Factors Affecting the Effectiveness of Litter Management Practices: An Overview

Abstract

In terms of selection, pig farmers have aimed to maximize profitability for decades by increasing litter sizes through choosing gilts with seven or more pairs of teats. This has resulted in sows with numerically large litters, an increase in within-litter birth weight variation, and more piglets with a low birth weight. For newborns, the amount of colostrum consumed within the first 24 h after birth is particularly important for thermoregulation, passive immunity and adequate growth during lactation. The colostrum must be consumed early, for at least the first 6 h after birth. Only after consuming the mother’s colostrum, i.e., within 24 to 48 h after birth, is performing cross-fostering (CF) recommended, whereby light piglets are placed with another sow. Alternatively, farmers can choose a one- or two-step CF technique with nurse sows. If there are surplus piglets and substitute mothers are not available, the option of artificially rearing the piglets with liquid (milk replacer) and, later, solid feed remains, which is nevertheless a suboptimal method compared to CF. One way or another, all breeder interventions and different management strategies for large litters are associated with reduced piglet welfare.

1. Introduction

Selection for improved sow prolificacy has increased litter sizes, resulting in pig herds with hyperprolific sows that give birth to more piglets than they have functional teats [1]. Functional teats are mammary glands that produce milk and can be suckled by piglets, so it is desirable that the number of teats matches the number of live-born piglets. This is important for piglet survival, as it reduces mortality during lactation and results in more weaning [2]. As the number of functional teats has a significant positive effect on piglet survival in the pre-weaning period [2,3], such a selection path poses various challenges for breeders. Furthermore, numerically large litters of primi- and multiparous sows show greater variability in terms of piglet birth weight. Larger litters usually have more piglets with low birth weights, putting them at greater risk of poor growth and survival. Due to low vitality, a piglet birth weight of 1000 g or less poses a high risk to survival during lactation [4]. Larger litter often increases competition among piglets, making it harder for those that are smaller or weaker to obtain enough colostrum and milk. Accordingly, a low birth weight has a long-term impact on piglet growth patterns, namely lower growth intensity during lactation and later in the post-weaning and fattening period [5,6]. The longer the birth time, lesser the colostrum available—in terms of both quantity and quality—during the second half of farrowing [7].

In addition, the composition of the colostrum begins to change within a few hours after birth. Compared to milk, colostrum has a high concentration of proteins, especially immunoglobulins, some microelements (copper, iron, iodine, and zinc), and vitamins and growth factors [8].

In this review article, we attempt to provide an overview of the latest findings on the importance of the timely intake of adequate amounts of colostrum, how and when piglet fostering should be carried out, what impact this management intervention has on altering piglet behavior and growth performance, and the effects of various management issues on piglet welfare in large litters. In addition, this review also provides research-based information on the key areas of focus in large-scale litter management practice in order to avoid potential failures.

2. The Importance of Consuming Colostrum

Colostrum, its composition and its intake timing all play very important roles in the early postpartum period. It is therefore necessary to adjust litter management to ensure that all piglets born alive receive the appropriate quantity and quality of colostrum [3]. In general, piglets that are heavier at birth also have a higher colostrum intake per kg of birth weight, achieving more intensive growth during lactation and a higher survival rate until weaning [4]. Piglets with a lower birth weight have a lower survival rate, accompanied by a lower colostrum intake, a lower daily gain in the first 24 h, a lower rectal temperature, lower IgG and glucose concentrations and a higher cortisol concentration in plasma [9]. This is especially important when considering that piglets are born with an unfavorably high body surface to volume/mass ratio [3], i.e., piglets have a relatively large body surface area (through which body thermal energy is dissipated) per unit of birth weight. Piglets do not have large energy reserves in their body composition because sows do not have mechanisms for supplying glycogen to their young. Furthermore, piglets have no brown adipose tissue, which is known to have an important thermoregulatory function in newborn mammals that do not hibernate, allowing non-shivering thermogenesis [10]. As such, their birth weight, genotype and colostrum intake are extremely important for thermoregulation [11]. Furthermore, piglets are born from a sterile intrauterine environment with a constant supply of nutrients into an environment rich in microorganisms. Nutrients are delivered to the body by suction via the gastrointestinal tract [12].

In the first 24 h after birth, glycogen and colostrum cover the energy requirements of live-born piglets [13], ensure immune defense, and have a long-term effect on their growth [9]. Most of the colostrum is produced before birth, followed by a gradual conversion of colostrum into transient, which is later transformed into mature milk. Colostrum is defined as the mammary secretion that is consumed in the first 24 h after the birth of the first piglet and up to 34 h after farrowing. Transient milk is produced from around 34 h after birth until around the fourth day after farrowing, while mature milk is produced by the glands from day 10 until the end of lactation [14].

The specific structure of the epitheliochorial placenta prevents the piglets from absorbing large immunoglobulin molecules from the mother’s blood during gestation [15]. After birth, they are therefore in a situation where their own immune system is not yet activated, and they must absorb immunoglobulins from the colostrum produced by the mother’s mammary glands. In this way, piglets receive metabolic products from the sow’s colostrum that enable them to remain healthy and grow properly in this early postpartum phase [16]. However, newborn piglets’ gastrointestinal tract is only permeable to immunoglobulins from the colostrum in the first 24 h [17,18]. By consuming colostrum, the piglets receive passive immune protection that is effective for the next four weeks of life. Piglets with a lower birth weight, however, use a large proportion of the ingested colostrum proteins as a substrate for energy production and thermoregulation, and only a small amount for body growth [4].

What if farrowing takes a long time and the last piglets are born after a long delay?

Piglets born first probably ingest more colostrum, implying lower survivability in late-born piglets. However, this aspect is methodologically problematic to study, as piglet birth order in modern “unicolored” breeds is difficult to phenotypically distinguish. If we assume that newborns are not touched, in large litters, it is practically impossible to study which piglet had the most teats available and which ingested the most colostrum. Nevertheless, there are exceptional opportunities for providing some insight into the actual influence of birth order in the context of early colostrum ingestion, such as the recently reported 12 h birth delay of two piglets [19]. Despite the theoretical assumption that missed colostrum suckling in the first 24 h can be fatal for piglets, these piglets survived. A possible—albeit speculative—explanation could be that, in piglets born so late, the low colostrum intake and IgG absorption could trigger earlier IgG synthesis as a part of active immunity. This is supported by findings that IgG content in piglets with lower growth at weaning did not differ from those with more intensive growth [20].

Low colostrum intake and high birth order negatively affect passive immunity transfer from sows to piglets [21], contributing to poor gastrointestinal and reproductive tract development in piglets at 23 days of age [22]. The concentrations of immunoglobulins (IgG, IgA, IgM) in piglet plasma at day 3 are positively associated with colostrum intake and negatively correlated with birth order [21]. Additionally, IgG concentration tends to decrease 48–72 h after birth in relation to piglet birth order [23].

Last- or middle-born piglets often occupy the last teat pairs of the sow’s mammary complex [24]. Low-birth-weight piglets, late-born piglets, and those suckling only posterior teats tend to obtain less colostrum; however, most piglets receive enough colostrum to achieve normal serum immunoglobulin concentrations [25]. Morton et al. [26] found that the heaviest piglets consumed more colostrum and gained more body weight during lactation, but did not have higher immunocrits (a measure of circulating immunoglobulins). They concluded that both birth weight and order affect prenatal growth and survival, though the former is more closely related to colostrum intake and the latter affects immunocrit. Estimated colostrum intake was higher for first-born piglets with high vitality scores [27] during the first 12 h after parturition onset, a period with higher colostrum quality, resulting in first-born piglets potentially having immunity and robustness advantages later in life [28]. Krolikowski et al. [29] confirmed that piglet immunocrit was negatively correlated with birth order; however, those that consumed more colostrum (≥230 g) in the first 24 h after birth had greater immunocrit and greater weight gain up to 42 days of age. These findings are supported by Le Dividich et al. [30], who showed that piglet mortality in the early period of life is more influenced by low birth weight and insufficient colostrum intake (energy) than by birth order. However, colostrum intake is one of the main factors influencing piglet survival within 3 days after birth and pre-weaning growth [31].

2.1. The Amount of Colostrum Produced and Its Composition

After farrowing, young sows have higher nutritional requirements, as they are not yet physically mature and have not reached their adult body size. At the same time, they generally have litters of piglets with lower birth weights and growth intensities than those of multiparous sows. In fact, piglets with a lower birth weight born from primiparous sows also have a less developed gastrointestinal tract, probably due to poorer maternal nutrition during the suckling period [32]. At the end of lactation, primiparous sows can be in lower body condition, causing further problems for successful reproduction.

To optimize investment and improve weaklings’ survivability and development, it is therefore very important that piglet fostering is carried out in such a way that light piglets from the litters of primiparous sows are transferred to those of multiparous sows. Indeed, studies have shown that the growth and humoral immune response of fostered piglets from primiparous litters improve when they are transferred to multiparous sows [33]. In fact, this shift can have a positive effect on foster piglets’ growth; when suckling on multiparous sows, such piglets apparently reach a higher body weight at 20, 28 and 42 days of age [20]. Furthermore, despite their low body weight, which would seemingly imply non-competitiveness in establishing a suckling position in the new litter, it was shown that body weight difference does not influence (deteriorate) foster piglet’s integration into the new suckling environment [34].

It is estimated that primiparous sows produce between 2.37 and 4.5 kg of colostrum [35], or between 2.8 kg and 4.6 kg, within 24 h after farrowing [36]. Moreover, some authors also reported that the colostrum amounts yielded were between 1.9 kg and 5.3 kg; these amounts were affected by parity, although they were not related to litter size [37]. Sows in second to fourth parity had the highest estimated amount of colostrum of around 7 kg; thereafter, those in fifth and sixth parity produced less colostrum, about 5.9 kg [38]. Parity thus influences the colostrum yield [37], but litter size and piglet birth weight do not [35]. In general, sows producing lower amounts of colostrum are reported to have a leaky mammary epithelium and lower lactose production, as well as slightly delayed hormonal changes in the pre-farrowing period [36].

Contrary to the amount of colostrum per se, parity has no effect on the overall immunoglobulin content (IgG, IgM and IgA) [14,39]. However, a recently published study investigated the development of early immunoontogenesis in piglets and found that sow parity influences the immunomodulatory effect of colostrum, as well as the effect of mitogen-stimulated lymphocytes [40].

Colostrum composition is thought to vary due to hormones, the environment and nutrition [37]. Primiparous sows have significantly higher dry matter and percentages of fat, lactose and non-fat solids in colostrum than those in higher parity [41]. With increasing parity, the fat content in colostrum thus decreases, while the protein content remains unchanged. The authors also report that colostrum composition changes drastically in the first 24 h after farrowing (

Table 1. Changes in colostrum composition within 24 h after farrowing.

	Dry Matter (%)	Protein (%)	Fat (%)	Lactose (%)	Reference
Farrowing 0 h	29.3	18.3	6.1	3.0	[41]
	27.3	17.7	5.1	3.5	[14]
After 24 h	22.9	8.7	8.8	4.2	[41]
	20.6	8.6	6.9	4.4	[14]

).

The dry matter decreases from 29.3% to 22.9%, the protein content from 18.3% to 8.7% and the non-solid fat content from 23.1% to 14.1%; however, during this period, the fat content increases from 6.1% to 8.8% and the lactose content from 3.0% to 4.2%. Another study shows the same tendencies [14]. The protein content of colostrum—measured at farrowing (0 h) and 24 h later—shows a decrease in protein content from 17.7% to 8.6% and dry matter from 27.3 g/100 g to 20.6 g/100 g, while the fat content increased from 5.1% to 6.9% and lactose content from 3.5% to 4.4% [14]. In fact, the first significant changes in colostrum composition occur very early, about 6 h after farrowing. A study examining the change in immunoglobulins at different time intervals showed a characteristic change in colostrum composition 6 h after birth, mainly due to a marked decrease in IgG, IgM and IgA [39]. The results of the most recent studies show the same trend: IgG decreased from 94.3 mg/mL to 70.6 mg/mL and IgA from 9.9 mg/mL to 8.2 mg/mL, while the IgM content remains unchanged (5.0 mg/mL, 4.7 mg/mL) [41].

Therefore, piglets must receive colostral immunoglobulins and other bioactive substances as soon as possible within the first 5 to 6 h, providing them with passive immune resistance (and thus resistance to disease or even death) and greater survivability during lactation and the post-weaning period. Furthermore, the energy content in colostrum is crucially related to newborn piglets’ thermoregulatory function. The energy in the colostrum ingested increased from 260 to 346 kJ/100 g within 24 h [14]. In light of this, and despite the general notion of avoiding CF before the first 12 h after farrowing [42,43], it has been suggested that CF be carried out within the first 5 to 6 h, after the future foster piglets have already consumed colostrum from their biological mother. Moreover, if CF was carried out within the first 5 h after birth, before piglets had their first suckling, biological and adopted piglets had the same IgG concentration in the first 24 h after birth, regardless of which teat was suckled from [44].

2.2. Amount of Colostrum Consumed

The amount of colostrum consumed by piglets, an important factor affecting survival, can range from 200 g [3,45] to 250 g [45] and even to 300 g [37].

If a piglet consumes at least 200 g of colostrum in the first 24 h after birth, the pre-weaning mortality rate decreases significantly and even allows a slight increase in body weight [3,45]. However, piglet survival and growth intensity until weaning is not only influenced by colostrum intake, but also by birth weight. Piglets from large litters, e.g., 17 offspring, are born lighter—and are still lighter at 28 days of age—than those from litters of 14 piglets, despite suckling and the use of milk replacer [46].

In light- (1.1–1.2 kg birth weight) and medium-weight (>1.2–1.3 kg) piglets, survival depends more on the amount of colostrum ingested than in piglets with a higher birth weight (>1.3–1.7 kg) [20]. If colostrum consumption in light- and medium-weight piglets is sufficient, 200 to 250 g/24 h after birth, there are no differences in body weight up to 42 days of age; piglets that had consumed at least 250 g or more colostrum were still heavier, even on the 42nd day of life [45].

Furthermore, piglet birth weight, cumulative farrowing interval and litter size have a significant influence on colostrum consummation [47]. It was found, for example, that piglets born 5 or more hours after the start of farrowing generally had lower colostrum consumption. About one third of piglets from large litters (≥18 piglets) were found to have inadequate colostrum consumption compared to piglets from those with 7–13 piglets. Moreover, a significant relationship between litter size and colostrum consumption was observed in piglets with low birth weight (<1000 g). One of the most recent studies came to similar conclusions, showing colostrum consumption in piglets as being negatively affected by birth order, low birth weight and asphyxia [21].

Colostrum intake also depends on other factors, such as the specific shape of a piglet’s head. This mainly refers to the dolphin-like head shape seen in cases of intrauterine growth restriction (IUGR) [1]. Piglets with this head shape consumed too little colostrum and had a lower glucose level measured 24 h after birth and, consequently, a lower survival rate [48]. Piglets with mild IUGR have twice the risk of dying during the suckling phase than normal piglets [49]. Piglets with defects such as splay leg and those with respiratory problems are also considered to be limited in their colostrum consumption [37]. Finally, under “artificial” rearing conditions, colostrum shortage can also be mitigated by offering piglets 25 mL of porcine or bovine colostrum per hour; in this way, piglets can gain the same amount of plasma immunoglobulins in 6 h feeding periods as those consuming natural sow colostrum [50].

3. Piglet Fostering and Behavior

3.1. Piglets’ Preference for a Particular Teat or Pair of Teats

Immediately after birth, first-born piglets can access colostrum from all mammary glands. Piglet suckling looks somewhat chaotic at first glance, but there is a certain group cohesion between members of the litter, maintaining inter-individual distance at the sow’s mammary complex as a precursor for the suckling order formed later [51]. The development of the suckling order in this early phase is also influenced by the physical limitation of the mammary complex, which affects the distribution of piglets to the teats and/or suckling pairs [52]. In the first days after birth, piglets already develop a preference for a particular teat or pair of teats, as they can distinguish more productive from less productive ones [53].

During lactation, this type of behavior increases in stability and culminates in the so-called suckling or teat order. This order is established within the second week of age and remains stable until the end of lactation. The piglet occupies a particular teat to reduce competition for resources within the litter [54]. They return to the same teat or pair of teats each time they suckle and, of course, fight for it when necessary. Fighting is especially fierce right after birth due to intense competition for limited colostrum [55]. This kind of aggressive behavior is evolutionary justified because the colostrum/milk from the mammary gland is the only and most important source of nutrition until breeders start feeding piglets creep feed at 10 to 14 days of age. It is therefore very important for litter management that CF is carried out at the optimal time and that the breeder recognizes potentially aggressive behavior either between piglets from the same litter or in litters containing fostered offspring. This is especially true in cases when CF is performed; however, techniques are also known whereby piglets from different litters can mix spontaneously, allowing them to move freely between farrowing pens [56].

3.2. Maternal Recognition by Piglets and Vice Versa

When performing CF, one of the basic premises we must be aware of is that the sow is able to distinguish her own piglets from others, both by smell and voice [57]. In addition, piglets are also able to recognize their mother’s odor as early as 12 h of life [58] and distinguish it from non-maternal odors. For example, even biological piglets do not suckle when odors thought to be maternal pheromones are removed from their nursing mother’s skin [59]. In cases of late CF, the sow may also be aggressive towards fostered piglets in her litter, as she is able to discriminate them from her own by odor as early as 12 to 24 h post-partum [60].

Furthermore, in first 24 h, piglets can recognize their home environment and, after 36 h post-partum, their mother’s vocalization [61]. At this early age, e.g., if fostering was initiated before 9 h of age, successful suckling within 6 h of fostering has been reported. In addition, it could be concluded that, during this early life period, a strong bond between the piglets and their mother has not yet been developed [62]. In this stressful situation, transferred piglets draw attention to themselves by whimpering, delaying or even abstaining from suckling, walking around in the farrowing pen, and often disturbing the sow by vocalizing; in turn, this creates a high risk of piglets being crushed [42].

However, late CF at the age of seven days has a negative effect on the growth of both fostered piglets and those from the original litter. At this age, the issue is not only that teat order formation is disrupted, but also that piglets are able to recognize their littermates [60,61].

Nest building influences sow maternal behavior during farrowing and early lactation [63]. It is a natural pre-farrowing behavior that has not been altered by domestication [64]. Providing nest-building material before parturition increases piglet survival and reduces pre-weaning mortality [65]. Nest-building activity is thought to prepare the sow for motherhood, and her communication with piglets during this activity (sniffing, nudging, and grunting) leads to lower postnatal piglet mortality in the pre-weaning period [66]. Additionally, nest-building during farrowing significantly reduces the frequency of negative communication (such as pushing, threatening barks, and biting) between sows and piglets and improves sow nursing performance [63].

3.3. Influence of Age, Piglet Body Weight and Litter Size at the Time of CF on Their Aggressive Behavior

The relationship between CF and its influence on piglet behavior has been studied in very different post-partum time periods: in the first 24 h [67,68,69,70,71,72] between 24 and 48 h [72,73], on the 6th day [74,75], between the 4th and 7th day [57], on the 7th day [72,76], and up to the 8th [34] and 10th day of life [77]. Roughly speaking, CF can be divided into early—until the piglets are 48 h old—and late, which coincides with start of teat order formation on the mammary complex.

In CF, breeders must pay attention to the group of piglets formed in terms of their birth weight and potential behaviors, as these affect their growth and survival rate [68]. In this study, three homogeneous litters with light piglets (0.8–1.25 kg), or combinations of six light- and six medium-weight piglets (1.40–1.60 kg) and six light and six heavy piglets (>1.70 kg) in a litter, were created. Most missed nursings were observed on the first and second day in the light piglets of the heavy group. Later, around day 4, the light piglets still had more missed nursing’s than the medium-weight and heavy piglets in the litter. Piglets with an intermediate body weight were involved in the most fights for teats during suckling on the first day; however, on the second day, they showed more playful behavior with high-body-weight piglets. Similar conclusions were drawn in another study; piglets with a lower birth weight in large litters “missed’ more milk nutritional suctions and spent more time “teat fighting” with medium- or high-birth-weight piglets than their heavier littermates [78].

The implementation of CF around day 5–6 is only acceptable to a limited extent, as it chronologically overlaps with the intensive establishment of the teat order. The CF process can be disruptive, as the entry of new piglets into the litter implies increased teat competition and destabilizes the developing order.

It is therefore recommended that CF be applied somewhere up to the second day of age, after piglets have already consumed sufficient colostrum from their biological mother but while the teat order is still loose enough. Fostered piglets reach only 76% of the body weight of non-adopted piglets and this difference remained until the 45th day of age [74].

Accordingly, so-called late CF after 7 days was found to have a negative effect on teat order stability in the autochthonous Luchuan breed, leading to elevated cortisol levels— a stress marker—in piglets [72]. Late CF influenced the position of fostered piglets, as they suckled more on the rear teats [72].

The success of integrating piglets into a foster litter is also influenced by their age and the size of the litter they were born in, as well as the age and litter size of piglets they are integrated with [34]. An important conclusion of the study is that piglet body mass has no significant effect on integration into the “new” litter in terms of their involvement in the milk suckling process. The study shows that piglets transferred to a 2-day-older litter were generally inactive during the first suckling’s after CF. Piglets transferred to litters of the same size or slightly larger than their original litter successfully participated in suckling, while piglets transferred to a smaller litter (one piglet less) were much more passive in the above activities. Furthermore, D’Eath [77] reported that if piglets were fostered at 10 days of age, in addition to aggressive littermate behavior toward unfamiliar piglets, the sows responded to their presence by standing up more often and by lunging and snapping.

3.3.1. Reducing Aggressive Piglet Behavior Using Multi-Suckling Pens

Teat fights are an integral part of piglet behavior in the early phase after farrowing during lactation. However, when transferred within the first 48 h, fights can still intensify; it was shown that fostered piglets fight more frequently than those in the control, non-fostered group [74]. Furthermore, when the litter remains unchanged after birth, Heim et al. [67] noted less aggressive interactions involving fewer piglets involved in litters formed with 50% unadopted and 50% adopted and 100% adopted piglets. One of the options for reducing aggressive interactions between piglets is the use of multi-suckling [57] and/or PigSAFE farrowing pens [73]. Sows farrow in pens, where they stay with their piglets for 1–2 weeks after birth. Later, they are moved to a multi-suckling pen, where they remain with other sows and piglets until weaning [57]. In the multi-suckling pen system, 84% of the piglets suckled between zero and four times, 16% of the piglets suckled more than five times, and about 34% at least once from another sow. The piglets move from sows and teats with lower milk yields to those with higher yields [57]. The authors also note that piglets in a multi-suckling pen are able to naturally cross-suckle; however, despite this, occasional fighting still occurs between piglets.

Multi-suckling pens provide sows with more movement and the opportunity to express behaviors such as maternal care and social interaction [79]. However, this system also has limitations, including an increased rate of crushing. Another significant limitation is the higher piglet mortality rate when piglets are group-housed at one week of age [80]. Compared to the conventional single-litter system, piglets in multi-suckling pens experience higher pre-weaning mortality rates [81]. Conversely, it has been reported that sows in multi-suckling pens initiate more nursing and tend to lose fewer piglets than those in crates [82]. A recent study shows that multi-suckling pens have a beneficial effect on animal welfare; however, the system is nevertheless associated with high variability in piglet body weight gain during the pre-weaning period [83,84].

3.3.2. Reducing Aggressive Piglet Behavior Using PigSAFE Farrowing Pens

A possible solution that alleviates the stress of piglet transfer is the use of the Piglet and Sow Alternative Farrowing Environment or PigSAFE farrowing pen [73]. This was developed to allow sows and piglets to engage in a wide repertoire of behaviors during the different farrowing and lactation phases, with the aim of optimizing welfare and production [85,86]. Social network analysis showed that piglets fostered in such crates formed a better bond with their foster litter than those fostered in conventional crates [73].

3.4. Piglets Fighting for Teats

Aggressive interactions between piglets also take on a broader character. Furthermore, it has been reported that repeated CF during lactation results in reduced growth across all piglets in the litter and increases sow aggression towards fostered offspring [87]. In both CF cases, it can be concluded that there is a detrimental effect on the welfare of both the sow and piglets [42].

4. Influence of CF on Piglet Growth Performance

In commercial rearing, litters are often standardized according to piglet size [67] and birth weights [88]. When a litter is adjusted according to birth weight, this weight is not related to the level of daily gain during the three weeks of lactation [88]. It was also found that CF within 24 h after birth into a different litter composition (100% adopted piglets (A), 100% biological piglets (B) and 50% A and 50% B) had no negative effects on the growth or survival of adopted piglets [67].

It is important not to introduce light piglets into litters of heavy piglets, as this reduces their survival rate [68]. Low-birth-weight piglets in small litters with high or average birth weight littermates have been reported to achieve significantly higher 21-day weights and have a greater chance of survival compared to low-birth-weight piglets in large litters [78]. Additionally, piglets weighing less than 1 kg can be moved into smaller litters with fewer than nine light piglets [89]. However, smaller litters may have a more rigid—and thus, “closed”—teat order, which might prevent foster piglets from successfully suckling quickly after CF [34].

Piglet litter standardization can also be achieved using CF based on piglet birth weights, which are divided into uniformly light (≤1.25 kg), heavy (1.5–2.0 kg) and mixed litters [90]. The authors found that piglets with a high birth weight compensated for the lower milk intake by a slightly increased intake of creep feed. Heavy piglets in heavy piglet groups try to compensate for their insufficient milk intake by increasing their intake of creep feed; however, they still grow at a lower intensity than heavy piglets in mixed groups. An increased intake of creep feed before weaning leads to improved growth in the post-weaning period [90].

In addition, another study shows that, regardless of piglet birth weight, piglets are ideally fostered by mid-parity (parity 3 to 5) sows [91]. In this way, piglets can achieve the highest daily growth and the highest consumption of creep feed. Monitoring cross-fostered piglet behavior has shown that CF leads to more facial and carpus wounds, diarrhea and more frequent antibiotic treatments [92]. In the period from 2 to 4 days before farrowing until CF, it is recommended that the sow be confined in the farrowing crate, as this can successfully reduce instances of piglet crushing [93].

5. Different Management and Nutritional Strategies and Their Association with Lower Welfare in Piglets

In general, the aim of all strategies is to reduce light piglet loss in large litters, increase piglet growth up to weaning, and thus influence subsequent growth in the post-weaning and growing-finishing period. Therefore, some of the suggestions in review articles focus on developing specific strategies for achieving these goals (

Table 2. Different strategies for managing light piglets from large litters.

Management and Selected Nutrition Strategies	Reference
Three avenues to increase colostrum intake: - Increasing piglets’ suckling ability; - Reducing within-litter birth weight variation; - Increasing the quantity of colostrum that the sow produces.	[45]
Preventive strategies at the sow and piglet level - Optimization of intra uterus environment via supplementation during gestation; - Interventions at the piglet level (adequate colostrum and milk intake, supplementing piglets, split-Nursing and-weaning, and cross-fostering).	[94]
Management strategies in farrowing houses - Cross-fostering; - Creep feeding; - Human–animal interactions.	[3]
Six key principles for piglet survival and growth - Piglets suckle colostrum after birth (split suckling); - Fostered piglets may not exceed the number of functional teats available in the sow; - Primiparous sows receive as many piglets as the mammary complex allows; - In the case of older sows, it is necessary to assess their rearing ability; - Fostering in the first 12 to 24 h after birth; - Fostering is generally minimized.	[43]
Five approaches to increase piglet survival - Optimizing the farrowing environment; - Supervising farrowing and assisting newborn piglets; - Using cross-fostering techniques; - Providing nurse sows; - Providing artificial milk.	[95]
Strategies to minimize piglet fallback - Cross-fostering; - Creep feeding; - Weaning an older pig.	[89]
Selected nutrition and management strategies - Pain relief provision for sows around farrowing; - Piglet split-suckling; - Administration of supplements (energy feed additives, liquid and dry feed); - Promotion of gut structure and function; - Modulate gut microbiota.	[96]

). There is no universally valid strategy, but rather a range of different approaches and proposed solutions. Different authors, each from their own perspective, detail the various factors that influence the creation of conditions that would enable lightweight piglets to have the best possible chance of survival in the first days of life. Most losses occur within the first three days.

Large litters have resulted from decades of intensive selection for increased sow fertility. To address the gap between the number of piglets born and the number of functional teats available, various strategies and solutions have been developed [3,43,89,94,95]. As the problem is multi-layered and complex, one proposal is to increase colostrum quantity and quality and improve milk yield in lactating sows [45,94]. Another approach is to improve maternal nutrition during gestation and lactation [45] and enhance piglet nutrition in the pre-weaning period [3,89,95,96]. Artificial piglet rearing solutions are also being explored, particularly the use of milk replacers [95,96].

Since 1990, selection pressure has not only increased litter size, but also piglet mortality [97]. In Danish pig production, several million piglets die each year, with losses estimated at almost 23% by weaning [98] (Christensen and Pedersen 2023). The unusually high mortality rate was observed in CF of low-birth-weight piglets when mixing more than two litters. Such a criterion does not seem to be justified from the point of view of both pig farming and animal welfare [99]. A tool called Milking Score (MS) was recently developed to evaluate the foster performance of lactating sows, taking into account traits such as CF, litter size, individual piglet weights and piglet mortality during lactation [100]. The results suggest that breeders prefer slightly smaller litters, as they achieve lower piglet mortality and higher birth and foster weights during lactation. Finally, the moral and ethical aspects of breeding outcomes must be kept in mind. The inclusion of the five domains of animal welfare in intensive pig farming and piglet rearing is refocusing attention and care in order to improve rearing conditions. In essence, this is not just about preventing thirst, hunger and disease, but also about creating an environment in which they can express their complex natural behaviors and ensure their mental health, as well as, of course, making sure that the animals do not suffer or feel pain [101].

With sentient animals such as pigs, we want to provide them with as many positive experiences as possible, so that their offspring can also express positive emotional states [102]. In addition, animals should not be afraid of the breeder. When moving piglets during fostering or when artificially rearing piglets, it is extremely important that there is a positive human–animal relationship [3,42]. The breeder must be gentle, careful and attentive when handling the piglets, understand and know the animals’ needs and thus minimize stress for both the sow and her piglets.

Can CF and artificial piglet rearing fulfill all the animal welfare criteria?

The answer is perhaps not straightforward. If piglet nursing is considered an important aspect of animal welfare, it could be addressed through various fostering methods, ranging from the use of nurse sows to double nursing.

Artificial piglet rearing without the presence of a mother (biological or non-biological) is far more difficult, particularly when using milk replacers in an artificial environment, with “familiar” or even unknown piglets in a small group. Both methods are associated with a lower level of animal welfare, as they prompt the development of undesirable behaviors (

Table 3. Factors to consider in implementing artificial piglet rearing and its impact on welfare.

Item	Factors	References
Five-domain model of farm welfare for young pigs		[101]
Domain1	Nutrition
Domain 2	Physical environment
Domain 3	Health
Domain 4	Behavior
Domain 5	Mental state
Animal welfare level of artificial piglet rearing
Poor	More fights, less play, belly nosing, oral manipulation of ears and tails of littermates, short resting time. Plastic trough, a nipple, and a nipple with an added bag of sterile water.	[103] [76] [46,104] [105]
Improved with dummy use	Combination of massage–suckling dummy reducing undesirable behavior and prolonging resting time.	[106]
Pen and feeding system
Rescue deck	Enclosure for artificial rearing (piglets are feed artificially).	[76]
Automatic feeding system	Automatic system for feeding piglets with colostrum.	[50]
Piglets feeding	Milk replacer
	3 days	[107]
	3–6 days	[103]
	7 days	[76]
	7 and 21 days	[105]

). Undesirable behavior manifests itself in the form of aggression, more frequent fights for the sow’s teats (whether from a biological or foster mother), and a greater number of injuries to the body and head. Under the conditions of artificial rearing, undesirable piglet behavior, leading to belly nosing and injuries to the ears and tails of littermates, is emphasized.

5.1. Split-Suckling

In large litters, split-suckling enables all piglets in the litter to be supplied with sufficient colostrum [108]. It is a special type of litter management, in which all the piglets do not have access to the mammary complex all the time, namely in the first 24 h [96] (Table 2) or even in the first 72 h after birth [109]. The litter can be divided into two or more groups, where lighter piglets are allowed to suckle or reach the glands with colostrum and at different times to heavy littermates [94]. Some of the heavier piglets from the litter are removed for a certain time, for example, 1 [96] or 3 h [109], so that the remaining piglets can suckle undisturbed. As the composition of the colostrum changes rapidly and the intestinal wall permeability for large molecules is also time-sensitive, it makes sense to perhaps perform split-suckling in the first 12 h after birth and then perform CF within 24 to 72 h after farrowing [109].

5.2. Nurse Sow

In case of sow death or agalactia, an alternative strategy is the use of a nurse sow [43]. Likewise, in the case of hyperprolific sows, there is a continuous excess of piglets in relation to available teats, so an alternative solution is required. One- and two-step nurse sow systems provide possible solutions [42] (

Table 4. Abstract of recommendations from the aforementioned literature for managing large litters with low-birth-weight piglets.

Item	Factors
Number of piglets born alive: number of functional teats	Number of piglets born alive must match number of functional teats.
Light piglet birth mass	<1000 g
Colostrum yield per litter	2.4 kg to 7 kg
Colostrum intake per piglet	≥200 g in 24 h
Make a decision on the further management strategy for sows and their piglets	Within-litter variability in birth mass (light piglets’ problem) and/or surplus piglets.
Time of colostrum intake postpartum (hours)	The first 6 h are very important; then, the composition of colostrum changes.
Split-suckling	Allow low-birth-weight piglets access to the teats to suck colostrum; at the same time, the other piglets are then unable to suck the mammary glands.
Cross-fostering (hours postpartum)	It is performed between 24 and 48 h postpartum, after the piglets have sucked colostrum from their biological mother. Piglet number equalization. Piglet size (birth weight) equalization. If possible, transfer piglets from primiparous to multiparous sows.
Nurse sow	A sow that has already weaned her own piglets; the sow nurses foreign piglets until they are weaned. Transfer piglets to mid-parity nurse sow (parities 3 to 5) to avoid poor reproductive results in younger sows in the next cycle. Total lactation should not last more than 40 days. The procedure can be carried out in two steps.
One step	A sow nurses her own piglets up to 21 or 28 days. On the 28th day of lactation, the sow then receives piglets from another sow that are at least 12 h old.
Two steps	A sow nurses her piglets up to 21 or 28 days. On the 28th day of lactation, she then receives piglets from another sow that are 4 to 7 days old; the sow who birthed these piglets receives a surplus of newborn piglets from other sows.
Double nursing	Double nursing sows simultaneously nurse two litters alternately from birth to weaning: their own litter, for example, half the day and newborn piglets from another litter or weaker ones during the rest of the day.

). In one-step management, at least twelve-hour-old, large foster piglets from large litters are relocated to a lactating nurse sow; after a minimum 21 days of lactation, her piglets are weaned and moved to a weaner facility. The nurse sow rears this second litter for at least the next 21 days; the piglets are then weaned, and the sow returns to a dry unit. Attention must be paid to prolonged lactation length, perhaps up to the 40th day and especially in young sows, so that their condition—and thus the reproductive success of the next cycle—does not deteriorate. The consequences of prolonged lactation will manifest in the next cycle as an increase in unproductive days due to reproductive failure [110,111].

In addition to one-step rearing, the two-step nurse system is another possible means of saving so-called surplus piglets [42,69,95]. In this strategy, the sow nurses her litter until at least the 21st or 28th day. After weaning, the sow then continues lactation with 4–7 days old piglets from another so-called interim sow, who receives a “surplus” of newborn piglets. This two-step system is not ideal; it is controversial because, as we have described in previous chapters, the bond between a mother and her piglets is already forming during this period. On the other hand, between the 5th and 7th day of life, the piglets have already established a teat order. All this intervention and mixing is problematic from an animal welfare point of view.

5.3. Double Nursing

Double nursing sows simultaneously nurse two litters alternately from birth to weaning: their own litter, for example, for half the day and newborn piglets from another litter, or weaker piglets, during the rest of the day [112] (Table 4). It has been noted that the mortality rate of piglets decreased slightly using this strategy, but double nursing had negative effects on the sow’s reproduction in the next cycle. Such sows had a smaller litter at the next farrowing.

5.4. Artificial Rearing

When sows give birth to more piglets than they have functional teats available for, so-called artificial rearing for surplus piglets is also a possibility. This technique means that, from a certain day after birth, piglets are reared entirely without the presence of the mother or other sows; instead, they are fed with milk replacers.

The use of milk replacers allows for reduced piglet mortality in the first days after birth, ensuring adequate nutritional supply for large litters. According to the results, this can be a good alternative to the use of nurse sows [46]. However, it has been observed that piglet behavior-related problems can be created when using this method, indicating poorer welfare status (Table 3). However, giving piglets milk replacer in addition to their natural suckling does not reduce suckling motivation. In large litters, however, fights over teats and suckling pairs are more frequent, disrupting the natural model of suckling. This is also associated with injuries to piglets and sows’ teats [104]. To reduce this type of behavior as much as possible, similar pig mother gland dummy adaptations can be used (Table 3). After suckling from biological mothers for 3 days, piglets were transferred to artificial rearing conditions: a plastic trough, a nipple and nipple with an added bag of sterile water [107]. This last form of artificial rearing enables piglets to suck and massage teats, reducing littermate-oriented behavior and leading piglets to rest more quickly [106]. Another study on piglets aged 3–6 days housed in artificial systems (Rescue Deck) (Table 3) showed that they exhibited more undesirable behaviors such as belly-nosing in comparison to those reared with a sow in a loose farrowing pen [103]. In this study, it was found that the frequency of these behaviors increased over time, with piglets showing less play and more aggression and spending less time resting.

The fact that artificial rearing has negative effects on piglet behavior in the pre-weaning period is also confirmed in another study, where 7-day-old piglets were fed milk replacer until they were 26 days old. During this time, their behavior was studied and compared with groups of piglets reared with sows [76]. This study also showed that piglets reared with milk replacer showed more belly-nosing, more visits to the milk cup and oral manipulation of the ears and tails of littermates than those in litters with a sow. Moreover, they reached a lower body weight than piglets reared with their mother (6.53 kg vs. 7.97 kg). Artificial rearing can also involve older piglets. In the study conducted by Han et al. [105], 7- and 21-day-old piglets were fed with milk replacer until 35 days of age. Those in the 7-day-old group consumed milk less frequently, explored their environment and played less, and were observed to more often engage in belly-nosing and oral manipulation with other piglets. In both cases, 7- and 21-day-old piglets admitted to the artificial rearing system showed the negative impact of this intervention on behavior in artificially reared piglets.

In practice, the manual preparation and feeding of milk replacer has many limitations: the temperature of the replacer is often room temperature, rather than sow body temperature. Furthermore, soon after mixing, thick particles start to settle at the bottom of the container, blocking the passage of the artificial teat. It is thus difficult to ensure the smooth suction of fresh preparations. Ultimately, although there are many kinds of milk replacers, some do not have a taste or appearance that is appealing to piglets. In addition to this manual system, there is also an automatic milk replacer preparation and feeding system. Such a system consists of several parts, with milk powder mixed with water in a central tank before being transported via pipes to the farrowing pens for piglet consumption [96].

Piglets fed only milk replacers were found to have a leaner body composition at weaning [113], suggesting that these are not fully adapted to their needs. The authors also note that no major differences in inflammatory cytokine concentrations and the number of cells involved in the immune response were found between piglets reared by a sow and those reared artificially. However, even these results cannot be properly compared, as the piglets that consumed milk replacer also received antibiotics due to diarrhea.

The rearing of piglets in an artificial rearing system with milk replacer is by no means a complete substitute for a sow and the nutrients contained in the milk she secretes.

Animal Welfare in Artificial Technologies

In order to manage large litters, breeders must implement technologies that make it difficult to adequately address all areas of animal welfare. The five-domain model for on-farm welfare for young pigs was recently proposed [101] (Table 3): domain 1: nutrition, domain 2: physical environment, domain 3: health, domain 4: behavioral interaction, and domain 5: mental state.

It can be argued that all five postulates are, to some extent, undermined in artificial piglet rearing. Piglets are separated from their mother after birth; are placed in an unfamiliar environment; are given a different type of food (milk substitute); are unable to develop normal behaviors with their mother and peers; display altered behavior associated with less play and more aggressiveness; and develop stereotypical behaviors associated with belly-nosing. All of these can be classified into domains 1, 2 and 4. Together, these domains are most likely linked to piglet health (domain 3), particularly to their mental state (domain 5). Similarly, many studies report problematic piglet behavior in these artificial rearing systems, which consequently indicates a poorer level of animal welfare (Table 3). In practice, it is very difficult to keep track of the five proposed domains, as well as to implement them in their entirety. Many farms come close to meeting animal welfare requirements; however, we are by no means able to meet all the requirements everywhere, including in artificial piglet rearing.

Therefore, ongoing refinement of piglet rearing environments and large litter management strategies is essential in order to meet the holistic welfare standards outlined in the five-domain model.

6. Conclusions

This review article examines the complexity of litter management in domestic pigs. Such complexity mainly arises from past intensive selection for larger litters, resulting in a surplus of piglets relative to the number of functional teats available and increased litter variability. Various factors that must be considered in litter management, as well as possible strategies to address them, are presented, including their respective advantages and disadvantages. One crucial aspect that should not be neglected when managing litters is colostrum—its production, composition, function, timely ingestion, and overall vital role for piglets. Accordingly, from a practical perspective, it is important that litter management does not prevent piglets from suckling sufficient colostrum in the first hours after birth, while the intestines are still fully permeable to immunoglobulins and adequate immune resistance and higher survival rates can be ensured. Once the piglets have consumed the colostrum, interventions in the litter—such as cross-fostering, split suckling, or other methods discussed in this review—should be carried out within 24 to 48 h of farrowing, with fostered piglets preferably transferred to sows of mid-parity. Furthermore, litter characteristics (such as litter size and body mass variability) and piglet properties (such as age and body mass) should be considered before making any changes to the original litter. Finally, an important issue in litter management is animal welfare. In this regard, some solutions, such as artificial piglet rearing, have been found to significantly compromise welfare. Therefore, improving welfare by optimizing litter management remains a challenge for future pig production systems. Ultimately, selection to increase litter size should be reconsidered because, in addition to unresolved technological and animal welfare issues, oversized litters with high mortality rates, usually among light piglets, raise ethical and moral concerns.