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Among the physiological limitations of piglets post-weaning is poor secretion of hydrochloric acid, which is essential for protein digestion. This, combined with large meals, makes it difficult to achieve a gastric pH between 2 and 3, creating a high risk that the protein will reach the intestine undigested, providing a substrate for the growth of pathogenic bacteria and consequently leading to digestive problems.
Therefore, when it comes to feed presentation, we look for ingredients that provide nutrients and also functionality, with the aim of:
- increasing gastric retention time to facilitate protein hydrolysis,
- increasing contact between feed and enzymes,
- and reducing the fermentation of undigested protein in the intestine.
To achieve this, we can consider several factors related to feed presentation: 1) viscosity, 2) particle size and distribution, 3) pellet size, and 4) pellet hardness and durability.
1) Viscosity: To increase gastric retention, the rheological properties of raw materials, such as viscosity, can be modified to achieve a balance between digestive safety and satiety levels. If the diet is too dense (low viscosity), the pig will feel full and reduce its intake.
Thermal processing allows the viscosity of ingredients to be modified, although its effect varies depending on the type of ingredient.
For example, under the same heat treatment, the viscosity of barley and wheat increased, while no significant changes were observed in corn (Huting et al., 2021; Figure 1). This is due to the higher content of soluble non-starch polysaccharides present in barley and wheat compared to corn. Due to their higher solubility, these compounds increase viscosity after thermal processing, which could lead to longer gastric retention time.
However, Martens et al. (2019) demonstrated that the gastric digestion process alters the physicochemical properties of feed, complicating the prediction of its behavior in the gastrointestinal tract. In a recent study, the potential to predict retention time and digesta segregation throughout the gastrointestinal tract based on the rheological properties of feed was evaluated. The results obtained indicated that the water retention capacity of feed can be useful for predicting the segregation of digesta in the stomach, while the rheological properties of feed failed to predict its behavior throughout the intestinal tract (Dorado-Montenegro et al., 2025).
2) Particle size: Controlling feed particle size is important because it influences gastric and intestinal function, digestibility, and gastrointestinal health. Recent scientific studies show that, at the start of the nursery phase, coarsely ground feed can be beneficial in promoting gastric retention and reducing the risk of digestive disorders. But what do we mean by coarsely ground? Warneboldt et al. (2016) obtained gastric pH values of 2.5 with a coarse meal diet containing 53% of particles > 1 mm, while with a fine pellet diet containing 54% particles < 0.2 mm, the gastric pH was 5. Other studies have observed that, in terms of intestinal health, the inclusion of 4% coarsely ground wheat bran (1088 µm) reduces E. coli adhesion during the 2 weeks post-weaning and increases the production of short-chain fatty acids in the distal small intestine (Molist et al., 2010).
To control particle size and distribution, it is recommended to work with diets containing between 7.5-10% particles > 1.5 mm to promote gastrointestinal health in piglets after weaning.
During the pelleting process, particle size is reduced due to physical and thermal forces. Naeem et al. (2024) observed a 124% increase in particles smaller than 0.5 mm after the pelleting process.
Analyzing post-pelleting particle size distribution is complex but crucial for digestive health and productivity.
Naeem et al. (2024) developed an equation for predicting post-pelleting particle size based on pre-pelleting particle size, although its practical application is difficult due to the variability in the quality of raw materials. On the other hand, Vestyllands Andel created “VA SizeMatters” in 2021, an automatic particle size measurement system for each production batch. Thus, we have tools such as the prediction equation and the automatic particle distribution measurement system, which can be useful when combined.
3) Pellet size: Most of the swine industry provides small pellet feed (1.8 mm in diameter) during the first days post-weaning. However, scientific studies show that piglets prefer a 12-13 mm pellet compared to a 2-3 mm pellet (Van den Brand et al., 2014; Clark et al., 2015), and higher growth and intake with lower mortality at 21 days of age have been observed in piglets fed 9x12 mm pellet feed compared to 4x4 mm feed (Craig et al., 2020).
4) Pellet hardness and durability: According to the results mentioned so far, weaned piglets prefer coarse-ground pellets with a large pellet size, while also meeting quality standards for durability and hardness, with values of >95% and 1.8-2.3 kg/mm, respectively. In fact, higher intake has been observed throughout weaning as feed hardness decreased from 3.1 to 1.8 kg/mm (Molist et al., 2021).
At this point, the following question arises regarding pellet quality: Can we acheive a large, high-quality pellet with coarse particles? If coarse grinding of the cereals is done to reduce the fine particle content after pelleting, the quality of the pellets could be negatively affected because the coarse particles create “weak spots” in the pellet structure (Thomas and van der Poel, 1996).
One possible strategy for achieving structure is combining a hammer mill with a roller mill. It has been observed that pellet durability decreases with coarser grinding of corn in a hammer mill, compared to grinding in a roller mill at different sizes (Vukmiroic et al., 2015, Figure 2).
Another strategy for achieving high-quality pellets could be to increase the space between the roller and the die. On a practical level, distances of 0.3-0.5 mm are currently being used, but it has been observed that increasing this distance to 1.15 and 2 mm can maintain pellet quality while increasing the hammer mill diameter from 3 to 6 and to 9 mm (Vukmiroic et al., 2016, Figure 3). However, this increase in distance can lead to problems with clogging the press, reduced manufacturing speed, and increased manufacturing energy requirements.
General reflections
During the first few days after weaning, it is essential to encourage feed intake while simultaneously providing a nutrient-rich feed with functional properties that promote gastrointestinal health and nutrient digestion and absorption. According to recent studies, and in terms of feed presentation, a feed that promotes gastric retention and stimulates post-weaning intake, while also protecting digestive health, translates into a large-sized pelleted feed with an intermediate hardness of 1.8-2.3 kg/mm and a coarse grind profile (7.5-10% of particles larger than 1.5 mm). However, this type of feed presents a manufacturing challenge.
