hegglin, 1997; Marchant et al., 2001; Valros. et al., 2003), and weaning and nest leaving (Bøe, 1991, 1994; Pajor et al., 2000; Pitts et al., 2002). In the present study, we have chosen to focus on how to measure the variability in the willingness of sows to nurse as one out of several important elements describing the maternal ability of sows. Normally a sow will lie down to nurse every 45–80 min depending on the stage of lactation (e.g., Jensen et al., 1991; Spinka et al., 1997; Valros. et al., 2002). The sow’s willingness to nurse is influenced by the balance between several factors. There are factors related to the sow’s own condition such as hunger level and time since last nursing and factors related to the piglets, like their activity and especially activity directed towards the sow which again is influenced by time since their last meal (Pajor et al., 2000). Also stimulation from other sows (Wechsler and Brodmann, 1996) and human disturbance affect when the sow chooses to nurse. A low nursing motivation will probably lower the nursing frequency, which have been shown to lower the milk intake of the piglets (Spinka et al., 1997; Valros. et al., 2002). The motivation to nurse decreases during lactation and the piglets gradually takes over the role of initiating nursings from their second week of life (Valros. et al., 2002; Damm et al., 2003). A mother–young conflict is escalated and the sow hereby saves energy for future reproduction (e.g., Fraser et al., 1995). In the first 2 weeks after farrowing it is, however, important that the sow is motivated to nurse as the sow represents the only food resource for the piglets. Especially in the first days after farrowing sows with a low nursing motivation put their piglets at risk; hungry, unthriving piglets are a problem in itself, but these piglets are also more prone to crushing. Presumably weak piglets do not react as readily to the postural changes of the sow as their well-fed and thriving littermates, as suggested by Bauman et al. (1966). Studies have shown, that piglets with a lower weight gain than their littermates, stay closer to the sow nuzzling her udder, thereby enhancing the risk of being crushed (Weary et al., 1996b). A test for willingness of sows to nurse would be a useful tool to select sows and has already been used by Herskin et al. (1999). The test involves a conflict situation between eating and nursing and is performed within the first week after farrowing. In a pilot study, the test was performed without evoking eating motivation by feed provision, which made the sows start to nurse readily after test start with very little variation between sows. Furthermore, the motivation to nurse inevitably is balanced against other motivations, e.g., eating motivation. When put in the conflict between eating and nursing the sow has to choose between providing for herself and her offspring, which is a realistic choice in modern pig production with few
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daily feedings. The outcome of this conflict could be interpreted as the willingness of the sow to nurse. When eating motivation is part of the test complex it is necessary to validate the effect of different hunger levels on the test response. The aim of the present study was to validate the existing test and to find a test situation in which maximum variation in the willingness of sows to nurse is displayed. Interval since last nursing, level of hunger of the sow and her piglets as well as the piglet activity during the test was considered.
2. Material and methods 2.1. Animals and housing Fifty-two Danish Landrace/Yorkshire sows ranging from parity 1 to 7 were recruited from the experimental herd of Research Centre Foulum. They were stalled in crates from 1 week before farrowing and fed at 07:30 and 14:30 h according to Danish standard. In the home crate, a trapdoor could be lowered between the sow and the piglet area, separating the sow and the piglets, however, still allowing auditive and visual contact. The experimental buildings were insulated and thermostatically controlled to approximately 18 8C. 2.2. Design The experiment had a 2 3 factorial design including level of hunger (HL1 and HL2) and interval since last nursing (70, 100 or 130 min). On the day of testing the sows on hunger level 1 (HL1) were fed normally at the morning feeding, whereas the sows on hunger level 2 (HL2) only received 1/3 of the normal ration (normal ration at the morning feeding: 2 kg) in order to induce a higher level of hunger in the HL2 treatment. 2.3. Test for willingness to nurse The test was performed on either day 5, 6 or 7 pp at 13:30 h. The fixed intervals since last nursing were achieved by manipulating the sows to nurse at specific times. If an interval of 100 min was required the piglets were separated from the sow at 10:50 h and reunited with her after 1 h (11:50 h), which almost always made the sow start a successful nursing right away. The nursing was evaluated as successful if the sow and piglets went through all the phases of a successful nursing (Whittemore and Fraser, 1974). After approximately 1 min of final massage, the sow and piglets were separated for 100 min, after which the test began at 13:30 h. The time schedule for the three different nursing interval treatments is illustrated in Fig. 1. If the sow did not stand up she was made to do so before the start of the test. The piglets were reunited with the sow simultaneously with her being introduced to 1 dl of her normal feed. Latency until she resumed lateral recumbency was recorded within a maximum test time of 10 min. The behaviour of the sow and the piglets was recorded by instantaneous sampling in 15 s intervals (Table 1) until lateral recumbency was assumed by the sow or until the maximum 10 min was reached. There were 20 crates in the stall and therefore sometimes other sows would nurse during the test. The sound of other nursing sows
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Fig. 1. The course of the experiment. The sows were fed at 08:00 and 14:30 h and the test was always performed at 13:30 h. The sows were manipulated into having a successful nursing (SN) at either 11:20, 11:50 or 12:20 h.
stimulates sows to start at a nursing as found by Wechsler and Brodmann (1996) and therefore a tape recording of the sounds of nursing sows was played back during the test to standardise the stimulation from neighbouring sows. 2.4. Statistics Latency to assume lateral recumbency was analysed by Cox’s proportional hazards model for survival data using the PHREG procedure of the computer software Statistical Analysis System (SAS Institute, 1996). This model allowed an unspecified form of the underlying survivor function. Due to dependence of covariates, it was not possible, by the usual graphic methods to test whether the assumption of proportionality between response intensities of all animals was fulfilled. Thus, the proportionality was assumed. In all analyses the model included the two factors: hunger level (1 and 2), and interval since last successful nursing (70, 100 and 130 min) and the covariates: piglet behaviour (active/ lying), parity of the sow, litter size and the test day (day 5, 6 or 7 pp). All elements in the model were reduced when the P-value exceeded 0.05, except for test day. In case of no response, the observations were treated as censored as is usual for survival analysis (Kleinbaum, 1997). The results are presented by the median of response from the survival curve including specific covariates, the inter-quantile range, the x2-value from the asymptotic Wald test and the P-value. Table 1 Behavioural categories used for instantaneous sampling in 15 s intervals during the test Sow behaviour Standing idle Standing and eating Standing in contact with piglet Lying in sternal recumbency
The sow is standing without any other activity The sow is standing while eating from the trough The sow is standing while touching a piglet (<5 cm) with her snout The sow is lying on her udder thereby preventing the piglets from access to the teats
Piglet behaviour (>75% of the litter engaged in the activity) Sow oriented activity Trying to nuzzle the udder or other parts of the sow Lying Lying Non-homogenic activity Less than 75% are lying and less than 75% are directing their activity towards the sow.
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Piglet and sow behaviour during the test was analysed separately for each hunger level by the Kruskal–Wallis test (Siegel and Castellen, 1988) using the Npar1way procedure of SAS (SAS Institute, 1996), based on ranks. The results will be presented by the median, the inter-quantile range, the x2-value from the asymptotic Wald test and the P-value.
3. Results In sows on HL1, the latency to assume lateral recumbency was significantly shorter when the interval from last nursing was 130 compared to 70 min (x2 = 6.13, P = 0.01; Fig. 2). On the contrary, interval from last nursing did not affect latency to assume lateral recumbency in sows on HL2. After 130 min nursing interval, sows on HL1 were significantly faster to assume nursing posture compared to those on HL2 (x2 = 5.87, P < 0.05; Fig. 2). The latency to assume lateral recumbency was shorter the higher proportion of time the piglets spent active (x2 = 6.17, P = 0.01). The activity of the piglets depended on the hunger level of the sows. Piglets from HL2 sows did not alter the proportion of time spent performing different behavioural elements according to the interval since last nursing (Fig. 3b). On the contrary, piglets from HL1 sows spent more time in sow directed activity and less time in ‘‘non-homogenic activity’’ when 130 min had passed since last nursing, compared to piglets experiencing the shortest interval of 70 min (x2 = 6.58, n = 26, P < 0.05 and x2 = 8.12, n = 26, P < 0.05; Fig. 3a). Apart from the latency to assume lateral recumbency the sow behaviour during the test did not vary according to the different treatments.
Fig. 2. The latency to assume lateral recumbency shown for sows at hunger level 1 (HL1) or hunger level 2 (HL2) after a nursing interval of 70, 100 or 130 min, respectively. The bars illustrate the inter-quantile range. Columns with different letters differ significantly.
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4. Discussion The present study shows that the sows’ willingness to nurse their piglets could be measured in a test based on a conflict between feeding and nursing motivation, assuming the hunger level of the sow and the piglets is standardised and not too high. In sows on the lowest hunger level 1, the latency to assume lateral recumbency decreased as the time, since last successful nursing increased. However, the same relationship was not present in the data from sows on the highest hunger level 2. The piglets’ behaviour was affected by the state of hunger of the sow. In HL1 sows, the piglets showed more sow-oriented behaviour the longer since last nursing, whereas piglets from HL2 sows did not. Apart from the latency to assume lateral recumbency the behaviour of the sow during the test was not affected by the treatments. The relationship between duration of the separation time and latency to nurse in HL1 sows suggests that the test, under these circumstances, actually measures the sows’ willingness to nurse. However, when using the test to illustrate the variation between sows, the interval between the last successful nursing and the test should not be too long, as the motivation to nurse will be too strong in most sows to display the individuality, as evident from the reduced inter-quantile range for HL1 sows shown in Fig. 2. When the sows are as
Fig. 3. The behaviour of the piglets from either HL1 (A) or HL2 (B) sows during the test. The bars illustrate the inter-quantile range. Columns with different letters within the same category differ significantly.
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Fig. 3. (Continued ).
hungry as HL2 the eating motivation seems to be so strong, that the motivation to nurse the piglets looses in the competition between the two motivations. This means that the hunger level in the sows should be standardised at an optimal level when performing the test and that HL2 is too high. The dependence of the outcome on hunger level also implies that the test measures the willingness to nurse rather than motivation to nurse. We can standardise the hunger level at test time to a certain point, but there will always be individual differences in hunger that we cannot separate from the individual differences in nursing motivation, as we are measuring the outcome of the conflict between two motivations. However, in terms of maternal quality, willingness to nurse may be a key factor rather than the motivation, as a very strong motivation to eat could be just as unwanted as a low nursing motivation, not only in the test situation but also in the undisturbed nursing situations. It could be argued that the test, to some extend, measures hunger of the piglets, and that the changes in the behaviour of the sow in relation to the interval since last nursing are a consequence of more stimulation from the piglets. The latency to assume nursing posture was indeed affected by the piglets’ behaviour; the more active the piglets the sooner the sow assumed nursing posture. Sow-directed activity could be interpreted as begging as suggested by Jensen et al. (1998). Some studies on pigs demonstrate that elements of sowdirected piglet behaviour increase with time since last nursing (Jensen et al., 1998; Pajor
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et al., 2000), although these and other studies (Lewis and Hurnik, 1986; Appleby et al., 1999) indicate that this mechanism depends both on the specific set-up, the level of hunger as well as the possibility for performing nursing behaviour. It has been shown that the young of several bird species beg more (vocalisation and postural changes) when the hunger level is higher (Redondo and Castro, 1992; Leonard et al., 2003) and the surrounding temperature is lowered (Leonard and Horn, 2001a). In pigs, it has been shown that piglets vocalise more when hungry and cold (Weary et al., 1996a, 1997). In the present study, the piglets presumably felt more hungry after a long nursing interval and it is likely that this ‘‘need’’ was communicated to the sow, making her respond sooner. In the HL1 sows, it could be seen that the piglets made more sow directed activity with an increasing interval from the last nursing and that the sows decreased their latency to nurse. It has been shown that sows react stronger to piglets in need due to either hunger or being cold (Weary et al., 1996a). Many examples of this relationship have been shown in birds, with a higher feeding rate towards young in need, showing more begging (Leonard and Horn, 1998, 2001b), but also in marmosets the young receives the caretaking in relation to their signalling of need (Geiss and Schrader, 1996). The observed effect of piglet activity underlines the importance of standardising the hunger of the piglets by controlling the interval since last nursing. The hunger state of the sow seemed, however, also to influence the behaviour of the piglets during the test. As opposed to piglets from HL1 sows, the piglets from HL2 sows did not increase their proportion of sow directed behaviour with increased nursing interval. These results suggest that the willingness to nurse influences the behaviour of the piglets. It could be that the hunger level of the sow affects the communication between sow and piglets. To our knowledge this has not been investigated. We suggest that the supposed more intense feeling of hunger in the HL2 sows is communicated to the piglets. Instead of inviting them to nurse, the sows perhaps signal that the piglets should stay away or maybe they simply stop communicating and focus on the small amount of feed they received at the start of the test. In the present study, no detailed observations of the vocalisations of sows or piglets or how the sow positions herself in relation to the piglets were made. Inclusion of these elements would be relevant in future studies of nursing motivation. In conclusion, the present test measures the sows’ willingness to nurse at least at a moderate hunger level. The biggest variation between sows was displayed when the interval from last nursing was 70 and 100 min, and intervals longer than this should be avoided in a test situation. However, sow hunger disrupts the test result and influences the behaviour of the piglets during the test and therefore hunger should be optimized and standardised during testing. A practical implication of the study is to prevent having too hungry sows in the production, as it could lower their willingness to nurse.
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