Reproductive endocrinology and postweaning performance in the multiparous sow

Theriogenology 58 (2002) 1519±1530

Reproductive endocrinology and postweaning performance in the multiparous sow Part 2. In¯uence of nursing behavior Fredrik HulteÂna,*, Anna Valrosb, Margareta Rundgrenc, Stig Einarssona a

Department of Obstetrics and Gynaecology, Centre of Reproductive Biology in Uppsala, Swedish University of Agricultural Sciences, Uppsala, Sweden b Section of Animal Hygiene, Faculty of Veterinary Medicine, University of Helsinki, Helsinki, Finland c Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, Uppsala, Sweden Received 19 April 2001; accepted 14 February 2002

Abstract The reason for variation in postweaning reproductive performance among multiparous sows is to a large extent unknown. In the present study, the in¯uence of nursing behavior was explored. Blood samples were collected during lactation and after weaning from 18 multiparous sows for cortisol, LH, estradiol-17b (E2), and progesterone analysis. Sow and piglet behavior was videotaped. The sows were fed according to litter size and slaughtered after the second postweaning estrus. The sows were divided into two groups based on average values for the different behavioral parameters. Sows with a long average nursing duration (long group) had lower average and basal LH levels on Day 14 and 21 of lactation as compared to the sows having a short average nursing duration (short group). In the long group, concentrations of E2 were lower the day after weaning, but on Day 15 and 21 of lactation no differences were noted between the two groups. Postweaning performance seemed impaired in the long group, though, differences were not signi®cant. The sows in the long group were heavier and tended to lose less weight during lactation. To conclude, nursing duration seems to in¯uence the extent to which reproductive functions are inhibited during lactation. # 2002 Elsevier Science Inc. All rights reserved. Keywords: Sow; Lactation; Nursing; Ovulation rate; Estrus

* Corresponding author. Tel.: ‡46-18-672507; fax:‡46-18-673545. E-mail address: [email protected] (F. HulteÂn).

0093-691X/02/$ ± see front matter # 2002 Elsevier Science Inc. All rights reserved. PII: S 0 0 9 3 - 6 9 1 X ( 0 2 ) 0 1 0 6 0 - 9

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1. Introduction During lactation sows are normally anestrus but in immediate association with weaning increased activity in the hypothalamus±pituitary±ovarian (HPO) axis occurs, leading to ovulation shortly after weaning. High productivity is dependent on the weaning to ®rst estrus interval being short and a suf®cient number of vital oocytes being shed at ®rst estrus. Recruitment of oocytes commences during late lactation and this process seems to be in¯uenced by nutritional status of the sow [1]. Nutritional status during lactation may also affect the viability of the fertilized oocytes [2]. However, there is large variation in postweaning reproductive performance among sows equally nourished during lactation [3,4]. Additionally, the relationship between nutritional status during lactation and reproductive performance has mainly been indicated for the primiparous sow [5,6]. Regarding the multiparous sow, there is little evidence for such a relationship [7±10], demonstrating that the causes for variation in postweaning reproductive performance in the multiparous sow need to be further elucidated. The suckling stimuli is considered to be the main factor inhibiting ovarian activity during lactation [11,12]. Suckling frequency seems to decline as the lactation period progresses and this circumstance is considered to explain the simultaneous increase in HPO activity [11,13]. However, the extent to which suckling frequency actually declines during Week 3±5 of lactation seems highly variable, and changes during the suckling periods may be negligible [14±16]. Nursing behavior in the sow is a complex process constituted of visual, olfactory, and auditory interactions; massage phases; and a milk letdown phase [17]. Other components in this process, apart from the suckling frequency, may be associated with reproductive functions. Considerable variation with regard to the effects on reproductive function is noted in trials where intervention in the suckling behavior has been conducted by means of separating the sows and piglets during lactation. When applying 12 h daily separation, the occurrence of lactational estrus varies between 0 and 100% [18±20]. These ®ndings suggest that further knowledge is needed regarding the in¯uence of different aspects of the nursing process on reproductive function. It is well known that stress induces behavioral changes in the sow and also in¯uences reproductive function [21±23]. Thus, to explore the interaction between nursing behavior and reproduction, stress-induced bias has to be taken into consideration, for example by measuring of cortisol. The aim of this study was to elucidate the in¯uence of different components of the nursing behavior on reproductive endocrinology and postweaning reproductive performance in the multiparous sow. 2. Material and methods Parts of the data set presented in this study have been used previously to assess the in¯uence of catabolic rate on postweaning reproductive performance and the in¯uence of sow behavior on piglet performance [10].

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2.1. Animals and housing Data concerning animals and housing has previously been presented [10]. The study included 20 multiparous (3rd to 8th parity) pure bred Yorkshire sows and the study period was from 3 days after parturition until 2 days after second postweaning estrus. The sows farrowed in ®ve separate batches, each consisting of four sows. During gestation the sows were group housed in a deep-litter system and 1 week before expected farrowing, they were provided a jugular vein catheter and moved to the experimental farrowing unit. The sows were weaned at Day 35 (2) of lactation, and after the second postweaning estrus, they were slaughtered by use of a bolt gun. All four pens in the farrowing unit were separated by aisles, and pen size was 6.2 m2 including a creep area provided with a heat lamp. The sows were kept loose in the farrowing pens throughout lactation but to prevent crushing of piglets, the pen was provided with steel pipes running along the walls about 15 cm above the ¯oor. Room temperature was kept between 15 and 20 8C, arti®cial light was provided 14 h per day, and dim light was kept on during the night to allow 24 h videotapings. Background noise was about 65 dB. The pens were cleaned twice daily and provided plenty of fresh straw. After a 35-day long (2 days) lactation period, the sows were separated from their piglets and moved to a dry section where they were kept in individual pens with an adult boar until slaughtered. The sows had free access to water during the experimental period. The feed provided during lactation contained 12.4 MJ metabolizable energy (ME)/kg, 15.5% crude protein, and had a lysine content of 6.9 g/kg, whereas energy and crude protein level in the feed given before and after the lactation period was 11.5 MJ ME/kg and 13%, respectively. During the gestation period preceding the experiment, they were individually fed 2.8 kg/day; during lactation they were given 2 kg ‡ 0:6 kg per piglet; from weaning to ®rst estrus daily ration was 3.5 kg and thereafter 3 kg. Sow feed consumption was recorded during lactation. From Day 22 postpartum, the piglets were given ad libitum access to creep feed. Water was available to the piglets from valves throughout the lactation period. During lactation, the rectal temperature and health status of the sows were checked every morning. Piglets were marked individually with an ear tattoo on the day after farrowing. On the fourth day after farrowing, all piglets were supplemented with iron i.m. and the male piglets were castrated. The sows and piglets were weighed at time of farrowing, on Day 15 of lactation and at time of weaning. 2.2. Surgery and care of catheter One week before expected farrowing, the sows were given general anesthesia and a permanent jugular vein catheter was surgically ®tted [24] in order to enable frequent blood sampling without disturbing the animals. Throughout the study period, the catheter was rinsed once a day with 5 ml of NaCl (9 mg/ml) solution containing 0.5% Heparin 5000 U/ml (Leo Pharma, Malmo, Sweden) and 1% Bensylpenicillin (Novocillin vet1 0.32 g/ml Boehringer Ingelheim Vetmedica, Ingelheim, Denmark). In conjunction with frequent blood sampling, the catheter was rinsed with approximately 3 ml NaCl (9 mg/ml) solution containing 0.5% Heparin 5000 IU/ml (Leo Pharma).

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2.3. Blood collection Samples for LH, estradiol-17b (E2), and cortisol analysis were collected in heparinized tubes on Day 14 and 21 of lactation and on the day after weaning. The samples were drawn every 15 min from 09:00 to 15:00 h. Analysis of LH was performed on all samples, cortisol on every second sample, and E2 on every fourth sample. Samples for progesterone analysis were drawn every second day from Day 14 of lactation until 10 days after ®rst postweaning estrus. The blood samples were kept at room temperature for 30 min and then centrifuged for 10 min, 2100  g at 4 8C. After that, the plasma and serum were harvested and stored in a freezer ( 20 8C) until assayed. 2.4. Hormone analyses 2.4.1. Progesterone and E2 Plasma progesterone and E2 concentrations were determined using an enzyme immunoassay kit (Amerlite, Kodak Clinical Diagnostics Ltd., Amersham, England). The kit was used according to the manufacturer's instructions with modi®cations as described by Rojkittikhun et al. [25]. The intra-assay coef®cients of variation, based on the precision pro®les of 20 assays, were 10.3 and 5.6% for low and high progesterone concentrations, respectively. The corresponding inter-assay coef®cients of variation were 8.0 and 7.1%. The intra-assay coef®cients of variation were 21.2 and 6.8% for low and high E2 concentrations, respectively. The corresponding inter-assay coef®cients of variation were 21.8 and 7.1% for low and high assay controls, respectively. The sensitivity was below 1 pmol/l and 2 nmol/l for the E2 and progesterone assay, respectively. 2.4.2. LH The LH concentrations were measured using a heterologous RIA validated for porcine plasma [26]. The intra-assay coef®cients of variation, based on the precision pro®les of 20 assays, were 24.8 and 3.4% for low and high LH concentrations, respectively. The corresponding inter-assay coef®cients of variation were 25.6 and 11.2%. The sensitivity was 0.3 ng/l. The LH peaks and pulses were identi®ed according to the method described by Tsuma et al. [27]. The average LH level and 1 S.D. was calculated for each sow and sampling occasion. A new mean and S.D. was calculated after excluding values higher than the mean plus 1 S.D. The procedure was repeated until there were no values higher than the mean plus 1 S.D. A single value higher than the new mean plus 2 S.D. was considered a peak, while a pulse was de®ned as two or more consecutive peaks. The LH base level was de®ned as the mean plus 1 S.D. of all values excluding the peak(s). 2.4.3. Cortisol Plasma cortisol was determined by luminescence immunoassay according to a method described previously [28]. The intra-assay coef®cients of variation, based on the precision pro®les of 20 assays, were 9.7 and 6.6% for low and high cortisol concentrations, respectively. The corresponding inter-assay coef®cients of variation were 5.9 and 6.9%. The sensitivity was 7.1 nmol/l.

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2.5. Behavioral observations Sow and piglet behavior was videotaped with time lapse recorders for 24 h, starting at 08:30 h, on Day 3, 6, 13, 20, and 30 of lactation. From these videotapes, continuous observations of nursing behavior were made, including nursing frequency and nursing duration, and the proportion of suckling occasions terminated by the sow. If the sow rolled over on her belly or stood up she was considered to have terminated the nursing, while the piglets were considered to have done so if at the end of a nursing period they stayed resting or moved away from the sow as she remained lying on her side. A nursing was considered to start when more than half of the piglets were actively manipulating the udder, and to end when more than half of the litter had left the udder or was inactive by the udder. A nursing was only recorded when duration exceeded 60 s. 2.6. Estrus detection and ultrasonographic examination From the day after weaning, estrus detection was performed twice daily. The sows were considered to have reached the proestrus phase when signs of vulva swelling and reddening were noted. From that time, detection of the onset of estrus was performed every 4 h using the back-pressure test in the presence of a boar. The onset of estrus was estimated as midway between last refusal and ®rst acceptance to stand. During both estrous cycles, the sows were subjected to transrectal ultrasonographic examination to determine the time of ovulation. An annular array sector scanner (Scanner 250, Pie Medical b.v., Maastricht, The Netherlands) with a multi-angle probe (5 MHz) was used. The examination technique has previously been described [29]. Ultrasonography was performed once daily from the onset of proestrus until 20 h after the onset of standing estrus, and thereafter every 4 h until ovulation had occurred. The time of ovulation was de®ned as 2 h before the ®rst time follicles were no longer visible, or 2 h before the time when there was a marked reduction in the number of follicles. 2.7. Determination of ovulation rate On Day 2 after second postweaning estrus, the sows were slaughtered and the genital tract was immediately removed for examination. The number of corpora lutea was recorded on each ovary. By counting both most recent and the older corpora lutea, the ovulation rate in the ®rst and second estrus could be determined. 2.8. Statistical analyses All analyses were performed using the SAS software package [30]. Analysis of variance was applied to data concerning sow weight, litter performance, and postweaning reproductive performance, using the GLM procedure in SAS. The analyses regarding the litter data were based on mean values within sow. Since the behavioral parameter did not show a linear distribution but the sows instead clustered into more or less well-de®ned groups, associations with reproduction traits were not explored by correlation analyses. Instead, the sows were divided into two groups (long and short) for each of the parameters (nursing frequency, nursing duration, and proportion of nursing terminated by the mother). Since the

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values obtained on Day 13, 20, and 30 were strongly correlated (0.50±0.78), average values from these sampling occasions for each sow and parameter were used to form the two groups. The few sows showing values in between the two groups were allocated to the most appropriate group and the cutoff limits were set to create equally large groups. The statistical model included the effect of nursing behavior group (two for each parameter), and the regression on parity number and weight loss during lactation. The CORR procedure in SAS was used to calculate correlations between the different nursing parameters, though distribution of data was not optimal for this analysis. The GLM procedure for repeated measurements was used to analyze progesterone. This model included the ®xed effects of nursing group, week of sampling (Week 3, 4, and 5 of lactation and Week 1 and 2 postweaning), the interaction between group and week of sampling, and the regression on parity number. Sow within group was used as error term in the test of differences between groups. Repeated measurements GLM analysis was also applied to LH, estradiol, and cortisol data. The model was the same as that used to analyze progesterone, with the exception that day of sampling (Day 14 and 21 of lactation and the day after weaning) was included as ®xed effect instead of week of sampling. 3. Results Two sows were excluded from the study due to catheter failure or to postsurgical infection. The other 18 sows were clinically healthy throughout the study period. Average nursing duration was equally long on Day 3, 6, and 13 of lactation, and decreased continuously thereafter (Table 1). The same pattern was noted regarding nursing frequency. On Day 3 of lactation, only a few nursings were terminated by the sow, but this proportion rose markedly as lactation progressed. As discussed in Section 2, the sows were divided into two groups for each parameter. Sows were separated into a long and a short group with regard to average suckling duration. In the long group (n ˆ 9), mean suckling duration per 24 h was 23 min and in the short group (n ˆ 9), it was 168 min (P < 0:001). Parity number was equal, and there were no differences regarding litter size between the two groups (Table 2). The sows in the long group were signi®cantly heavier throughout lactation but tended to lose less weight (Table 2). Feed consumption was about the same (P ˆ 0:86) in the short group (7.9 kg/day) and the long group (7.8 kg/day). Litter weight Table 1 Nursing behavior assessed during 24-h observation periods (LSM  S:E:M:) Day of lactation

Duration of nursings (min/24 h)

3 6 13 20 30

244 247 243 205 173

(21)a (20)a (19)a (19)a,b (20)b

Nursings terminated by the sow n (%) 25 38 53 59 64

(8)a (8)a,b,c (8)b,c (8)c (8)c

Values within column with no superscript in common differ signi®cantly (P < 0:05).

Nursing frequency per 24 h 25.6 28.7 27.5 25.4 23.3

(1.6)a,b (1.5)a (1.5)a (1.5)a,b (1.5)b

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Table 2 Sow weight, age, and litter performance in the short (mean 168 min/24 h) and long (mean 236 min/24 h) nursing duration group (LSM  S:E:M:) Short duration group

Long duration group

5.2 ()

5.2 ()

Parity number Litter size Live born Total born Day 15 At weaning

11.7 12.7 10.0 9.8

Sow weight (kg) Day after farrowing Day 15 of lactation At weaning

() () () ()

11.6 13.3 9.6 9.6

() () () ()

Between group difference P-value 1.0 1.0 0.64 0.56 0.67

256 () 242 () 229 ()

287 () 277 () 272 ()

0.02 0.02 0.003

27 ()

15 ()

0.09

Weight loss farrowing to weaning (kg)

and piglet growth rate did not differ signi®cantly between the two groups at any stage of lactation (Table 3). Progesterone concentrations were low in both groups during lactation, showing that no ovulation had occurred (Table 4). The second week after weaning, progesterone levels rose to equally high levels in both groups, indicating the occurrence of ®rst postweaning estrus and the formation of corpora lutea. Average LH concentrations were signi®cantly lower in the long group on Day 14 and 21 of lactation (Table 5). The same relationship was noted regarding basal LH level on Day 21 of lactation. Furthermore, on Day 14 of lactation, basal LH level tended to be lower in the long group. The day after weaning, the concentration of E2 was signi®cantly lower in the long group. Cortisol concentrations were similar in the two groups during lactation and on the day after weaning (Table 5). The interval between weaning and ®rst estrus seemed longer in the long group and ovulation rate at ®rst estrus seemed lower in this group, but the Table 3 Litter weight and weight gain in the short (mean 168 min/24 h) and long (mean 236 min/24 h) nursing duration group (LSM  S:E:M:) Short duration group

Long duration group

Between group difference P-value

Litter weight (kg) At birth Day 15 of lactation At weaning

17.6 () 50.2 () 112.9 ()

17.3 () 44.4 () 105.9 ()

0.90 0.29 0.60

Daily growth per piglet (g) Farrowing to Day 15 Day 15 to weaning Farrowing to weaning

246 () 330 () 295 ()

217 () 327 () 280 ()

0.12 0.88 0.43

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Table 4 Average progesterone levels (nmol/l) during Week 3±5 of lactation and the first 2 weeks postweaning, in the short (mean 168 min/24 h) and long (mean 236 min/24 h) nursing duration group (LSM  S:E:M:) Progesterone

Lactation week 3

Short duration group Long duration group

Weeks postweaning 4

a

1.8 (0.6) 1.8 (0.7)a

5 a

1.6 (0.7) 1.6 (0.8)a

1 a

2.0 (0.6) 2.3 (0.7)a

2 a

73.8 (5.6)a 72.7 (5.4)a

6.4 (0.5) 4.4 (0.5)a

Values within columns with different superscripts differ signi®cantly (P < 0:05).

differences between the two groups were not signi®cant (Table 6). Ovulation rate at second estrus was similar in the two groups. A negative correlation was noted between nursing duration and the proportion of nursings terminated by the sow (r ˆ 0:72, P < 0:001). However, there was no correlation Table 5 LH, E2, and cortisol levels in the short (mean 168 min/24 h) and long (mean 236 min/24 h) nursing duration group (LSM  S:E:M:) Short duration group

Long duration group

LH average (mg/l) Day 14 of lactation Day 21 of lactation Day after weaning

1.4 (0.4)a,1 1.4 (0.4)a 1.6 (0.5)a

0.8 (0.3)a 0.8 (0.3)a 1.1 (0.4)a

0.03 0.01 0.20

LH basal (mg/l) Day 14 of lactation Day 21 of lactation Day after weaning

1.2 (0.05)a 1.3 (0.05)a 1.5 (0.05)a

0.8 (0.03)a 0.7 (0.03)a 1.0 (0.04)a

0.08 0.03 0.14

LH peaks2 (mg/l) Day 14 of lactation Day 21 of lactation Day after weaning

6.3 (1.5)a 5.0 (1.5)a 6.3 (2.0)a

3.4 (1.2)a 5.0 (1.2)a 4.4 (1.5)a

0.22 0.99 0.49

LH pulses2 (mg/l) Day 14 of lactation Day 21 of lactation Day after weaning

1.2 (0.4)a 1.4 (0.4)a 1.8 (0.5)a

0.8 (0.3)a 1.2 (0.3)a 0.8 (0.4)a

0.54 0.87 0.24

E2 (pmol/l) Day 14 of lactation Day 21 of lactation Day after weaning

7.3 (2.8)a 7.6 (2.8)a 30.7 (3.7)b

4.2 (2.4)a 4.3 (2.4)a 18.2 (2.9)b

0.52 0.48 0.03

Cortisol (nmol/l) Day 14 of lactation Day 21 of lactation Day after weaning

72.2 ()a 70.1 ()a 87.2 ()a

66.2 ()a 72.8 ()a 83.7 ()a

0.65 0.84 0.82

1 2

Between group difference P-value

Values within columns and parameters with no superscript in common differ signi®cantly (P < 0:05). Average pulse and peak frequency per 6 h.

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Table 6 Postweaning reproductive performance in the short (mean 168 min/24 h) and long (mean 236 min/24 h) nursing duration group (LSM  S:E:M:)

Interval Weaning to 1st estrus (h) Onset of estrus to 1st ovulation (h) Weaning to 2nd estrus (h) Onset of 2nd estrus to ovulation Number of corpora lutea 1st estrus 2nd estrus

Short duration group

Long duration group

Between group difference P-value

92 () 35.5 () 504 () 42.7 ()

112 () 37.7 () 510 () 42.4 ()

0.20 0.37 0.67 0.95

22.0 () 20.0 ()

18.7 () 22.4 ()

0.12 0.22

between nursing duration and nursing frequency (r ˆ 0:15, P ˆ 0:52) and between nursing frequency and proportion of nursings terminated by the mother (r ˆ 0:03, P ˆ 0:88). With regard to the behavioral parameters ``nursing frequency'' and ``proportion of nursings terminated by the mother'', no signi®cant (P > 0:05) associations to sow, litter, and reproductive performance were noted, and results concerning these parameters are, therefore, not further presented. 4. Discussion The results of the present study show that nursing duration in¯uences LH secretion during lactation. Signi®cantly, lower levels of LH during lactation and higher levels of E2 the day after weaning, among sows with a long average daily suckling duration, support this conclusion. Further indications of this relationship are given by the fact that the frequency of LH pulses and peaks before and after weaning seemed lower among sows with a long nursing duration, although these differences were not signi®cant. The interval between weaning and ®rst estrus seemed longer among sows with a long nursing duration and ovulation rate seemed lower, as compared to the sows with a short nursing interval. However, since no signi®cant results regarding the effect on postweaning reproductive performance were noted, these indications have to be tested in future studies. To our knowledge, this is the ®rst study where the relationship between reproductive function in the sow and different components of the suckling process has been explored. Previous studies have given strong indications that suckling is the main factor responsible for the occurrence of lactational anestrus in sows [11,12] and nursing frequency has been suggested to be crucial in this context [31]. Manipulation of nursing behavior by separating the piglets from the sow for some hours daily is associated with increasing LH concentrations [32]. In contrast, inconsistent results were reported regarding the effect of fractionated weaning on LH secretion [33,34]. When using this latter design, suckling frequency may remain stable, which could explain, in accordance with our results, that LH secretion might not be altered. By separating the sow and litter, both suckling frequency and duration are likely manipulated, implying that the potential in¯uence of each of these parameters

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cannot be estimated from these studies. However, according to our results suckling frequency has no in¯uence on reproductive functions during or after the lactation period. The fact that in the present study nursing duration and the proportion of nursings terminated by the sow were correlated would suggest that reproductive performance is also affected by this latter parameter, however, this effect was not shown in the present study. Although the correlation coef®cient between these parameters was quite high (0.72), the r2 is only 0.52, indicating that variation between these two parameters is considerable. The sows in the long nursing duration group were heavier at time of farrowing but lost less weight during lactation. Several previous studies have shown that farrowing weight is positively related to weight loss during lactation [8,35,36], and this relationship is considered to be due to variation in appetite. In the present study, the sows in the two suckling duration groups ate equal amounts of feed during lactation. In addition, litter weight and piglet growth rate, which in¯uences sow nutritional level [10], were equal in the two groups. Studies in dogs and sows have shown that suckling is associated with hormonal changes that promote utilization of glucose by the mammary gland [37,38], and to changes that increase digestive capacity [39±42]. Thus, the lower weight loss in the long nursing duration group could indicate that these sows are better able to utilize ingested nutrients as compared to the sows in the short nursing duration group. In the primiparous sow, high weight loss during lactation is associated with a prolonged weaning to estrus interval [43] whereas in the multiparous sow this relationship seems less evident [8]. In the present study, postweaning reproductive performance seemed better among sows having a short nursing duration, despite the fact that these sows tended to lose more weight than the sows with a long nursing duration. These results indicate that for the multiparous sows, nursing behavior is an important explanatory factor for variation in postweaning reproductive performance. No relationship between cortisol concentrations and nursing duration was noted, suggesting that variation in nursing duration is not stress related. Weaning is associated with a transient rise in cortisol concentration, with the highest values at the day of weaning and a marked decrease the day after weaning [27,32]. In our study, the cortisol levels remained at a high level the day after weaning, suggesting that the weaning associated stress reaction had not yet subsided at that time. The fact that at time of weaning sows were transported a short distance to the dry sow section might explain this prolonged reaction. The conclusions from this study should be regarded with some degree of uncertainty due to the fact that sample size was limited. Keeping this in mind, a long nursing duration appears to be associated with a stronger inhibitory in¯uence on reproductive function during lactation, and might impair postweaning reproductive performance. Additionally, a long nursing duration seems to be associated with a decrease in sow weight loss during lactation, while piglet performance is unaffected. Variation in nursing duration seems not to be stress related. If these ®ndings are con®rmed by future studies, nursing behavior may have to be taken into account in future work on improving postweaning reproductive performance in the multiparous sow. Acknowledgements This work was ®nancially supported by the Swedish Meat Farmers Research Program.

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