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ANIMAL PRODUCTION |
* Applied Research Division of Animal Sciences Group, Wageningen UR, 8203 AD Lelystad, The Netherlands;
and
Wageningen Institute of Animal Sciences, Wageningen UR, 6700 AH Wageningen, The Netherlands;
and
Biometris, 6700 AC Wageningen, The Netherlands; and
and
Nutreco Agriculture Research and Development, 5830 AE Boxmeer, The Netherlands
Abstract
The objective of this experiment was to study the effects of feeding group-housed gestating sows a diet with a high level of fermentable nonstarch polysaccharides (NSP; approximately 45% sugar beet pulp as fed) ad libitum on the development in individual feed intake characteristics and reproductive performance during three successive reproduction cycles. Performance of the ad libitum-fed sows was compared to the performance of sows that were fed a conventional diet restrictedly. Feed intake characteristics during gestation were only measured in the ad libitum-fed sows. One hundred and nineteen sows were assigned to one of two gestation feeding regimens. Gestating sows were fed a conventional Dutch diet restrictedly or a diet with a high level of fermentable NSP ad libitum. During lactation, sows were given free access to a commercial lactation diet from d 6 after parturition onward. The ad libitum-fed sows ate 1.3 kg/d more during gestation than the restrictedly fed sows (P < 0.001), resulting in higher body weight and backfat gains during gestation (P < 0.05). Sows that were fed ad libitum during gestation lost more body weight and backfat during lactation (P < 0.001) than sows that were fed restrictedly during gestation. Feed intake during lactation, however, did not differ between sows that were fed restrictedly or ad libitum during gestation. The numbers of total piglets born, live-born and stillborn piglets, piglet birth weight, weaning-to-estrus interval, and percentage of sows that returned to estrus after first insemination were not affected by gestation feeding regimen. Mean daily voluntary feed intake (as-fed basis) over the three reproduction cycles in the ad libitum-fed gestating sows was 4.2 kg/d. Depending on the number of preceding reproduction cycles during which a sow was fed ad libitum, the maximum voluntary feed intake was reached in Parity 3, 4, or 5 and then remained stable in subsequent parities. Mean daily feed intake of the ad libitum-fed sows increased from wk 2 to 6 of gestation and then decreased to wk 15 of gestation. The mean number of daily visits with feed intake over the three reproduction cycles was 13.8. On average, ad libitum-fed sows spent 90 min/d on eating. This study shows that it is possible to feed gestating sows a diet with a high level of fermentable NSP ad libitum during three successive reproduction cycles without negative effects on reproductive performance.
Key Words: Feeding • Fiber • Gestation • Polysaccharides • Reproduction • Sows
Introduction
Feed restriction is identified as one of the major factors associated with the development of stereotypic behavior in gestating sows (Terlouw et al., 1991
). Ad libitum feeding of gestating sows is very effective in reducing stereotypic behavior (Bergeron et al., 2000). However, ad libitum feeding with the conventional low-fiber diets is not a viable option because of the high-feed intake capacity of sows and its concomitant obesity (Petherick and Blackshaw, 1989). Diets with a high level of fermentable nonstarch polysaccharides (NSP) can limit ad libitum feed intake to acceptable levels (Brouns et al., 1995). However, data on the development in individual feed intake characteristics during gestation and in successive parities of group-housed ad libitum-fed gestating sows are lacking. Reproductive performance of sows of Parities 1 and 2 does not appear to be influenced by providing diets containing a high level of fermentable NSP ad libitum from d 28 of gestation (Whittaker et al., 2000). From a Danish field study, there are some indications that ad libitum feeding of a diet with a high level of fermentable NSP during gestation may decrease piglet birth weight compared with restrictedly feeding a low-fiber diet (Fisker and Sørensen, 1999). This may be explained by a decreasing feed intake in the last month of gestation in ad libitum-fed sows. Long-term consequences of feeding gestating sows ad libitum from d 1 of gestation are not known.
The first objective of this experiment was to study the development in individual feed intake characteristics of group-housed gestating sows that are fed a high-NSP diet ad libitum over three successive reproduction cycles. The second objective was to determine the effects of feeding a high-NSP diet ad libitum compared to feeding a conventional diet restrictedly on reproductive performance and development in body weight and backfat thickness during three successive reproduction cycles.
Materials and Methods
Animals, Experimental Design, and Diets
A total of 119 crossbred (Dutch Large White x Dutch Landrace) nulliparous, primiparous, and multiparous sows was allotted to the experiment during three successive trials. In each trial, on the day of weaning, 40 sows were blocked by parity number and body weight and assigned to one of two gestation feeding regimens. The same treatment was applied over three successive reproduction cycles. Sows were fed a conventional diet typical for The Netherlands restrictedly or a diet with a high level of fermentable NSP ad libitum (Table 1). To comply with the Dutch law (National Reference Center, 1998), the conventional diet contained 34% NSP. The high-NSP diet contained 50% NSP. Estrus was checked twice daily in the presence of a mature boar, using the back pressure test. Sows that showed estrus were inseminated each day of standing estrus with a commercial dose of semen (3 x 109 sperm cells) of a Dutch Large White boar. Sows that returned to estrus after first insemination were rebred. Rebred sows were moved to the farrowing room and weaned 3 wk later than the other sows in the group (sows that did not return to estrus) and were therefore replaced by another sow at the day of group’s weaning. If a sow was culled during the experiment, another sow replaced her and was added to the group at the day of weaning. Thus, part of the sows that were initially allotted to the experiment were followed for only one or two reproduction cycles. Moreover, replacement sows were followed for only one or two reproduction cycles. The numbers of sows on trial in reproduction Cycles 1, 2 and 3 are presented in Table 2. The care and treatment of the sows were conducted according to Dutch animal welfare legislation. The Institutional Animal Care and Use Committee of the Wageningen University approved all experimental protocols.
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). The IVOG-feeding stations consist of a single space feeder placed on load cells that weighed within a range of 0 to 50 kg. Each weighing was rounded in steps of 10 g. To identify individual animals, each sow had an electronic ear transponder that was read by two antennas in the IVOG-feeding station. The IVOG-feeding station recorded feeder weight and time at the beginning and end of each visit to the feeder. In reproduction Cycle 3, Trials 1 and 2, sows were fed ad libitum with a dry feeder because, for practical reasons, the facility could not be scheduled. Therefore, it was not possible to feed sows with the IVOG-feeding station and, consequently, individual feed intake characteristics could not be measured. This means that individual feed intake characteristics during gestation were measured on 74, 53, and 13 sows in reproduction Cycle 1, 2, and 3, respectively. Approximately 10 d before the expected time of parturition, irrespective of feeding regimen during gestation, sows were moved to farrowing rooms, each having 10 pens of 2.20 x 1.80 m in dimension. The concrete solid floor (1.20 x 1.80 m) was equipped with floor heating. Piglet cross-fostering took place within 3 d after parturition and occurred only among sows of the same experimental treatment. Cross-fostering was used in cases where large or small litter sizes occurred. Piglets were weaned at an average age of 28.3 d (SD = 3.6). All rooms were equipped with computer-controlled heating and mechanical ventilation systems.
Feeding
In the electronic sow feeding system, the feeding cycle started at 0730 in Pen 1 and at 1400 in Pen 2. Sows were free to consume their daily ration all at once or to divide it into more portions. Daily amount (as-fed basis) of the conventional diet increased during gestation (nulliparous and primiparous sows: d 0 to 35 = 2.3 kg/d; d 36 to 85 = 2.6 kg/d; d 86 to day of transfer to farrowing room = 3.0 kg/d; multiparous sows: d 0 to 35 = 2.6 kg/d; d 36 to 85 = 2.8 kg/d; d 86 to day of transfer to the farrowing room = 3.4 kg/d). The ad libitum-fed sows had free access to the high-NSP diet until the day of transfer to the farrowing room. In the farrowing room, all sows were fed the same commercial lactation diet (Table 1
) twice daily (0800 and 1500). In the farrowing room, sows were fed the lactation diet at 3.0 kg/d (nulliparous and primiparous sows) or 3.4 kg/d (multiparous sows) before parturition. During lactation, sows were fed on an ascending scale from parturition until d 6 after parturition and were given free access to the lactation diet from d 6 after parturition onward. On the day of weaning, sows were fed 0.5 kg of the lactation diet in the farrowing room at 0800. The day after weaning, sows were returned to their gestation diets. During the weaning-to-estrus interval (WEI) sows were fed the gestation diets at a level of 3.0 kg/d. All feeds were given as dry pelleted feeds. All sows were given free access to drinking water. Piglets were given free access to a commercial creep feed from d 11 after birth until weaning.
Measurements
Feed and Feed Intake.
Experimental diets were sampled weekly. The weekly samples were pooled within 2-mo periods. Feed was analyzed every 2 mo for DM, ash, CP, crude fat, and crude fiber. All samples were analyzed in duplicate. The contents of DM, ash, CP, crude fat and crude fiber were analyzed according to ISO standards 6496 (ISO, 1999b), ISO 5984 (ISO, 1978), ISO 5983 (ISO, 1979), ISO 6492 procedure B (ISO, 1999a), and NEN 5417 (NEN, 1988), respectively. Feed intake of the sows was recorded in the gestation room and during lactation.
Feed Intake Characteristics.
The IVOG feeding station recorded feeder weight and time at the beginning and end of each visit to the feeder. The total number of visits per day and the number of visits in which feed was consumed were calculated together with the feed intake (grams), time (seconds), and feeding rate (grams/minute) per visit in which feed was consumed. Before feed intake characteristics were calculated, the IVOG data were screened. There were two exclusion criteria for possible erroneous data. Firstly, a negligible decrease in feeder weight may cause the recording of a decrease of 10 g due to rounding (Eissen et al., 1998). Therefore, feed intake values of -10 g were tolerated, whereas greater negative values were excluded. The second exclusion criterion was based on the calculated feeding rate per visit (grams/minute). Visits with a feeding rate of 
400 g/min were excluded, as they were considered unrealistic; this feeding rate was arbitrarily chosen.
Culling Rate.
During the experiment, the culling of sows was recorded. Sows were culled for the following reasons: severe lameness; aggressive behavior; endometritis; and reproductive failure, including return to estrus for the second time in one parity and abortions in the last month of gestation.
Body Weight and Backfat Thickness.
Individual BW and backfat thickness of the sows was measured at the day of transfer to the gestation room, in wk 8 after insemination, at the day of transfer to the farrowing room, and at weaning. Backfat thickness was measured ultrasonically at three points 5 cm left of the median as described in Vesseur et al. (1997). The distance between the shoulder and last rib was divided in three equal parts to create four points. At the last three points, backfat thickness was measured.
Reproductive Performance.
Number of total piglets born (= live-born piglets + stillborn piglets + mummies) and individual weights of live-born piglets were recorded within 24 h after parturition. At weaning, the number of weaned piglets and the individual weights of the weaned piglets were recorded. Weaning-to-estrus interval, number of sows that returned to estrus after first insemination, and farrowing rate after the first insemination were recorded for sows in all three reproduction cycles.
Statistical Analysis
Total number of visits per day at the IVOG station, number of visits in which feed was consumed, duration of a visit with feed intake, feed intake per visit, feeding rate, and feed intake per day were analyzed using mixed models for the fixed effects of trial, initial parity number (parity number at the start of the trial), reproduction cycle and the interaction between initial parity number and reproduction cycle, and the random effect of sow. To study the development of daily feed intake per week of gestation, the model was extended with the fixed effect of week of gestation and an additional random error term for sow differences within a reproduction cycle to describe dependencies between weekly measurements on the same sow. Body weight and backfat thickness of the sows; changes in BW and backfat thickness; number of total piglets born; piglet birth weight; live piglet litter weight at birth; piglet weaning weight; WEI; and feed intake during lactation were analyzed using mixed models for the fixed effects of trial; initial parity number; reproduction cycle, gestation feeding regimen, and the interaction between reproduction cycle and gestation feeding regimen; and the random effects of sow and pen. Nonsignificant interactions (except for the interaction between reproduction cycle and feeding regimen) were omitted from the model. The random effects of sow, sow within reproduction cycle and pen were considered to be normally distributed with mean 0 and variance equal to 
2sow, 2sow•reproduction cycle, and 2pen, respectively. Weaning-to-estrus interval was log-transformed before analysis to stabilize the variance. Live-born piglets and stillborn piglets were analyzed as a fraction of total piglets born. Weaned piglets were expressed as a fraction of live-born piglets after fostering. Live-born piglets, stillborn piglets and weaned piglets were analyzed using logistic regression (McCullagh and Nelder, 1989). The percentage of sows that returned to estrus after first insemination, and farrowing rate after first insemination, were analyzed using logistic regression for binomially distributed data. Initially, response variables were only analyzed in sows that were followed for three reproduction cycles. Additionally, response variables were analyzed in sows that were followed for two or three reproduction cycles and in sows that were followed for one, two, or three reproduction cycles. Because the results of these three analyses were very similar, the results of the analysis in sows that were followed for one, two, or three reproduction cycles are presented. Estimates of fixed effects and components of variance in the mixed models were obtained using the residual maximum likelihood (REML) procedure. Fixed effects were assessed using 
2 for the Wald statistics. Pairwise differences between treatment means were tested using a t-test. All analyses were performed using the statistical program GenStat (2000).
Results
General
Initially, a total of 119 gilts and sows were assigned to the experiment. Additionally, another 36 sows (16 in the restrictedly fed group and 20 in the ad libitum-fed group) (Table 2) were assigned to the experiment to replace culled sows and sows that returned to estrus. During the experiment, 12 restrictedly fed sows and 16 ad libitum-fed sows were culled. Main reasons for culling were lameness and reproductive failure.
Feed Intake Characteristics of Ad Libitum-Fed Sows
Feed intake during gestation of the ad libitum-fed sows increased with increasing reproduction cycle (Table 3). However, there was an interaction with initial parity number (Table 4; P < 0.05). In sows with initial Parity Number 1, 2, 3, or 4, feed intake during gestation increased with increasing reproduction cycle, whereas in sows with initial Parity Number 5, feed intake was similar in reproduction Cycles 1, 2, and 3. The total number of visits to the IVOG feeding station and visits with feed intake were higher in reproduction Cycles 2 and 3 than in reproduction Cycle 1 (Table 3). Time per visit was lowest and feeding rate was highest in reproduction Cycle 2. Feed intake per visit did not increase with increasing reproduction cycle. Mean daily feed intake over the three reproduction cycles in the ad libitum-fed sows was dependent on the week of gestation (Figure 1; P < 0.001). Mean daily feed intake increased from 3.49 kg/d in wk 2 of gestation to 4.76 kg/d in wk 6 of gestation. Then it decreased to 3.75 kg/d in wk 15 of gestation.
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). In reproduction Cycle 1, BW at d 7 of gestation was similar in the ad libitum-fed and restrictedly fed sows, whereas, in reproduction Cycles 2 and 3, the ad libitum-fed sows were heavier at d 7 of gestation (10.9 and 19.6 kg, respectively) (Table 5). The same tendency was seen for backfat. In reproduction Cycle 1, the ad libitum-fed sows had 1.0 mm less backfat than the restrictedly fed sows, whereas, in reproduction Cycles 2 and 3, they had 1.1 and 2.4 mm, respectively, thicker backfat at d 7 of gestation (Table 5). Averaged over all three reproduction cycles, BW gains in the first 8 wk of gestation were 25.3 and 44.0 kg (P < 0.001; Figure 2), respectively, in the restrictedly and ad libitum-fed sows, whereas, in the latter half of gestation, BW gains were 32.8 and 31.3 kg (P = 0.65), respectively. Backfat gains in the first 8 wk of gestation and in the latter half of gestation were 3.4 and 5.1 mm (P < 0.05) and 2.7 and 2.7 mm (P = 0.90), respectively, in the restrictedly and ad libitum-fed sows (Figure 3).
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). Moreover, WEI, the percentage of sows that returned to estrus after first insemination, and farrowing rate were not affected by gestation feeding regimen (Table 7).
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). These effects were consistent during the whole lactation period. However, the ad libitum-fed sows lost more BW and backfat in the farrowing room than the restrictedly fed sows (Table 8). In reproduction Cycle 1, BW and backfat thickness at weaning were similar in the ad libitum-fed and restrictedly fed sows. In reproduction Cycles 2 and 3, the ad libitum-fed sows were 14.9 and 25.5 kg heavier at weaning, respectively, and had backfat that was 0.9 and 2.1 mm thicker.
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Feed Intake Characteristics of ad Libitum-Fed Sows
Mean daily feed intake during gestation over the three reproduction cycles was 4.2 kg, which is in agreement with the voluntary feed intake levels as published by Whittaker et al. (2000) (4.1 kg/d of a diet containing 60% sugar beet pulp) and Brouns et al. (1995; 4.1 kg/d of a diet containing 50% sugar beet pulp). Higher voluntary feed intakes were reported in gestating sows that were fed a conventional diet (7.2 kg/d; Bergeron et al., 2000) or diets containing fiber sources other than sugar beet pulp (7.1 kg/d; Brouns et al., 1995). Thus, diets containing at least 45% sugar beet pulp can decrease voluntary feed intake during gestation. It is not clear whether this reduction results from physical properties of the feed or from effects on metabolism during gestation. Sugar beet pulp has a high water-holding capacity (Bertin et al., 1988), it delays gastric emptying (Guérin et al., 2001), and it results in higher postprandial glucose levels that induce long-lasting effects on satiety (Vestergaard, 1997) compared to a conventional diet.
In sows with initial Parity Number 1, 2, 3, or 4, voluntary feed intake during gestation increased with increasing reproduction cycle (Table 4). In sows with initial Parity Number 5, feed intakes were similar in reproduction Cycles 1, 2, and 3. It seems that voluntary feed intake in gestating sows is increasing from Parity 1 to 5 and then remains stable in subsequent parities. Moreover, it seems that the voluntary feed intake level depends on the number of preceding reproduction cycles over which the sow is fed ad libitum. The voluntary feed intake of a third-parity sow that is fed ad libitum during all preceding reproduction cycles equals that of a fifth-parity sow, suggesting that this third-parity sow already reached the maximum feed intake. A third-parity sow that is fed ad libitum for the first time has not reached her maximum feed intake yet because her voluntary feed intake is increasing in the next reproduction cycles. Thus, it seems that the parity in which sows reach the maximum feed intake depends on parity number and on the number of preceding reproduction cycles over which the sow is fed ad libitum.
Mean daily feed intake of the ad libitum-fed sows increased from wk 2 to 6 of gestation and then decreased to wk 15 of gestation. Bergeron et al. (2000) also reported a decreasing feed intake during gestation in sows that were fed ad libitum with a conventional diet. In general, restrictedly fed sows are fed a constant or an increasing amount of feed during gestation. Thus, it seems that the development in feed intake during gestation in ad libitum-fed sows differs from that in restrictedly fed sows.
The mean number of daily visits with feed intake in the ad libitum-fed sows was 13.8. To our knowledge, individual feed intake characteristics in ad libitum-fed group-housed gestating sows are not measured in other studies. However, in trials with ad libitum-fed weanling pigs (Bruininx et al., 2001) and growing and finishing pigs (Ramaekers, 1996), the mean number of daily visits with feed intake was 12.5 and 11.6, respectively. It seems that the eating frequency in ad libitum-fed weanling pigs, growing and finishing pigs, and gestating sows is very similar. On average, the ad libitum-fed gestating sows spent 90 min/d eating. Brouns and Edwards (1994) reported similar results. In gestating sows that are fed a conventional diet or a high-fiber diet restrictedly, mean eating times were 16 and 51 min/d, respectively (Ramonet et al., 1999). Thus, eating time increases two- to fivefold by feeding gestating sows a high-NSP diet ad libitum. Mean feeding rate of the ad libitum-fed sows was 54 g/min, which is much lower than reported in other studies with restrictedly fed sows (95 to 152 g/min) as summarized by Rijnen et al. (2003). Ad libitum feeding of a high-NSP diet affected development in feed intake during gestation, eating frequency, time spent on eating, and feeding rate in gestating sows. Therefore, not only feed intake but also feed intake pattern in ad libitum-fed sows (sows are free to choose at what time they eat and how often and how much they eat) differs from those in restrictedly fed sows, where sows are forced to eat their daily ration in one or two meals.
Effects of Gestation Feeding Regimen on Body Weight and Backfat Thickness
The ad libitum-fed sows ate 1.3 kg/d—that is, 4.79 MJ NE/d (4.2 kg/d x 7.03 MJ NE - 2.9 kg/d x 8.53 MJ NE)—more during gestation than the restrictedly fed sows. Therefore, the ad libitum-fed sows gained more BW (+20.1, +20.2, and +14.7 kg in reproduction Cycles 1, 2, and 3, respectively) and more backfat (+2.2, +1.6, and +1.5 mm in reproduction Cycles 1, 2, and 3, respectively) in the gestation rooms than the restrictedly fed sows. These results are in agreement with the results of Whittaker et al. (2000). The differences in BW gain and backfat gain already occurred in the first 8 wk of gestation. In the first half of gestation, the differences in daily energy intake between the restrictedly fed and ad libitum-fed sows were higher than in the latter half of gestation (Figure 1) and this probably explains the greater differences in BW gain and backfat gain in the first 8 wk of gestation.
Sows that were fed ad libitum during gestation lost more BW (48.5 vs. 33.7 kg; P < 0.001) and backfat (5.6 vs. 3.9 mm; P < 0.001) in the farrowing room than those that were fed restrictedly during gestation. However, feed intake during lactation was similar in sows that were fed restrictedly or ad libitum during gestation. The higher BW loss in the farrowing room in ad libitum-fed sows can partly be explained by the loss of gut fill when sows are switched from a high-NSP gestation diet to a commercial lactation diet. Van der Peet-Schwering et al. (2003) calculated that the extra gut fill caused by a high-NSP compared to a starch diet during gestation is about 6 kg. The higher BW and backfat losses might also be explained by a higher BW and backfat of the ad libitum-fed sows at the day of transfer to the farrowing room (279.5 and 253.7 kg and 22.5 and 20.4 mm, respectively, in the ad libitum and restrictedly fed sows; P < 0.001). Yang et al. (1989) reported that fatter and heavier sows at parturition lose more BW and backfat during lactation than leaner sows. A third explanation might be the higher total litter gain during lactation in the ad libitum-fed sows. Mean total litter gain during lactation was 74.6 and 78.1 kg in the restrictedly and ad libitum-fed sows, respectively. For a 1-kg litter gain, 4.2 kg milk is needed (Everts et al., 1995). Thus, a 3.5-kg higher litter gain means a 14.7-kg higher milk production of the ad libitum-fed sows resulting in higher BW and backfat losses. To produce 14.7 kg of milk containing 73.5 MJ NE (Everts et al., 1995), 1.1 mm of backfat (i.e., 2.1 kg of lipid; Everts et al., 1994) will be mobilized (efficiency of utilization of energy from body reserves for milk production is 0.88; Noblet et al., 1990).
Similar levels of feed intake during lactation in sows that were fed restrictedly or ad libitum during gestation were not expected because the ad libitum sows were fatter at the day of transfer to the farrowing room (22.5 vs. 20.4 mm; P < 0.01). It is often demonstrated that fatter sows at parturition eat less during lactation (Dourmad, 1991; Revell et al., 1998; Prunier et al., 2001) than lean sows. Conversely, Jørgensen et al. (1996) showed that pigs that were fed high-fiber diets had an extended gastrointestinal tract and a longer colon, which might stimulate feed intake during lactation. Matte et al. (1994) and Van der Peet-Schwering et al. (2003) indeed reported higher feed intakes during lactation in sows that were fed high-fiber diets during gestation. It might be that these two opposite effects on feed intake counterbalance each other, resulting in no effect on feed intake. Whittaker et al. (2000), however, found lower feed intakes during lactation in second-parity sows but not in first-parity sows that were fed a high-fiber diet ad libitum during gestation. In their trial, mean daily NE intake during gestation in the restrictedly and ad libitum-fed second-parity sows was 23 and 34 MJ, respectively. In our research, the difference in daily NE intake during gestation between the restrictedly and ad libitum-fed sows was 4.8 MJ. It might be that this difference in NE intake was too small to negatively affect feed intake during lactation. After three reproduction cycles, the ad libitum-fed sows were 25 kg heavier and had deposited 3 mm more backfat than the restrictedly fed sows; this was due to an average 19.3% higher NE intake during gestation.
Reproductive Performance
In spite of higher feed intakes during gestation, resulting in heavier and fatter sows at parturition and greater losses in BW and backfat during lactation, ad libitum feeding of gestating sows during three successive reproduction cycles did not affect reproductive performance. In all three reproduction cycles, the numbers of total piglets born, live-born piglets, and stillborn piglets were similar in sows that were fed a conventional diet restrictedly or a high-NSP diet ad libitum. This is in agreement with the results of Whittaker et al. (2000), who did not find effects on litter performance in Parity 1 and 2 sows that were fed ad libitum from d 28 of gestation. Moreover, the greater losses in BW and backfat during lactation in the sows fed ad libitum during gestation did not result in a prolonged weaning-to-estrus interval, which is in contrast with results of Baidoo et al. (1992) and Zak et al. (1997). Aherne et al. (1990) and Whittemore (1996) suggested that, besides the amount of tissue mobilized during lactation, the amount of body reserves at farrowing and at weaning are critical factors affecting the weaning-to-estrus interval. The ad libitum-fed sows were heavier at farrowing and weaning than the restrictedly fed sows, and this might explain why no effect on the weaning-to-estrus interval was found. Another explanation might be that there was no difference in feed intake during lactation between the restrictedly and ad libitum-fed gestating sows.
Piglet birth weight and total litter weights at birth were not affected by gestation feeding regimen. This is in agreement with the results of Whittaker et al. (2000) but in contrast with the results of Fisker and Sørensen (1999), who found lower birth weights of piglets in sows that were fed a high-NSP diet ad libitum compared with sows that were fed a conventional diet restrictedly, and suggested that this might be explained by a deficiency in glucogenic energy at the end of gestation. As in both their and our research, ad libitum-fed gestating sows received the same daily amount of starch and sugar (approximately 650 g/d), it does not seem likely that a deficiency in glucogenic energy explains the lower birth weights. However, a decreasing feed intake in the last month of gestation in ad libitum-fed sows might be an explanation. The higher energy intake in the ad libitum-fed sows also did not result in heavier birth weights of the piglets. The energy intake of the restrictedly fed sows is presumably high enough to maximize piglet birth weight.
Implications
In this study, gestating sows that were fed ad libitum with a diet containing a high level of fermentable nonstarch polysaccharides (approximately 45% sugar beet pulp on an as-fed basis) during three successive reproduction cycles ate 1.3 kg/d more during gestation than sows that were fed a conventional diet restrictedly. Although the higher feed intake during gestation resulted in heavier and fatter sows at parturition and greater losses in BW and backfat during lactation, ad libitum feeding of gestating sows during three successive reproduction cycles did not negatively affect reproductive performance. Moreover, lactation feed intake was similar in sows that were fed a diet with a high level of fermentable nonstarch polysaccharides ad libitum or a conventional diet restrictedly during gestation. Therefore, ad libitum feeding of gestating sows with a diet containing a high level of nonstarch polysaccharides during three successive reproduction cycles is possible without negative effects on reproductive performance.
Footnotes
1 This research was financially supported by the Product Boards for Livestock, Meat, and Eggs. 
2 Correspondence: P.O. Box 2176 (phone: +31 320293211; fax: +31 320241584; e-mail: carola.vanderpeet{at}wur.nl).
Received for publication July 18, 2003. Accepted for publication December 23, 2003.
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) and ad libitum-fed (
) gestating sows.
