The weaning transition is a challenge in swine breeding husbandry. To maximize profits, early-weaning strategies at 3–4 weeks of age have been widely adopted on commercial farms. Such sudden transition of diets predisposes piglets to diarrhea, aberrant behaviors, growth stasis, and even more detriments associated with weaning stressors, eventually leading to production loss (Lallès et al., 2007; Summers et al., 2019). To address these concerns, creep feed is provided to familiarize the piglets during the suckling period with the solid feed after weaning and partly release the pressure for the lactating sows (Miller et al., 2012; Novotni-Dankó et al., 2015).

Although consumption of creep feed can lead to higher feed intake after weaning with concomitant greater growth performance (Bruininx et al., 2002; Pluske et al., 2007; Muns and Magowan, 2018), the application of creep feed still comes with variable feed intake, limited impact on the gut maturation during the pre-weaning period or the later life phases of piglets (Sulabo et al., 2010; Van den Brand et al., 2014). Meanwhile, the presence of maternal fecal semiochemicals in farrowing crates is also appealing to piglets (Aviles-Rosa et al., 2020), and gastrointestinal microbiota is primarily influenced by maternal and environmental factors (Lührmann et al., 2021; Nowland et al., 2021). Therefore, there is room to improve the early solid nutrition of piglets to activate their appetite and gut maturation.

In a semi-natural environment, piglets living outdoors already begin chewing straw and rooting during the second week of life (Schouten, 1985; Petersen, 1994). Our previous research on feral piglets showed spontaneous solid feeding intake from the first week of life, primarily leaves and stems, accounting for 83% of their stomach contents. This intake of fibrous plant matter was associated with a stronger stomach development (Van Hees et al., 2022). Compared to feral piglets, farm-raised piglets are usually only able to access creep feed formulated as being nutrient-dense and digestible. Therefore, the inclusion of fibrous content in creep feed is a promising avenue to explore, and it was reported to support the piglets’ behavior, including recognition memory, and frequent exploration and interaction with littermates (Clouard et al., 2018; Fleming et al., 2019). Meanwhile, in terms of physiology, a study by our group showed that the addition of dietary insoluble fiber in creep feed activated the colonic fermentation of short-chain fatty acids (SCFAs) and increased large intestinal size and fill, which strengthened the health of the digestive tract (Van Hees et al., 2019). These studies included the fiber sources as a ground ingredient mixed in the creep feed, which is obviously different from the intake of structure-rich plant parts in the wild. Grass hay is rarely fed to farmed pigs, but a common part of the natural diet of wild boar (Groot Bruinderink et al., 1994).

Because of the above, we wanted to mimic a part of the natural piglet diet by providing grass hay to suckling piglets. The objective of this study was to investigate whether a grass hay supplement would stimulate the appetite of piglets and foster development of a stronger gastrointestinal tract from an early age, enabling them to cope with weaning stress and achieve better performance in later phases.

The housing and rearing of experimental animals were in compliance with European Union Directive 2010/63/EU. The protocols and procedures of this study were approved by the Ethics Committee of Flanders Research Institute for Agriculture, Fisheries and Food (ILVO), with application number 2022/420, and all animal experiments complied with the ARRIVE guidelines 2.0.

Three piglets died within 3 days after treatment due to crushing and they were excluded from the results. Generally, according to daily monitoring, the animals in this study exhibited good clinical health throughout the experiment.

This study investigated the impact of a separate provision of grass hay on supporting suckling piglets development. Our findings showed that the inclusion of a separate feeder containing chopped grass hay in the farrowing crate modified the metabolic profile and growth of the gastrointestinal tract. Fiber enrichment has been extensively tested in pig feeding due to its potential benefits, and selective inclusion of fiber was shown to promote GIT development and modify gut microbiome and immune status in finishing pigs and sows feeding (Jin et al., 1994; Mcglone and Fullwood, 2001; Claus et al., 2007; Heinritz et al., 2016). Unlike other feeding phases, appetite to solid feed of piglets in the suckling phase is markedly variable and unpredictable not only between litters but also within the same litter, even when providing what is considered a palatable formula (Pajor et al., 1991; Sulabo et al., 2010; Middelkoop et al., 2019a).

Therefore, the consistent increase in creep feed intake upon grass hay provision, is an important finding for practice. Other studies on inclusion of fiber in creep feed so far only showed numerical changes in creep feed intake (Hanczakowska et al., 2008; Van Hees et al., 2019), suggesting that the physical form of the grass hay in addition to its chemical traits may be important to affect voluntary solid feed intake in young piglets. Other types of insoluble fiber, such as wheat bran, have been observed to speed up the gastric mobility. This acceleration plays a crucial role in the gut–brain axis, regulating appetite signals to activate the feed intake and behaviors, which might contribute to the result in the present study (Benini et al., 1995; Inui et al., 2004; Li et al., 2023). Furthermore, it also might be interesting to investigate if the impact of feeding hay depends on the birth weight of piglets in a study involving more litters. The presence of extra feeders with diverse feed resources in the farrowing crate is capable of stimulating the suckling piglets’ exploring and foraging behaviors (Middelkoop et al., 2019b). This, in turn, might lead to a reduction in the competition among piglets for teats access, as it is known that piglets with heavier birth weight are known to consume more milk per suckling event than their lighter littermates (Campbell and Dunkin, 1982). However, some other previous studies demonstrated that the supplementation with dietary fiber could suppress the appetite (De Leeuw et al., 2008; Jarrett and Ashworth, 2018). It may be attributed to the fact that fermentation of SCFAs in the colon can stimulate the secretion of gut–brain peptides and mediate the appetite signals via the vagus nerve (Bolognini et al., 2021). Whether this pathway is primarily activated via the fermentation in colon rather than other sections of large intestine needs to be fully elucidated, and non-pronounced change of colonic microbiome occurred in this study, playing a key role in regulating host’s appetite, might also erase the decreased feed intake. Therefore, the effects seen in the GH group may go beyond the direct impact of fiber consumption.

The weight of visceral organs can reflect the piglet’s maturation process (Elefson et al., 2021), but the grass hay provision induced no effect on stomach, liver, and spleen weight. Len et al. (2009) studied fiber levels and sources in weaned piglets and also found no effect on liver weight, yet a higher stomach weight after 33 days of feeding. Studies on the effect of fiber on spleen weight are limited, but in general, the development of spleen and liver remains closely related to the body weight growth from birth to weaning (Lanferdini et al., 2018; Elefson et al., 2021). It is worth noting that a sufficient duration of exposure to the diet might also play a role in its effect. For instance, pigs provided with high-fiber diet for an extended period had heavier weights of liver and kidney weights compared to those on a lower fiber diet, but this effect was not as pronounced with a shorter feeding duration (Pond et al., 1988; Anugwa et al., 1989). Given the fairly short length of the suckling phase combined with the low intake in commercial farming, this aspect is hard to cope with. The response on stomach weight to insoluble fiber appears to be faster than other organs due to the bulking effect in weaned piglets (Rijnen et al., 2001; Len et al., 2009), although—as in our study—supplementing suckling piglets with cellulose and other fibers in creep feed did not alter the empty stomach weight (Van Hees et al., 2019; Choudhury et al., 2021).

Growing pigs fed high-fiber diets typically exhibit a larger gastrointestinal tract than those fed a common diet (Jørgensen et al., 1996; Freire et al., 2003; Pluske et al., 2003). We here demonstrated that this also occurs in suckling piglets when fed grass hay. This enlargement might be attributed to water-retentive and “bulking” properties of (insoluble) fiber (Tungland and Meyer, 2002; Dhingra et al., 2012), as observed in previous studies applying cellulose and oat hulls in creep feed (Van Hees et al., 2019). The absence of a significant difference in relative weight of small and large intestine might also suggest that the increased weight of intestine may not be sufficient to influence the overall carcass weight. Not only the macroscopic anatomy but also the microscopic architecture was affected, mainly shown by an increased ileal villus:crypt ratio, likely resulting from increased digestion of nutrients in the jejunum (Nigam et al., 2019). This observation may therefore reflect increased proximal nutrient digestion and absorption, rendering less challenge of undigested matter to the distal intestinal sections, hence explaining its higher villus:crypt ratio. Some of the changes in the villus:crypt ratio may originate from a response to intestinal injury induced by various agents, such as digestive pressure, promoting crypt hyperplasia (Da Cunha Ferreira et al., 1990; Potten, 1990; Pizarro et al., 2000).

A well-developed large intestine is essential for piglets absorbing fluid, electrolytes, and remaining nutrients, including SCFA, from the proximal sections efficiently. The physical barrier of the intestine also helps against microbial invasion (Williams et al., 2001; Xu and Cranwell, 2003). The 12% heavier and 10% longer large intestine due to grass hay thus is an interesting trait. Moreover, it coincides with increased production of SCFA, generating additional fuel for large intestinal enterocytes and cell proliferation (Scheppach, 1994). Further study is needed to identify the cause of the more proximal shift in fermentation site due to grass hay, but it is well-studied that cellulose-rich material on itself is poorly fermentable in pigs (Bachmann et al., 2021), especially young individuals. Hence, the likely explanation for the fermentation shift is an effect of the grass hay on the passage of the fermentable fractions of the ingested creep feed and milk oligosaccharides.

Abimosleh et al. (2012) have reported that induced colitis was accompanied by deeper crypt depth in rats, and imbalanced cell proliferation is also a risk factor of neoplasia (Deschner and Maskens, 1982). However, one trait such as crypt depth within a complex population of intestinal cells may not represent the entire function of the LI (Snippert, 2016). While this perspective is insightful regarding the current study, further investigation is required for confirming the association between the structural characteristics of LI and its optimal functioning, such as cell proliferation, nutrient absorption, and other relevant factors.

Despite marked changes in intestinal anatomy and morphology, consumption of grass hay did not exert major changes in colonic microbiota. Plenty of studies have reported the effect of fiber on the microbiota on weaning or fattening pigs (Kraler et al., 2016; Onarman Umu et al., 2018; Fouhse et al., 2019). Unlike older pigs, observing changes in microbiota composition in the suckling piglet intestine is challenging due to various factors such as creep feed patterns, the genital tract and feces of sow influence, genetic interactions, and pen environment, all of which contribute to the complex microbiota community (Kubasova et al., 2017; Chen et al., 2018). The trend of a lower alpha-diversity with GH in the colon may relate to the proximal shift of fermentation discussed higher. At the same time, the abrasive function of insoluble fiber may slough the epithelial mucus together with the microbes (Molist et al., 2009; Chen et al., 2013). The trend of increased abundance of Lactobacillus in the colonic microbiota when fed grass hay is in line with observations in rats and humans fed insoluble fiber (Gibson et al., 1995; Zhong et al., 2015), but in general, it is surprising that the microbiome was hardly affected by the grass hay intake.

Although there are still many lingering questions regarding long-term effects, feeding grass hay to suckling piglets shows promise and warrants further exploration in practical settings. Larger-scale studies will need to clarify if performance differences during suckling as well as throughout the entire fattening phase will appear.

Piglets readily consume grass hay in the pre-weaning period. Access to chopped grass hay during the suckling period in a separate feeder stimulated the feed intake of creep feed and promoted intestinal growth. Moreover, the consumption of grass hay exerted a more proximal shift in intestinal fermentation without prominent changes in microbial communities.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation. The sequencing data are available in the NCBI Sequence Read Archive (SRA) under accession: PRJNA1043179.

The animal studies were approved by Ethics Committee of Flanders Research Institute for Agriculture, Fisheries and Food (ILVO), with application number 2022/420 and all animal experiments complied with the ARRIVE guidelines 2.0. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent was obtained from the owners for the participation of their animals in this study.

RY: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Writing – original draft. AC: Conceptualization, Data curation, Formal Analysis, Investigation, Writing – review & editing. HH: Methodology, Writing – review & editing. KC: Methodology, Writing – review & editing. AM: Methodology, Writing – review & editing. MA: Methodology, Writing – review & editing. DM: Methodology, Supervision, Writing – review & editing. GJ: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Writing – review & editing.